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NATURAL GAMMA ACTIVITIES INT GLACIMARINE SEDIMENTS: CORRELATIONS
WITH TERRESTRIAL SOURCE DATA
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
To those who endured it with me
First, I would like to thank Dr. John Jaeger for his patience and help throughout the entire
proj ect. I would also like to thank those on the R/V Alpha Helix science crew who did the
sampling for this study, and to Gillian Rosen for her countless hours of consult. I would like to
thank Dr. Mike Perfit and Warren Grice for their help with petrographic analysis, and Dr. Guerry
McClellan for help with XRD analysis. A special thank you goes to my family, friends, and
those at Geohazards, Inc. for their support throughout this process. Lastly I want to give a
sincere thank you to Scott Purcifull and Nicole Yonke, without whom this proj ect never would
have been finished.
TABLE OF CONTENTS
ACKNOWLEDGMENTS .............. ...............4.....
LIST OF TABLES ............. ....._.. ...............7....
LIST OF FIGURES .............. ...............8.....
AB S TRAC T ............._. .......... ..............._ 1 1..
1 INTRODUCTION ................. ...............13.......... ......
2 BACKGROUND .............. ...............24....
Regional Geology .............. ...............24....
Icy B ay............... ...............25..
Resurrection Bay .............. ...............25....
Regional Glaciation .............. ...............25....
Sedimentation .............. ...............26....
Transport and Deposition .............. ...............27....
Yakataga Formation ................. ...............28................
Valdez Group ................. ...............29.................
Mineralogy ................. ...............29.................
Radioisotopes ................. ...............30.................
3 METHODS .............. ...............36....
Sampling ................. ........... ...............36.......
Radioisotope Evaluation............... ...............3
Grain Size Separation .............. ...............39....
Mineral ogy ............... .........__ ...............40....
Microscopic Evaluation............... ...............4
X-Ray Diffraction............... ..............4
4 RE SULT S .............. ...............46....
Radioisotopic Analysis .........____............. ...............46.....
Mineralogy and Physical Properties ................ ...............47......_.__....
Core 249PC .............. ...............47....
Core 223 BC .............. ...............47....
Grain Size .............. ...............48....
5 DI SCUS SSION ............. ...... .__ ...............65..
G rain Size .............. ...............65....
Com position............... ..............6
Elemental Concentrations ............. ...... .__ ...............67...
238U .............. ...............68~~~~
232Th ............. ...... ._ ...............68...
Clay M ineralogy .............. ...............71....
Th/K Ratios............... ... .. .. ...........7
Possible Alteration/Biasing of Signal ........._.. ....___........_. ...........7
Correlation with Aeroradiometric Data ................. ...............74................
Correlation with Geochemical Data .............. ...............75....
6 CONCLU SION................ ..............7
LIST OF REFERENCES ................. ...............82......_.._ ....
BIOGRAPHICAL SKETCH .............. ...............90....
LIST OF TABLES
2-1 Half lives and average abundances of relevant radioisotopes ................. ............... .....35
3-1 Precision data for associated each radionuclide measured. The largest mean
deviation about the mean measured was chosen to obtain greatest accuracy. ...................41
3-2 Elemental concentration data normalized to clay percent. The measured (original)
concentrations are also listed for comparison. Only bulk sample measurements are
presented. ............. ...............44.....
4-1 Radionuclide concentration data and associated error of each bulk sample within
each interval ............. ...............49.....
4-2 Specific concentrations of elements within each core .............. ...............53....
4-3 Percent of clay-, silt- and sand-sized fractions from the two cores. The averages for
each core are included at the bottom............... ...............64.
LIST OF FIGURES
1-1 Satellite image of Alaska coastal margin showing extent of glaciation and sample
environments (modified from MODIS Rapid Response Proj ect at NASA/GSFC, Gulf
of Alaska Science Plan 2004) ................. ................ ......... ........ ......... 17
1-2 Location and geological map of area surrounding core 223BC. Sediments within
core 223BC are sourced by the Guyot Glacier (courtesy of John Jaeger). ................... .....18
1-3 Location and geological map of area surrounding core 249PC. The core contains
sediment sourced from Bear Glacier (modified from Bradley and Donley 1995,
U SG S). .............. ...............19....
1-4 Published uranium geochemical data in southern Alaska from lake and river
sediment samples (modified from Weaver 1983) ................. ..............................20
1-5 Published thorium geochemical data of southern Alaska river and lake sediment
samples. (modified from Weaver 1983). ............. ...............21.....
1-6 Published potassium geochemical data from southern Alaska lake and river sediment
samples (modified from Weaver 1983). ............. ...............22.....
1-7 Aeroradiometric data of study area. Source area surrounding core 249PC shows
elevated K and Th, while the environment near core 223BC shows elevated U and K.
Drainage basins are outlined. (modified from Saltus et al. 1999). ............. ...................23
2-1 Structural formations on the southern Alaska margin (modified from Plafker et al.
1994). ............. ...............3 2....
2-2 Generalized sketch of the cross section near core 223BC. ................ .......................33
2-3 Generalized sketch of the cross section surrounding core 249PC. ................ ................33
2-4 Mean annual precipitation for the state of Alaska. Note that the area surrounding
Resurrection Bay (core 249PC) has a range of 1501-5000 mm/yr while the area
surrounding Icy Bay (core 223BC) shows levels from 3001-13000 mm/yr. (reprinted
with permission from Spacial Climate Analysis Service, Oregon State University
2000). ............. ...............3 4....
3-1 Decay series of the 238U radioisotope relevant to this study. ................ ............. .......42
3-2 Decay series for the 232Th isotope. .............. ...............43....
4-1 Concentration of uranium with depth in core 249PC. ........._._......___ ........._.....50
4-2 Concentration of thorium with depth in core 249PC. ......___ ....... ...............50
4-3 Radioisotopic concentration of potassium with depth in core 249PC. ........._.._.. .............51
4-4 Concentration of uranium with depth in core 223BC. ......___ .......___ ........._.....5 1
4-5 Concentration of thorium with depth in core 223BC. ....._._._ .... ... .__ ........._......52
4-6 Concentration of potassium with depth in core 223BC. ....._._._ .......___ ........._....52
4-7 Concentration of uranium with respect to percent clay for all intervals. After ~60%
clay-sized material, there is a general increase of uranium concentration with
increasing clay-sized material ...........__......___ ...............54....
4-8 Concentration of thorium with respect to percent clay for all intervals. .........................54
4-9 Concentration of potassium with respect to percent clay for all intervals. ......................55
4-10 Mineralogy of sand fraction within core 249PC interval 0-10 cm............... ..................55
4-11 Mineralogy of sand fraction within core 249PC interval 11-20 cm............... .................56
4-12 Mineralogy of sand fraction for core 249PC interval 21-30 cm. ............. ....................56
4-13 Mineralogy of sand fraction from core 223BC interval 15-16 cm............... ..................57
4-14 Mineralogy of sand fraction from core 223BC interval 20-21 cm. ................. ..............57
4-15 Typical image of core 249PC interval 0-10 cm showing mostly rock fragments
(designated RF) with associated quartz. Field of view approximately 0.8 mm. ................58
4-16 Image of biogenic material (designated BM) among rock fragments in core 249PC
interval 1 1-20 cm. Field of view approximately 0.8 mm. ......____ ..... ...._ ...........58
4-17 Oxidized coating on grain from core 249PC interval 11-20 cm. This interval was the
only one exhibiting coated grains. Field of view approximately 1.25 mm. ................... ....59
4-18 Images of biotite (designated B) and accessory minerals from core 249PC interval
21-30 cm. Field of view approximately 1.25 mm. ............. ...............59.....
4-19 Typical picture of core 249PC interval 21-30 cm showing large rock fragments and
quartz (designated Q). Field of view approximately 1.25 mm. ............. ....................60
4-20 Image from core 223BC interval 15-16 cm. Rock fragments dominate but there is an
increase in quartz and accessory minerals. Sand particles in this core are also more
angular in shape. Field of view approximately 1.5 mm ................. ................ ....__60
4-21 Typical image from core 223BC interval 15-16 showing elevated abundances of
plagioclase and amphibole, as well as increased quartz (relative to core 249PC)
among the dominant rock fragments. Field of view approximately 1.0 mm. .................. .61
4-22 Image of core 223BC interval 15-16 cm silt fraction. Field of view approximately
1.5 m m ............. ...............61.....
4-23 Image of biotite among rock fragments and quartz grains from core 223BC interval
20-21 cm. Field of view approximately 0.8 mm. ............. ...............62.....
4-24 Image of core 223BC interval 20-21 cm silt fraction. The silt fragments are larger in
general size as compared to the 15-16 cm interval of this core. Field of view
approximately 1.5 mm. ............. ...............62.....
4-25 XRD data for both cores with associated mineralogy. Core 249PC is offset (raised)
to better illustrate variations between cores ................. ...............63...............
5-1 Th/K ratio from core 249PC. ............. ...............77.....
5-2 Th/K ratio for core 223BC. .............. ...............77....
5-3 Overlay of Th/K ratio for both cores .............. ...............78....
5-4 Th/K ratio with percent clay for cores 223BC and 249PC showing little variation
between them ............. ...............78.....
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
NATURAL GAMMA ACTIVITIES INT GLACIMARINE SEDIMENTS: CORRELATIONS
WITH TERRESTRIAL SOURCE DATA
Chair: John M. Jaeger
Major Department: Geology
The provenance analysis of Eine-grained sediments is particularly important in continental
margin environments where Eine particles dominate the stratigraphic record. One area receiving
voluminous quantities of fine-grained material is the tectonically active southern Alaska margin,
where sediment derived by glacial erosion is accumulating at some of the highest rates globally.
Although the magnitude and rate of sediment delivery is known, little work has been done to
determine the terrestrial sources and surfieial processes responsible for spatial heterogeneities in
Natural gamma activities (238U, 232Th, and 40K) and mineralogy of two cores were
examined at differing locations within the Gulf of Alaska (GOA) in an attempt to distinguish
them using only these techniques. Cores were chosen based on their differing lithologies, one
core being comprised of material derived entirely from the Valdez Group of the Chugach terrane,
the other being comprised entirely of material from the Yakataga Formation (a ~5km thick
marine and glacimarine plastic deposit of unknown origin). Natural gamma activities were
measured on a Canberra UltraLow Background Planar-Style germanium detector, mineralogy
was determined by both XRD and petrographic analyses. In addition to bulk sample analysis,
samples were separated into sand-, silt- and clay-sized fractions to examine the association of
grain size and radioistopic activity. Elemental concentrations of 238U, 232Th and 40K were
compared to published geochemical river and stream sediment data.
Measurements in this study fall well within ranges of parent source material, revealing
their accuracy as a provenance tool. Radioisotopic activity measurements from each size
fraction reveal an association of 238U, 232Th, and 40K with the fine-sized fraction, particularly of
40K with clay-sized fraction. The similarity between both natural gamma activities and
mineralogy between cores suggest that sediments of both cores have the same source material.
The Valdez Group is a well-established member of the Chugach terrane, implying that the
glacially-derived sediments of the Yakataga Formation are also derived for the Chugach terrane.
Sediment source (provenance) identification is important in geological and environmental
management fields for basin analysis. It is an important tool in tectonic reconstructions and in
understanding weathering and transport processes, which help shape the topography observed
today. Sediment provenance is used to constrain the sedimentary processes from erosion to final
deposition, with the goal being to reconstruct parent-rock assemblages of sediments and the
climatic and physiographic conditions under which these sediments formed (Augustsson,
Fanning, Munker, Bahlburg, and Jacobsen 2003; Weltje and Eynatten 2004). The provenance
analysis of fine-grained sediments is particularly important in continental margin environments,
where these fractions dominate the stratigraphic record. One area receiving voluminous
quantities (~250 x 106 tons/y; Jaeger, Nittrouer, Scott, and Milliman 1998) of fine-grained
material is the Gulf of Alaska (GOA) margin, where sediment derived by glacial erosion is
rapidly accumulating (Figure 1-1). Although the magnitude and rate of sediment delivery is
known, there has been little provenance work done to determine the terrestrial sources and
surficial processes responsible for the spatial heterogeneities in accumulation patterns.
Geochemical characteristics of sedimentary rocks are known to provide important clues
to their provenance and depositional environments. During the last few decades, geochemical
study of sedimentary rocks has grown, particularly in the area of provenance and source
composition investigations (McLennan, Taylor, and Kroner 1983; Fedo, Eriksson, and Krogstad
1996; Kampunzu, Cailteux, Moine, and Loris 2005). Many provenance studies focus on sand
fractions or bulk sediment samples, with heavy mineral analysis and rare earth element (REE)
patterns being the dominant "fingerprinting" techniques applied (Basu 2002; Kampunza et al.
2005; Kairyte, Stevens, and Egidijus 2005; Nyakairu and Koeberl 2001). Single grain
techniques are used, but are only effective if their results can be firmly connected to the bulk
mass transfer (Andersen 2004; Kairyte et al. 2005). Other criteria employed to identify sources
include detrital thermochronology (using zircons or apatite), bulk composition analysis, and
analysis of magnetic properties (Boggs 2001; Hounslow and Morton 2004; Liu, Zhu, and Li
2003; Watkins and Maher 2003). Sediments from many depositional environments, however, do
not contain enough sand to make statistically significant petrographic determinations.
Geochemical approaches to sedimentary provenance analysis are therefore especially useful
where coarse sediment is scarce (McDaniel, McLennan, and Hanson 1997; Andrews and
Principato 2002; Kairyte et al. 2005).
Whereas REE and other trace elemental analyses are a preferred method of studying the
provenance of fine-grained sediments (Basu 2002; Weltj e and Eynatten 2004), they are time-
consuming and expensive, and thus are not ideal for higher spatial resolution studies of sediment
cores. For decades, the oil industry has used several wire-line logging tools (e.g., spectral gamma
ray, photoelectric index) to provide high-resolution, continuous proxies of elemental abundances
and mineralogy (Doveton 1994). Recently, Carter and Gammon (2004) used continuous gamma-
ray spectroscopy on cores from ODP site 1 119 on the Canterbury Margin of New Zealand to
show climatically controlled variability in the delivery of fine-grained 40K-rich glacial rock flour
from the Southern Alps. Geochemical provenance studies have been successful in using isotopic
data to determine provenance and paleoclimate (Lang Farmer, Ayuso, and Plafker 1993;
Schnyder, Deconick, and Boudin 2005) and fine-grained sediments have proved to record
accurately global paleoclimate evolution (Fabres, Calafat, Canals, Barcena, and Flores 2000).
Southern Alaska has been referred to as an in situ natural laboratory to study the
interaction of glacial and orogenic processes, tectonics, and continental margin sedimentation
(Jaeger et al. 2001). The focus of this research is to use geochemical data at two separate
locations (Figs. 1-2 and 1-3) within the GOA region to determine (1) if grain-size and/or
mineralogy plays a role in controlling naturally occurring radioisotopic activities (238U, 232Th,
40K), and (2) the ability of these data to differentiate between two unique terranes in order to
establish provenance. Detailed fluvial sediment geochemical data from stream and river
sediment samples is available for comparison (Weaver 1983). (Figs. 1-4 through 1-6) Also
available is an aeroradiometric survey map (Saltus, Riggle, Clark, and Hill 1999). (Figure 1-7)
The hypothesis is that 238U is associated with zircons and heavy minerals being carried
predominantly in the coarse (>63 Cm) fraction, and 232Th and 40K are concentrated in the fine-
sized fraction due to the presence of clays (illite, chlorite) and mica. If this is true, then bulk
rocks within the source area for core 249PC (which is composed of more fine-grained,
metasedimentary, flysch material of the Valdez Group) should contain lower 238U values and
higher 232Th and 40K. The data from core 223BC should contain more coarse material (from the
Yakataga Formation) and, therefore, contain higher levels of uranium relative to potassium and
thorium. Other factors such as clay mineralogy and diagenesis are expected to play a role in
radioisotopic activity, but will be minimal relative to grain size. If grain size does in fact play a
role in controlling radioisotopic activities, then it could be possible to determine provenance
based on this technique.
Core location is important due to possible influences from other sources, such as changes
in clay mineralogy, which might change the geochemical signature. The two cores selected are
each sourced by only one drainage basin. Core 223BC was taken 4 km from the ice front in Icy
Bay, and core 249PC was taken just outside the mouth of Resurrection Bay near the termination
of Bear Glacier. Based on core locations and associated drainage basins, material associated
with a source other than the Yakataga Formation for core 223BC or the Valdez Group for core
249PC is minimal. (Figure. 1-7)
Figure 1-1. Satellite image of Alaska coastal margin showing extent of glaciation and
sample environments (modified from MODIS Rapid Response Proj ect at
NASA/GSFC, Gulf of Alaska Science Plan 2004).
rT Gaore 0-300 m
Co Glac;E 1501180
-UPN GULFOF al pna Rver5101-5400
/C Braided Streams
10 0 10 20 km :
r 110 I(RI tI( I. DEPOSITS Alluvial. glacial. lake~. estuarine-. swamlp. landslide. 11...iJ p aln. and lar~h
U CPPER TERTIA\RY ROCK;S Sandstone, siltstone. shale. mudstone sand conglomerate of the \:la..uarga
0 Formation. M~iocene to PI~o..erie- in.u;.x
rmT.. XIDDLE TERTIARY' ROCK;S Silisiorne salnd-l.r, .In1J or ,ansUI shale of the Poul Cr~eek Formation.
e~ ~ ~ Oligacene to Mliocene in age.
(U ~ 1T ~LOW.ER TERTIARY. ROCKiS Intensely defrmend manri and contmentld clastic rock~s ofthe~
O F Kulthizth and Haydon Peak; Frm~ations, Paleocene to Eocene in age.
LOWER TERTIARY~ ALLFIC YOLCArNIC ROCKS Intensely deformed marine mudstones and
r, CRE'TACEO L S ROCKS Cou.~ lil. I :1 11! ch anld IInC ;iange mousel: unC isll Of the Yiakatart Giroup.
O ICRETACEOUIS AND L PPER JI'RASSIC AfETAMLORPHICS Connsist of unnamed phyllite slate,
Q,.. schist. gr~eeschist. amlphibolite. gaciss, and migmatite in St. ELias Mlountains.
Figure 1-2. Location and geological map of area surrounding core 223BC. Sediments within
core 223BC are sourced by the Guyot Glacier (courtesy of John Jaeger).
I Studv Area
GEOLOGIC MAP OF
rilionlal Park. lantal "
Meiugllool~uplcKll irciac lotec(;accous)
P'illow hasalt (Terdam? about 57 Ma)
B~asl shulemed dikes I lertionr.Lbout S7 Ma)
1 Glsabbr oler Ieingvko s7Ma) I a
IIMeamerphiiu maloks offa lhnd (Crelmnus?)~
rI IL I
- 2-19 PCI
il~l i .1r 1
0 5 10 MILES iI
O 5 10 15 KIL.OMPTERS
Figure 1-3. Location and geological map of area surrounding core 249PC. The core contains
sediment sourced from Bear Glacier (modified from Bradley and Donley 1995,
Figure 1-4. Published uranium geochemical data in southern Alaska from lake and river
sediment samples (modified from Weaver 1983).
Figure 1-5. Published thorium geochemical data of southern Alaska river and lake sediment
samples. (modified from Weaver 1983).
Figure 1-6. Published potassium geochemical data from southern Alaska lake and river sediment
samples (modified from Weaver 1983).
Figure 1-7. Aeroradiometric data of study area. Source area surrounding core 249PC shows
elevated K and Th, while the environment near core 223BC shows elevated U and K.
Drainage basins are outlined. (modified from Saltus et al. 1999).
Southern Alaska is a geologically complex area of accreted terranes representing relict
Paleozoic, Mesozoic, and Cenozoic arc-trench systems, oceanic plateaus, and flysch basins
(Figure 2-1). The landward side of the Pacific Plate boundary transform is a continental
assemblage of five fault-bounded terranes that were accreted to the North American plate in the
Mesozoic and Cenozoic. Additionally, the Yakutat terrane lies to the west of the Fairweather-
Queen Charlotte fault and is currently being accreted to southern Alaska (Monger and Berg
1984; Dobson, O'Leary, and Veart 1998). Underthrusting and accretion of the Yakutat oceanic
crust is apparent in a series of northeastward- to northward-dipping thrust faults. These include
the Chugach-St. Elias, Contact, and Border Ranges fault systems (Plafker 1987; Mazzotti and
The accretion of southern Alaska is summarized in steps by Hillhouse and Coe (1994).
The core of Alaska was produced by the collision of the Wrangellia and Peninsular terranes with
the Nixon Fork and Yukon-Tanana terranes during the interval 100 to 55 million years ago (Ma).
This produced the crust of south-central Alaska, the ensuing Kula plate motion then likely
provided the means to close the latitude gap between Wrangellia and the mainland. The
counterclockwise rotation of southwestern Alaska most likely occurred 68 to 44 Ma as the
latitude gap was closing. Volcanic complexes in the southern margin of the Chugach and Prince
William terranes were added to Alaska after 55 Ma, carried by the Kula, then Pacific plates,
respectively. Lastly, the ongoing accretion of the Yakutat microplate beginning around 30 Ma,
has led to the uplift of the Chugach-St. Elias ranges bordering the GOA. It is currently amongst
the most seismically and tectonically active regions in the world (Jaeger et al. 2001; Plafker,
Nockelberg, and Lull 1989). Interactions between the Pacific plate and overlying Yakutat
microplate with the North American plate near the coastal margin have produced regions of high
elevations and steep topography (Mazzotti and Hyndman 2002).
Icy Bay has a complex geometry due to the recession of the Guyot Glacier, which occupied
the bay until approximately 100 years ago. The Guyot, Yahtse, and Tyndall Glaciers have all
been receding since 1904 (Jaeger and Nittrouer 1999). The recession has opened up four smaller
fj ords within the bay that had previously been filled with ice, and has left a moraine in the lower
reaches of the bay. Sediment deposition within Icy Bay was studied extensively by Jaeger and
Nittrouer (1999) who found that sediment from meltwater streams of Malaspina Glacier draining
directly into the lower half of the bay greatly influence sediment input there. The drainage basin
sits almost entirely among sediments of the Yakataga Formation (Figure 1-2). The Guyot
Glacier is a tidewater glacier depositing directly into Icy Bay and sourcing the site for core
The Resurrection Bay area is characterized as a fjord coastline. Resurrection Bay is a deep
glacially eroded segment of the GOA coastline. Broad alluvial fans were built by several creeks
and the Resurrection River. Within a few hundred feet of shore steep slopes plunge hundreds of
feet to the ocean bottom. Sediments in this fj ord are derived from Bear Glacier, which overrides
topography comprised almost entirely of rocks of the Valdez Group. The 249PC core site is
located just outside the mouth of Resurrection Bay. (Figure 1-3)
Glacial activity has been an intricate part of forming the topography throughout Alaska. It
is described by Molnia and Hein (1982) as the single most important process controlling
sediment distribution in the GOA continental margin environment. The GOA area is bordered
by high coastal mountains, which trap abundant moisture off the north Pacific. The abundance
of glacial meltwater and rapid glacial motion lead to some of the highest erosion rates on the
planet (10' tons km2 -1l, Hallet, Hunter, and Bogen 1996). Glaciation in the area is extensive
and glaciers currently cover about 74,705 km2 (5%) of Alaska, half of which occurs in the Kenai,
Chugach, and St. Elias Mountains rimming the northern GOA (Calkin, Wiles, and Barclay 2001;
Sauber and Molnia 2003). Most glaciers in southern Alaska are characterized as surging
glaciers. The glaciers are more temperate compared to the Polar North Atlantic (Jaeger et al.
2001; Dobson et al. 1998).
Sedimentary deposits are instrumental in recording the geologic and climatic evolution of
modern environments. The history of uplift and glaciation in southern Alaska is recorded in
sedimentary deposits throughout the Gulf region (Plafker 1987; Martin 1993). High basal debris
loads (up to 1.5 m thick, Powell and Molnia 1989) and rapid glacial flow combine to produce
large volumes of siliclastic glacimarine sediment. Sedimentation rates from the coastal
mountains of southern Alaska have been estimated as the highest globally (Hallet et al. 1996;
Hunter, Powell, and Lawson 1996; Powell and Molnia 1989). This rapid sedimentation is due to
vigorous tectonic uplift, weakened bedrock, and heavy precipitation (Powell 1984; Hallet et al.
1996). Sediment delivery to the Gulf in southern Alaska is dominated by meltwater plumes in
fj ords and rivers emptying onto the shelf (Curran et al. 2003; Jaeger and Nittrouer, accepted;
Sharma 1979). Many streams originate at the termini of active valley glaciers and carry
sediment loads of up to >1g/1 (Molnia and Hein 1982). Dominant controls on tidewater
sedimentation (relevant to Icy Bay) are driven by seasonal fluctuations in meltwater discharge
Transport and Deposition
It is important to consider the history of the sediment to help constrain possible
environmental effects. Generalized images of the two sites are presented. (Figs. 2-2, 2-3) The
transport history of the particles analyzed from these two sites are relatively similar, because
both are from a temperate fjord environment in the GOA. Both contain sediments characterized
as rock flour, which were likely carried in meltwater from the glacier bed in a relatively dark,
cold, vegetation-free environment (Anderson, Longacre, and Kraal 2003). Initial weathering of
the source rock (Valdez Group for core 249PC, Yakataga Formation for core 223BC) preceded
erosion. Heavy storms and high rates of precipitation (in the form of rain and snow) increase
physical weathering in the Gulf environment, facilitating rapid erosion and transport of the
sediment. Much of the sediment was incorporated into the respective glaciers which continually
ground and crushed material as it moved down slope. Additional sediment was eroded from the
valley walls and incorporated as the glacier moved. Sediment fluxes into temperate fjords such
as this are generally controlled by meltwater discharge and calving (Jaeger and Nittrouer 1999).
Glacial meltwater containing the rock flour was likely released as an englacial or subglacial jet
and rose as a turbulent plume, which mixed with ambient water until it finally settled out (Powell
and Molnia 1989; Syvitski 1988). In this environment, the coarsest material settles out quickly
(often within 1 km of glacier terminus, Cowan, Powell, and Smith 1988) while the bulk of the
fine sediment moves away from the fjords. Much of the fine-sized fraction is often carried in
suspension onto the outer shelf (Sharma 1979).
Mean annual precipitation for the area near core 223BC is 100-200 cm higher than that of
the area sourcing core 249PC (Figure 2-4). Rainfall amounts may affect sedimentation and
transport processes including residence time, and have been shown to cause large variations in
meltwater runoff from glaciers in southern Alaska (Cowan et al. 1988; Gustayson and Boothroyd
1982). The environment near Resurrection Bay likely has a slightly more complex depositional
history relative to Icy Bay. The presence of a moraine and proglacial lake (Bear Lake) at
Resurrection Bay between the Bear Glacier terminus and the Gulf may act as a trap for sediment.
Seasonal precipitation and extremely high sediment discharge rates in the Gulf area make
specific determination of residence times difficult, and beyond the scope of this proj ect. The
residence times within small drainage basins in the GOA environment are known to be short, as
sediment is rapidly transported to the ocean (Jaeger et al. 1998).
Once material is deposited into the ocean, residence times of particles in the water column
for both core sites are also estimated to be short based on observations of floc settling rates in
other Alaskan tidewater fj ords (Jaeger and Nittrouer 1999; Hill, Syvitski, Cowan, and Powell
1998). Re-suspension of bottom sediment allowing for increased residence in the water column
is negligible because cores are taken in 145 m and 161 m water depth, and the wave orbital
velocities necessary to re-suspend silt-sized bottom sediment only applies to depths < 40 m
throughout the year and < 60 m for most of the year (Jaeger and Nittrouer, accepted).
The Guyot Glacier is the source of the maj ority of sediments at core 223BC in Icy Bay.
(Figure 1-2) Sediment sampled within the northern part of Icy Bay is derived principally from
Upper Tertiary (Miocene to Pleistocene) rocks of the Yakataga Formation. The Yakataga
Formation is located in the middle of an area of convergence and uplift on the GOA margin and
is composed of interbedded terrestrial, marine, glacimarine, and glaciofluvial deposits that can
locally exceed 5 km thickness (Bruns and Schwab 1983; Hamilton 1994). It represents rapid
deposition of sediments consisting predominantly of sandstones, mudstones, siltstones, shale and
conglomerates (Mazzotti and Hyndman 2002; Sauber and Molnia 2003). Glaciation recorded by
the Yakataga Formation is attributed to orogenic uplift and increased precipitation resulting from
the collision of the Yakutat terrane with the North American plate (Turner 1992). Clay mineral
suites (illite, kaolinite, chlorite and smeetite) within it are relatively similar to those of the
modern shelf (Molnia and Hein 1982). Accumulation of recycled sedimentary material
comprising the Yakataga Formation is estimated to have begun near mid-Miocene.
The other study site is in the Resurrection Bay area of the GOA. (Figure 1-3) The
sediments here are supplied from the Bear Glacier, which sits entirely with rocks of the Valdez
Group of the Chugach terrane. This group, a series of arc-derived slope and trench plastic
deposits that comprise the vast maj ority of the outer Kenai Peninsula, is part of a flysch sequence
which forms the southern part of the Chugach terrane (Ward, Moslow, and Finkelsteinl 987;
Nockleberg et al. 1994). The sedimentary rocks that compose it have been derived largely from
a Phanerozoic continental margin arc complex characterized by igneous rocks (Plafker, Moore,
and Winkler 1994). Latest Cretaceous to early Paleocene arc-continent collision resulted in off-
scraping and accretion to the continental margin of the flysch, mixed flysch and basaltic tuff, and
basalt which principally comprise the Valdez Group (Lang Farmer et al. 1993; Lull and Plafker
1989). Precambrian crustal material is present, possibly derived from late Proterozoic or older
metasedimentary and metaigneous rocks (Lang Farmer et al. 1993).
Most maj or glacimarine depositional systems are siliclastic (Powell and Molnia 1989).
The clay mineral content is controlled principally by 1) climate and relief, 2) type (mineralogy)
of weathered source material, 3) chemical composition of weathering solutions, and 4) later
diagenesis within the depositional environment (Brownlow 1996; Schnyder et al. 2005). The
most common clay minerals in soils, sediments, and sedimentary rocks are kaolinite, illite,
smeetite clays, and chlorite clays (Brownlow 1996). The average clay-sized (<2 Clm) sediment
in southern Alaska (Molnia and Hein 1982) is predominantly kaolinite + chlorite (61 %),
intermediate illite (37 %), and low smeetite (2 %), representing an immature sediment
characterized by rapid mechanical weathering and little chemical alteration. Analysis of the non-
clay mineralogy of the clay-sized fraction by Molnia and Hein (1982) identified the presence of
accessory minerals which include but aren't limited to quartz, feldspar, amphibole, and calcite.
Gamma-ray measurements are non-destructive, efficient methods of formation evaluation
and can be a valuable tool in both the environmental and engineering Hields (Nir-El 1997, Ayres
and Theilen 2001). A study by Schnyder et al. (2005) notes the use of radioisotopes in a variety
of geological applications, including sequence stratigraphy (van Wagoner et al. 1990), reservoir
characterization, diagenesis and mineral characterization (Hurst 1990), and source-rock
evaluation. Gamma-ray measurements detect variations in natural radioactivity originating from
changes in concentrations of the trace elements uranium (U) and thorium (Th), as well as the
more common rock-forming element potassium (K). The abundance and half-lives of U, Th, and
K (Table 2-1) make these three elements the dominant sources of gamma-rays detected, and thus
the most important natural radionuclides for many geological studies (Ruffell and Worden 1999;
Ayres and Theilen 2001). Decay of the parent radioisotope 238U gives rise to one of the uranium
decay series. The isotopic composition and concentrations of uranium, thorium (and their
associated daughter products) and potassium have previously been used as a dating and
Eingerprinting tool (Blum 1995; Harlavan and Erel 2002; Blum and Erel 1997). Particularly for
the purposes of this thesis, it is important to note that clay mineralogy is controlled primarily by
weathered source rock, climate, transport, and deposition, which then influence the spectral
gamma-ray (SGR) response of the sediments (Schnyder et al. 2005).
Uranium and thorium have many host minerals in sedimentary rocks including clays,
feldspars, phosphates, and zircons (McLennan et al. 2003; Weltje and Eynatten 2004). Thorium,
which is widely distributed in igneous rocks, is considered at least partially insoluble and thus is
often concentrated in sediments during weathering (Schnyder et al. 2005). Both uranium and
thorium tend to be highly concentrated in trace accessory minerals such as zircon, monazite,
apatite, and sphene (Blum and Erel 1997). Potassium is abundant in sediments and is
concentrated particularly in alkali feldspar and biotite, it is considered soluble in aqueous
solutions (White et al. 1999; Ruffell and Worden 1999). The amount of 238U in natural uranium
accounts for 99.27 % of total uranium, and 232Th accounts for almost all (assumed 100%) of total
thorium. 40K compromises an average of 0.01 18% of total potassium, which is actually very
significant because potassium is one of the ten most surface-abundant elements on earth (Irwin,
VanMouwerik, Stevens, Seese, and Basham 1997; Hutchison and Hutchison 1997).
IW Io nO mann
I ALsKA CANADA
OF .4LAS A 4
Figure 2-1. Structural formations on the southern Alaska margin (modified from Plafker et al.
G~uyot Gldr er
Figure 2-2. Generalized sketch of the cross section near core 223BC.
Figure 2-3. Generalized sketch of the cross section surrounding core 249PC.
(Ice Rafted Debris)
~ Precipitation (rnr) Bol -soo
ra I 1301-350 2001-3000
J 351-400 3001-5000 .
l~p~q I 1401-450 5001- 7000
., ~ .Ta I 1 451-500 7001-13000
Service. Oregon State University
Figure 2-4. Mean annual precipitation for the state of Alaska. Note that the area surrounding
Resurrection Bay (core 249PC) has a range of 1501-5000 mm/yr while the area
surrounding Icy Bay (core 223BC) shows levels from 3001-13000 mm/yr. (reprinted
with permission from Spacial Climate Analysis Service, Oregon State University
Table 2-1. Half lives and average abundances of relevant radioisotopes
Rad ioisotope 40K 232Th 238U
Half-life (billion years) 1.277 14.05 4.468
Upper continental crust
Elemental abundance (ppm) 28000 10.7 2.8
Activity (Bqlkg) 870 43 35
Activity (nCilkg) 23 1.2 0.9
Activity (kCilkm3) 66 3.3 2.6
(mg/Iiter) 399 1 x1 0 0.0032
Activity (Bqlliter) 12 4x1 07 0.04
Activity (nCilliter) 0.33 1 x1 0 0.0011
Elemental abundance (ppm) 17000 5 1
Activity (Bqlkg) 500 20 12
Activity (nCilkg) 14 0.5 0.3
Piston core 249PC and box core 223BC were chosen for this study based on the differing
lithologies of their sediment sources. Also considered was the similar proximity to glacial
termini (~4 km), with similar water depths (~150 m). Using glacier-proximal core sites allows
for the assumption that post-depositional physical and chemical alteration is minimal (e.g.,
slumping, turbidity flows, biological activity). The proximal location of these cores also
increases the likelihood that these sediments accurately represent source material, relative to
cores taken further out onto the shelf. Core samples used in this study were collected on the RVY
Alpha Helix during June and July 1995.
Core location is important because it is necessary to minimize influences from other
sources. Core 223 was taken in 145 m depth water approximately 4 km from the ice front in Icy
Bay, a fj ord located on the eastern side of the southern Alaska GOA margin. Core 249PC was
taken close to Resurrection Bay in 161 m water depth approximately 4 km from the coast, near
Bear Glacier. These locations allow for minimal influence of material associated with a source
other than the Yakataga Formation for core 223BC or the Valdez Group for core 249PC. (Figs 1-
Core 249PC was separated into 10 cm intervals (0-10 cm, 11-20 cm, and 21-30 cm) at the
University of Florida. During sampling a box core was sub sampled with a 15 cm-diameter, 50
cm-long subcore, creating core 223BC. Core 223BC had been previously segmented into 1 cm
intervals and placed in whirlpak bags. Intervals 11-12 cm, 15-16 cm, 20-21cm, and 31-32 cm
were chosen based on availability and similarity to depths of core 249PC. The core 223BC site
experiences higher sedimentation rates (>100 cm y l; Jaeger and Nittrouer 1999) than those at the
site of core 249PC (~ 1 cm y l; Jaeger et al. 1998). Sediment depositional rates at the site of core
249PC in Resurrection Bay have not been as tightly constrained as those in Icy Bay but can
assumed based on observations from surrounding areas (Jaeger et al. 1998). The sediments
within core 249PC near Resurrection Bay, therefore, represent a longer time period than
sediment retrieved in the Icy Bay core.
Radioactivity measurements of 232Th, 238U and 40K were performed on dried and powdered
sediment. These samples were counted on a Canberra UltraLow Background Planar-Style
germanium detector at the University of Florida. The amount of sample used varied by
availability, but averaged 15 g for core 249PC and 12 g for core 223BC. Count times ranged
from 80,240 to 160,993 seconds but averaged 90,245 (about 25 hours), in order to accurately
measure activity and minimize error. Raw gamma spectroscopy data was processed by
analyzing photopeaks generated using Gamma Genie software.
Background levels were determined by running an empty sample jar and subtracting the
background value for each region of interest in the sample spectra. Efficiency was determined
by counting a sample (NIST standard) with known activity and comparing it with the amount
detected on the instrument at the University of Florida. Self-absorption correction factor
calculations were made for radioisotopes with gamma decay energies of less than 200 keV,
which for this study affects only measurements of 234Th (related to 238U activity) at the 63 keV
photopeak. This technique involves direct gamma transmission measurements on sample and
efficiency calibration standards (see Cutshall, Larsen, and Olsen 1983 for further explanation).
Radionuclide activity determinations were made by converting the raw data from counts
per minute (cpm) to decays per minute (dpm), then dividing by sample weight. The standard
form of conversion from activity (dpm/g) to concentration (ppm) is a process requiring the
conversion of activity to atom quantity. Concentration determinations for these analyses were
done using efficiencies previously established from the Buffalo River and an estuary (with
known concentrations) for the radioisotope of interest. The cpm value is determined by dividing
the net peak area by counting time. The 63 keV photopeak corresponding to 234Th activity was
examined to determine 238U activity (Figures 3-1, 3-2). A similar technique was used to obtain
232Th activity by measuring photopeaks associated with activity of the daughter 228Ac (half life =
6.13 d). Though no specific measurement was made, the daughter is assumed to be in secular
equilibrium with 232Th (half life = 1.4x1010 yr), since it has a significantly shorter half life and
we assume minimal loss of 228Ra. For a more accurate measurement, weighted averages of two
peaks associated with 228Ac (a high energy gamma-ray at 911 keV as well as the 338 keV ray)
were used. Multiple photopeaks are often averaged for more accurate 228Ac measurements (Nir-
Secular equilibrium is the condition in which the rate of decay of the daughter is equal to
that of the parent, and most commonly occurs when the daughter has a significantly shorter half
life than the parent. For 232Th and 238U, the half lives are significantly longer than those of their
daughters (Table 2-1), satisfying necessary conditions to enable this type of analysis (Faure
1986). 40K was measured directly at the 1461 keV photopeak.
Precision was determined by running two random samples on three separate occasions and
determining the mean deviation from the mean. Due to the small data set and associated scatter,
it is more appropriate to use this deviation as opposed to a standard deviation in order to better
represent error. This procedure established errors for the radionuclides (238U f0.7 ppm, 232Th
f0.4 ppm, and 40K f0.1 %). The largest deviation for each particular element was selected.
Grain Size Separation
Grain size separation in preparation for radioisotopic analysis was done at the University
of Florida using sieve and Sedigraph analyses (Lewis and McConchie 1994; Syvitski 1991). Dry
sediment samples weighing approximately 10 g (core 249PC) or 2.5 g (core 223BC) were
homogenized then put into 120 milliliter (ml) glass jars and soaked in a 0.05 % sodium
metaphosphate (Na(PO4)5) Solution overnight in order to help disaggregate particles. Those
showing signs of flocculation were soaked an additional day in 1.0 % Na(PO4)5 Solution.
Samples were placed in an ultrasonic bath for a minimum of 10 minutes before being wet-sieved
through a 63 Clm sieve in order to isolate the sand-sized fraction which was then dried and
weighed. The silt fraction was isolated by adding de-ionized water and diluting the clay/silt
mixture to improve settling velocity. The mixture of approximately 500 ml was then agitated
and allowed to settle in a water column based on the application of Stoke's Law (in accordance
with Lewis and McConchie 1994). After the designated time, the remaining liquid was siphoned
leaving only the silt fraction. The clay fraction was separated by siphoning followed by either
centrifugation or drying in a low-temperature (< 600 F) oven. Random samples were selected to
run on the Sedigraph as a check to determine if any remaining silt was left in suspension, and
was found to be negligible (<1%).
Radioisotopic activity is normalized to the mass of the counted sample (i.e., dpm/g). The
sand-sized (>63 Clm) fraction was separated and weighed, then divided by the original mass to
get percent sand. The fine-sized fraction was then mixed with 0.05 % Na(PO4)5 and run on a
Sedigraph 5100 analyzer to determine percent silt and percent clay (error was found to be less
than 0.6 % in each interval). Additionally, all intervals were normalized according to mass
percent clay to eliminate biases associated with increased mass due to increased sand content
Mineralogy of the sand-sized fraction was determined in part by analysis of smear slides
constructed using techniques of the I.O.D.P. and Flemings et al. (2006). Slides were created by
sprinkling a small amount of homogenized sediment on a 2.5 cm x 7.5 cm glass slide and
dispersing it over the slide with a drop of deionized water. The sample was then dried on a hot
plate at a low temperature for approximately 5 minutes. A drop of Norland optical adhesive and
a 2.5 cm x 2.5 cm cover glass were placed over the sample. The slide was then put under an
ultraviolet light to dry and set. Point count data was done on a Nikon petrographic microscope
with an integrated automatic point counter. Slides were analyzed at 1000 counts per slide spaced
at approximately 1 mm.
Bulk mineral analysis of homogenized sediment was prepared for conventional powder
mount x-ray diffraction (XRD) in accordance with Lewis and McConchie (1994) and done at the
University of Florida. Approximately 2 g of sediment was taken from a central interval in each
core (11-20 cm for core 249PC, 15-16 cm for core 223BC).
Table 3-1. Precision data for associated each radionuclide measured. The largest mean
deviation about the mean measured was chosen to obtain greatest accuracy.
U (ppm) Th (ppm) K (%)
run 1 3.5 5.9 1.5
run 2 2.8 5.4 1.6
run 3 2.1 6.6 1.5
Average 2.8 5.9 1.5
mean deviation 0.7 0.4 0.04
run 1 3.2 3.7 2.1
run 2 3.2 3.2 2.4
run 3 3.7 2.7 0.1
Average 3.4 3.2 2.2
mean deviation 0.2 0.3 0.1
Figure 3-1. Decay series of the 238U radioisotope relevant to this study.
Figure 3-2. Decay series for the 232Th isotope.
Table 3-2. Elemental concentration data normalized to clay percent. The measured (original)
concentrations are also listed for comparison. Only bulk sample measurements are
Concentrations (normalized to clay)
DEPTH U (ppm) Th (ppm) K (%) U (ppm) Th (ppm) K (%)
Core 249PC exhibited comparable concentrations throughout the core for each element
(Table 4-1, Figs. 4-1 through 4-6). The uranium concentration (averaged from replicates of each
interval) in core 249PC ranged from 4.2 ppm to 4.4 ppm, whereas in core 223BC it ranged from
1.7 ppm to 3.7 ppm. Thorium concentration in core 249PC ranged from 4.6 ppm to 5.9 ppm as
opposed to the core 223BC range of 0.6 ppm to 4. 1 ppm. The potassium percentage also showed
more consistency in core 249PC, ranging from 2.3 % to 2.4 %, whereas core 223BC ranged from
0.3 % to 2.0 %. Although core 223BC did show increased variability in ranges of element
concentrations, there remained a consistent overall decrease of activity with depth. Uranium
showed a relatively linear trend of decreasing concentration with increased depth. The thorium
concentration in core 223BC decreases in general, the exception being interval 20-21 cm, which
exhibited low thorium and potassium. This interval was different from all other intervals in that
it exhibited a significantly higher uranium (2.4 ppm) concentration relative to extremely low
thorium (0.6 ppm) and potassium (0.3 %).
Detailed concentration data on separated size fractions is shown in Table 4-2. There is
little evidence that elemental thorium concentrations are enhanced within a particular size
fraction for either core, it is at times highest in each of the three size fractions. The concentration
of uranium is not associated with the sand-sized fraction. It is always highest in either the clay-
or silt-sized fraction, but varies between the two. For potassium there is a distinct correlation of
concentration and grain size throughout both cores. The clay (<2 Clm) fraction contains the
highest concentration relative to silt and sand at every interval. The potassium percentage is also
lowest in the sand fraction at every interval. Each element is plotted against clay percent. (Figs.
4-7 through 4-9) Concentration data once normalized to clay, which assumes 100 percent clay-
sized material, is shown compared to bulk concentration. (Table 3-2)
Mineralogy and Physical Properties
Mineralogic data for all three intervals of core 249PC showed a higher abundance of rock
fragments (ranging from 78-86%) when compared to core 223BC (61-68%). (Figs. 4-10 through
4-14) There was also an appreciable amount of quartz in core 249PC (9-1 1%) at all three
intervals. Biotite and amphibole were the next most common minerals. Accessory minerals
compromising less than 1% of the sample include, but are not limited to, pyroxene, garnet,
biogenic material, opaque minerals (hematite, ilmenite), and glass (Figs. 4-15 through 4-24).
Due to the limited amount of sand available for core 223BC intervals 11-12 cm and 31-32
cm, no smear slides were made. Point count data for core 223BC on intervals 20-21 cm and 15-
16 cm (the only two slides for core 223BC) show an overall decrease in rock fragments and
increase in quartz [relative to 249PC]. (Figs. 4-13, 4-14) Quartz abundances for core 223BC (22
% and 27 %) were at least twice that of those observed in core 249PC. The core 223BC interval
containing fewer rock fragments (15-16 cm) had a corresponding increase in quartz fragments
(27%). Biotite occurrence is at 4% for both intervals, while amphibole and accessory minerals
show a slight (1% to 2%) increase in the 15-16 cm segment.
Relative to core 249PC, core 223BC contains more quartz and fewer rock fragments, with
a more angular shape. Mineralogy of the fine-sized fraction is very similar between cores
(Figure 4-25). From XRD analysis, the most significant peak corresponds to quartz at 26.67 (at
26cU), and is noted again at secondary peaks (e.g., 50.21). The peaks corresponding to illite,
chlorite, and kaolinite are all elevated in core 249PC relative to core 223BC. This is expected
due to the higher percentage of clay in core 249PC relative to 223BC. Core 223BC contains one
additional mineral (likely a feldspar) which is not present at core 249PC.
Grain size separation shows an average of ~66% clay, ~26% silt, and ~8% sand for core
249PC from 0-30 cm, with specific intervals ranging from 57 % to 71 % clay. (Table 4-3) There
is an increase in silt with depth (22% to 31%), and an overall increase in sand (8% to 12%). The
core 249PC interval 21-30 cm shows a significant decrease in percent clay and increase in
percent sand (57% clay, 12% sand).
Grain size averages for core 223BC are 62% clay, 33% silt, and 3% sand, with clay
ranging from 58 % to 67 %. There is an overall slight decrease in percent clay with depth (67%
to 61%). Contrastingly, there is an overall slight increase of both percent silt (3 1% to 38%) and
percent sand (2% to 6%) with depth. The exceptions are interval 15-16 cm which exhibits a
slight decrease in percent silt from the interval above it, and interval 31-32 cm which exhibits
distinctly low sand content.
Table 4-1. Radionuclide concentration data and associated error of each bulk sample within each
23U (ppm) error 23Th (ppm) Error 4K (%) error
0-10Ocm 4.2 10.7 4.6 10.4 2.4 1.
11-21cm 4.3 10.7 4.7 10.4 2.4 1.
21-30cm 4.4 10.7 5.9 10.4 2.3 1.
11-12cm 3.7 10.7 4.1 10.4 2 1.
15-16cm 3.2 10.7 3.4 10.4 1.8 1.
20-21 cm 2.4 10.7 0.6 10.4 0.3 1.
31-32cm 1.7 10.7 2.1 10.4 0.9 1.
Note: For core 249PC, the three bulk samples were averaged
Figure 4-2. Concentration ofthorium with depth in core 249PC.
10 20 30 40
50 60 70 80
Figure 4-1. Concentration of uranium with depth in core 249PC.
10 20 30 40
50 60 70 80
Figure 4-3. Radioisotopic concentration of potassium with depth in core 249PC.
05 10 15 20 25
30 35 40 45 50
I r 1
Figure 4-4. Concentration of uranium with depth in core 223BC.
05 10 15 20 25
30 35 40 45 50
I H I
Figure 4-5. Concentration of thorium with depth in core 223BC.
Figure 4-6. Concentration of potassium with depth in core 223BC.
Table 4-2. Specific concentrations of elements within each core
DEPTH 238U (ppm) 232Th (ppm) 40K (%)
249 PCa 0-10 cm 4.5 3.1 2.5
249 PCb 0-10 cm 4.0 5.8 2.3
249 PCc 0-10 cm 4.2 5.0 2.4
249 PCclay 0-10 cm 3.9 4.3 2.7
249 PCsilt 0-10 cm 4.4 6.0 1.7
249 PCsand 0-10 cm 3.1 3.6 1.8
249 PCa 10-20 cm 4.0 4.4 2.4
249 PCb 10-20 cm 4.3 4.3 2.4
249 PCc 10-20 cm 4.6 5.2 2.4
249 PCclay 10-20 cm 6.0 4.0 2.8
249 PCsilt 10-20 cm 4.2 4.3 2.2
249 PCsand 10-20 cm 3.5 5.9 1.5
249 PCa 20-30 cm 4.1 5.3 2.2
249 PCb 20-30 cm 5.0 5.8 2.3
249 PCc 20-30 cm 4.1 6.6 2.3
249 PCclay 20-30 cm 5.3 4.4 2.7
249 PCsilt 20-30 cm 3.8 4.2 1.4
249 PCsand 20-30 cm 3.9 4.0 1.6
223 BC 11-12 cm 3.7 4.1 2.0
223 BCclay 11-12 cm 0.1 4.2 1.2
223 BCsilt 11-12 cm 2.3 0.8 0.8
223 BCsand 11-12 cm N/A N/A N/A
223 BC 15-16 cm 3.2 3.4 1.8
223 BCclay 15-16 cm 2.0 2.6 1.7
223 BCsilt 15-16 cm 2.9 5.1 1.2
223 BCsand 15-16 cm 0.2 3.9 0.3
223 BC 20-21 cm 2.4 0.6 0.3
223 BCclay 20-21 cm 2.2 2.8 1.7
223 BCsilt 20-21 cm 3.9 4.0 1.2
223 BCsand 20-21 cm 3.2 4.9 0.8
223 BC 31-32 cm 1.7 2.1 0.9
223 BCclay 31-32 cm 3.2 3.7 2.1
223 BCsilt 31-32 cm 1.9 3.2 0.3
223 BCsand 31-32 cm 0.1 0.3 0.1
9 249PC O-10
H 249PC 11-20
X 223BC 11-12
X 223BC 15-16
+ 223BC 31-32
0% 10% 20% 30% 40% 50% 60% 70% 80%
% clay-sized material
Figure 4-7. Concentration of uranium with respect to percent clay for all intervals. After ~60%
clay-sized material, there is a general increase of uranium concentration with
increasing clay-sized material.
X 223BC 11-12
0% 10% 20% 30% 40% 50% 60% 70% 80%
% clay-sized material
Figure 4-8. Concentration of thorium with respect to percent clay for all intervals.
* 249PC 0-10
H 249PC 11-20
X 223BC 11-12
X 223BC 15-16
+ 223BC 31-32
0% 10% 20% 30% 40% 50% 60% 70% 80%
% clay-sized material
Figure 4-9. Concentration of potassium with respect to percent clay for all intervals.
2% 2% 1%
g rock frags
Figure 4-10. Mineralogy of sand fraction within core 249PC interval 0-10 cm.
g rock frags
Figure 4-11. Mineralogy of sand fraction within core 249PC interval 11-20 cm.
Figure 4-12. Mineralogy of sand fraction for core 249PC interval 21-30 cm.
g rock frags
Figure 4-13. Mineralogy of sand fraction from core 223BC interval 15-16 cm.
3% 4% 2%
g rock frags
Figure 4-14. Mineralogy of sand fraction from core 223BC interval 20-21 cm.
Figure 4-15. Typical image of core 249PC interval 0-10 cm showing mostly rock fragments
(designated RF) with associated quartz. Field of view approximately 0.8 mm.
Figure 4-16. Image of biogenic material (designated BM) among rock fragments in core 249PC
interval 11-20 cm. Field of view approximately 0.8 mm.
Figure 4-17. Oxidized coating on grain from core 249PC interval 11-20 cm. This interval was
the only one exhibiting coated grains. Field of view approximately 1.25 mm.
Figure 4-18. Images of biotite (designated B) and accessory minerals from core 249PC interval
21-30 cm. Field of view approximately 1.25 mm.
Figure 4-19. Typical picture of core 249PC interval 21-30 cm showing large rock fragments and
quartz (designated Q). Field of view approximately 1.25 mm.
Figure 4-20. Image from core 223BC interval 15-16 cm. Rock fragments dominate but there is
an increase in quartz and accessory minerals. Sand particles in this core are also more
angular in shape. Field of view approximately 1.5 mm.
Figure 4-21. Typical image from core 223BC interval 15-16 showing elevated abundances of
plagioclase and amphibole, as well as increased quartz (relative to core 249PC)
among the dominant rock fragments. Field of view approximately 1.0 mm.
Figure 4-22. Image of core 223BC interval 15-16 cm silt fraction. Field of view approximately
Figure 4-23. Image of biotite among rock fragments and quartz grains from core 223BC interval
20-21 cm. Field of view approximately 0.8 mm.
Figure 4-24. Image of core 223BC interval 20-21 cm silt fraction. The silt fragments are larger
in general size as compared to the 15-16 cm interval of this core. Field of view
approximately 1.5 mm.
~~r11 I 'P''"""
1sZ~~l II ( II IFDIF~4~r
10 %& d 40; S1 ;60
123<507$90~ 1"" ~~1 a~~"l~'''""I"""" I '" ~~t"
Figure 4-25. XRD data for both cores with associated mineralogy. Core 249PC is offset (raised)
to better illustrate variations between cores.
Table 4-3. Percent of clay-, silt- and sand-sized fractions from the two cores. The averages for
each core are included at the bottom.
Total % clay Total % Silt Total % Sand
0-10cm 70 22 8
11-20cm 71 25 4
21-30cm 57 31 12
11-12cm 67 31 2
15-16cm 66 30 4
20-21cm 58 36 6
31-32cm 61 39 0.3
Avg. clay %
Avg. silt %
Avg. sand %
Core 249PC has a similar percentage of fine-sized sediment in the 0-10 and 1 1-20 cm
intervals, with a decrease in sand due to a slight (1% and 3%) increase in clay and silt. The 21-
30 cm interval exhibits a significant drop in clay and increase in sand from the two intervals
above it. This influx of sand can be attributed to the 1964 earthquake, which corresponds to that
interval given the sediment accumulation rate (~1 cm y^l, Jaeger et al. 1998). The earthquake
epicenter was located in nearby Prince William Sound and accounted for extensive redistribution
of sediments by tsunamis (Jaeger et al. 1998).
Sand percentage at core 223BC increases steadily downcore (2%, 4%, and 6%) until the
31-32 cm interval where it drops to 0.3 %. Due to the extremely high sedimentation rate in the
northern Icy Bay location (>0.3 cm d- Jaeger 2002) and lack of steady-state deposition,
inconsistencies with accumulation and grain size at depth are expected. Decreased sand could be
attributed to increased precipitation, which may substantially increase meltwater discharge and
associated velocity, and allow for deposition of the sand-sized fraction further from the glacial
The two sites reveal variations in transport environments, which may affect grain size
distribution. The presence of a moraine and Bear Lake between Bear Glacier and the
Resurrection Bay core 249PC site may act as a trap for grains silt-sized and larger, whereas
Guyot Glacier is a tidewater glacier, which deposits sediment directly into Icy Bay (Figs 2-1, 2-
2). This may also explain the lower relative clay percentages in core 223BC compared to core
249PC. Guyot Glacier is connected to the water body and contributes sediment directly into Icy
Bay, allowing for extended suspension and distribution of the finer particles.
Mineralogy of the two cores is surprisingly similar. Both cores are from a temperate
glacial environment in the GOA, which produces predominantly glacimarine rock flour,
therefore, a general similarity in particle size and common rock-forming mineralogy (relative to
other parts of the world) is expected. The almost identical results from both petrographic and
XRD analyses are not expected based on differing source lithology and geological environments
(Figs. 1-2, 1-3). The XRD patterns, when viewed together, are distinguished only by more well-
defined peaks from the Icy Bay sample, and one or two additional minerals (likely feldspars,
McClellan, verbal communication) at the same site (Figure 4-25). Differences in peak intensities
are partly due to variations in clay mineralogy as well as increased overall clay percentage in the
249PC sample. Higher proportions of clay-sized particles produce a less intense, muted
appearance in graphs (Moore and Reynolds 1997). The mineralogic analyses (4-10 through 4-
14) show nearly identical sediment compositions, even with regard to accessory minerals. The
variation is principally in relative percentages, the exception being the increase in biogenic
material under accessory minerals in core 249PC interval 11-20 cm. These findings indicating
that the source rocks presented in the two core sites may not be as different as initially thought.
No source has currently been determined for the Yakataga Formation. It is possible that the
Chugach terrane, which is the source material for core 249PC sediments, is also the source for
the Yakataga Formation sediments of core 223BC.
The point count data for core 249PC shows an overall decrease in rock fragments with
depth, and a slight elevation in amphibole, biotite, and accessory minerals for intervals 11-20 cm
and 21-30 cm. The coarse-size fraction of both cores contains predominantly rock fragments,
with the next most common occurrence being quartz (though core 223BC consists of nearly
twice the quartz of core 249PC). Both cores also contain biotite, amphibole, and accessory
minerals in amounts of less than 5 percent. The weathering of differing source rock types
surrounding each basin should hypothetically produce differing clay-mineral percentages (Hein
et al. 2003). Though relative clay-mineral percentages vary between the cores, it is only a slight
variation which can be attributed to differences in sedimentation processes between core sites, or
seasonal sediment discharge fluctuations. Elevated illite content at the Resurrection Bay area
relative to the Icy Bay area seen in the XRD analysis was also recorded by Molnia and Hein
(1982). A single depositional or periodic event might alter source of sediment, and is a possible
reason for elevated illite (e.g., flooding at an illite-rich drainage basin or rapid draining of a
Elemental concentrations are predominantly due to mineralogy, diagentic changes of clay
mineralogy, and adsorption processes (Ayres and Theilen 2001). Elevated clay mineral contents
(illite, chlorite) and overall clay-sized material percentages in core 249PC (Figure 4-25, Table 4-
3) correspond to higher isotopic concentrations. Radioactivity is often associated with clay- or
fine-sized particles (van Wijngaarden et al. 2002, Anderson 2004, Naidu, Han, Mowatt, and
Wajda 1995). Relatively high concentrations of K have been recorded in marine sedimentary
rocks of the Valdez Group near Bear Glacier (Goldfarb and Borden 1982). The Aialik pluton
outcrops discontinuously around the mouth of Resurrection Bay, it is locally biotite-rich and may
contribute to increased potassium levels (Kusky, Bradley, Donley, Rowley, Haeussler 2003).
In an attempt to better represent elemental concentrations, samples from this study were
normalized to percent clay. (Table 3-2) Due to the high amount of clay initially in most of the
intervals, relative concentrations of normalized data are similar to initial bulk concentration data,
with an increase in specific concentrations. Core 249PC interval 21-30 cm has a high elemental
concentration (in each of the three elements) with respect to the relatively low sand percentage
(Figs 4-7 through 4-9). This may be due to the catastrophic nature of the 1964 earthquake event
that deposited the sediment. Material of a wide range of sizes was moved at an extremely rapid
rate, which would allow for minimal disaggregating of clay particles before deposition and
Though the averaged concentrations for uranium in core 249PC show an increase in
activity with depth, it is not definitive. (Figure 4-1, Table 4-2). Based on the concentration data
and associated errors (Figure 4-1), it is impossible to conclude there exists an increased uranium
concentration with depth. There is, however, a general consistency of uranium elemental
concentration in all intervals from core 249PC, even at the 21-30 cm interval, which contains
considerably less clay. By analyzing a homogenized 10 cm sample, as was done for core 249PC,
fluctuations in sedimentation could be minimized, producing more consistent results.
The uranium concentrations in core 223BC decrease with depth in an almost linear
manner. The decrease in concentration corresponds to a decrease in clay and, therefore, supports
the correlation of activity of this element with grain-size. At the 20-21 cm interval uranium is
elevated relative to thorium (Tables 4-1, 4-2). This interval is the most similar to the hypothesis
put forth regarding an increase in uranium with associated decrease in potassium and thorium,
and is the only interval where this behavior is seen. The hypothesis stated this might be due to
heavy minerals concentrated in the sand fraction. Based on the mineralogy observed in this
study that conclusion is unlikely.
Core 249PC shows an increase in the average concentration of thorium with depth. When
errors are taken into account (Figure 4-2) the increase becomes unclear. This results in thorium
exhibiting a general consistency in elemental concentration among all intervals, much like the
uranium concentration in this core. There is an increase in silt content (Table 4-3) with depth
that may support the association of thorium with the silt-sized fraction. Due to lack of
consistency, it is more accurate to associate thorium with the more general fine-sized fraction
(<63 Cpm) than to specify either the clay- or silt-sized fraction. Thorium elemental abundance in
core 223BC shows the same general decrease with depth as seen with uranium, except at the 20-
21 cm interval where concentration significantly lower than the other intervals. Thorium
abundance in both cores is higher than uranium abundance, and can be related to initial
mineralogy, since thorium is more abundant in the earth (10ppm) than uranium (2ppm) (Ruffell
and Worden 1999). This interval exhibits the highest amount of sand (6 %) within the core,
supporting the association of the fine-sized fraction with thorium elemental concentration. The
elevated thorium concentration relative to uranium at both the Resurrection Bay and Icy Bay
core sites is also seen in published geochemical data. (Figs. 1-4 through 1-6)
The percentage of potassium in core 249PC fluctuates very little (<1 %) and, therefore,
does not specifically show a decrease with depth. Potassium concentration only varies by 0.1%
in each of the three intervals in the 249PC core. The concentration is highest in the clay fraction
and lowest in the sand fraction at every interval. The consistency seen in potassium elemental
abundance among samples for core 249PC is similar to uranium and thorium. The sediment
associated with core 249PC shows slightly elevated overall concentrations of all radioisotopes
examined when compared to core 223BC, but is particularly noticeable with potassium.
Core 223BC potassium concentrations range from 0.3 % (20-21 cm interval) to 2.0 % (11-
12 cm interval). Core 223BC shows an overall decrease in potassium with depth, similar to the
uranium and thorium concentration with depth seen in this core. The exception is a very low
concentration at the 20-21 cm interval which corresponds to the lowest amount of clay (58 %)
and highest amount of sand (6 %) in the core.
There is an obvious association of potassium with the clay fraction for both cores. Glacial
meltwater is known to be relatively high in potassium (Anderson 2004). Sediment discharged
into the GOA is predominantly clay-sized and thus is the principal potassium source, since
potassium is locked in the clay mineral lattice and relatively immobile. Physical grinding of
biotite grains during abrasion in this type of glacial environment also exposes the inner layer
(potassium) cations (Anderson 2004). The release of potassium relative to plagioclase is
promoted in colder climates due to this type of biotite weathering (White et al. 1999; Blum and
Erel 1997) and thus contributes to overall potassium. The potassium radioisotope is spread
through many rock-forming minerals (e.g., feldspar) as well as heavy minerals (Asadov,
Krofcheck, and Gregory 2001), so a uniform signal even after separation into size fractions is not
In general, core 249PC exhibits a different (elevated) elemental abundance from that of
core 223BC, particularly when normalized to mass percent clay (Table 3-2). The distinction can
be associated with an elevated percentage of clay-sized grains at core 249PC (Table 4-3), since
there is an association of concentration with the fine-sized fraction. Note that the low sand
content in the 11-20 cm interval does not result in a low uranium concentration or elevated
thorium and potassium. A higher accessory mineral content was recorded for this interval, but
was largely due to increased diatom tests and biogenic material. The initial hypothesis suggested
that 238U (uranium) in the cores was associated with zircons or other heavy minerals which are
resistant to weathering and, therefore, concentrated predominantly in the coarse (>63 Clm)
fraction, and that 232Th (thorium) and 40K potassiumu) are associated with clays (illite, chlorite)
and mica. Uranium abundances are similar to thorium and potassium abundances at almost
every interval. The intensity of the glacial abrasion can promote the release of dissolved
uranium from rocks into the waters where it would be incorporated into the fine-sized fractions
(Taboada, Cortizas, Garcia, and Garcia-Rodeja 2006; Hodson 2002).
The decrease in percent clay at the core 223BC 20-21 cm interval is a likely explanation
for lower 232Th and 40K activities. Gamma-ray activity should be a function of grain size
(Asadov et al. 2001), and appears to be recorded here. With regard to potassium in particular,
there seems to be an association with the fine-sized fraction, (the dominant sediment mode in the
GOA), and the clay-sized fraction specifically. Changes in clay mineralogy (decrease in
potassium-rich illite clay relative to smeetite clays) are thus likely responsible for decreasing
natural gamma activities not associated with decrease in clay-sized sediments.
Clay mineral assemblages play an intricate role in controlling radioactivity and are
particularly informative of source rock composition (Naidu et al. 1995). This is primarily due to
the fine-sized fraction comprising the maj ority of sediment discharged in the GOA and the
association of potassium with this fraction (Jaeger et al. 1998; Molnia and Hein 1982; Anderson
2004). Clays in the GOA are characterized by high amounts of illite and chlorite with traces of
expandable clay minerals and little to no kaolinite (Naidu et al. 1995; Molnia and Hein 1982).
Based on XRD in this study there is a presence of kaolinite that is considered to be high relative
to previous studies. This is attributed to kaolinite having a tendency to flocculate and
concentrate in shallow marine successions close to shore (Ruffell and Worden 1999). Kaolinite
and expandable clays such as montmorillanite contain significantly less potassium (and thorium)
relative to illite clays (Ayers and Theilen 2001; Ruffell and Worden 1999). Thus, if a decrease
in the relative abundance of an element (e.g., potassium) does not coincide with a significant
decrease in the fine-sized fraction, it may simply be related to the clay minerals present, as well
as the mineralogy of non-clay minerals in the clay-sized fraction.
It is suggested that the mobility of potassium and uranium and the relative concentration of
thorium during weathering should result in clays with elevated Th/K and Th/U ratios (Schnyder
et al. 2005). The Th/K ratio is used to recognize clay mineral, feldspar, and mica associations
(Ruffell and Worden 1999). Clay mineral analyses and Th/K ratios help to distinguish long-term
transgressive events as well as short-term flooding (Ruffell and Worden 1999). When these
ratios are plotted, core 249PC exhibits a mix of chlorite and illite, with most points falling close
to each other due to the very consistent potassium concentration. (Figs 5-1 through 5-3) Core
223BC shows a very consistent ratio of 2:1 for thorium and potassium. Core 223BC interval 20-
21 cm records the lowest concentration of both thorium and potassium. When the Th/K ratio is
plotted against percent clay, there is little distinction between the two cores. (Figure 5-4) The
similarity of clay mineral percentages, as well as depositional processes would produce similar
Th/K ratios once normalized to clay.
Possible Alteration/Biasing of Signal
It is important for this study to understand the potential extent of chemical weathering in
the glacial environment, particularly for this type of study, which assumes initial source material
is represented accurately in the sedimentary record. The geochemistry of a sedimentary deposit
is often influenced by many variables other than parent rock composition (Fralick 2003).
Weathering can be the dominant process affecting the geochemistry of sedimentary rocks, and
physical weathering is the dominant process in glacial environments. Mobility of elements is
particularly hard to constrain in these types of cold weather environments. Uplift and erosion are
actively occurring in the GOA, and they are a driving function for geochemical cycling.
Additionally, runoff and temperature are two of the most important parameters controlling
chemical weathering rates (Dessert, Dupre, Gaillardet, Francois, and Claude 2003; Derry and
France-Lanord 1996). The nature of the temperate glaciers within the study area makes chemical
alteration a possibility relative to colder climates due to the presence of water at the glacier base.
This allows the glacier to erode its bed and thus provides conditions necessary for accelerated
weathering (Anderson, Drever, Frost, and Holden 1999).
The potential of a difference in uranium concentration between parent-rock and sediment
deposited is greater than that of thorium or potassium in this type of environment (Ruffell and
Worden 1999). Conditions on the southern Alaska margin are considered oxidizing and there is
very little organic matter present. Under sufficiently oxidizing conditions uranium is commonly
soluble in water (as U+6), while thorium has low solubility (Faure 1986). The mineralogy of the
sand fraction did not reveal heavy minerals typically associated with uranium and therefore must
be broken down prior to deposition and incorporated into all size fractions. This even
distribution could occur by dissolved uranium in the water column being transported in
proglacial rivers and streams. Thorium and potassium are both considered to be locked in the
mineral lattice, and relatively immobile. The exception is thorium, which may be somewhat
mobile in the water column.
The GOA is considered an oxidizing environment, it is likely that reducing conditions
would exist only after deposition. Post-depositional alterations are very unlikely considering the
rapid accumulation of sediment at each site, and relatively short time-period represented in each
core. Previous studies show that glacial meltwater is likely the dominant factor governing
elemental fluxes (White and Blum 1995; Anderson 2004). There are englacial and supraglacial
flow paths transmitting water quickly to outlet streams, allowing little opportunity to interact
with rocks and sediments (Anderson et al. 1999; Collins 1979). Mountain ranges in very close
proximity to the sea such as in southern Alaska minimize terrestrial storage, and sediments
within the Gulf have previously been characterized as having undergone mild chemical
weathering (Jaeger et al. 2001; Anderson 2004). Alteration is possible, but considered unlikely
and very mild given the extremely high discharge rates.
Correlation with Aeroradiometric Data
Aerial gamma-ray surveys measure the flux of gamma-rays emitted by the radioactive
decay of the elements 40K potassiumum, 238U (uranium), and 232Th (thorium). These elemental
abundances can be used as proxies for studies, because different rocks and soils generally
contain different amounts of these elements. Thus the aeroradiometric measurements obtained
can be useful for locating intrusive rocks and mapping rock units with a distinctive radioelement
signature (Duval, Cook, and Adams 1971). The National Uranium Resource Evaluation (NURE)
program was conducted by the U.S. Government to assess radioelement data (Duval 2001). The
program included airborne gamma-ray spectrometry and magnetic data collection along with
extensive geochemical sample collection and processing. Aeroradiometric surveys of 98 10 by
30 quadrangles were flown in Alaska between 1975 and 1980. The data, collected in 15 surveys
flown approximately 400 feet high and spaced approximately 6 miles apart, were done by Texas
Instruments (T.I.), Lockwood, Kessler and Bartlett (LKB), and AeroServices (Aero) under
contract with the U.S. Government. The surveys typically penetrate the upper 2 feet (Duval et al.
There is little correlation with aeroradiometric data in this study. (Figure 1-7) Elevated
uranium measured in sediments at the Resurrection Bay site (relative to Icy Bay) are not depicted
on the aeroradiometric map. There is a small area on the survey map near Resurrection Bay
which shows the presence of uranium, though it would not seem to be enough to influence the
elevated concentration recorded in the sediment here, particularly when compared with those of
the Icy Bay core. The aeroradiometric survey map shows an absence of thorium in the Icy Bay
environment with a highly elevated uranium concentration, also not recorded in the measured
One possible explanation for the elevated uranium concentration near Icy Bay on the
aeroradiometric map is the preferential sorting of heavy minerals by aeolian transport processes.
Aeolian transport has been shown to be effective at zircon enrichment and produce very high Zr
and Hf contents in loess deposits when compared to continental crust (Taylor 1983; McLennan et
al. 2003). These zircon enrichments on the surface near the Icy Bay environment would cause
elevated uranium unrelated to parent rock material or the coarse fraction. There might also be
biases in the aeroradiometric map due to extrapolation of data. The 6-mile spacing between
flight-lines would require assumptions to be made about the areas not directly measured. The
analysis then is that there is no overall correlation of aeroradiometric maps with measured
concentrations, and they should not be used for comparison with core material in the southern
Correlation with Geochemical Data
Geochemical data from river and stream samples (Figs. 1-4 through 1-6) reveal very
similar elemental abundances to those determined at the core sites. The thorium data suggest a
concentration at both core locations with a range from <3.7 to 8.7 ppm. (Table 4-1) Thorium
concentrations were slightly lower at core 223BC relative to core 249PC, but still fall within
published ranges. Potassium concentrations of both published data and that of this study show
elevated concentrations in the core 249PC environment relative to those at core 223BC.
Uranium is also slightly elevated in both this study and published data at the 249PC site. Core
223BC published data suggests a concentration ranging from approximately 1.4 to 3.8 ppm,
which is very close to what this study observed. The environment near core 249PC shows a
range of 1.9 to 7.3 ppm in published data, which is also almost identical to what was observed in
this study. Though the ranges for core 223BC are large relative to 249PC ranges, they are well
within the concentrations of published geochemical data.
* 249Pc 0-10cm
00 05 10 15 20 25 30 35 40 45 50
K (% )
Figure 5-1. Th/K ratio from core 249PC.
4 223BC 11-12cm
X 223BC 31-32cm
00 05 10 15 20 25 30 35 40 45 50
K (% )
Figure 5-2. Th/K ratio for core 223BC.
16.0 Mixed layep
1.0 1.5 2.0
Figure 5-3. Overlay of Th/K ratio for both cores
H 249PC 11-20cm
X 223 BC 11-12cm
m 223BC 15-16cm
+ 223BC 31-32cm
0 10 20 30 40
50 60 70 80
Figure 5-4. Th/K ratio with percent clay for cores 223BC and 249PC showing little variation
The hypothesis stated previously was that 238U was associated with zircons or other heavy
minerals carried predominantly in the coarse (>63 Cm) fraction, and that 232Th and 40K were
associated with clays (illite, chlorite) and mica in the fine-sized fraction. This was expected to
produce an elevated 238U concentration in core 223BC, and an elevated 232Th and 40K
concentration at core 249PC, since 249PC contains more fine-grained material. Though 249PC
did show elevated 232Th and 40K concentrations due to a higher clay percentage, the 238U was
also increased. This is indicative of the 238U radioisotope also being carried predominantly in the
This study shows no correlation between concentration and the sand-sized fraction. The
sand fraction has been shown to display a wide range of radioactivity and often produces results
similar to those in this study (Blum and Erel 1997; Ayers and Theilen 2001). The variability of
238U and 232Th with each interval makes it difficult to discern precisely where it is being carried,
though it is can be generally associated with the fine-sized fraction. There is particularly good
association of the 40K radioisotope with the clay-sized fraction, it is highest there for each
interval of both cores. Natural gamma activity in this study is thus controlled primarily by grain
size and not mineralogy, though clay mineral assemblages do play a role in determining relative
amounts of potassium. Previous studies have determined that the radioactivity from potassium
often dominates the natural activity of the sediment and can be used as a provenance tool (Ayers
and Theilen 2001).
The second part of the hypothesis suggested that if grain size did play a role in controlling
radioisotopic activities, then it could be possible to determine provenance based on the
radioisotope analysis of each core. Though grain size does show a strong correlation with
potassium, the similar overall clay percentage and mineralogy of the two cores make a distinct
provenance determination difficult. Overall concentrations of core 249PC are higher than those
of core 223BC are therefore distinguishable, but not convincingly. It is very possible that the
Chugach terrane associated with the Chugach and St. Elias Mountains is influencing the source
material at both locations, thus disrupting the unique geological terrane characteristics. This
would imply that the Valdez group and the Yakataga formation are composed of material from
the Chugach terrane. Specific determination source material for the Yakataga formation is
beyond the scope of this thesis, however, the similarity in XRD and petrographic analyses
supports a more similar source material than originally estimated. The slightly elevated
radioactivity at core 249PC is then attributed to a combination of elevated fine-sized sediment
and slightly elevated initial potassium content of the rock assemblages near Bear Glacier and
A secondary goal of the study was to test the validity of aeroradiometric data from the
southern Alaska region. The noticeably high uranium in core 223BC relative to 249PC
illustrated in Figure 1-7 was not seen. This is likely due to the misrepresentation of parent rock
material by aeroradiometric data. The high uranium levels recorded are attributed to
concentrated near-surface coarse material deposition both by receding glacial activity and
aeolian processes. This would concentrate heavy minerals that weren't incorporated into the
finer fraction (such as zircons and monazite) in the coarse fraction while the fine-sized fraction
was transported to the Gulf. Certain storm-induced flooding resulting in high sediment discharge
will still carry some heavy minerals to the Gulf (possibly recorded in core 223BC interval 20-21
cm), however would not be significant enough to prevent a bias in aeroradiometric
measurements at certain depositional environments. Inferring sediment provenance from the
final product is anything but straightforward since it evolves as it is transported from the source,
this affects the near-surface sediments measured by the aeroradiometric surveys in particular
(Weltje and Eynatten 2004).
This study shows that, in general, there is a correlation of radioisotope activity with grain
size. The inconsistencies observed reveal there are other factors, such as mineralogy and surface
adsorption (particularly for 232Th), contributing to overall activity. Activity of the 40K isotope is
the least affected by factors other than grain size, and correlates well with the amount of clay-
sized material. The similarity of source mineralogy makes distinction between locations
difficult. Results do show a slight distinction the two cores, and thus it may be possible to use
this technique for provenance determination between two more unique environments. Th/K
ratios are consistently near 2: 1 at core 223BC, whereas the ratio at core 249PC is variable. Also,
the minimum 40K values of core 249PC are higher than the maximum ones at 223BC. Natural
gamma activity in this study is then primarily controlled by amount of clay minerals and the
potassium content of the clay mineral assemblages, which has been recorded in other studies
(Ayres and Theilen 2001; Carter and Gammon 2004). Naturally it would be ideal to apply a
suite of current techniques to obtain the highest accuracy and precision for provenance
determination. These techniques can be very time consuming and expensive. Geochemical data
from stream and river samples (Weaver 1983) for the GOA margin correlates well with
radioisotopic concentrations measured in this study, proving that it is a non-destructive and
efficient way of accurately determining radioisotope concentration. Given two more unique
environments, this technique could be a very valuable provenance tool.
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Alice Hildick was born in Vermont on February 16, 1979. She grew up in Florida, where
she graduated from Clearwater Central Catholic High School. She got her bachelor' s degree in
geology from the University of Florida in 2001, worked for Geohazards, Inc., then returned to
the University of Florida to get her Master of Science degree. She spent time working at
GeoSierra, LLC in Atlanta for a brief period during her Masters research. Alice' s favorite things
include traveling, surfing, and wine.