a 700-year-long record of glacial surging and...
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A 700-YEAR-LONG RECORD OF GLACIAL SURGING AND ASSOCIATED FLOODING: BERING GLACIER, ALASKA
By
BRANDEN JAMES KRAMER
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
2008
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© 2008 Branden James Kramer
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ACKNOWLEDGMENTS
A special thanks to my family, Barbry, Danny-boy, Justbry, and Kel-Kel, for all of the
support they have shown. Thanks to my girl friend, Erin, for keeping me sane during the stress
caused by writing this masterpiece. I thank my advisor, John Jaeger, and the rest of my
committee (i.e. Ellen Martin and Joann Mossa) for all of the help and guidance that they have
provided me in the last few years. I would like to say thanks to all of my friends in the
department, especially Derrick, David, and Mike. Last but not least I would also like to thank
Gilly for her time spent teaching me how things worked in the lab.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS ...............................................................................................................3
LIST OF FIGURES .........................................................................................................................6
LIST OF ABBREVIATIONS..........................................................................................................8
ABSTRACT.....................................................................................................................................9
CHAPTERS
1 INTRODUCTION ..................................................................................................................11
2 BACKGROUND ....................................................................................................................14
Gulf of Alaska Climate...........................................................................................................14 Bering Glacier.........................................................................................................................15 Preservation of Event Layers..................................................................................................16 GOA Shelf Sedimentation ......................................................................................................17
3 METHODS.............................................................................................................................25
Core Logging Data .................................................................................................................25 X-ray Radiographs..................................................................................................................26 Grain Size Data.......................................................................................................................26 Age Model ..............................................................................................................................27
4 RESULTS...............................................................................................................................28
Chronology .............................................................................................................................28 Lithologic Description ............................................................................................................28 Grain Size ...............................................................................................................................29
5 DISCUSSION.........................................................................................................................39
Chronology .............................................................................................................................39 Lithofacies ..............................................................................................................................40
Upper Unit .......................................................................................................................41 Middle Unit .....................................................................................................................42 Lower Unit.......................................................................................................................43
Lithofacies Interpretations ......................................................................................................44
6 CONCLUSIONS ....................................................................................................................50
LIST OF REFERENCES...............................................................................................................52
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BIOGRAPHICAL SKETCH .........................................................................................................56
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LIST OF FIGURES
Figure page 2-1 The position of the Aleutian Low (AL) pressure system is one of the dominate
controls on climate in the Gulf of Alaska. .........................................................................18
2-2 Records of the Pacific Decadal Oscillation (PDO) index and air surface temperatures from Cordova for the Gulf of Alaska appear to be correlated, which relates a strong or eastern Aleutian Low (AL) to warm temperatures. .......................................................19
2-3 A paleo-temperature record for the Gulf of Alaska is based on tree ring data. .................20
2-4 A reconstruction of three termini positions of the Bering Glacier for the past 850 yrs were derived from radiocarbon dates of buried forest. ......................................................21
2-5 Map of the Bering Glacier, showing its 2001 terminus position that terminates into proglacial Vitus Lake.........................................................................................................22
2-6 The correlation of the 1977 – 1988 Wolverine Glacier positive mass balance with an eastern/strong AL depicted as negative values for the Aleutian Low Pressure Index and positive values for the Pacific Decadal Oscillation Index. .........................................23
2-7 The variables affecting the preservation potential of an event bed include the sediment accumulation rate (t), event bed thickness (Ls), and depth of bioturbation (Lb). ...................................................................................................................................24
4-1 210Pb and 226Ra activity from core 81MC and 82TC and a schematic of core 82TC show decreases in 210Pb activity correlate to low-density beds. ........................................30
4-2 AMS radiocarbon ages for 82JC show a decrease of age with depth. ...............................31
4-3 The shipboard GRA bulk density and volume magnetic susceptibility for core 82JC are illustrating the physical properties of each lithologic unit. ..........................................32
4-4 X-radiograph postivies of parallel right and left u-channels were extracted along the center of sections from core 82JC......................................................................................33
4-5 A characteristic section of lithology with in 2 – 10 m section showing interbedded low- and high-density beds with some bioturbation (483 – 490 cm). ...............................34
4-6 The lithology below 10 m depth in core has rare laminations and is dominated by bioturbated intervals (1027 – 1048 cm). ............................................................................35
4-7 The grain size distributions for low- and high-density beds show low-density beds have a finer signature then high-density beds....................................................................36
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4-8 Grain size distribution of the three low-density beds from Figure 16 showing the upper 2 m of core has a finer signature than the remainder of the core.............................37
4-9 The distribution of grain-size down core for 82JC is shown by a) relative percent mass of sand, silt, and clay, and b) modal sand and mean sortable silt. ............................38
5-1 A non-steady state age model was developed with the use of 210Pb and lithologic based accumulation rates. The increased accumulation rates during 300 – 500 cal yr BP correspond to the timing of the LIA.............................................................................47
5-2 Sediment lithologies are correlated with known changes in regional climate...................48
5-3 An aerial photograph shows the Malaspina Glacier, which is located east of the Bering Glacier at the mouth of Yakutat Bay. ....................................................................49
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LIST OF ABBREVIATIONS
BP Before Present
GOA Gulf of Alaska
LIA Little Ice Age
MSCL Multi-Sensor Core Logger
GRA Gamma Ray Attenuation
GPS Global Positioning System
dpm disintegrations per minute
AL Aleutian Low
NPI North Pacific Index
PDO Pacific Decadal Oscillation
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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
A 700-YEAR-LONG RECORD OF GLACIAL SURGING AND ASSOCIATED FLOODING: BERING GLACIER ALASKA
By
Branden James Kramer
August 2008
Chair: John M. Jaeger Major: Geology
Bering Glacier, Alaska is one of the largest glaciers in North America, and is largely
known for its dramatic surging events, five of which have occurred in the last century. A primary
late Holocene history of the Bering has been previously established from on-shore studies of
glacial termini position and evidence of glacial advances, but the Little Ice Age (LIA) record of
glacial surging and associated flooding has not been examined. A 14 m-long jumbo core
collected on the adjacent continental shelf reveals a 700-year-long record of flood deposition.
The core was dated using 210Pb chronology and five radiocarbon dates, and can be separated into
three distinct lithologic units based on examination of x-radiographs and physical properties: 1)
the uppermost unit dates from ~120 yr BP to the present and is characterized by bioturbated mud
interbedded with faintly laminated, thick (5-20 cm) low-density beds, 2) the middle unit dates
from ~120-500 yr BP and includes abundant laminated-to-interbedded low- and high-density
beds with some evidence of bioturbation, and 3) a lowermost unit post dates 500 yr BP and is
composed of rare laminated beds grading into mottled to massive mud. In each of these units,
the laminated lithofacies from this oxic mid-shelf location indicates both flood and gravity flow
deposition. Based on previous terrestrial studies, from 850-400 yr BP, the terminus was at a
slightly advanced position relative to the present, and it was at its Holocene Neoglacial
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maximum extent, which was close to the modern coastline, during the LIA (200-350 yr BP). The
thick low-density, clay-rich beds in the uppermost unit correlate with historic outburst floods
associated with known surge events. During the LIA, bioturbated intervals are rare and thin
while laminated intervals are common. Given average radiocarbon-supported sedimentation
rates of 2-3 cm yr-1, this style of interbedding indicates frequent, if not annual, deposition at the
core’s location. This would suggest that melt water plumes and redistribution by winter storms
are more prevalent during the LIA rather than outburst flood deposits typical of the past century.
The infrequent deposition of event layers in the lowermost unit could be attributed to the
enhanced diversion of glacial drainage to the eastern Kaliakh, Tsiu, and Tsivat Rivers instead of
present day Seal River. The observation of thinner flood beds during the LIA differs appreciably
from the past century, suggesting that the Bering Glacier was not surging at a multi-decadal
interval, and its position during this period allowed for increased accumulation rates and
preservation of annual melt water flood deposits.
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CHAPTER 1 INTRODUCTION
The dynamic behavior of ice and its relationship to global climate change have become a
major environmental concern. Glaciers in particular are experiencing increased ice discharge
and unprecedented rates of retreat (Joughlin et al., 2008; Nesje et al., 2008; Throst and Truffer,
2008). The retreat is likely climate driven (Joughlin et al., 2008; Nesje et al., 2008; Throst and
Truffer, 2008; Wiles et al., 2008), but there is a poor understanding of glacial response to climate
change over millennial time scales (Brown and Rivera, 2007; Wiles et al., 2008; Zumbühl et al.,
2008). An understanding of dynamic glacial behavior especially the time scale between melting
and/or advance events will provide insight to changes observed in modern day. The study of
modern glacial behavior has been enhanced with the establishment of new techniques for
determining rates of retreat using high-precision GPS (Kumar et al., 2008), ice thickness using
laser altimetry (Arendt et al., 2002; Kumar et al., 2008), and changes in ice mass using Gravity
Recovery and Climate Experiment (GRACE) satellites (Arendt et al., 2007). Observations at this
scale allow for the production of high resolution, temporal and spatial data, but the comparison
of modern data to paleo-data are needed to determine whether the modern observations reveal
anomalous activity or are within the range of previous fluctuations.
The history of Holocene glaciation, in southern Alaska, has been constrained by glacial
termini positions (Calkin et al., 2001; Wiles et al., 2008) and summer air surface reconstruction
for the GOA from tree-ring data (Wilson et al., 2006). Interpreting climatic change from the
terminus position of glaciers is best done using a composite record of glacial terminus histories
to reduce the affects of reworking by the most recent glacier advance (Wilson et al., 2008). The
collection of multiple incomplete or fragmented records could be eliminated by the recovery of a
more complete record. One such environment with the potential to preserve a more complete
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record is the continental shelf. A continental shelf environment proximal to a glacier with (1)
high sediment accumulation rates, (2) episodic deposition, and (3) reduced rates of bioturbation
provide an ideal location to preserve a high resolution stratigraphic record. Studies like
STRATAFORM on the northern California shelf have shown that sediment transport processes
(Wheatcroft, 2000), oceanic storms (Fan et al., 2004), fluvial floods (Wheatcroft et al., 1997;
Wheatcroft and Borgeld, 2000), and anthropogenic influences (Sommerfield and Wheatcroft,
2007) can all be recorded within the marine shelf strata.
For this study, the sediment record on the shelf adjacent to the Bering Glacier in the Gulf
of Alaska (GOA) was used to identify glacial dynamics. The Bering Glacier is known to
experience quasi-periodic abrupt increases in ice-flow velocity, known as surging. The most
recent surge event occurred from 1993 to 1995 (Molnia and Post, 1995) and caused the Bering
Glacier to advance ~9 km within several months into Vitus Lake (Molina et al., 1994). Although
the exact triggering mechanism is unknown, it has been suggested that the timing between surges
is climatically driven, and the events occur at a period of enhanced ice accumulation (Tangborn,
2002; Lingle and Fatland, 2003). Large outburst floods that transport sediment and water in the
form of buoyant surface plumes to the Gulf of Alaska (GOA) are associated with glacial surging.
Jaeger and Nittrouer (1999) were able to identify outburst flood facies and correlate each deposit
to known surges for the last century. The 20-30 yr periodicity of historic surges and shifts in the
Aleutian Low (AL) pressure system could influence the conditions favorable for producing a
positive mass balance in ice accumulation. As the AL shifts from strong/east to weak/west, the
GOA experiences changes in air surface temperature and precipitation. This suggests that a 20-
30 yr period of warm and moist conditions could produce the positive mass balance needed to
trigger a surge, and Wilson et al. (2006) have identified other such multi-decadal periods of
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warm and moist conditions during the past 1300 yrs, suggesting that surging could be a persistent
process of the Bering Glacier.
In this study, a record of shelf sedimentation was examined to provide an extended record
of glacial surging. I hypothesize that Bering Glacier surging will be more frequent when positive
annual mass balance (i.e., higher temperature /moisture delivery) periods persist for several
decades coupled with a lack of colder periods that promote freezing. To test this hypothesis an
age model for a 14 m-long core, EW0408 82JC, was developed using 210Pb geochronology and
five radiocarbon dates. X-radiographs of the core were used to identify sediment facies down
core. The physical properties of the core including gamma ray attenuation (GRA) bulk density,
volume magnetic susceptibility, and grain size were also used to identify and interpret
lithofacies. The interpreted climatic conditions for the GOA from Wilson et al. (2006) were used
in conjunction with the age model to compare the timing of climatic events to lithologic
properties to test the hypothesis.
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CHAPTER 2 BACKGROUND
Gulf of Alaska Climate
Climatic changes in the Gulf of Alaska (GOA) are primarily controlled by the position of
the Aleutian Low (AL) pressure system. The AL dominates climate in spring and winter months
while during the summer months it is dominated by the North Pacific High pressure cell (Wiles
et al., 1998). The AL is also known to experience decadal scale variability, which has been
observed in climate indices such as the North Pacific Index (NPI) (Trenberth and Hurrell, 1994)
and the Pacific Decadal Oscillation (PDO) (Mantua et al., 1997). The NPI measures the change
in sea level pressure to observe the position and intensity of the AL (Trenberth and Hurrell,
1994). At an eastern or strong position, the AL provides increased precipitation to the coast
(Anderson et al., 2005) as winds bring warmer surface waters into the GOA (Figure 2-1a).
During the western or weak position, the coast experiences drier conditions as winds are directed
along shore (Figure 2-1b). Along with the shifts in moisture, the air surface temperature is also
observed to fluctuate along with the phase shifts of the AL. For the past 80 yrs a strong phase in
the AL can be associated with warmer air surface temperatures as the weak phase is associated
with cooler temperatures (Figure 2-2). A multi-decadal shift between the strong and weak phase
of the PDO can also be observed for the past 80 yrs (Figure 2-2). Wilson et al. (2006) developed
a 1300 yr record of coastal temperatures for the GOA based on tree ring data, which revealed the
multi-decadal variability also occurred during ~800-950, ~1080-1100, and ~1300-1400 yr AD
(Figure 2-3). One notable period when the multi-decadal variability is not present is during the
little ice age (LIA; Figure 2-3), which occurred in the GOA region ~1650-1850 yr AD (Wiles et
al., 1999).
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Bering Glacier
The Bering Glacier is one of the largest temperate glaciers in North America with an area
of 5,200 km2. It extends down from the Bagley Ice Field in the St. Elias Mountains where its
lower piedmont lobe is currently composed of the Stellar and Bering lobes and is more than 40
km wide. During the mid-Holocene however the terminus of the Bering Glacier was located 50
km or greater inland, creating a broad bay and narrow fjord where the Bering currently
terminates (Mueller and Fleisher, 1995). It began to advance from this inland position around
5,000 BP building the Neoglacial foreplain observed today. Wiles et al., (1999) were able to
identify three major termini positions of the Bering Glacier during the late-Holocene (850-0 yr
BP). These positions include a more advanced terminus than modern day (850-400 yr BP), its
Neoglacial maximum that terminated at the coast (400-200 yr BP), and its modern position (200-
0 yr BP) (Figure 2-4). The Bering lobe is currently terminating into proglacial Vitus Lake which
drains into the GOA via the Seal River (Figure 2-5).
The Bering Glacier is known to experience quasi-periodic increase in ice flow, known as
glacial surging. The first indications of glacial surging were observed from aerial observations
of folding in the medial moraines of the Bering Glacier (Post, 1972). For many glaciers, surges
appear to follow decadal-long periods of slow flow and thickening. Eventually, an instability
threshold is reached leading to sudden acceleration of flow and rapid ice thinning. The system
then returns to slow, quiescent flow (Meier and Post, 1969; Raymond, 1987). Some surging
glaciers in Alaska (e.g., Bering Glacier, Variegated Glacier) appear to follow this cycle. Eisen et
al. (2001) reconstructed the annual mass balance for the Variegated Glacier, and found that the
length of time between surges was due to the variability in ice accumulation rates prior to the
surge. Also, there is evidence that Bering Glacier surges are triggered by several consecutive
years of above-normal winter balances (Tangborn, 2002; Lingle and Fatland, 2003). A period of
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positive mass balance for the Wolverine Glacier has been observed to correlate with an
eastern/strong phase of the AL (Figure 2-6). The periods of an eastern/strong AL for the past 80
yrs can be correlated to the five historic surges observed for the Bering Glacier (Figure 2-2).
The Bering Glacier experiences two different types of flooding; typical summer meltwater
floods and outburst floods. Associated with Bering Glacier surges are large outburst floods that
discharge ~70% more then a typical summer seasonal ablation (Merrand and Hallet, 1996). Both
types of floods transport large volumes of suspended sediment, most of which is deposited into
Vitus Lake (Merrand and Hallet, 1996; Molnia et al., 1996). The finer grained sediment not
deposited in Vitus Lake is transported to the Gulf of Alaska in a buoyant surface plume. A study
by Jaeger and Nittrouer (1999) recognized that outburst floods are deposited as thick low-density
beds that are internally laminated and were rapidly deposited, based on 234Th data. Jaeger and
Nittrouer (1999) were able to correlate outburst facies to a historical surge event using 210Pb
geochronology on the core. In contrast, the meltwater floods are not well preserved in the
sedimentary record.
Preservation of Event Layers
The preservation of an outburst facies or any event bed in the sedimentary record is
controlled by three factors (1) sediment accumulation rate (t), (2) thickness of the event layer
(Ls), and (3) the thickness of the bioturbation layer (Lb) (Figure 2-7). To preserve the
sedimentary fabric of an event layer in a steady state system, Ls needs to be thicker than Lb
(Nittrouer and Sternberg, 1981). A thicker Ls produces rapid burial of the event layer base and
allows complete preservation. High preservation is also found from the rapid accumulation of
sediment (Nittrouer and Sternberg, 1981). It can be assumed that areas experiencing episodic
deposition have a larger potential to preserve sedimentary fabric (Wheatcroft and Drake, 2003)
than in a steady state system (Bentley et al., 2006). The constant flux of sediment for a steady
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state system produces an Ls << Lb where as episodic deposition can create a situation where Ls >
Lb. The preservation of episodic deposition is also related to the residence time (Tr = [Lb –
Ls/2]/t) or amount of time required to bury the bed below Lb (Wheatcroft and Drake, 2003).
Residence time can be decreased if multiple event beds are deposited consecutively essentially
making Ls >> Lb.
GOA Shelf Sedimentation
The southeastern coastline of Alaska is tectonically active as the Pacific plate subducts
underneath of the North American plate. The combination of high coastal relief and large
amounts of precipitation contribute to one of the largest annual shelf sediment accumulation
rates (250 x 106 tons yr-1) in North America (Jaeger et al., 1998). The majority of the sediment
entering the GOA occurs during the late summer and fall months as elevated glacial melting and
increased precipitation increase the amount of meltwater and runoff (Stabeno et al., 2004). The
input to the GOA of large volumes of fresh water runoff and along-shore winds produces the
Alaskan Coastal Current (ACC) (Weingartner et al., 2002; Stabeno et al., 2004). The ACC flows
in a western direction causing buoyant surface plumes entering the shelf to be re-directed to the
west and remain inshore of the 50 m isobath (Jaeger and Nittrouer, 2006). Also inshore of the 50
m isobath, bottom wave orbital velocities are the highest. The combination of the ACC and
wave orbital velocities inshore of the 50 m isobath creates a high energy environment with
minimal mud accumulation (Carlson et al., 1977). Other currents available to transport sediment
across shelf are downwelling currents produced by eastern alongshore winds during the winter
months (Stabeno et al., 2004). Also during the winter, strong storms produce higher wave
energies than in the summer months causing the resuspension of muddy sediment in water depths
of < 100 m (Jaeger and Nittrouer, 2006).
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a
b Figure 2-1. The position of the Aleutian Low (AL) pressure system is one of the dominate
controls on climate in the Gulf of Alaska. A) The eastern/strong phase produces a warm and moist climate. B) The west/weak phase produces cooler and drier conditions.
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Figure 2-2. Records of the Pacific Decadal Oscillation (PDO) index and air surface temperatures
from Cordova for the Gulf of Alaska appear to be correlated, which relates a strong or eastern Aleutian Low (AL) to warm temperatures. The five shaded regions represent the five historical surges for the Bering Glacier, which occur within 5 yr after a multi-year period of increased warming or strong AL. The climate record and the surges both exhibit a multi-decadal periodicity.
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Figure 2-3. A paleo-temperature record for the Gulf of Alaska is based on tree ring data.
Periods of multi-decadal variability in mean summer air surface temperature are highlighted in blue. Also highlighted is the Little Ice Age (gray), which is not associated with the multi-decadal shifts in temperature. The horizontal bars represent overall mean temperature with no significant difference for the given time period. [Adapted from Wilson, R., Wiles, G., D’Arrigo, R., and Zweck, C., 2006, Cycles and shifts: 1,300 years of multi-decadal temperature variability in the Gulf of Alaska: Climate Dynamics, v. 28(4), p. 425-440.]
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Figure 2-4. A reconstruction of three termini positions of the Bering Glacier for the past 850 yrs
were derived from radiocarbon dates of buried forest.
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Figure 2-5 Map of the Bering Glacier, showing its 2001 terminus position that terminates into
proglacial Vitus Lake. Coloring of the terrestrial landscape represents uncovered ice (blue), debris covered ice (red), and ice free areas (green). The jumbo piston core 82JC is located ~16 km from the mouth of the Seal River in 150 m of water. The location of other cores collected during a 1994-95 cruise are also shown. Bathymetric data at the core location shows surface morphology of varying depth of the shelf and trough. We gratefully acknowledge the USGS for the Landsat image of the Bering Glacier, and Larry Mayer and the staff at the Center for Coastal and Ocean Mapping (CCOM)/ Joint Hydrographic Center (JHC) for collecting, processing, and sharing these swath bathymetry data.
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Figure 2-6. The correlation of the 1977-1988 Wolverine Glacier positive mass balance
(Josberger et al., 2007) with an eastern/strong AL depicted as negative values for the Aleutian Low Pressure Index and positive values for the Pacific Decadal Oscillation Index.
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Figure 2-7. The variables affecting the preservation potential of an event bed include the
sediment accumulation rate (t), event bed thickness (Ls), and depth of bioturbation (Lb). An exponential curve shows the rate of biological mixing (α) is the most intense at the sediment water interface and decreases with depth. [Adapted from Bentley, S. J., Sheremet, A., and Jaeger, J. M., 2006, Event sedimentation, bioturbation, and preserved sedimentary fabric: Field and model comparisons in three contrasting marine settings: Continental Shelf Sedimentation, v. 26, p. 2108-2124.]
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CHAPTER 3 METHODS
During the 2004 R/V Ewing cruise, a 0.5 m-long multicore, 2 m-long trigger core, and 14
m-long jumbo piston core (e.g., EW0408 81MC, 82TC, and 82JC) were respectively collected
~16 km southwest from the mouth of the Seal River on the adjacent continental shelf in 150
meters of water. The collection of three cores allowed for complete recovery with minimum loss
of sediment from the sediment water interface to depth down core. Duplicate Ocean Drilling
Program (ODP)-style u-channels (2 x 2 x 150 cm) were extracted in parallel from the core
archived at Oregon State University. The physical properties and sedimentary structures of the
core were examined using a multi-sensor core logger (MSCL), x-ray radiographs, and grain size
analysis. An age model for the core was constructed using 210Pb geochronology and radiocarbon
dates.
Core Logging Data
After collection at sea, the cores were sectioned into 150 cm lengths and immediately
processed on a GeoTek Multi-Senor Core Logger (MSCL) for the following physical properties;
gamma-ray attenuation (GRA) bulk density, and volume magnetic susceptibility. These data
were processed at a coarse-scale resolution (1 cm). After extraction, ODP-style u-channels were
re-analyzed with an MSCL, at UF, at a finer-scale (0.5 cm) for GRA bulk density and at 1-cm
resolution for magnetic properties.
The depth scale from the shipboard MSCL data was determined by the shipboard party to
represent the most accurate depth scale for all cores, so the u-channel depths were corrected to
the shipboard data. Each section’s depth was corrected by adding an additional 0.5 cm to the
core logging data of the u-channel to match that of the shipboard data. Depth correction to the
shipboard data allows for uniform depths among all cores collected during the 2004 cruise.
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X-ray Radiographs
X-radiographs of each u-channel (~150 cm) were taken in 30 cm segments; 15 cm-long
segments were scanned onto a computer and were spliced into continuous sections using Adobe
Photoshop. To verify that segments were placed together properly, the core logging data were
used to match low density beds from the logging data with low density beds in the x-ray.
Correcting the x-rays with the core logging data also ensures that x-rays have the same depth
scale as the shipboard data. The x-rays were then inverted from negatives to positives so dense
objects appear opaque and less dense objects appear brighter. The contrast of each section was
enhanced to make fainter contacts between beds more apparent. The lithologic properties of the
core were then described and interpreted from the x-radiographs. The x-radiographs were also
utilized in the extraction of grain size samples by printing out true scale versions of the x-
radiographs and placing them next to the actual u-channel to ensure proper sample selection.
Grain Size Data
Numerous grain-size samples were collected from the entire core to provide a detailed
distribution of sediment size. The core logging data were used to determine the relative grain
size of the sediment (e.g., low-density and magnetic susceptibility = fine grain, high-density and
magnetic susceptibility = coarse grain) and to pick samples based on the following properties;
low-density and low-susceptibility, low-density and high-susceptibility, high-density and low-
susceptibility, or high-density and high-susceptibility. The core logging properties were used to
provide a variety of different lithologic attributes found throughout the core, but still have the
ability to compare those samples. Subsamples were extracted at 1.5 cm intervals from the u-
channel, homogenized, and disaggregated so they could be wet sieved at 53 microns. The mud
fraction was then analyzed on a 5100 Micrometrics Sedigraph (Coakley and Syvistski, 1991) and
the sand portion with a settling column (Syvitski et al., 1991). The separate sedigraph and
27
settling column data were then combined by normalizing the mud and sand fraction to their
relative masses to form a complete distribution from sand to clay size particles.
Age Model
A chronology for the core was developed using 210Pb radioisotopes for the trigger core and
AMS radiocarbon dates for the jumbo core. The 210Pb samples were analyzed by gamma
spectroscopy using a low intrinsic germanium detector. Samples were powered and placed into
plastic counting jars to ‘age’ for three weeks to capture 222Rn gas and ensure equilibrium
between 226Ra and daughters 214Pb and 214Bi (Goodbred and Kuehl, 1998). The samples were
then counted for a 24 hr period. 210Pb activities were determined from the isolated gamma rays
at 46.6 keV while 226Ra was based from the decay of daughters 214Pb (295 keV and 351 keV) and
214Bi (609 keV) assuming secular equilibrium. The 210Pb activity was used to develop an
accumulation rate for the past ~100 years (4-5 half lives) by using the first appearance approach
(Goodbred and Kuehl, 1998).
Whole articulated bivalves were collected from the jumbo core and from within the u-
channels for the AMS radiocarbon dating. After extraction they were rinsed with de-ionized
water and dried in a 50°F oven for 24 hours. The samples were analyzed at the National Ocean
Science AMS (NOSAMS) facility for radiocarbon ages.
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CHAPTER 4 RESULTS
Chronology
A chronology for the trigger core was determined using 210Pb radioisotope geochronology
while AMS radiocarbon dates were derived from the jumbo core samples. The profile of 210Pb
and 226Ra activity shows that 210Pb activity decreases from 10-1 dpm g-1 while supported levels
of 226Ra remain close to 1 dpm g-1 with depth in the core (Figure 4-1). The 210Pb profile can be
characterized by a non-steady state profile (Jaeger et al., 1998). This is evident as activity
decreases to supported levels at 28 and 98 cm depth. Both of these depths are associated with
low density beds, which are interpreted to represent flood beds (Jaeger and Nittrouer, 1998). It
should be noted that the 210Pb activities have not yet reached supported levels at 148 cm depth.
Based on the first appearance approach an average accumulation rate of 1.5 cm yr-1 was
calculated (Goodbred and Kuehl, 1998). The AMS radiocarbon dates increase with depth from
~900-1300 14C yr BP (Figure 4-2).
Lithologic Description
The core logging data show large variations in the upper 2 m of core (Figure 4-3). The
GRA bulk density has a range of 0.6 g cm-3 in the upper 2 m while it typically only ranges by 0.3
g cm-3 for the remainder of the core. X-radiographs show that the strata of the upper 2 m are
composed of thick (5-20 cm), faintly laminated low-density beds interbedded with dense
bioturbated beds (Figure 4-4). The low-density beds in the x-radiographs correlate with low
values in the GRA bulk density and volume magnetic susceptibility, and the dense bioturbated
beds correlate with higher GRA bulk density and magnetic susceptibility values (Figure 4-4). It
is also evident that the susceptibility data only show large scale variations in the core while the
density data correlate well with changes observed in the x-radiographs (Figure 4-4). Down core
29
the thickness of the low-density beds observed in x-radiographs decrease to 1-3 cm, and they
become more frequent. The lithology at a depth in core of 2-10 m is characterized by
interlaminated-to-interbedded low- and high-density beds with occasional bioturbation (Figure 4-
5). The alternating low- and high-density beds produce a smaller range in GRA bulk density (0.3
g cm-3), but show more small scale variability from 2-6 m depth in core than in the upper 2 m.
Below 10 m, the laminations become rare and are separated by thick (5-20 cm) bioturbated beds
that grade into mottled and massive mud beds (Figure 4-6). The lack of pronounced density
changes in the x-radiographs still creates the same range of values as from 2-10 m (0.3 g cm-3),
but the amount of small scale variability is less apparent (Figure 4-3). Common throughout the
core are low-density, low-susceptibility beds, but they are thick (5-20 cm) in the upper 2 m, but
are thinner (3-10 cm) below 2 m (Figure 4-3).
Grain Size
Throughout the core, sequences of low- and high-density beds are observed, and these
sequences can be divided in three distinct lithologic sections of the core as described in the
section above. The differences in the grain size distribution of a low- and high-density bed have
been compared from each of the three lithologies. The low-density beds observed in any of the
sections were finer grained then the high-density bed (Figure 4-7), and the low-density bed in the
upper 2 m of core was finer than low-density beds below 2 m (Figure 4-8).
The low density beds found throughout the core are clay-rich with greater than 60% clay
by mass while the dense beds have lower clay percentages (< 60%) and increased silt and sand
percentages (Figure 4-9a). The percentage of sand for the majority of the core remains relatively
low (< 10%), although at 4.5, 6.0, 6.3, 6.8, 9.1, and 11 m depth sand percentages were > 20%.
The percentage of silt shows the least amount of variability (20-40%) as it parallels sand for the
length of the core. The distribution of modal sand diameter shows little variation as it ranges
30
within 1 φ for the entire length of core (Figure 4-9b). The mean sortable silt is the average size
fraction that ranges from 10-63 μm (4-6.5 φ) and is an indicator of flow speed for the depositing
current (McCave et al., 1995). The mean sortable silt does show variation down core, but no
distinct patterns or trends were observed (Figure 4-9b).
Figure 4-1. 210Pb and 226Ra activity from core 81MC and 82TC and a schematic of core 82TC
show decreases in 210Pb activity correlate to low-density beds. Accumulation rates calculated from the 210Pb data were ~1.5 cm yr-1.
31
Figure 4-2. AMS radiocarbon ages for 82JC show a decrease of age with depth.
32
Figure 4-3. The shipboard GRA bulk density and volume magnetic susceptibility for core 82JC
are illustrating the physical properties of each lithologic unit. Depths experiencing a low in density and susceptibility have been highlighted in gray. It should be noted that large shift in the density and susceptibility data in the upper 2 m are not observed with depth downcore. Section breaks in the core are delineated by dashed lines.
33
Figure 4-4. X-radiograph positives of parallel right and left u-channels were extracted along the
center of sections from core 82JC. The x-radiographs were stretched horizontally to accentuate bedding features. Dark beds represent high density and light beds are low density. The SI units for magnetic susceptibility are times 10-5.
34
Figure 4-5. A characteristic section of lithology with in 2-10 m section showing interbedded
low- and high-density beds with some bioturbation (483-490 cm). Ls is typically 1 – 3 cm thick compared to 5-20 cm in the upper 2 m, but events beds are still preserved. The magnetic susceptibility data is not resolved well enough to show the small scale variations associated with the thin alternating event beds although they are present in the GRA bulk density data.
35
Figure 4-6. The lithology below 10 m depth in core has rare laminations and is dominated by
bioturbated intervals (1027-1048 cm). Similar to the middle core interval, the magnetic susceptibility does not record change of small scale variation in the lithology.
36
Figure 4-7. The grain size distributions for low- and high-density beds show low-density beds
have a finer signature then high-density beds.
37
Figure 4-8. Grain size distribution of the three low-density beds from Figure 16 showing the
upper 2 m of core has a finer signature than the remainder of the core.
38
Figure 4-9. The distribution of grain-size down core for 82JC is shown by a) relative percent
mass of sand, silt, and clay, and b) modal sand and mean sortable silt. Clay-rich (> 60%) low-density, low-susceptibility beds are represented by the highlighted gray areas. The modal sand diameter remains relatively constant down core, but the mean sortable silt size does show some variation although there is no discernable down core trends.
39
CHAPTER 5 DISCUSSION
Chronology
To provide age constraint on the lithologic data within the core, an age model was
developed from 210Pb radioisotopes, five radiocarbon dates, and interpretations of the lithology
from x-radiographs. The 210Pb activity was used to calculate a first appearance accumulation
rate by identifying the depth at which 210Pb activities reach supported levels of 226Ra activity.
When supported levels of 226Ra activity have been reached, the 210Pb activity is produced solely
by the decay of 226Ra. At this depth, excess 210Pb that scavenged onto the sediment prior to
deposition has decayed by 4-5 half lives, which represents ~100 yr. In the trigger core supported
levels were never reached, but were trending towards supported near which the deepest
measurement (148 cm) was assumed to be at supported levels. The average accumulation rate
for the upper 2 m of core is ~1.5 cm yr-1. This rate is associated with episodic deposition and a
non-steady state system. Episodic deposition is evident as a decrease in 210Pb activity at 28 and
98 cm (Figure 4-1) suggesting rapid deposition (Dukat and Kuehl, 1995; Jaeger et al., 1998;
Nittrouer et al., 1979). When a particle is rapidly deposited it spends less time in the water
column were excess 210Pb can scavenge onto that particle, reducing the amount of activity
(Nittrouer et al., 1979). Both decreases in 210Pb activity correlate with the base of a low-density
bed, which corresponds with an outburst flood (Jaeger and Nittrouer, 1999).
The radiocarbon ages were calibrated with CALIB 5.0 (Stuvier and Reimer et al., 1993) for
a 700 yr reservoir correction. The 700 yr reservoir correction was derived using paired shell-
wood ages in the GOA (Tom Ager, pers. com.). Calibration to calendar ages of the five 14C ages
produced large ranges for each respective age. These large ranges limit the age model to broad
40
interpretations, but when coupled with the 210Pb data they provide an adequate chronology to
develop an age model.
A curve was fit to the 14C chronology by estimating ages based on accumulation rates
(Figure 5-1). The ages were calculated by dividing a constant depth interval by the 210Pb or
lithologic based accumulation rate. The 210Pb derived accumulation rate (1.5 cm yr-1) was used
for the upper 2 m of the core while the accumulation rates for the remainder of the core were
assumed based on lithologic properties. The lack of bioturbation and preservation of physical
strata between 2-10.4 m depth indicate accumulation rates of > 2-3 cm yr-1 (Jaeger and Nittrouer,
2006). Below 10.4 m depth, bioturbated beds become more dominant while physical strata is not
well preserved suggesting that accumulation rates decrease to ~1 cm yr-1 (Jaeger and Nittrouer,
2006). The resulting age model indicates that the upper 2 m of thicker interbedded deposits
formed between 120 cal yr BP and the present. The age model also indicates that the middle
interval (2-10 m) of thinner but more frequent interbeds formed from 120-500 cal yr BP, and that
the lower bioturbated beds formed between 500-700 cal yr BP.
Lithofacies
The distinct lithologies of the core that were used to determine the accumulation rates for
the age model can be used to separate the core into three distinct lithologic units. The age ranges
for each lithologic unit, determined from the age model, can be correlated with three major
changes in the position of the Bering Glacier terminus (Figure 2-4) (Wiles et al., 1999). The
position of the terminus likely affects the routing of glaciofluvial drainage and ultimately the
transport of sediment to the GOA. In this section, each unit will be discussed with respect to the
positions of the terminus, and how sedimentation is affected at the core site.
41
Upper Unit
Episodic deposition is noted in the strata of the upper 2 m by interbedded high- and low-
density beds (Figure 5-2d). The high-density beds are characterized by bioturbated mud that
typically lack sedimentary fabric. The low-density beds correspond to thick clay-rich deposits
with sharp basal contacts and internal laminations. The preservation of the low-density beds (Ls
= 5-20 cm) is a product of their thickness compared to the thickness of the zone of active
bioturbation (Lb ~3-7 cm; Jaeger and Nittrouer, 2006). These low-density beds have been
correlated to the historic surges and associated outburst floods of the Bering Glacier, and the
bioturbated intervals correlate to the quiescent periods between surges (Jaeger and Nittrouer,
1999).
For the past century, the Bering Glacier has terminated into proglacial Vitus Lake (Figure
5-2b). The lake acts as a sediment trap for coarse grains only allowing silt and clay sized
particles to be transported via the Seal River to the GOA (Merrand and Hallet, 1996). The
sedimentary evidence of this are the low-density beds observed in the upper unit that have a finer
grain size distribution compared to low-density beds in the lower two units (Figure 4-8), a period
when Vitus Lake is not present. This increase can also be observed in the percentage of clay by
mass (> 60%) in the upper unit (Figure 4-9a). As an outburst flood propagates from the Bering
Glacier, the Seal River acts to focus the flood plume out onto the shelf. The focusing of the
plume along with the ACC (Stabeno et al., 2004) cause deposition to be concentrated within the
50 m isobath. The large supply of suspended sediment could create a high concentration regime
where bottom wave orbital velocities allow for the sediment to remain suspended at the seafloor
until a critical concentration (10 g l-1) is reached producing a fluid mud (Fan et al., 2004), and the
resulting deposits produce thick low-density, clay-rich outburst flood facies (Figure 5-2d).
Observation of dense internal laminations within the low-density bed suggests fluctuations in
42
transport energy during the process of deposition, which can span over a few months (Jaeger and
Nittrouer, 1999). After a surge, the glacier experiences a 20-30 yr-long quiescent period.
During this period, annual meltwater floods produce plumes, but flood facies are not evident in
the sediment record. The majority of sediment from annual melt water floods is probably
deposited in Vitus Lake, which has recorded a maximum accumulation rate from seismic profiles
of 10 m yr-1 since 1967 (Molina et al., 1996). The annual meltwater events most likely produce a
smaller Ls (< 3 cm) on the shelf, which is reworked by bioturbation and not preserved. The
bioturbated dense layers can be found overlying outburst flood facies as organisms begin to re-
establish the bioturbated layer (Lb).
Middle Unit
Strata at these depths in core are interlaminated to interbedded high- and low-density beds
that are separated by bioturbated beds (Figure 5-2d). The high- and low-density deposits differ
from the upper unit because they are thinner (1-3 cm) and more frequent. Although Ls is reduced
the preservation of these deposits suggests that there was an increase in the overall sediment
accumulation rate (3 cm yr-1, Figure 5-2c) (Bentley et al., 2006; Jaeger and Nittrouer, 2006).
This is evident in the thin or non-existent bioturbated layers. The increase in sedimentation rates
and lack of bioturbation suggests sedimentation has changed from modern deposition, which is
dominated by thick low-density beds interbedded with bioturbated beds.
The middle unit was deposited around the time when the Bering Glacier reached its
Neoglacial maximum during the LIA and terminated on the coastline (Figure 5-2b). From this
position glacial drainage and floods would likely flow directly onto the continental shelf. When
the Being Glacier was at the coast it was possibly similar to the modern piedmont Malaspina
Glacier, in southern Alaska, with numerous discharge points to the ocean (Figure 5-3). A coastal
terminus would suggest that there was not a single outlet present to focus flow toward one
43
portion of the shelf rather a flood would be dispersed across a larger portion of the shelf, but still
directed westward by the ACC. The dispersal of the same volume of sediment across a larger
area would produce a thinner Ls (1-3 cm), as observed from x-radiographs. The large amounts of
sediment associated with modern outburst flooding and the absence of Vitus Lake to trap
sediment should produce thick flood deposits during this interval, but they are not observed in x-
radiographs. This could imply that the area of flood dispersal increased or that more frequent
floods resulted in lower sediment yields per event.
The absence of Vitus Lake would also allow for the distribution of a coarser range of grain
sizes to the shelf. This is evident in a coarser signature for the low-density beds of the middle
unit compared to the upper unit (Figure 4-8). It is also apparent in the increase of sand beds
observed from x-radiographs, but the modal sand size of these sand beds is similar to other sand
beds found throughout the core (Figure 4-9b). The range of modal sand varies between 3.1-3.8φ
and implies that similar shear stresses were available during transport to the core location. The
increase in the number of sand beds could be a result of increased storm activity causing the
frequency of sand deposition to increase. Another possibility could be related to the volume of
sediment the Bering is delivering onto the shelf while positioned on the coastline. This would
supply a large amount of sand, which can then be transported across shelf by normal winter
storms. With more sediment available for transport, Ls would increase allowing for the
preservation of more sand beds. The sand beds are less common in the upper and lower units,
indicating that these are periods of time when large volumes of sand were not being deposited at
the core site.
Lower Unit
The laminated deposits in this unit become less abundant and are interbedded with thick (≥
20 cm) beds of mottled mud (Figure 5-2d). A decrease in the preservation of physical
44
sedimentary fabric suggests a decrease in Ls (< 3 cm) to less than Lb or in sediment accumulation
rate. A thicker Lb would allow for episodic deposition to still occur, but not be preserved
because of lower accumulation rates (Bentley et al., 2006).
Coincident with the deposition of the lower unit, the Bering Glacier terminus was at a
more advanced position than modern day, but not on the coast (Figure 5-2b). The position of
this terminus caused glacial drainage to be diverted to a more eastern position as the Kaliakh,
Tsivat, and Tsiu Rivers built an outwash apron between 1300-200 yr BP (Muller and Fleisher,
1995). Similar to the middle unit, the dispersal of a constant volume of sediment over a larger
area, results in a deceased Ls. The decrease in Ls is evident in the x-radiographs as the number of
preserved event layers have decreased and the preservation of biological traces has increased.
The first 50 cm of the lower unit consist of low-density laminations interbedded with thick (> 15
cm) bioturbated beds, which suggests episodic deposition (Jaeger and Nittrouer, 2006). This
type of low-density deposit is similar to deposits observed in the middle unit having a coarser
grain size distribution, but they are less common. Below 11 m, the core is composed of a
mottled to massive mud facies indicating complete mixing from bioturbation and lower
accumulation rates (< 1.0 cm yr-1, Figure 18c) (Jaeger and Nittrouer, 2006).
Lithofacies Interpretations
The three units of the core have notable differences in thickness of the low-density event
beds and bioturbated intervals. The change in thickness observed down core is most likely
caused by the dispersal pathways of glacial drainage in relation to the position of the glacial
terminus, as described above. An understanding of glacial behavior with respect to temporal
climatic change can also be utilized to interpret the deposits observed within the core. This can
be done by calculating a recurrence interval for the low-density event beds from each unit. The
recurrence interval represents the amount of time that has elapsed between the deposition of low-
45
density event beds, and was calculated by dividing an average thickness of bioturbated strata
between low-density event beds by an assumed accumulation rate. In the upper unit, the average
thickness of bioturbated strata was 20 cm which are associated with an average accumulation
rate of 1.0 cm yr-1, which corresponds to a recurrence interval of 20 yrs. The middle unit has
thinner bioturbated intervals (5-15 cm) and higher sediment accumulation rate (1.5 cm yr-1)
giving it a recurrence interval of 3-10 yrs. The difference in sediment accumulation rates
between the upper and middle unit was determined based on the amount of preserved physical
strata observed in the bioturbated intervals. The upper unit displayed more prevalent biological
structures (e.g., burrows) with little to no preservation of physical strata while the middle unit
exhibits physical strata that were only partially destroyed.
The 20-30 yr recurrence interval corresponds to the quiescent period before the Bering
Glacier surges. During the quiescent period volumes of water and sediment are stored
underneath the glacier and if enough water is stored over time could produce the buoyant force
that is thought to trigger a surge event. This excess water is possibly produced during the multi-
decadal periods of increased temperature and moisture associated with a strong AL (Figure 5-
2a). The excess water and the positive mass balance created during a strong AL could produce
the instability required to allow a surge to occur. The stored water and sediment is then released
during and after the surge in an outburst flood. The large amounts of sediment and the focusing
of the outburst flood by Vitus Lake and the Seal River combine to help create the thick, low-
density deposits observed on the shelf.
It is evident from the lithology and grain size of the core that glacier surging, as manifested
by thick, low-density beds, is not as pronounced in the lower two units. The low-density beds in
the middle unit are thinner but more frequent while bioturbated intervals were less abundant.
46
The observation of more frequent low-density beds suggests that the glacier was flooding more
frequently, which would continually flush water and sediment from underneath the glacier. Not
allowing large volumes of water and sediment to build underneath the glacier would produce
smaller magnitude floods. This combined with the dispersal of sediment discharge over a larger
area would create a thinner deposit as observed. The recurrence interval between interbedded to
interlaminated low- and high-density sequences was 3-10 yrs. This recurrence interval is not as
prolonged as in the upper unit and may be related to the shorter time periods between shifts from
warm to cold conditions during the LIA (1850-1650 AD, Figure 5-2c). The bioturbated intervals
could represent a colder drier period when meltwater flooding was at a minimum, resulting in
small scale events not preserved in the strata. The interbedded low-density sequence on the other
hand would be produced during a warmer, moist period when the glacier was actively flooding
subglacial water was not being trapped underneath the glacier as observed in modern conditions.
This would suggest that during this time period the Bering Glacier was not surging on the
modern multi-decadal time scale.
Similar to the middle unit, the lower unit represents a period of time with multi-year long
periods between shifts from warm to cool temperatures, but the shifts were not as dramatic as
during the LIA (1700-1450 AD, Figure 5-2a). This would have produced less intense
precipitation during warmer periods compared to the LIA. Overall the lower unit was deposited
when the average climate was cooler and most likely drier causing smaller magnitude flood
events. The combination of a cooler climate and a lack of preserved event imply that this period
of time was not favorable for glacial surging.
47
Figure 5-1. A non-steady state age model was developed with the use of 210Pb and lithologic
based accumulation rates. The increased accumulation rates during 300-500 cal yr BP correspond to the timing of the LIA.
48
Figure 5-2. Sediment lithologies are correlated with known changes in regional climate. The three lithologic units have been
separated by distinct lithologic properties, but can also be described in terms of A) climate and B) glacial terminus position. The change in air surface temperature for the last 1300 yrs was derived from tree ring data collected from the Gulf of Alaska (Wilson et al., 2006). Three terminus positions for the Bering Glacier during the past 850 yr are shown youngest to oldest from top to bottom (Wiles et al., 1999). Also shown in this figure are C) core logging and accumulation rate data and D) x-radiographs of characteristic lithologies from each unit.
49
Figure 5-3. An aerial photograph shows the Malaspina Glacier, which is located east of the
Bering Glacier at the mouth of Yakutat Bay. A modern analog for the terminus position of the Bering Glacier during the LIA, the Malaspina has seven main tributaries draining into the GOA.
50
CHAPTER 6 CONCLUSIONS
The results of this study reveal that over the past 500 years changes in sedimentation on
the continental shelf seaward of the Bering Glacier correlate with historical records of surging
events, outburst floods, and with previously established paleo-temperature record based on
coastal tree-ring data. These correlations suggest that large infrequent outburst floods typical of
the past century are a result of multi-decadal periods of higher precipitation coupled with higher
mean summer temperatures. This study also reveals that the Bering Glacier terminus position
influences the delivery of sediment to the adjacent continental shelf.
Modern day sedimentation consisting of thick interbedded mottled and laminated mud
beds is present in the upper 2 m of core (0-120 yr BP), which was deposited during a period of
time when the Bering Glacier terminates in proglacial Vitus Lake. The lithofacies that dominate
this unit are thick, laminated outburst flood facies, which correlate with glacial surge events.
These low-density beds representing outburst flood facies have a ~20 yr recurrence interval,
which correlates with the prolonged multi-decadal warm trends (~7°C) in mean summer air
surface temperature for the past century. During these prolonged warmer climate periods excess
meltwater could be stored sub-glacially, potentially creating the buoyant force needed to trigger a
surge. Between 2-10 m depth in core (120-500 yr BP), a period when other investigators have
determined that the Bering Glacier terminated on the coast, thin interbedded to interlaminated
facies were preserved. These interbedded facies represent a period of more frequent flooding,
but this flooding is not likely related to modern multi-decadal scale glacial surging as flood beds
have a short recurrence interval (3-10 yr). The more frequent flooding could reduce sub-glacial
meltwater storage, potentially making it less likely to trigger a surge. Unlike climatic conditions
during the deposition of the upper unit, the middle unit did not have multi-decadal periods of
51
consistently warmer temperature. Instead, extreme warm (>7°C) to cold (< 4°C) temperature
changes are observed to occur on less then decadal time scales causing an overall constant mean
air surface temperature (1850-1700 yr AD). This would allow for more frequent flooding during
years with warmer temperatures while years with colder temperatures would be associated with
less discharge and lower sediment accumulation rates. From 10-13.6 m depth in core (> 500 yr
BP), the Bering Glacier terminated at a more advance position than modern day as established by
previous investigators. The lack of preserved physical stratigraphy observed in this unit suggests
bioturbation coupled with low sediment accumulation rates was the dominant condition and
correlates to an overall cooler and possibly drier climate as established by tree-ring records.
Under these drier conditions, meltwater production would be lower, and the conditions favoring
Bering Glacier surging were most likely not present.
This study indicates that the behavior of the Bering Glacier has varied during the late
Holocene. The record however did not extend back long enough to encompass another period of
multi-decadal variability (1300-1400 yr AD) in climate to accurately test the hypothesis.
Because of this reason it was difficult to make any interpretation or comparison on the influence
of the multi-decadal shifts in the Aleutian Low and their affect on the behavior of the Bering
Glacier. The collection of a deeper core at this site would allow for multiple periods of multi-
decadal shifts in the climate to be compared. The ability to compare these conditions could
provide greater insight into the potential influence of climate on glacial surging.
52
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BIOGRAPHICAL SKETCH
Branden Kramer grew up in the little town of Highland Heights, KY, and aspired to be a
marine biologist so he could be the best dolphin trainer in the world. To obtain his dream, he
consistently watched the Discovery channel to soak in the wonderful opportunities the ocean had
to offer. Once he graduated high school, he enrolled into the Marine Science program at Coastal
Carolina University. While at Coastal, Branden realized that the job market for dolphin trainers
was few and far between, so he decided to search for other opportunities within the marine
realm. After a few weeks of searching, he stumbled upon a marine geologist by the name of Eric
Wright that needed help collecting samples one summer. As summer ended, Branden acquired
an interest for the geosciences. Eric allowed him to continue working on various projects and
eventually led to a senior project where Branden used ground penetrating radar (GPR) to study
the stratigraphy and evolution of a barrier island system. Branden had finally found his calling
as a geologist. Knowing that he would want to continue his education, he applied to graduate
schools and was accepted to the University of Florida. During his three years at UF he studied
glacially influenced, continental shelf deposits in the Gulf of Alaska. He received his master’s
degree in August of 2008 and has moved onto the next step of his life.