A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska

Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska John M. Jaeger n, Branden Kramer Department of Geological Sciences, University of Florida, Gainesville, FL, USA

art ic l e i nf o

a b s t r a c t

Article history: Received 6 September 2012 Received in revised form 7 March 2013 Accepted 19 March 2013

The Bering Glacier System is the world's largest surging temperate glacier with seven events occurring over the past century under a range of north Pacific climatic conditions. Onshore records reveal changes in glacial termini positions and evidence of late Holocene glacial advances, but the Little Ice Age (LIA) record of potential glacial surging and associated flooding has not been examined. A 13.6 m-long jumbo core collected on the adjacent continental shelf reveals a 600-yr-long record of sedimentation associated with changing glacifluvial discharge. The chronology is based on 210Pb geochronology and five radiocarbon dates, and the core can be separated into three distinct lithologic units based on the examination of X-radiographs and physical properties: (1) an uppermost unit dating from ∼125 cal yr BP to the present characterized by bioturbated mud interbedded with laminated, thick (5–20 cm) low-bulk density clay-rich beds; (2) a middle unit dating from ∼120–400 cal yr BP that includes numerous interlaminatedto-interbedded low- and high-bulk density beds with infrequent evidence of bioturbation; thick laminated clay-rich beds are rare; (3) a lowermost unit that predates ∼400 cal yr BP and is composed of rare laminated beds grading down into mottled to massive mud. In each of these units, the laminated lithofacies from this mid-shelf location indicates both flood deposition and likely sediment transport in the wave-current bottom-boundary layer. The thick low-density, clay-rich beds in the uppermost unit correlate with historic outburst floods associated with known surge events. Based on previous terrestrial studies, the terminus was at its Holocene Neoglacial maximum extent close to the modern coastline at some point in the middle to late stages of the LIA in southern Alaska (100–350 cal yr BP). During the LIA, preservation of bioturbated intervals is rare while laminated intervals are common. This style of interbedding indicates frequent ( o10 yr recurrence interval) event-scale mud deposition, suggesting that frequent summer flooding and redistribution by winter storms were more prevalent during the LIA rather than the outburst flooding typical of the past century. Rare event-scale bedding indicative of outburst flooding and possible surge events is found within the middle unit, and may correspond to periods with similar climatic trends as in the 20th century. The infrequent deposition of event layers in the lowermost unit could be attributed to the less frequent flooding and/or enhanced diversion of glacial drainage to the eastern terminus instead of present day Seal River. The thickness and depositional frequency of event-scale bedding can be related to Gulf of Alaska tree-ring proxy temperature reconstructions, where more numerous event bed formation occurs when there are more frequent, higher-amplitude temperature excursions. These frequent fluctuations may have prevented the decadallong periods of positive mass balance required to enable numerous surge events during this period. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Bering Glacier Gulf of Alaska Glacial surge Little Ice Age Continental shelf Sedimentation

1. Introduction The dynamic behavior of continental ice and its relationship to global climate change have become a major concern because of the pronounced influence it has on regional climate, glacifluvial runoff, and eustacy. Strong evidence indicates that glaciers are experiencing n Correspondence to: Department of Geological Sciences, University of Florida, PO Box 112120, Gainesville, FL 32611-2120, USA. Tel.: +1 352 846 1381; fax: +1 382 392 9294. E-mail address: jmjaeger@ufl.edu (J.M. Jaeger).

increased ice discharge and accelerating rates of retreat that are likely climate driven (Arendt et al., 2002; Dyurgerov and Meier, 2000; Larsen et al., 2007; Oerlemans, 2005; Solomina et al., 2008). Interpreting the relationship between dynamic glacial behavior and climate forcing can provide a necessary perspective on larger spatial-scale changes observed over the past century (Davis et al., 2009; Oerlemans, 2005; Solomina et al., 2008). Surging glaciers experience quasi-periodic abrupt increases in ice-flow velocity and are found throughout the northern hemisphere under a range of thermal regimes (Barrand and Murray, 2006; Copland et al., 2003; Dowdeswell et al., 1995; Dowdeswell and Williams, 1997; Grant et al., 2009; Harrison and

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iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011

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Post, 2003; Kamb et al., 1985; Moon et al., 2012; Striberger et al., 2011). Although not areally predominant, surging glaciers can dominate temporal fluctuations in ice flow velocity for certain regions (Alley et al., 2006; Moon et al., 2012). Surges rapidly transfer ice from accumulation to ablation zones (Knudsen et al., 2007; Muskett et al., 2003, 2008; Roush et al., 2003; Sauber et al., 2000), releasing the stored water back into the fluvial and marine environment, which have both regional and global implications (Alley et al., 2006; Arendt et al., 2002; Fellman et al., 2010; Schroth et al., 2011). Although the exact mechanisms that lead to surge behavior are known at a rudimentary level (Alley et al., 2006; Eisen et al., 2001; Harrison and Post, 2003; Lingle and Fatland, 2003; Murray et al., 2003; Raymond,

1987), the role of climate change on surge dynamics is less well constrained, but must be related to changes in geometry that control internal shear stresses (Eisen et al., 2001; Harrison and Post, 2003). Glacier geometry variations (ice thickness, areal extent) are governed by climatically controlled changes in glacial mass balances (Dowdeswell et al., 1995; Eisen et al., 2001; Harrison and Post, 2003; Tangborn, 2002). The temporal variability in surge frequency influenced by changes in mass balances has been noted for both temperate and polythermal systems, where negative mass balances lead to longer quiescent periods between surges and positive mass balances result in the opposite (Dowdeswell et al., 1995; Eisen et al., 2001). The Little Ice Age (LIA; ca. cal yr AD 1200–1900) is generally considered a

143°40’W

143°20’W

143°00’W

Wolverine Gl.

60°20’N

Stella

r Lob

e

Bering Lobe

Kaliakh R.

60°10’N

Tsivat R.

Vitus Lake

Tsiu R.

Seal R. 50 m

60°0’N 100 m

82JC

150

m

-75 m 82JC

Malaspina Glacier

-165 m

1

0.5

0

1km

10

0

10km

Fig. 1. (A) Bering Glacier and location of EW0408-82JC (red circle). The jumbo piston core 82JC is located ∼13 km from the mouth of the Seal River in ∼150 m of water. White rectangle is extent of image in B. Solid white lines are 50-m interval bathymetric contours adapted from (Molnia and Post, 2010a). Dashed white line around glacier indicates Little Ice Age maximum position of ice margin (Crossen and Lowell, 2010). Base false-color Landsat Thematic Mapper (bands 2, 4, 6) image taken 10 September 2001. Image courtesy of U.S. Geological Survey. (B) Inset showing swath bathymetric data at core location revealing a relatively flat seafloor. Darker, pixelated stripes are artifacts of data collection. Data courtesy of Center for Coastal and Ocean Mapping (CCOM)/Joint Hydrographic Center (JHC). (C) Malaspina Glacier 150 km east of Bering Glacier from MODIS image taken on 22 August 2003 [http://visibleearth.nasa.gov, catalogue number 5723]. The Malaspina Glacier terminus is currently within 1 km of the coastline and this configuration may be an analogue for the Bering during its LIA maximum extent. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011

Reconstructed Temperature (°C)

J.M. Jaeger, B. Kramer / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 9

LIA-E

LIA-M

3

LIA-L

8 7 6 5 4 3 2 1200

Temp 5-Yr Mean Surge Events 1300

1400

1500

1600

1700

1800

1900

2000

Calendar year A.D. Fig. 2. A January–September paleotemperature reconstruction for the Gulf of Alaska based on tree-ring data (Wilson et al., 2007). Horizontal bars represent periods of significant (95% confidence limits) shifts in values with vertical position of bars representing mean value for time period (Wilson et al., 2007). Highlighted in gray are the early (E), middle (M), and late (L) stages of the Little Ice Age (LIA) in southern Alaska (Wiles et al., 2008). A five-year running mean highlights periods of large temperature excursions during the middle to late LIA. Paleotemperature reconstruction data from http://www.ncdc.noaa.gov/paleo/recons.html.

time of global glacier expansion (Grove, 2004, 2008). For surging glaciers, the general correlation between positive mass balances and surge frequency would imply that surging should have been more frequent during the LIA. However, given the remoteness of most surging glaciers, historical accounts of their behavior during the LIA are lacking. The temperate glacial dynamics of southern Alaska are noteworthy. This area includes the piedmont Bering and Malaspina Glacier systems and is the most extensive glacierized area in continental North America (Molnia and Post, 2010a) (Fig. 1). Within this region, the majority of ice is stored within the Bering Glacier system, the world's largest temperate surging glacier. The most recent major surge event occurred from AD 1993 to 1995 and caused the Bering Glacier to advance ∼9 km within several months into its adjacent proglacial lakes (Fleisher et al., 2010; Molnia and Post, 2010b; Molnia et al., 1994). Holocene changes of the Bering Glacier system have been constrained by glacial termini positions and secondary depositional features, which reveal that the LIA advance of the glacier was the most extensive since the Last Glacial Maximum (Barclay et al., 2009; Calkin et al., 2001; Crossen and Lowell, 2010; Wiles et al., 1999, 2008). Missing from these terrestrial studies is documentation of Bering Glacier dynamics during the LIA, including evidence of surge dynamics resulting from the presumptive positive mass balances during this period. For this study, the sediment record on the continental shelf adjacent to the Bering Glacier in the Gulf of Alaska is examined to document late Holocene glacial dynamics. Large outburst floods associated with glacial surging transport sediment and water in the form of buoyant surface plumes to the Gulf (Molnia and Post, 2010a,b), and Jaeger and Nittrouer (1999) were able to identify outburst flood facies and correlate each deposit to known surges for the last century. It has been suggested that the timing between Bering Glacier surges is climatically driven, with surges occurring after a period of enhanced ice accumulation, similar to other surging glaciers (Eisen et al., 2001; Harrison and Post, 2003; Lingle and Fatland, 2003; Striberger et al., 2011; Tangborn, 2002). Perhaps coincidentally, the 20–30 yr periodicity of historic Bering surges is similar to the multi-decadal periodicity in regional climate known as the Pacific Decadal Oscillation (PDO). Mantua and Hare (2002) and Bitz and Battisti (1999) documented how decadal-scale PDO forcing has a notable impact on the mass balances of maritime glaciers in Alaska. If surge periodicity is related to the rate at which a critical cumulative mass balance is attained (Eisen et al., 2001; Tangborn, 2002), we hypothesize that enhanced periods of positive mass balance at the Bering, such as during the LIA, should lead to more frequent surges. In this study, we describe changes in shelf sedimentation that provide an extended record of sediment discharge from the Bering

Glacier that can be used as a proxy for glacial surges and other processes responding to changes in ice dynamics. To assess this, a multicore, trigger core, and jumbo core were collected at a location that preserves a record of 20th century surge events (Jaeger and Nittrouer, 1999). An age model is developed using 210Pb geochronology and five radiocarbon dates. X-radiographs, physical properties of the core, and grain size distributions are used to identify and interpret lithofacies. The reconstructed climatic conditions for the Gulf of Alaska from Wilson et al. (2007) are used in conjunction with the age model to compare the timing of climatic events to lithologic properties that reflect changes in glacial sediment discharge.

2. Background 2.1. Gulf of Alaska climate The climate in the Gulf is primarily controlled by the position of the Aleutian Low (AL) pressure system, which dominates in spring and winter months, whereas during the summer months, climate is influenced by the North Pacific High pressure cell (Bond et al., 2003; Stabeno et al., 2004). The AL experiences decadal-scale variability in position and intensity as seen in the North Pacific Index (NPI) (Trenberth and Hurrell, 1994) and the Pacific Decadal Oscillation (PDO) (Mantua and Hare, 2002) climate indices. At an eastern or strong position (+PDO), the AL provides increased precipitation to the coast (Anderson et al., 2005; Barron and Anderson, 2011; Bitz and Battisti, 1999; Stabeno et al., 2004) as southerly winds advect warmer surface waters and air masses into the Gulf. During the western or weak position (−PDO), the coast experiences drier conditions as winds are directed along shore. Accompanying these shifts in moisture, surface air temperature fluctuates with the phase shifts of the AL. For the past 80 yr, a strong phase in the AL is associated with warmer air surface temperatures and the weak phase is associated with cooler temperatures (Stabeno et al., 2004). There is no correlation between summer temperature and precipitation but there is a positive relationship between wintertime temperature and precipitation (Stabeno et al., 2004). These changes in atmospheric circulation strongly impact the winter balance of the maritime glaciers in Alaska, which dominate the net balance fluctuations with the greatest influence through changes in precipitation (Bitz and Battisti, 1999; Hodgkins, 2009; Josberger et al., 2007; Neal et al., 2010). Eisen et al. (2001) were able to demonstrate that the recurrence interval of Variegated Glacier surging depended on reaching a rather constant cumulative mass balance (43.5 m ice equivalent), where the annual mass balance could be predicted

iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011

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EW0408 82TC Radioisotope Activity (dpm g-1)

AH-181 111KC

GRA Density(g cm-3)

0 1 2 3 4 5 6 7 8 9 10 1.5 1.6 1.7 1.8 1.9 2.0

0 10 0

20

1994

1994

1994

30 40 50 60 50

70

1966±3

1965

1953±4

1957

1938±6

1938

1917±8

1920

Depth in Core (cm)

Depth in Core (cm)

80 90 100 110 120 130

100

140 150 160 150

170 180 190 200 210

81 MC Total 210 Pb 81 MC 226 Ra 82 TC Total 210 Pb 82 TC 226 Ra

1899±10 1900

200

220

KEY Higher Density Mud Lower Density Mud Mottling

Bivalves

1874±13

Laminae Burrows and Tubes

240

Fig. 3. (left) Radioisotope and gamma-ray attenuation (GRA) bulk density data from cores EW0408 81MC and 82TC. Due to episodic nature of sediment accumulation revealed by X-radiographs (Fig. 6), corresponding steady-state accumulation rates cannot be calculated from 210Pb data. (right) Generalized stratigraphy from core AH181111KC that is co-located with 82TC. Correlation based on density between 111KC and 82TC allows for estimation of chronology for 82TC (1994 surge event ties point is shown). Laminated, lower-density mud layers in both cores are formed by surge-related outburst floods followed by periods of slower sediment accumulation of bioturbated higher-density mud. Modified from Jaeger and Nittrouer (1999).

within 18% rms error from summertime mean minimum temperatures and wintertime total precipitation (dominant control) as measured at nearby Yakutat, along the coast. Eisen et al. (2001) found that the length of time between surges was due to the variability in ice accumulation rates prior to the surge. The recurrence interval was shorter (o15 yr) when the PDO was in a positive phase (1920–1948, after 1977) than when in the negative phase (1948–1977). Tangborn (2002) created a mass balance model for the Bering Glacier and documented that surges tend to follow several consecutive years (5–10) of normal to abovenormal winter balances followed by a period of high temperatures that create more ablation and/or rainfall rather than snowfall. Over century time scales, coastal temperatures for the Gulf have been reconstructed from tree ring data. D'Arrigo et al. (1999) found colder sea-surface temperatures during the LIA, with warming since 1890 and cooling in 1960s and 1970s (−PDO). Wilson et al. (2007) developed a 1300-yr proxy record of coastal January–September temperatures, which revealed that, in addition to the past ∼100 yr, multi-decadal (PDO-like) variability also occurred during ∼AD 800–950, ∼1080–1100, and ∼1300–1400. One notable period when the multi-decadal variability is not

evident is during the LIA (Fig. 2), which occurred in the circumGulf region during three notable periods of glacial advance: early LIA (∼AD 1200–1300); middle LIA (∼AD 1650–1750); late LIA (AD 1820–1900) (Barclay et al., 2009; Wiles et al., 1999, 2008). The middle and late phases of the LIA are the most prominent ice advance in the Bering Glacier region during the late Holocene, with widespread glacial till deposition after AD 1600 (Crossen and Lowell, 2010; Wiles et al., 1999, 2008). Although the LIA generally was not a time of multi-decadal temperature shifts like the 20th century, it was a period when temperatures were as warm as the past century and temperature transitions of 4 1C occurred over o10 yr (e.g., ca. AD 1690–1700) (Fig. 2). 2.2. Bering Glacier The Bering Glacier system extends down from the Bagley Ice Valley in the St. Elias Mountains forming the Stellar and Bering piedmont lobes (Molnia and Post, 1995, 2010a). During the midHolocene, 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 (Crossen and Lowell, 2010; Molnia and

iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011

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Post, 1995, 2010a; Muller and Fleisher, 1995). It began to advance from this inland position ∼5000 cal yr BP building the Neoglacial foreplain observed today (Crossen and Lowell, 2010). Before ∼AD 1600, the Bering Foreland was dominated by gravel outwash plains and lakes (Crossen and Lowell, 2010; Muller and Fleisher, 1995; Wiles et al., 1999). During the middle to late LIA, the Bering reached its Neoglacial maximum, terminating within 1 km of the modern coast (Crossen and Lowell, 2010). Since then, the terminus has receded with notable exceptions during surges in ∼1900, ∼1920, 1938–1940, 1957–1967, 1993–1995, 2008–2011 (Molnia and Post, 2010b). The Bering lobe is currently terminating into proglacial Vitus, Tsiu, and Tsivat Lakes, which drain into the Gulf via the Seal River (Molnia and Post, 2010a; Fig. 1). During the Little Ice Age, it was likely that a larger portion of Bering meltwater discharged through the eastern Kaliakh, Tsiu, and Tsivat Rivers directly into the Gulf (Crossen and Lowell, 2010; Molnia and Post, 2010a; Muller and Fleisher, 1995; Wiles et al., 1999). Melting of the Bering Glacier results in two different types of flooding; typical summer meltwater peaks in discharge and outburst floods (Molnia and Post, 2010a). Associated with surges are large outburst floods that discharge ∼70% more than 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) with additional accumulation in Tsviat and Tsiu Lakes (Fleisher et al., 2003). The finer-grained sediment not deposited in these lakes is transported to the Gulf of Alaska in a buoyant surface plume (Molnia and Post, 2010a). Jaeger and Nittrouer (1999) recognized that outburst floods are deposited as thick low-density beds that are internally laminated and were rapidly deposited (Fig. 3). Using 210Pb geochronology, they related the outburst facies to historical surge events. In contrast, distinct strata representing annual meltwater floods during non-surge years were not observed in these cores. 2.3. Gulf of Alaska shelf sedimentation The majority of the sediment entering the Gulf 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 of large volumes of freshwater runoff to the Gulf coupled with strong along-shore winds produces the Alaskan Coastal Current (ACC) (Stabeno et al., 2004; Weingartner et al., 2002). The ACC flows in a western direction causing buoyant surface plumes entering the shelf to be directed to the west and generally remain inshore of the 50 m isobath (Jaeger and Nittrouer, 2006). The combination of the ACC and elevated wave orbital velocities shallower than 50 m creates a high-energy inner shelf environment with minimal mud accumulation (Carlson et al., 1977). Downwelling currents are produced by eastern alongshore winds during the winter months (Stabeno et al., 2004) and may contribute to across-shore sediment transport (Jaeger and Nittrouer, 2006). Also during the winter, strong storms produce higher wave energy than in the summer months causing the resuspension of muddy sediment in water depths of o100 m (Jaeger and Nittrouer, 2006).

3. Methodology During the 2004 R/V Maurice Ewing cruise EW0408, a 0.5 mlong multicore EW0408‐81MC1 (591 56.5729′N, 1431 43.6805′W, 166 m water depth), a 13.6 m-long jumbo piston core EW0408‐ 82JC (591 56.6094′N, 1431 43.3545′W, 154 m water depth) and its 2 m-long trigger core EW0408‐82TC were collected ∼16 km southwest from the mouth of the Seal River on the continental shelf

5

(Fig. 1). This site is the same location where core AH-181-111KC was collected as described in Jaeger and Nittrouer (1999), a generally flat area west of Bering Trough (Fig. 1B) as seen in multibeam data also collected in 2004 using a bottom-mounted EM1002 bathymetric system (data available from the National Geophysical Data Center, http://map.ngdc.noaa.gov/website/mgg/ multibeam/viewer.htm). Processed data in grid format were provided by the Center for Coastal and Ocean Mapping (CCOM)/Joint Hydrographic Center (JHC). Physical properties and sedimentary structures were examined using a multi-sensor core logger (MSCL) and X-radiographs. Upon collection at sea, the 10-cm diameter jumbo cores were sectioned into 150 cm lengths and immediately processed on a GeoTek Multi-Senor Core Logger (MSCL) for gamma-ray attenuation (GRA) bulk density, and volume magnetic susceptibility recorded at a 1-cm resolution. Duplicate Ocean Drilling Program (ODP)-style plastic u-channels (2
2
150 cm3) were extracted in parallel from the trigger and jumbo cores archived at Oregon State University. After extraction, ODP-style u-channels were re-analyzed with a custom-made magnetic susceptibility track at UF (Thomas et al., 2003) to provide higher-resolution (3-cm horizontal response function) data. The depth scale from the shipboard MSCL data was determined by the shipboard party to represent the most accurate scale for all cores, so the u-channel depths were corrected to the shipboard depths by matching patterns in physical properties and adjusting the depth scale of the u-channels as necessary. X-radiographs of each ∼150-cm long u-channel were taken in ∼30 cm segments with an Acoma PX-15HF x-ray generator. X-radiograph negative films (45 cm-long segments) were scanned at 300 dpi with an Epson Expression 1650 and were spliced into continuous sections in Adobe Photoshop. To ensure that X-radiographs have the same depth scale as the shipboard data, distinct density changes in the logging data were matched with corresponding density changes in the X-radiographs. The X-radiographs were inverted from negatives to positives so dense objects appear opaque and less dense objects appear brighter, and the contrast of each section was enhanced to make fainter sedimentary features more apparent. The lithologic properties of the core were described and interpreted from the X-radiographs (Supplementary material). The X-radiographs also were utilized in the locating of samples from u-channels for grain size analysis. Sediment texture was evaluated using qualitative and quantitative methods. The core logging data were used to determine the relative grain size of the sediment (e.g., low GRA density and low magnetic susceptibility¼ fine grain; high GRA density and high magnetic susceptibility¼ coarse grain) (Dunbar et al., 2009; Gilbert et al., 2002; Rack et al., 1996). Subsamples for quantitative size analyses were extracted in 1.5 cm-thick intervals from the u-channel, weighed, homogenized, disaggregated and wet sieved at 53 μm. A small aliquot of the homogenized sample was dried to determine water content, which was then used to establish the equivalent dry mass used in the particle size analysis. The mass of the sand fraction was determined after wet sieving. The mud fraction was analyzed on a 5100 Micrometrics Sedigraph (Coakley and Syvitski, 1991) and the distribution of the sand portion when more than 10% by weight was determined using a settling column (Syvitski et al., 1991). The separate Sedigraph and settling column data were combined by normalizing the mud and sand fraction to their relative masses to form a complete distribution from sand to clay size particles. A chronology for the core was developed using 210Pb geochronology for the multi and trigger cores and AMS radiocarbon dates for the trigger and jumbo cores. The 210Pb samples were analyzed by gamma spectroscopy using a low-background intrinsic germanium detector. Dried samples were powdered and placed into sealed plastic jars for three weeks to allow for equilibrium to develop between 226Ra and daughters 214Pb and 214Bi (Goodbred

iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011

J.M. Jaeger, B. Kramer / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Table 1 Radiocarbon dates from Bering Glacier continental shelf. The youngest sample is from trigger core, all others from jumbo core. 95.4% (2s) Cal age ranges

Relative area under distribution

Median probability (cal yr AD)

Calibration data

Cal Cal Cal Cal Cal Cal Cal Cal Cal Cal

0.05 1.00 1.00 0.01 1.00 1.00 1.00 1.00 1.00 1.00

1842 1842 1775 1775 1605 1605 1551 1551 1472 1472

Reimer et al. (2009)

OS61559

2.83

Small, whole gastropod

990 7 40

OS62212

8.13

Disarticulated bivalve

1140 7 30

OS61560

10.88

Disarticulated bivalve

12107 30

OS61547

13.18

Disarticulated bivalve

13007 30

yr yr yr yr yr yr yr yr yr yr

AD AD AD AD AD AD AD AD AD AD

and Kuehl, 1998; Sommerfield and Nittrouer, 1999). The samples were then counted for a 24 h period. Total 210Pb activities were determined from the isolated gamma rays at 46.6 keV while 226Ra activity was calculated from the decay of daughters 214Pb (295 keV and 351 keV) and 214Bi (609 keV) assuming secular equilibrium. The 210Pb activity was used to estimate chronologies for the past ∼100 yr (4–5 half lives, t½ ¼ 22.3 yr) by using the first appearance approach (Goodbred and Kuehl, 1998). 137Cs activity was not observed above the detector's minimum detectable activity limit of ∼0.15 dpm g−1, suggesting that the sediments had not acquired noticeable 137Cs activity prior to deposition. Mollusks shells (gastropod and whole articulated or disarticulated bivalve fragments) were collected from the jumbo core and from within the u-channels for AMS radiocarbon dating. After extraction they were rinsed with deionized water and dried in a 50 1C oven for 24 h. The samples were analyzed at the National Ocean Science AMS (NOSAMS) facility. The radiocarbon ages were calibrated with CALIB 6.1.0 (Stuiver and Reimer, 1993), using a Delta R value of 440 yr 780 yr, which is based on a weighted mean of the pre-bomb mollusk data from the Gulf of Alaska region (Reimer et al., 2009). Calibrated dates are expressed as calendar years AD.

4. Results 4.1. Chronology The 210Pb activity profile reveals a general decrease in activity with depth while 226Ra activity remains close to 1 dpm g−1 (Fig. 3). The 210Pb profile can be characterized as a non-steady state profile (Jaeger et al., 1998), which is evident as relative lows in activity near supported levels at 28 cm and 98 cm depth. Both of these depths are associated with low-density beds. 210Pb activities decay close to supported levels (4–5 half-lives, or ∼100 yr) at 1.48 m depth. Based on the first appearance of excess activity approach (Goodbred and Kuehl, 1998), average sediment accumulation rates are ∼1.5 cm yr−1. The AMS radiocarbon dates increase with depth from ∼900–1300 14C yr BP. (Table 1). Calibrated ages range from modern to ∼600 cal yr BP. (Table 1 and Fig. 4). 4.2. Physical properties and lithofacies The shipboard whole-round core logging data show large variations at decimeter to sub-decimeter scale throughout (Fig. 5). Comparison of the shipboard trigger and jumbo core GRA density reveals patterns in high and low values over the upper ∼2 m, with some compaction seen in the jumbo core at ∼0.5 m and 1.0 m. Correlation of the two records indicates that ∼20 cm of compaction/loss occurred in the jumbo core (1.65 m depth in JC¼ 1.85 m depth in TC). Volume

1773–1783 1699–1949 1677–1875 1593–1609 1517–1678 1457–1806 1475–1627 1414–1695 1396–1548 1330–1639

0

Unit 1

cm

895 7 30

.5

Disarticulated bivalve

~1

2.11

2

Unit 2

4 /y

OS61558

/y

C age yr BP

cm

14

.5

Description

~2

Depth in core (m)

Depth in Core (m)

Lab code

6

8

10

Unit 3 /y

1.5

12

cm

14 LIA- M

LIA-E

1100

1200

1300

1400

1500

1600

1700

LIA-L

1800

1900

2000

cal Year A.D. Fig. 4. Calibrated 14C ages of mollusks from 82TC (2.11 m) and 82JC (2.83, 8.13, 10.88, 10.18 m). Two-sigma probability distributions for each sample are shown. A non-steady age-depth model is shown by dashed line and is based on estimating sediment accumulation rates from 210Pb data for 0–1.8 m and the relative preservation of primary sediment fabric below ∼2 m following criteria in (Jaeger and Nittrouer, 2006). Highlighted in gray are the three stages of the Little Ice Age (LIA) in southern Alaska (Wiles et al., 2008).

magnetic susceptibility ranges 60
10−6 SI units in the upper 2 m, but only ∼30 units in the lower ∼12 m. Likewise, 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. A compaction trend can be removed from the GRA bulk density data, which better reveals the larger amplitudes in density apparent over the upper 2 m. Z-scores calculated from the residual density data can be used to delineate anomalous (41 or o−1) density values, which correspond in Xradiographs to less dense, laminated beds (o−0.07 g cm−3) or denser, occasionally cross-laminated beds (40.07 g cm−3) (Figs. 5–8). Higher magnetic susceptibility and GRA density values can occur in coarser sediment due to the higher concentration of iron-bearing trace minerals in the sand fraction and the lower porosity of sandier units, respectively (Dunbar et al., 2009; Evans and Heller, 2003; Gilbert et al., 2002; Rack et al., 1996). X-radiographs reveal that the strata are dominantly physically stratified, becoming more prevalent upcore (Figs. 6–9; Supplementary material). The uppermost 1.8 m are composed of thick (5–20 cm), faintly to well-laminated low-density beds (lighter in

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Volume Magnetic Susceptibility (x10-6SI units) 20

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82JC GRA Bulk Density (g cm-3) 120 1.4 1.6 1.8 2.0 2.2

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z =-0.36*e(-0.39*z)+2.04 2 R =0.66

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5 6 7 8 9 Unit 2 Unit 3

10 11 12 13 14

Fig. 5. Shipboard GRA bulk density and volume magnetic susceptibility for cores 82JC and 82TC illustrating the physical properties of each lithologic unit. Trends in density values in 82JC and 82TC generally match, with the JC values altered by compaction and stretching, resulting in a ∼0.2 m vertical offset between cores. A compaction trend (equation and R2 value shown) can be fit to density data resulting in residual density values that better highlight depth-variable trends in density. Residual density values o 1s are highlighted in gray and hatched pattern, depending on interpreted lithofacies. Dashed lines delineate section breaks in the core, and density and susceptibility values around these breaks may be spurious.

tone in X-radiograph positive images) interbedded with denser (darker), laminated to bioturbated beds (Fig. 6). The low-density beds in the X-radiographs correlate with low GRA bulk density and low magnetic susceptibility values, and the denser beds correlate with higher values (Fig. 6). It is also evident that the susceptibility data correlate with decimeter-scale variations in lithology while the density data correlate well with cm-scale changes observed in the X-radiographs. The lithology from 1.8–10 m is characterized by interlaminated-to-interbedded low- and high-density beds with occasional bioturbation (Figs. 7 and 9). Downcore, the thickness of the low-density beds observed in X-radiographs generally decrease to 1–3 cm, and occur at a high frequency (2–3 beds per decimeter), especially in the interval from 2.50 to 6.40 m. Exceptions are decimeter-scale fine-grain sometimes interlaminated beds with sharp lower contacts found at 3.60, 5.20, 5.65, and 8.50 m depth. The alternating low- and high-density beds produce a smaller range in GRA bulk density (0.3 g cm−3), and have more small-scale vertical variability from 2–6 m depth in core than in the uppermost 2 m. Below 10 m, the laminations become rare and are separated by thicker (5–20 cm) bioturbated beds that grade into mottled and massive mud beds (Figs. 8 and 9). The amount of small-scale variability in GRA density and X-radiograph density is less apparent. The bioturbated facies range from totally mottled (∼10.40 m, Fig. 8), to partially bedded, where remnants of depositional fabric are still evident (Fig. 9A). Throughout the core, denser

layers in X-radiographs also exhibit cross-lamination and interlamination with cross-strata of variable bulk density (Fig. 9B). In total, the core can be divided in three distinct lithologic units depending on the relative thickness and frequency of low-density beds: Unit 1 (0–1.8 m); Unit 2 (1.8–10 m); Unit 3 (10–14 m). 4.3. Grain size The most distinctive downcore changes in grain size are related to the alternation between low- and high-bulk density lithologies. The low-density beds observed in all of the three units of the jumbo core were finer grained than the high-density beds (Fig. 10), and the low-density beds in the upper 1.8 m of core were finer than low-density beds below 1.8 m. The low-density beds throughout the core are clay-rich (i.e., high concentration of clay-sized material) with generally 460% clay by mass whereas the denser beds have lower clay concentrations (o60%) and are increased in silt and fine sand. The percentage of sand for the majority of the core remains relatively low (o 10%), although at certain intervals sand percentages were 420% (Fig. 11). The percentage of silt shows the least amount of variability (20–40%) as it roughly parallels sand for the length of the core. The concentration of sand and clay are strongly inversely related. The modal diameter of the sand fraction shows small variation ranging within 1φ for the entire length of core. There is a general tendency

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Residual GRA Density (g cm-3) -0.3 0.85

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Fig. 6. Physical property and lithology representative of Unit 1 lithofacies. GRA data are from shipboard measurements and volume magnetic susceptibility data are from u-channels. X-radiograph positives of parallel right and left u-channels extracted from core 82JC. The X-radiographs were stretched horizontally to accentuate bedding features. Dark beds correlate with higher-density and higher-susceptibility, with the inverse seen for lighter beds. Decimeter-scale bedding is most apparent in this unit.

for finer sand to occur in more clay-rich intervals, but there is no correlation between the modal sand diameter and the total sand concentration. Some very sand-rich intervals (∼9.10–9.11 m) are composed of very fine sand (mode ¼3.7φ), whereas others (∼6.30– 6.31 m; Fig. 9B) are composed of somewhat coarser sand (mode¼3.2φ). The sortable silt is the 10–63 μm (4–6.5φ) size fraction and is a rough indicator of the turbulent shear stress within the bottom boundary layer during deposition (McCave and Hall, 2006). The mean sortable silt diameter does show variation down core, and generally is finer in more clay-rich beds and coarser in sandier intervals (Fig. 11).

5. Discussion 5.1. Chronology An age model was developed from 210Pb radioisotopes, five radiocarbon dates, and interpretations of the lithology from Xradiographs. Episodic, non-steady sediment deposition is evident as low excess 210Pb activity values at 28 cm and 98 cm (Fig. 3), suggesting rapid deposition and minimal scavenging activity in the water column (Dukat and Kuehl, 1995; Jaeger et al., 1998; Kniskern et al., 2010; Sommerfield and Nittrouer, 1999). Both of these decreases in 210Pb activity correlate with the base of a lowdensity bed. A minimum first-appearance sediment accumulation

rate of ∼1.5 cm yr−1 is comparable with rates (1.6–2.0 cm yr−1) determined for the same location by Jaeger and Nittrouer (1999) (Fig. 3), indicating that the upper ∼1.8 m of the seabed accumulated in the past ∼100–120 yr. Given the relatively large (300 yr) 2s range in the calibrated 14C ages, a definitive age-depth model based on 14C cannot be established for the core with the exception that the uppermost 1.8 m (Unit 1) was deposited since AD 1600 and the lowermost 4 m (Unit 3) was deposited before AD 1600 (Fig. 4). A simple agedepth model can be constructed by estimating accumulation rates from 210Pb activity and sedimentary fabric. The 210Pb-derived accumulation rate (∼1.5 cm yr−1) was used for the upper 1.8 m of the core while the accumulation rates for the remainder of the core were assumed based on sedimentary fabric as seen in X-radiographs. The lack of bioturbation and preservation of physical strata between 1.8 and 10.0 m depth indicate accumulation rates of 42–3 cm yr−1 (Jaeger and Nittrouer, 2006). Below 10.0 m depth, bioturbated beds become more dominant while physical strata is not well preserved suggesting that accumulation rates decrease to o2 cm yr−1 (Jaeger and Nittrouer, 2006). The resulting age-depth model suggest that the upper 1.8 m of thicker interbedded deposits formed after AD 1875 (∼125 cal yr BP) and the present, the middle unit (1.8–10.0 m) of thinner but more frequent interbeds formed from AD 1600–1875 (∼125– 400 cal yr BP), and that the lowermost unit formed before AD 1600 (400 cal yr BP). These ages fall within the 2s calibrated ages

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Residual GRA Density (g cm-3) -0.3 4.55

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900

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Fig. 7. Physical property and lithology representative of Unit 2 lithofacies. Data and images were created the same as shown in Fig. 6. Physical property values are less variable in this unit compared to Unit 1 (Fig. 6). Bedding is thinner than seen in Unit 1, with lower-density layers more abundant per length of core.

of measured 14C samples (Fig. 4). Because the shell fragments may have been reworked from shallower depths, these age estimates are maximums for each of the three lithologic units. 5.2. Lithofacies The three lithologic units and corresponding depositional ages can be correlated with major changes in the position of the Bering Glacier terminus from the Little Ice Age to the present (Fig. 1) (Crossen and Lowell, 2010; Wiles et al., 2008, 1999). The terminus was in a more retreated position relative to its LIA maximum during the time Unit 3 was deposited, which was prior to the middle LIA (Fig. 4). Unit 2 was deposited during the middle to late LIA, at which the terminus was at the coast (Figs. 1 and 4). Following the LIA-maximum extent, retreat of the terminus beginning around ∼AD 1900 resulted in the formation of Vitus Lake and the likely creation of the Seal River (Crossen and Lowell, 2010). The position of the terminus affects the routing of glacifluvial drainage and ultimately the transport of sediment to the Gulf. In this section, each unit will be discussed with respect to the positions of the terminus, and how subsequent sedimentation is affected at the core site. 5.2.1. Unit 1 Episodic deposition is noted in the uppermost unit by interbedded high- and low-density beds. The high-density beds are

often characterized by sandier, sometimes bioturbated mud that can lack physical 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 laminated event beds (event layer thickness, Ls, ¼ 5–20 cm) is a product of their increased thickness relative to the thickness of the zone of active bioturbation (surface mixed layer thickness, Lb, ∼3–7 cm; (Bentley et al., 2006; Jaeger and Nittrouer, 2006; Wheatcroft and Drake, 2003). These low-density clay-rich 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 (Fig. 3) (Jaeger and Nittrouer, 1999). In Gulf of Alaska shelf strata, greater perseveration of primary physical stratification is driven by high sediment accumulation rates ( 41 cm yr−1) and is associated with a reduced sedimentary indications of bioturbation as seen in X-radiographs (Bentley et al., 2006; Jaeger and Nittrouer, 2006; Wheatcroft and Drake, 2003). Similar types of clay-rich, laminated facies related to flood deposition have been noted for many modern shelves (Drexler and Nittrouer, 2008; Kniskern et al., 2010; Milligan et al., 2007; Sommerfield et al., 2002; Sommerfield and Nittrouer, 1999). The coarser, denser intervals in Unit 1 likely represent wintertime periods of intense downslope reworking of sand from the inner shelf (o50 m water depth) (Drake, 1999; Fan et al., 2004). The relative deep location of the core site (∼150 m) places it outside the depth of normal wintertime wave base (∼100 m) (Jaeger and Nittrouer, 2006; Powell and Molnia, 1989).

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10

Residual GRA Density (g cm-3) -0.3 10.20

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Fig. 8. Physical property and lithology representative of Unit 3 lithofacies. Data and images were created the same as shown in Fig. 6. Physical property values are less variable and in general higher in this unit compared to Unit 1 (Fig. 6). Bedding is less pronounced and a bioturbated fabric is most common in this unit.

The deposition of these sandier layers immediately above the summertime-emplaced outburst flood beds supports a wintertime period of formation. For the past century, the Bering Glacier has terminated into proglacial Vitus Lake, which is a sediment trap for coarser grains, with primarily silt- and clay-sized particles transported via the Seal River to the Gulf (Merrand and Hallet, 1996; Molnia and Post, 2010a). The sedimentary evidence of this is reflected in the more clay-rich character of the low-density beds observed in the upper unit (postLIA) relative to the low-density beds in the two lower units (Fig. 10), a period when Vitus Lake was likely absent. The trapping by Vitus Lake also can be observed in the generally higher concentration of claysized material by mass (460%) and the lower magnetic susceptibility and GRA density in the upper unit (Figs. 3, 5, 6, and 11). Low susceptibility values are observed in other examples of finer-grained outburst flood facies (Gilbert et al., 2002). As an outburst flood propagates from the Bering Glacier, the Seal River acts to focus the flood plume out onto the shelf (Molnia and Post, 2010b). Because of the larger discharge during an outburst flood, the plume extends seaward beyond the 50-m isobath, inboard of which is where the normal summertime discharge plume is observed (Jaeger and Nittrouer, 2006; Molnia and Post, 2010a). The denser/coarser internal laminations within the low-density beds suggests fluctuations in transport energy within the Seal River and surface plume during the process of deposition, which can span over a few months (Jaeger and Nittrouer, 1999; Molnia and Post, 2010b). Coarser interlaminations in fine-grain glacial outburst flood deposits also are observed in Greenland fjords (Gilbert et al., 2002).

After a surge, the Bering experiences a 20–30 yr-long quiescent period. During this period, annual meltwater floods produce plumes, sometimes quite small (Molnia and Post, 2010b), and facies within Unit 1 that could be indicative of annual floods (thin, low-density beds) are not obvious, although some lower-density intervals (e.g., 0.3–0.7 m, Fig. 3) could be remnant flood beds that have been subsequently bioturbated. 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 (Molnia et al., 1996). The annual meltwater events most likely produce a smaller annual event layer (Ls o3 cm) on the shelf, which is reworked by bioturbation and not preserved. Bioturbated denser and coarser layers are found overlying outburst flood facies and represent the recolonization of the seabed and reestablishment the bioturbated surface mixed layer (Bentley et al., 2006).

5.2.2. Unit 2 Strata in this unit are interlaminated to interbedded, high- and low-density beds that are separated by relatively infrequent bioturbated intervals. The high- and low-density deposits generally differ from the upper unit because they are thinner (1–3 cm) and more numerous. The finer-grain nature of these beds and sharp contacts with adjacent coarser layers (Figs. 7 and 9) suggest rapid deposition of mud from suspension or within high-concentration (fluid mud) layers moving downslope in the bottom boundary later (Falcini et al., 2012; Fan et al., 2004; Harris et al., 2005; Ogston et al., 2000; Scully et al., 2002; Sternberg et al., 1996; Traykovski et al., 2000, 2007; Wright

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Fig. 9. Representative X-radiograph positive images of main types of sedimentary fabric. (A) Bioturbated and mottled fabric, with partial preservation of primary bedding. (B) Cross-laminated bedding indicative of bottom-boundary layer transport. (C) Interlaminated low-density beds within Unit 2 that may represent outburst flooding. The X-radiographs were stretched horizontally to accentuate bedding features.

et al., 2002). The highly variable grain size within these beds suggests frequent changes in shear stress in the fluvial discharge points and/or water column during settling (Milligan et al., 2007; Wheatcroft et al., 2006). Although the thickness of these event layers (Ls) is reduced relative to Unit 1, the preservation of these deposits suggests that there was an increase in the overall sediment accumulation rate (43 cm yr−1) (Bentley et al., 2006; Jaeger and Nittrouer, 2006). The increase in accumulation rates and reduction of bioturbation suggests sedimentation differs from modern depositional conditions that are dominated by accumulation of 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 (Fig. 1). From this position, glacial drainage and floods would likely flow directly onto the continental shelf. When the Bering 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 (Fig. 1C). A coastal terminus would suggest that there was not a single outlet present to focus flow toward one portion of the shelf but rather an annual meltwater 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 in X-radiographs. The large amounts of sediment associated with modern outburst flooding and the absence of Vitus Lake to trap sediment should produce thick outburst flood deposits during this interval if they had occurred, but there are only a few thicker fine-grain beds in this unit (3.60, 5.20, 5.65, and 8.50 m) with similar characteristics (interlaminated beds with sharp lower contacts) to outburst flood beds seen in Unit 1 (Fig. 9C). The dominance of thinner fine-grain beds in Unit 2 could imply that the area over which outburst flood dispersal increased and/or that more frequent floods resulted in lower sediment yields per event. Without a larger geographic distribution of cores, it is not possible to address the first alternative, but the latter option is discussed below.

The absence of Vitus Lake during the LIA should also allow for coarser sediment discharge to the shelf. This is evident in a more silt-rich signature for the low-density beds of the middle unit compared to the upper unit (Figs. 10 and 11). Sandier beds observed from X-radiographs are more numerous within this unit, but the modal sand size of these beds is similar to other sand beds found throughout the core (Fig. 11). The increase in the number of sandier beds could be a result of increased storm activity causing an increased frequency of downslope sand reworking and deposition (Drake, 1999; Fan et al., 2004). The coarser sortable silt and observation of fine-scale cross-lamination (implying bottom boundary layer transport; Fig. 9B) indicates a higher-energy seabed at this time (Bentley and Nittrouer, 2003; Drake, 1999; McCave and Hall, 2006). With more sandier sediment under for transport, Ls would increase allowing for the preservation of more coarser beds that otherwise would be bioturbated. In contrast, the sandier/siltier beds are less common in the upper and lower units, indicating that these are periods of time when increased volumes of sand were not being deposited at the core site.

5.2.3. Unit 3 The laminated deposits in this unit become less abundant and are interbedded with thicker (≥20 cm) beds of mottled mud (Figs. 8 and 9A). A decrease in the preservation of physical sedimentary fabric suggests a decrease in sediment accumulation rates or in Ls (o 3 cm) to less than Lb. Episodic deposition may still occur but not be preserved because of lower accumulation rates and/or a thicker Lb (Bentley et al., 2006; Wheatcroft and Drake, 2003). Coincident with the deposition of the lower unit prior to the middle LIA, the Bering Glacier terminus was at a more retreated position relative to the LIA maximum. This terminus position corresponded to glacial discharge that was large in the eastern part of the glacial drainage as the Kaliakh, Tsivat, and Tsiu Rivers built an outwash apron between 700 and 1400 cal yr AD (Crossen and Lowell, 2010; Muller and Fleisher, 1995; Wiles et al., 1999).

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Unit 1

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Fig. 10. Representative grain-size distributions for low- and high-density beds as seen in X-radiograph positive images for each of the three lithofacies units. Low-density beds are more enriched in clay-size material than the siltier denser beds, and are most clay-rich in Unit 1. Denser layers are composed of more sortable silt (10–63 μm fraction) reflecting higher bottom boundary shear stress at time of deposition.

This suggests that sediment delivery from the glacier to the core site was greatly reduced during the formation of Unit 3. Assuming a similar volume of sediment discharge per melt season as in Unit 2, the dispersal and deposition of a over a larger inner shelf area should result in a decreased event layer thickness (Ls) at the core site. A decrease in Ls is evident from the X-radiographs as the number of preserved event layers has decreased and the preservation of biogenic fabric has increased.

5.3. Climatic influences on lithofacies formation and flood depositonal frequency The temporal transitions in lithofacies seen at this site reflect changes in the dispersal pathways of glacial drainage in relation to the position of the glacial terminus, and at a larger scale, climate forcing that controls glacial mass balances. A fundamental question that motivated this study was whether the connection

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Grain Diameter (phi units)

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14 Fig. 11. Downcore distributions of mass percent sand, silt, and clay; modal sand diameter of sandier (4 10%) layers; mean sortable silt diameter. Sand and silt concentrations generally co-vary and are inversely correlated with clay content. Lithology becomes more clay-rich up-section. No distinctive trends in modal sand size are noted, indicating relatively consistent source of sand transported from inner shelf. Mean sortable silt diameter co-varies with sand content, becoming coarser in sandier layers. Lines tying points together is done for clarity and do not imply trends between points.

between Gulf of Alaska climate and decadal-scale Bering Glacier dynamics that characterizes the historical record extended back into the LIA. For the past 100–200 yr, the climate in the Gulf derived from tree-ring proxy records has been characterized by decadal-long warm (wet) and cold (dry) periods (Fig. 2). Bering surges occur during both temperature regimes (Fig. 2), but the trigger for a surge appears to require 5 to 10 yr of near normal or above average snowfall (positive mass balance) (Tangborn, 2002), a similar condition observed at Variegated Glacier. Measurements and modeling of mass balances for the maritime Wolverine, Variegated and Bering glaciers using temperature and precipitation records at nearby stations at sea level indicate that there is net accumulation when sea-level temperatures are warm, which generally corresponds to more precipitation. At higher elevations, such as in the Bagley Ice Valley, this precipitation falls as snow, which would lead to a positive mass balance, priming the system for a surge (Tangborn, 2002). The decades before a Bering surge contain both cold and warm periods that last for several consecutive years, but the average range in temperature during the period expressed as a five-year running mean is not more than

∼1 1C (Fig. 2). Consequently, it can be postulated that any decadallong period of similar climatic conditions could be conducive to generating the mass balance conditions necessary to lead to a surge. Prior to the 20th century, several of these periods are evident (e.g., ∼AD 1500–1650, 1760–1820, Fig. 2), which would imply that surges should be occurring during this period. An analysis of the sedimentary units in 82JC can provide insight into whether this hypothesis is tenable. The thick fine-grain lowdensity beds at 3.60, 5.20, 5.65, and 8.50 m have many characteristics of 20th century outburst flood beds suggesting that these are surge-induced deposits. Although the age model for the core is equivocal, these beds fall within the ∼AD 1500–1650 and 1760– 1820 time periods. The overall lack of thicker, outburst facies within Unit 2 indicates that depositional conditions during this period were different than the modern. It is apparent that flooding events were more frequent and there was more cross-shelf transport of sand during the time this unit accumulated. The recurrence interval of flooding events represented by low-density event beds represents the amount of time that has elapsed between the deposition of

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beds, and is calculated by dividing an average thickness of bioturbated strata between event beds by an assumed accumulation rate. Unit 2 has thinner bioturbated intervals (5–15 cm) with some preservation of primary depositional fabric, indicating a higher sediment accumulation rate of these bioturbated strata (4 1–2 cm yr−1, (Jaeger and Nittrouer, 2006), which results in a recurrence interval of o10 yr. The observation of more numerous low-density flood beds in Unit 2 suggests that the glacier was flooding more frequently. The flood recurrence interval is not as prolonged as in the upper unit and may be related to the shorter time periods between shifts from very warm (47 1C) to cold (o4 1C) temperature changes that occur on less then decadal time scales during the Middle LIA (Fig. 2). The bioturbated intervals in Unit 2 may represent a colder, drier period when meltwater flooding was at a minimum, and some of the coldest reconstructed temperatures occur during this time period (Fig. 2). The interbedded low-density layers on the other hand would be produced during a much warmer, moister period when the glacier was actively flooding, meaning that the large volumes of englacial/ subglacial water needed to trigger a surge (Lingle and Fatland, 2003) were not being trapped. We speculate that the much warmer temperature excursions in this period may have prevented the 5–10 yr-long period of normal to above normal mass accumulation needed to trigger a surge. Together, this would indicate that during this time period, the Bering Glacier was not surging as it has during the 20th century. The lower unit represents a period of time with temperature changes of comparable magnitude and frequency to the 20th century, which could be conducive to generating the mass balances trends needed to trigger surging events. However, in the lowermost unit, bioturbated strata dominate indicating that event deposition at the core site is infrequent, with perhaps only a few events per century. Because the primary discharge point of the glacier may have been further east during this period (Crossen and Lowell, 2010; Molnia and Post, 2010a), it is possible that a larger proportion of any outburst or other type of flooding discharge was directed towards the head of the Bering Trough and was not sufficiently large enough to generate a thick-enough event bed that could be preserved at the core site.

6. Conclusions The results of this study reveal that over the past 400 yr, changes in sedimentation on the continental shelf seaward of the Bering Glacier correlate with historical records of surging events, outburst floods, and with Gulf of Alaska paleoclimate tree-ring proxy records. The Bering Glacier terminus position influences the delivery of sediment to the adjacent continental shelf. Sediment accumulation over the past century consists of interbedding of thick mottled to laminated mud beds in the upper 1.8 m of core, which were deposited during a period of time (∼125 cal yr BP to present) when the Bering Glacier terminus is in retreat and a large fraction of sediment discharge from the Bering is trapped in proglacial Vitus Lake. The dominant lithofacies in this unit is thick, laminated outburst flood deposits, which correlate with glacial surge events. These low-density beds representing outburst flood facies have a ∼20 yr recurrence interval within the sedimentary record and deposition of this unit correlates with temperature (and presumably precipitation as seen in instrumental records at sea-level) ranges that may be more conducive to creating positive mass balances necessary to trigger a surge. Accumulation of a second unit (1.8–10 m depth in core) correlates to ∼AD 1600 to 1875 (125–400 cal yr BP), which is the middle to late periods of the Little Ice Age when the Bering Glacier terminated at the coast. This unit contains thin interbedded to

interlaminated facies that represent a period of more frequent flooding (short recurrence interval o10 yr), but this flooding is not likely related to modern glacial surging that reoccurs at longer time intervals. A few examples exist in Unit 2 of event-scale bedding similar to surge-induced outburst flood beds seen in Unit 1 and their depositional period may have been during periods when climatic conditions were similar to the 20th century. However, for most of the time that Unit 2 was accumulating, climatic conditions were different than the upper and lowermost units, and warm (4 7 1C) to cold (o 4 1C) temperature excursions occurred on less then decadal time scales. These more rapid shortterm temperature fluctuations could allow for more frequent flooding during years with extremely warmer temperatures whereas years with colder temperatures would be associated with less discharge and lower sediment accumulation rates. A third sedimentary unit (Unit 3) is below 10 m depth in core and accumulated 4 400 cal yr BP, a period when the Bering Glacier terminated in retreated position relative to the LIA maximum. There is lack of significant physical stratification observed in this unit, suggesting that bioturbation coupled with lower sediment accumulation rates were predominant during this time period. Accumulation of Unit 3 correlates in time with climatic conditions similar to the past century, but no evidence exists at the core site of event scale bedding that could be related to surge-induced outburst floods. The results of this study of continental shelf stratigraphy reveal in greater temporal resolution the dynamic behavior and glacial chronology of the largest surging temperate glacial system during the Little Ice Age period.

Acknowledgments The authors appreciatively acknowledge the crew and science party of cruise EW0408 for assistance in sample collection, and J.E.T. Channell and Kainian Huang at the research center for Paleomagnetism and Environmental Magnetism at the University of Florida, for assistance in magnetic susceptibility measurements. We thank two anonymous reviewers for constructive improvements. 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 the swath bathymetry data. We thank Bobbi Conard and the OSU Marine Geology Repository for aid in core sampling. This research was supported by a grant from the U.S. National Science Foundation Award OCE0351043.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.csr.2013.03.011.

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iPlease cite this article as: Jaeger, J.M., Kramer, B., A continental shelf sedimentary record of Little Ice Age to modern glacial dynamics: Bering Glacier, Alaska. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.03.011