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Post-LGM Provenance and Flow Regime Reconstruction for the Bering/ Chukchi Seas through Sedimentological and Geochemical Evidence

Pelto, Ben M 1 ; Kocis , James J 2 ; Brigham-Grette, Julie 3 ; Petsch, Steven 4

1 University of Massachusetts Amherst
2 University of Massachusetts Amherst
3 University of Massachusetts Amherst
4 University of Massachusetts Amherst

The exposure of the Bering Land Bridge during the Last Glacial Maximum (LGM) cut off the connection between the North Pacific and Arctic Ocean. By extension, this closed the connection of the North Pacific and North Atlantic Oceans, ending Bering Strait (BS) throughflow. North Pacific Water (NPW) comprises a major portion of the fresh water input to the Arctic Ocean and is of vital importance to the formation of North Atlantic Deep Water formation (NADW) [Wijffels et al., 1992; Aagaard and Carmack, 1994; Keigwin and Cook, 2007]. The flooding of the Bering Strait is thought to have acted as an "exhaust valve" [De Boer and Nof, 2004] for North Atlantic freshwater anomalies when open by enabling more vigorous circulation, and thus quicker dispersion of a freshwater cap over the North Atlantic. Thus the inundation of the Bering Strait around 12-11 ka BP [Elias et al., 1996; Keigwin et al., 2006] and reestablishment of modern oceanography in the Bering and Chukchi Seas was a critical part of global climate change as the Holocene began. Pronounced shifts in sedimentological and geochemical data at crucial climatic intervals such as the Bølling-Allerød warm period, and Younger Dryas (YD) stadial, elucidate changes in sediment sourcing and regional oceanography. A suite of five cores were selected, two in the Chukchi, three in the Bering Sea, in order to bracket the Bering Strait for our examination of the reestablishment of modern oceanography following the LGM and Land Bridge flooding. The age control models (Figure 2) were developed using radiocarbon dates on forams and paired shells obtained by previous studies. Major and trace element geochemistry spanning the past ~20 ka were investigated using an ITRAX XRF core scanner. Biogeochemical investigation of organic matter (OM) sourcing included %TOC, and δ13Corg measured on a PDZ-Europa 20/20 Isotope Ratio Mass Spectrometer at Oregon State University. These data, in concert with the other sedimentological data (Corg/N ratios, δ15N, bulk grain size, bulk density, x-radiographs, and magnetic susceptibility), infer shifts in paleo-flow conditions and sediment provenance during this time period.
Core JPC3 is located on the western side of the Bering Sea Shelf Slope between Navarin and Pervenets Canyons. Elemental XRF data from JPC3 indicates geochemical change during the BA warming (~15 ka), and the opening of the BS (between 11 and 12 ka). During both of these periods there is a decrease in the relative concentration of Ti, Fe, K, and Ba with a corresponding increase in counts of Cl, Ca, and Br (Figure 3). Carbon isotope values (δ13Corg) post LGM in JPC3 are relatively enriched at around -22.5‰, and then drop precipitously to -25‰ at ~17.5 cal ka BP (Figure 3). These values rise slightly through the BA (with variability), and are lower during the YD. Following the YD, δ13Corg rises rapidly to below -22‰ by 10 ka BP. GDGT biomarker sea surface temperature estimates contribute our understanding of water mass changes over time. JPC3 records the transgression of sea level: terrestrial (δ13Corg relatively depleted) sources of OM become more distal, the site became an open water (relatively enriched) location, sea ice cover (relatively enriched) duration decreased, and North Pacific water was able to flow north over the site through the flooded Bering Strait. Interpreting shifts in relative elemental concentrations will require principal component analysis to determine covariance of sedimentologic and geochemical properties of these sediments. Our sedimentological and geochemical evidence from Bering and Chukchi Sea sediment cores both independently and in concert with biomarkers, allows for investigation of the oceanographic change that marked this region following the LGM, and its influence on global climate.

Aagaard, K., and E. C. Carmack (1994), The Arctic Ocean and Climate: A Perspective, in The Polar Oceans and Their Role in Shaping the Global Environment: American Geophysical Union, p. 5–20.

Blaauw, M. (2010), Methods and code for “classical” age-modelling of radiocarbon sequences, Quat. Geochronol., 5(5), 512–518.

Cook, M. S., L. D. Keigwin, and C. A. Sancetta (2005), The deglacial history of surface and intermediate water of the Bering Sea, Deep Sea Research Part II: Topical Studies in Oceanography, 52(16-18), 2163–2173.

De Boer, A. M., and D. Nof (2004), The exhaust valve of the North Atlantic, J. Clim., 17(3), 417–422.

Elias, S. A., S. K. Short, C. H. Nelson, and H. H. Birks (1996), Life and times of the Bering land bridge, Nature, 382(6586), 60–63.

Keigwin, L. D., and M. S. Cook (2007), A role for North Pacific salinity in stabilizing North Atlantic climate: PACIFIC SALINITY AND ATLANTIC CLIMATE, Paleoceanography, 22(3).

Keigwin, L. D., J. P. Donnelly, M. S. Cook, N. W. Driscoll, and J. Brigham-Grette (2006), Rapid sea-level rise and Holocene climate in the Chukchi Sea, Geology, 34(10), 861.

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Woodgate, R., Aagaard, K., Weingartner, T., Whitledge, T., & Lavrenov, I. (2004). Bering Srait: Pacific Gateway to the Arctic. Retrieved from http://psc.apl.washington.edu/HLD/Bstrait/bstrait.html

 

Fig 1.

Locations of the five core sites with bathymetry, present-day major currents, and present-day sea ice edge maximum and minimum extent. Modified plot from Danielson and Weingartner.

 

Fig 2.

JPC 3 Sedimentologic and geochemical data. GEOTEK core scanner data: Red, Magnetic susceptibility, dark brown, GRAPE bulk density from gamma ray attenuation on a split core. EA-IRMS data: Orange, %Corg and green , δ13Corg. Blue dots, Barbados Sea level curve relative to present day, from Fairbanks (1992), dashed blue line indicates the 50 m threshold where the Bering Strait would flood. XRF data from ITRAX core scanner: grey, titanium, light blue, chlorine, and light brown, potassium, red, iron, orange, strontium, and violet, calcium. XRF data are in raw counts taken on ITRAX XRF core scanner at 1000 µm in massive sediment sections, and 200 µm in laminated sections. General sediment lithology black line, 0=no recovery, 1=massive sediment, 2=laminated sediment. Teal, SSTs are GDGT based from Jim Kocis (UMass Amherst).

 

Fig 3.

Age Depth Model JPC3. Six radiocarbon dates on N. pachyderma (sinistral) by Cook et al.(2006). Model developed using Clam 2.2 [Blaauw, 2010] using ΔR = 400 and a spline smoothing curve.

 

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