Assessing Holocene Climate Variability in the Arctic Ocean
Morris, David, Environmental Sciences - Graduate School of Arts and Sciences, University of Virginia
Macko, Stephen, Environmental Sciences, University of Virginia
The western Arctic Ocean is a unique part of the world’s oceans, and is an area undergoing significant climate changes. The effects and possible climate feedbacks of these changes are not yet fully understood. Decreased sea ice cover could lead to increased productivity in the water column, or a reduction in total productivity through the loss of sea ice algal contributions. Climate change could also alter the patterns of terrestrial inputs from rivers and coastal erosion.
In this dissertation, carbon and nitrogen elemental and isotopic compositions from 19 surface sediment samples and 7 cores from 4 shelf to basin transects were analyzed, as well as a longer jumbo piston core and neutral lipid extracts, in order to better understand the balance of organic material inputs in the sediments of the Chukchi and Alaskan Beaufort Seas. Bulk δ13C, δ15N and compound specific isotope analysis on these samples were performed, and it was found that the bulk of the material was comprised of pelagic material, with distinct contributions from sea ice algae, degraded marine material and terrestrial organic matter.
The C:N ratios for surface sediments were between 5.3 and 11.5 (mean 9.0±1.3). The mean %TOC was 1.2±0.3% and mean %TN was 0.16±0.02%. The highest %TOC contents were observed in the Barrow Canyon transect, likely reflecting heightened overlying productivity. The δ13C of this preserved material varied from -22.1 to -16.7‰ (mean -19.4±1.3‰). A trend to depletion in 13C and higher C:N in eastern sites was observed. Among the surface samples, δ15N varied from 4.1 to 7.6‰ (mean 5.7±1.1‰).
The deeper core HLY 090501 provided a sediment record on Holocene timescales. We found that the majority of the organic material was marine, but that much of this marine matter had been reworked and advected to the core location. Mean δ13C was -23.2‰. We also observed a trend towards more depleted 13C downcore (to -24.9‰), which is indicative of a stronger terrestrial contribution to the oldest parts of the core, which was consistent with coastal retreat at the end of the last glacial maximum.
Using compound specific isotope analysis on alkane biomarkers, we found evidence of terrestrial material in the sediments. Due to the odd to even predominance in the distribution pattern and δ13C (mean -31.6‰) of the alkane biomarkers, we concluded that terrestrial material does contribute to the carbon cycle in the Western Arctic Ocean.
An evaluation of the inputs of organic matter to the western Arctic sediment was accomplished using bulk δ13C and δ15N values with the novel technique of a Bayesian analysis multi-source mixing model. This model was used to estimate proportional contributions of sea ice algae, water column phytoplankton and terrestrial organic matter. We conclude that water column productivity is the source of between 50 and 70% of the organic carbon buried in this portion of the western Arctic. The remaining 25-35% of carbon is mainly supplied by sea ice algal productivity, with at most 15% of sedimentary carbon derived from terrestrial inputs.
PHD (Doctor of Philosophy)
Chukchi Sea, organic geochemistry, stable isotopes, Bayesian modeling
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