The Ocean Circulation and Carbon Cycle during the Last Glacial Maximum Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bv73c506s

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  • During the Last Glacial Maximum (LGM, ∼ 21 ky before present) the atmospheric CO2 concentration was about 100 ppm lower than its pre-industrial (PI)value. The missing carbon from the atmosphere must have been stored in thedeep ocean during this period, but the mechanisms driving such re-distribution ofthe carbon cycle are still uncertain. LGM-PI changes in circulation, stratification,and or biogeochemistry have been suggested to enhance ocean carbon storage, butquantitative, three-dimensional, data-constrained estimates of these effects remainscarce.The most recent simulations from the Paleoclimate Model IntercomparisonProject 3 (PMIP3) predict an increase and deepening of the LGM Atlantic Meridional Overturning Circulation (AMOC) with respect to PI simulations, althoughthis is inconsistent with the interpretation of most sedimentary proxy data attributed to this period. The goal of this dissertation is to use an ocean model toconstrain the LGM global ocean circulation and biogeochemistry, and asses theireffects on the carbon cycle.We use a three dimensional global circulation model, coupled with a biogeochemical model that includes the interactive cycles of radiocarbon (14 C),15N and 13C. The inclusion of these three isotopes provides a powerful tool to constrainthe possible LGM scenarios, since model results can be directly compared to measurements of the same isotopes from the sediment records. Our physical LGMmodel set up includes changes in atmospheric CO2 , continental ice sheets, orbitalparameters and circulation.By varying meridional moisture transport of the model’s atmospheric component, we produce LGM circulations with different AMOC strengths and depths,from a collapsed state, to a strong state similar to PMIP3 models. We find that aweak (6 − 9 Sv) and shallow AMOC underlaid by a more voluminous and carbonrich Antarctic Bottom Water (AABW) best reproduces glacial δ 13 C and radiocarbon ages from sedimentary data. This configuration of water masses also maximizes the amount of remineralized organic carbon stored in deep waters. Wepropose that increased wind stress over the North Atlantic stabilizes the weakAMOC and prevents it from collapsing.We also evaluate effects of PI-to-LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean’s iron and carbon cycles. We find thatiron fertilization caused by enhanced dust deposition in the LGM is strongly countered by a decrease in sedimentary flux due to 125 m lower sea level, which tendsto decrease primary productivity. In an upper-limit estimation of Southern Oceanatmospheric iron fertilization, assuming an increase in soluble iron deposition ofthe order of ∼ 10 − 20 times its PI value for this region, combined with changes insedimentary flux, we obtain higher export production and enhanced accumulationof remineralized organic carbon in deep waters, which would lead to atmosphericcarbon sequestration. This bigeochemical state also improves the agreement withδ 13 C and δ 15 N reconstructions from the LGM.The combined results suggest that a weak and shallow AMOC and enhancediron fertilization conspired to maximize carbon storage in the glacial ocean, andproduce part of the glacial-interglacial variations in atmospheric CO2 concentrations.
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