What do paleo-geochemical tracers tell us about the deep ocean circulation during the last ice age?
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Paleo-tracers such as carbon 13 and cadmium show that the deep Atlantic was enriched in nutrients during the Last Ice Age. The conventionally accepted interpretation of these higher nutrient levels is that a reduction of the rate of formation of nutrient-depleted Lower North Atlantic Deep Water (Lower NADW) allowed nutrient-rich Antarctic Bottom Water (AABW) to push further north during the Last Glacial Maximum (LGM) (Boyle and Keigwin, 1982; 1987; Duplessy et al., 1988). The evidence for this interpretation is re-examined in this work, with an emphasis on the quantitative analysis of the paleo-data. An end-member analysis of the δ13C data indicates a larger volume of AABW and a smaller volume of Lower NADW during the LGM. It is not yet possible, however, to quantify the extent of the volume differences between the modern and the glacial distributions, because the LGM δ13C end-members are poorly known. The second issue examined in this thesis deals with the interpretation of the water mass distribution, inferred from paleo-tracers, in terms of the oceanic circulation. Using a dynamical inverse model of the North Atlantic and a kinematic inverse model of the South Atlantic, it is shown that a tracer distribution corresponding to a significantly reduced volume of Lower NADW does not necessarily correspond to a reduced flux of NADW. Indeed, a circulation almost identical to a modem ocean reference circulation is consistent with the available LGM δ13C and δ18O data A flux of Lower NADW reduced by 50%, though not needed to explain the LGM tracer distribution, is also consistent with the data Thus, the paleo-tracers δ13C and δ18O do not suffice to quantify the flux of NADW in the glacial ocean. The modem ocean circulation is one of many possible circulations consistent with the available δ13C and δ18O data.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1994
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