Glacial/Interglacial and deglacial changes in ocean circulation and their consequences for the global carbon cycle: A model study

Abstract

This dissertation documents model results that quantitatively address the physical, biological and chemical processes that caused the tight correspondence between the late Pleistocene glacial/interglacial climate cycles and concentration of atmospheric CO2. Central to most current hypotheses for the glacial/interglacial CO2 cycles is the ocean's "biological pump." To quantify the net contribution of distinct ocean changes to glacial CO2 drawdown, and to identify the geochemical changes that determine these net CO2 sensitivities, new diagnostic tools are applied to CYCLOPS carbon cycle model. Based on this analysis, it is inferred that the punctuated saw tooth pattern of the CO2 cycles is best explained by the successive activation of Antarctic, Subantarctic and North Atlantic changes, and that much of the deep ice age ocean was ventilated from the North Atlantic. Due to the preservation of radiocarbon signatures, the last deglaciation offers additional constrains on the state of the glacial ocean, and helps to test hypotheses about ocean circulation changes and carbon redistribution. In this deglacial context, this dissertation presents the first attempt to simulate deglacial radiocarbon anomalies found in some sites of the Indo-Pacific, which have been taken as evidence for the popular hypothesis that the deep glacial ocean stagnated to become a reservoir for the sequestration of CO2 from the atmosphere, and that this reservoir of radiocarbon-deplete carbon was vented during deglaciation. These simulations render a uniquely isolated glacial deep ocean unlikely, and strongly argue that the radiocarbon anomalies record local processes rather than a basin-scale release of deep ocean carbon, such that two episodes of abrupt deglacial atmospheric 14C/C decline demand an alternative explanation. While accounting for the uncertainty in the history radiocarbon production, it is found that observations can be closely matched by deglacial model experiments, with abrupt 14C/C decline caused by the repeated initiation of North Atlantic Deep Water formation instead of Southern Ocean changes that release CO2 to the atmosphere. Overall, the competition between North Atlantic and Southern Ocean over the ventilation of the deep ocean found to be central to the ice age CO2 cycles mirrors the "bipolar seesaw" of warming and cooling among the hemispheres.