Models for changes in atmospheric carbon dioxide, ocean geochemistry and circulation during the late Pleistocene

Abstract

This report provides a review of various hypotheses put forward so far as to the role of the oceans in controlling the glacial-interglacial climatic cycles through controls on the atmospheric carbon dioxide. The available paleochemical data appear to provide little support for the hypotheses involving changes in the overall oceanic nutrient inventory, but there are definite indications of substantial differences in the water mass distribution during the glacial and interglacial times. A shift in the vertical chemical structure (greater accumulation of the labile nutrients and metabolic CO@d2@@ in the deep ocean at the expense of the intermediate layers during the ice ages), strongly suggested by the available data, could cause significant increases in oceanic alkalinity (and hence a decrease in atmospheric CO@d2@@) due to CaCO@d3@@ compensation. A likely mechanism for this rearrangement could be an orbital-forced insolation related increase in biological production in the Southern Ocean. This, coupled with greater production of the Antarctic Bottom Water (AABW) as compared to the North Atlantic Deep Water (NADW), seems to explain most of the observations. However, it is still not entirely clear how the system actually works. For example, the role of the dissolved organic matter (DOM) is being appreciated just now, but the factors which constrain the size of the oceanic DOM pool remain hitherto unknown. One of the major shortcomings of the models proposed so far is that they do not take into consideration the regionally varying responses of primary productivity and water circulation to the climatic changes. For example, given the unique seasonally varying circulation pattern and an acute deficiency in dissolved oxygen at mid-depth, the feedback mechanisms operating in the North Indian Ocean could substantially modify the magnitude of the global responses. However, very little data are presently available on the paleoceanography of this region to assess these effects