Variability in the Atlantic Meridional Overturning Circulation During Marine Isotope Stage 3

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Paleoclimate data show that the Earth's climate varied dramatically during the last ice age. Abrupt transitions from cold to warm North Atlantic climate states, spaced several millennia apart, were generally associated with increases in atmospheric carbon dioxide concentrations. While surface climate also varied on shorter, millennial time scales, atmospheric carbon dioxide variability on this time scale was negligible. Changes in atmospheric carbon dioxide concentrations on the longer times scale, and surface climate on both time scales, are generally attributed to changes in circulation within the deep ocean, but our knowledge of deep ocean circulation variability during the last ice age is limited. Detailed knowledge of glacial circulation variability is required to understand the influence of deep ocean circulation variability on surface climate and atmospheric carbon. This project supports education and training of an undergraduate student and a post-graduate investigator. All data generated as part of this study will be made publicly available. The data will also be valuable to the climate modeling community, which uses paleoclimate data to test the same models used to predict future climate change.Bottom water temperature estimates, determined from elemental ratios of benthic foraminifera, from a tropical North Atlantic sediment core, will be used to identify times of deep ocean circulation change during the last 65,000 years. Models predict large and rapid warming at the location and depth (~550 m) of the sediment core in association with a weakening of deep Atlantic water originating from the North Atlantic region. The sediment core has high accumulation rates (30 cm/kyr), and variations in magnetic susceptibility that can be tied directly to Greenland ice core δ18O variations. Proposed radiocarbon and planktonic foraminiferal δ18O analysis will further refine the chronology. Reconstructing deep ocean variability during the last ice age is critical to understanding the role of deep ocean circulation in driving glacial atmospheric carbon dioxide variability, which generally varies significantly only on longer, multi-millennial, time-scale. On the other hand, confirming that deep ocean circulation did (or did not) vary on the shorter millennial time scale will help determine whether or not observed large climate shifts in, for example, tropical Atlantic hydrology, occurred in the absence of deep ocean circulation change. Such a finding would imply that episodes of North Atlantic cooling have significant and widespread climate impacts in the absence of changes in ocean dynamics.