IGPP Seminar Series

Discovering Ice on a "Greenhouse" Planet: Evidence for Glaciation in both Hemispheres from 44 to 30 Ma

by Aradhna K. Tripati, Sponsored by the Geoscience Initiative
Department of Earth Sciences , University of Cambridge


The transition from the extreme global warmth of the Eocene “greenhouse” to glacial conditions is one of the most prominent in Earth’s climatic evolution, yet one of the most poorly understood. It is widely accepted that large ice sheets first appeared ~34 Ma, coincident with decreasing pCO2 and a deepening of the calcite compensation depth. Glaciation in the Northern Hemisphere is thought to have begun much later, between 10 and 6 Ma, during a period of stable pCO2. I will present records of sediment and foraminiferal geochemistry covering the “greenhouse-icehouse” climate transition for the tropical Pacific, North Atlantic, and South Atlantic. These records indicate there was significant storage of ice in the Arctic and on Antarctica in the middle Eocene through earliest Oligocene, implying that glacial onset in both hemispheres may have been synchronous. <> The carbonate content of tropical Pacific and South Atlantic sediments contains evidence for oscillations in the calcite compensation depth in these basins from ca. 42 Ma, with a permanent deepening 34 Ma. Comparison with a new composite benthic foraminiferal oxygen isotope (d18O) stratigraphy based on records for three deep Pacific sites at different water depths shows that the most prominent carbonate compensation depth variations coincide with excursions in d18O. This correlation is consistent with changing shelf-to-basin fractionation of carbonate deposition due to glacioeustatic sea level variations. <> Benthic d18O records at all three sites exhibit synchronous changes of greater than 1‰, with peak middle Eocene d18O ratios that are similar to early Oligocene baseline values. The residual extracted by combining paleotemperatures derived from benthic foraminiferal Mg/Ca ratios with the d18O ratios indicates changes in seawater d18O of 0.80‰± 0.15‰. Estimates of the change in sea level have been calculated, and take into account potential errors including uncertainties arising from seawater heterogeneity, oceanic Mg/Ca ratios, carbonate ion concentration, and changes in the d18O of ice during the Cenozoic. If we assume these reconstructed changes in seawater d18O accurately reflect variations in global ocean d18O, then the record implies there were glacioeustatic sea level decreases of approximately 90 meters or greater, necessitating storage of ice in both hemispheres in the Middle Eocene through early Oligocene. This age for the onset of bipolar glaciation is about 30 million years earlier than previously identified. <> Sedimentological data indicates ice-rafted debris (IRD) is present in 43.4 to 30.5 Ma sediments from the Greenland Sea, direct evidence for glaciation in the North Atlantic during this much earlier time interval. Detailed evidence indicates that glaciers extended to sea level in the region during the late middle Eocene through early Oligocene, allowing icebergs to be produced. Ice-rafted debris may have been sourced from tidewater glaciers, small ice caps, and/or a continental ice sheet. Comparison of a moderate resolution record of ice-rafted debris abundance shows that peaks coincide with the timing of major glacial episodes inferred from the history of the carbonate compensation depth and seawater d18O.
Tuesday, 13 May 2008
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM