Global cooling and ocean circulation of the North Atlantic during the Middle and Late Eocene

One of the most extreme examples of rapid climate change in the geologic past is the greenhouse-icehouse transition at the end of the Eocene at ~34 Ma, when Antarctic ice caps rapidly grew to near-modern size in <100 kyr. The current understanding is that this climate transition was paced by orbital (Milankovitch) climate cycles, which triggered accelerated growth of ice-sheets once decreasing pCO2 levels crossed a threshold (DeConto and Pollard, 2003, Nature 421: 245-249). A decrease in pCO2 between 50-34 Ma has indeed been observed, however, the reason for this trend remains unclear (Beerling and Royer, 2011, Nature Geoscience 4:418-420). Detailed records for this period are therefore required to understand the processes that eventually led to the onset of global glaciation. The focus of this project is on the palaeoceanography of the North Atlantic. Ocean Drilling Program (ODP Leg) 171B (Blake Nose) was drilled in an area that is currently situated in the Deep Western Boundary Current (DWBC), a major component of the modern North Atlantic overturning circulation that was initiated at the end of the Eocene. However, there is evidence that a proto-DWBC was already active before that time. A pilot study on sediments from Leg 171B shows that there is considerable variation in both accumulation rates and mean grain-size, which points to winnowing and sediment focussing by contourite currents.
The student will analyse sediment grain-size, and foraminiferal d18O and trace metal content in a suite of samples from ODP Leg 171B drill cores spanning the period from 42-34 Ma. Samples will be collected in the ODP core repository in Bremen. The stable isotope record will be used to correlate the Leg 171B record to climate events recognised in other published drill sites. Grainsize data will be used to establish how current strength at intermediate water depth evolved in the North Atlantic. Combined d18O and trace metal data will be generated in selected intervals to determine if changes in circulation are due to variation in temperature or salinity. It is known that the Eocene was a period of long-term gradual cooling from ~50 Ma until the onset of Antarctic glaciation. High-resolution studies in specific intervals have shown the presence of apparently abrupt climate events that are superimposed on this long-term trend. However, their context in the global climate evolution has yet to be determined. The results from this project will elucidate the interaction between long-term climate trends, superimposed regular cycles, and exceptional events.