A high-resolution investigation of the eocene-oligocene climate transition: the dawn of the cenozoic icehouse world

Abstract

Global cooling and continental-scale expansion of Antarctic ice sheets occurred approximately 34 million years ago (Ma) during a period of climatic restructuring known as the Eocene-Oligocene Climate Transition (EOT). At this time, the Earth transitioned from the warm greenhouse climate state of the Eocene to the modern icehouse setting that still dominates today. Major changes in marine chemistry and ecosystems accompanied climate change and lowered sea levels, including deepening of the calcium carbonate compensation depth and accelerated turnover in marine plankton. Understanding of EOT climate change, however, is currently limited by the lack of welldated, continuous, and highly resolved marine geochemical records, especially from the Atlantic Ocean. New detailed EOT records are particularly needed to assess relationships between global climate change and potential driving mechanisms, such as changes in atmospheric CO2 and orbital configuration. Thus, with the aim of precisely evaluating the timing, magnitude, and pacing of the climate change leading up to and following the transition, I developed new high-resolution biostratigraphic, lithological, and foraminiferal stable isotope records from two sites in the Atlantic Ocean: (i) Integrated Ocean Drilling Program Expedition 342 Site U1411 on the Newfoundland Margin, Northwest Atlantic Ocean, and (ii) Ocean Drilling Program Leg 208 Site 1263 on Walvis Ridge, Southeast Atlantic Ocean. These records provide a new perspective both on the long-term and short-term climate evolution across the EOT, revealing a distinctive negative carbon isotope excursion at the onset of the EOT, high-amplitude climate cycles on short (~20–40 kyr) astronomical timescales, and a restructuring of the upper water column in the Northwest Atlantic across the EOT. In Chapter 2, a revised composite depth scale and astronomically tuned age model for Site U1411 is presented. The new age model is based on tuning of elemental records generated via Xray fluorescence (XRF) core scanning. Specifically, carbonate concentration and XRF-derived zirconium/rubidium ratios records are tuned in combination to the stable short-eccentricity (~110-kyr) astronomical cycle. The new composite depth scale and astronomical age model provides a detailed chronostratigraphic framework for Site U1411 and enables independent astronomical calibration of stable isotope records, magnetostratigraphic reversals, and planktic foraminiferal and calcareous nannofossil bioevents across the EOT. Strong cyclic (~100 kyr) variations in carbonate accumulation observed at Site U1411 could have been driven by expansion and contraction of the sub-polar gyre influencing nannofossil productivity in the Northwest Atlantic. Chapter 3 presents a new high-resolution (~1.5-kyr resolution) benthic foraminiferal δ18O and δ13C records from Site U1411. These records exhibit characteristic long-term δ18O and δ13C patterns across the EOT but also show prominent and distinctive short-term variability. A marked shift in the pacing of these high frequency cycles is recorded across the EOT, switching from predominantly precession-paced cycles (~20 kyr) throughout the late Eocene to longer-period



cycles (50–80 kyr) in the earliest Oligocene, synchronous with Antarctic ice sheet growth at 33.6 Ma.The results highlight the importance of the Antarctic ice sheet in imparting non-linear feedbacks in the climate system, causing shifts in global pacing of climate variations. Chapter 4 investigates the concomitant response of the surface ocean across the EOT at Site U1411 using multi-species planktic foraminiferal δ18O and δ13C analysis. The records indicate a complex surface-ocean response characterised by an initial collapse of the surface-to-deep thermal gradient at the onset of the EOT at ~34.1 Ma. This collapse is then followed by a gradual re-establishment of thermal stratification and a subsequent abrupt strengthening of thermal gradients synchronous with Antarctic ice sheet expansion at 33.6 Ma. The surface ocean response in the Northwest Atlantic indicate that ice sheet expansion across the EOT also strongly influenced hydrography and surface circulation even far away from the Antarctic continent. Finally, Chapter 5 presents a high-resolution benthic foraminiferal δ18O and δ13C record from Site 1263, which represents the most complete EOT record from the South Atlantic Ocean to date. A characteristic series of stable isotope chemostratigraphic events are recorded at Site 1263 across the EOT, including a prominent late Eocene negative δ13C excursion that is also recorded at Site U1411 and other sites globally. This event, designated the ‘Terminal Eocene Carbon Isotope Excursion’ (TECIE), can be correlated globally and represents a major perturbation in the carbon cycle at the onset of the EOT.

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ORCID for Steven Bohaty: orcid.org/0000-0002-1193-7398

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