Balance and imbalance: a middle Eocene perspective on coupled carbon cycle and climate change

Abstract

Earth’s global climate is linked to the amount of the CO2 in the atmosphere and oceans. Processes that add and remove CO2 to and from the ocean-atmosphere system should be in balance on timescales of millions of years to maintain habitable conditions at Earth’s surface. However, imbalances in this geological carbon cycle on shorter timescales of approximately 10,000 to several 100,000 years have led to dramatic perturbations of global climate in the past. Present-day global warming due to anthropogenic carbon emissions is a similar phenomenon, but occurs faster than even the most prominent perturbations known from geological history. In this thesis, the concepts of long-term balance and temporary imbalance in the geological carbon cycle are investigated. A large part of this thesis involves silicate rock weathering, which is an important mechanism responsible for long-term removal of CO2 from the atmosphere and has a stabilizing, thermostat-like effect on Earth’s climate. In the first part of this thesis, ways to better understand the weathering history and overall evolution of the carbon cycle across the Cenozoic are explored. In Chapter 2, the established proxies for reconstructions of global chemical weathering are reviewed and various weathering scenarios for the Cenozoic are modeled. Then, in Chapter 3, secular changes in Cenozoic carbonate chemistry are retrodicted and the robustness of published atmospheric CO2 estimates based on boron isotopes is tested. Next, in Chapter 4, estimates of neritic carbonate burial over the Cenozoic are derived with a model based on carbonate alkalinity mass balance. In the subsequent chapters, this thesis focuses on the enigmatic interactions between CO2, weathering and climate during the Middle Eocene Climatic Optimum (MECO), an episode of greenhouse warming that occurred approximately 40 million years ago. In Chapter 5, changes in global silicate weathering during the MECO are reconstructed and carbon cycle model simulations are performed to identify the most plausible cause for the event. In Chapter 6, the extent of warming and potential confounding factors in sea surface temperature proxies during the MECO are addressed by presenting high-resolution surface ocean temperature records from the North Atlantic based on multiple proxies. Finally, in Chapter 7, temperatures and environmental change across the MECO are reported from a continental shelf site in the Tethys realm. The model simulations for the Cenozoic presented here show that the temporal evolution of the carbon cycle and various other element cycles is becoming increasingly well understood. However, it remains difficult to establish a single, convergent weathering history for the Cenozoic through the inversion of marine isotope records due to uncertainties in isotopic compositions of weathering fluxes. A key finding of this thesis is that the extent of MECO warming was global and that the subsequent climatic recovery appears to have been delayed due to a diminished silicate weathering feedback because of a long-term decrease in continental weatherability over the Eocene. Finally, an episode of increased organic carbon burial in the Tethys realm may have acted as an alternative mechanism for CO2 removal and climatic recovery following the MECO.