Climate, carbon cycling and marine ecology during the Paleocene-Eocene Thermal Maximum

Abstract

The Paleocene and Eocene are characterized by strong greenhouse climates. Atmospheric CO2 concentrations and global temperatures were much higher than today. The period from 60 to 50 million years ago (Ma) is marked by a gradual warming trend of ~8 ºC in the deep ocean. The interval from 56 to 50 Ma is further characterized by several transient perturbations of the carbon cycle. Essentially, these perturbations, or “hyperthermal” events mark phases of rapid (103 – 104 years) warming, associated with a drop in the stable carbon isotopic composition of the global exogenic carbon pool, which provides strong evidence that these warming events are related to rapid anomalous carbon input from a reservoir outside the global exogenic carbon cycle. The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is the hyperthermal with the largest magnitude of global warming (+4 ºC) and negative carbon isotope excursion (CIE; >3‰). Ample evidence for large-scale environmental perturbations and biotic migrations have been recorded, such as the quasi-global acme of the subtropical dinoflagellate cyst (dinocyst) genus Apectodinium. As global temperatures increased, the hydrological cycle accelerated and oceans experienced lower oxygen concentrations, both in bottom and surface waters. Several outstanding questions regarding carbon cycling, biotic response, spatial characteristics of temperature change during the PETM remain and are addressed in this thesis. A first record showing Northern Hemisphere sea surface temperature (SST) evolution across the late Paleocene to Early Eocene Climate Optimum (EECO) is presented, based on a record from Siberia. Both the short and long-term warming and presence and abundance of thermophilic biota, notably Apectodinium, are strongly associated. In contrast, two tropical sections in the equatorial Atlantic are marked by a large drop in diversity and number of dinocysts, as well as the absence of planktonic foraminifera, as SSTs rise >36 ºC during the PETM. Since temperature is the only variable at the studied localities in the eastern equatorial Atlantic that may cause such an effect across so many different species and plankton groups, it is concluded that heat-stress negatively affects their abundance. The new data from the equatorial Atlantic, together with available literature also showed that temperature most likely rose before the CIE. This has important implications for the global carbon cycle, as it suggests that the CIE is the result of a positive feedback, in this case most likely methane hydrate dissociation, and not the driver of the entire warming. The question what caused the initial warming remains open, although enhanced volcanism in the North Atlantic is proposed as a potential carbon source. New dinocyst and isotope data from the North Atlantic shows that thermogenic methane was released during the ~70 kyr-long anomalously warm period of the PETM. This period, the so-called "body" phase, distinguishes the PETM from other early Eocene hyperthermals. Based on our new data and carbon cycle modelling, we propose thermogenic methane as an important carbon source during the PETM. Lastly, all the proxy and dinocyst assemblage data generated for this thesis was combined to resolve large scale ecological affinities of extinct dinocyst genera.