Palaeoclimatology of the late Palaeocene to middle Eocene: geochemical records of stable and transient climate states

摘要

The late Palaeocene to late Eocene period of Earth's history is characterised by remarkable change. Temperate ice free poles at the beginning of this period gradually cooled until permanent ice formed on Antarctica around 33.5 million years before present (Ma) and sea ice formed in the Arctic. The intervening time was not stable and data, despite relatively low resolution, appear to show that the Eocene climate was dynamic. This period was the most recent time when atmospheric pCO2 concentrations were as high as predicted by models simulating the effects of anthropogenic fossil fuel burning on Earths' climate. The ability to understand the mechanisms of climate change in the Eocene will help to understand potential climate impacts in the future. This thesis examines 3 contrasting periods of climate change. Geochemical data indicate that a 3.5 million year period of high biogenic silica deposition during the Eocene was climatically relatively stable in the Arctic basin with only infrequent communication to the world's oceans outside. This period is correlated with high organic burial in the basin and global siliceous rich deposits which acted to gradually draw down pCO2. This period of `quiet' climate compares to two periods of warming where significant carbon isotope perturbations may indicate the forcing of the Earth's climate into an alternative quasi-stable state. The Palaeocene { Eocene Thermal Maximum (PETM) represents a significant input of exogenic carbon into the atmosphere over the course of several thousand years and significant warming of the Earth. Records of bulk carbonate isotopes from a section in NE Italy show several other Delta13C perturbations both before and after the PETM event, albeit a quarter to a half of the magnitude of the PETM, and having durations of only 40 { 60 thousand years (kyr). These events are thought to be the result of a re-arrangement of the internal carbon cycle of the Earth - atmosphere and may represent orbitally forced changes in deep water ocean ventilation similar to controls seen on modern day glacial { interglacial cycles. These rapid changes in the carbon cycle are shown to be inverse at the middle Eocene Climatic Optimum (MECO), where gradual warming over 400 kyr is ended abruptly by significant cooling. From the first marginal marine section of this event rapid organic carbon burial occurs over 50 { 100 kyr and is associated with previously unrecorded low oxygen bottom water conditions and high organic burial. We hypothesize that if this burial was extended over significant shelf areas then this could rapidly have returned the middle Eocene to the general cooling trend of the Eocene.