| dc.description.abstract | The Paleocene-Eocene Thermal Maximum (PETM) was a global warming event that occurred ~56 Ma and was associated with a negative carbon isotope excursion (CIE). The injection of greenhouse gases during the PETM led to sharp temperature increases and had profound biological effects on both marine and continental ecosystems. The event, with its rapid onset (~3 kyr), is considered one of the best analogues in the geological record for today’s anthropogenic global warming. For this reason, the PETM has attracted the interest of the scientific community with the intent to understand and predict, within limitations, what bioecological changes might occur in the near future.
In this thesis, I present new high-resolution isotopic, geochemical, mineralogical and palynological datasets to reconstruct palaeoenvironmental changes around and within the North Sea basin. The North Sea is an important study area due to its proximity to an active Large Igneous Province (LIP) known as the North Atlantic Igneous Province (NAIP). It is believed that the NAIP emplacement led to uplift of the NW section of the North Sea with consequent strong regional basin restriction. One of the aims of the thesis is to discern whether previously observed precursor environmental perturbations were a response to pre PETM climatic change, or were rather driven by local topographic changes. Particular emphasis is on discriminating the causal mechanisms that led to the observed environmental shifts, and consequently to decouple the triggers and drivers of the PETM.
The thesis contains geochemical evidence that the North Sea became anoxic and euxinic just before the negative CIE. Bottom water oxygen depletion is commonly associated to climatic responses, such as eutrophication and water column stratification due to atmospheric warming and increased nutrient input during the PETM. Herein I attempt to discern whether anoxia before the CIE was driven by early climatic change or basin restriction. A decoupling of TOC and redox/basin restriction proxies just before the CIE, together with additional supporting evidence, suggests that euxinic conditions were triggered by tectonic uplift from NAIP emplacement that restricted the North Sea basin. The timing between carbon injection and basin restriction provides further evidence that NAIP activity might have been a causal trigger for the carbon cycle disruption.
In this thesis I further explore the possible triggers of the PETM by presenting high-resolution sedimentary mercury (Hg) data, a direct proxy for volcanic activity, from the North Sea. The Hg records show low background values during the pre PETM interval, followed by a build-up of sedimentary Hg around the CIE and throughout the main phase, providing further evidence of a causal link between NAIP volcanism and the PETM. For the first time, pulsed volcanic activity can be correlated between different sites in the North Sea providing insight on the nature and timing of NAIP volcanism: at least 4 Hg pulses are detected during the first ~35 kyr of the main phase. Each pulse displays a distinctive shape characterised by an abrupt Hg enrichment occurring in only hundreds of years, followed by a gradual decrease lasting few kyr. The peculiar shape of the Hg pulses might reflect rapid injection of magmatic sills in organic-rich sediments and gradual cooling.
Finally, I reconstruct marine and vegetation changes around the North Sea using palynological data. Vegetation and surface water changes before the PETM can be related to topographic uplift and relative sea level fall. Only at the CIE onset can the palaeoenvironmental changes be attributed to climate change as seen by the occurrence of tropical taxa. This thesis is the first record to explore marine and floristic changes during the recovery phases: interestingly, ecosystems never returned to pre PETM conditions throughout the studied interval. This signifies that environments can respond rapidly to climate change (few kyr), but the recovery can take hundreds of thousands of years. | en_GB |
