Investigating the triggering mechanisms of palaeoceanographic disturbance across the Frasnian–Famennian, Late Permian and the Paleocene–Eocene Thermal Maximum: insights from osmium isotopes and geochemistry

This thesis utilizes osmium (Os) isotope, together with other geochemical proxies, to investigate the paleoclimate and palaeoceanography of three Earth history intervals: the Wuchiapingian–Changhsing boundary (WCB), the Frasnian–Famennian (F–F) boundary and the Paleocene–Eocene (P–E) boundary. High-resolution Os isotope chemostratigraphy of four globally correlated WCB sections show two separate Os isotope excursions to less radiogenic compositions that are coincident with the carbon isotope excursions (CIEs). The Os isotope shift is interpreted to reflect increased unradiogenic Os input from basaltic magmatism in South China, possibly related to the Emeishan large igneous province. Volcanism may have provided the isotopically light carbon that drove the negative carbon isotope excursions across the WCB. Organic petrology, Os isotope stratigraphy, major and trace element analyses, and programmed pyrolysis analysis from five F–F sections from western New York, USA show evidence of a wildfire event at the F–F boundary and yield an estimated pO2 level of ~25% for the Late Devonian. Furthermore, the Os isotope records does not support an extra-terrestrial impact or volcanic event as a trigger for the F–F mass extinction. The inferred high O2 level supports the hypothesis that pCO2 drawdown and climate cooling may have caused the F–F mass extinction. A multiproxy geochemical study (Os isotope, mercury, sulfur, platinum group elements) on two P–E boundary North Atlantic Ocean records suggests that both a comet impact and major volcanic activity likely contributed to the environmental perturbations during the P–E interval. Approximately 0.4 Gt of carbon is estimated to have been derived from the comet, thus the impact cannot have been responsible for the full manifestation of the P–E CIE. Other sources of carbon may have jointly driven the P–E thermal maximum. Climate simulations indicate that stratospheric sulfate aerosols from the impact may have caused transient cooling and reduced precipitation prior to the onset of substantial P–E warming