C,S,Sr,and Hg Cycles In The Late Ordovician-early Silurian Oceans

Reconstruction of environmental and climatic conditions as well as oceanic carbon cycling during the periods of mass extinctions in the geological history is essential to understand the modern environment and biodiversity crises, provides perspective for promoting harmonious development between human and nature. In this dissertation,marine sedimentary rocks were used to explore the oceanic and atmospheric chemistry conditions during the Late Ordovician, as well as their potential links to the mass extinction event. The present dissertation mainly focuses on:1. Exploring the mechanism for perturbations of oceanic carbon cycling during the Late OrdovicianOn the basis of well-constrained biostratigraphy, we reconstructed oceanic carbon cycling and its underlying mechanisms during the Late Ordovician, using the coupled C and Sr isotopes systematics as well as trace elements of carbonates from Nevada,USA. The trace elements data indicate little or no diagenetic alteration and the original seawater isotopic signals are well preserved in Nevada. Our results reveal multiple 87Sr/86Sr shifts including two rapid negative excursions that are consistent with two prominent positive ?13Ccarb excursions of ?7‰ and?5‰ respectively, which coincide well with the two advances of the ice sheet in Gondwana. A numerical model is also used to identify the potential cause of the Hirnantian positive carbon isotope excursion. Enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level falling is required to produce such large positive ?13Ccarb excursions, indicating that the positive ?13Ccarb shifts during the Hirnantian were the result of increased rates of carbonate weathering that would elevate the ?13C of the riverine flux and subsequently marine ?13Ccarb values,possibly with or without increased organic carbon burial. The interglacial period of ice sheet recession that would result in sea level rise and submergence of the carbonate platforms, the resultant reduction of carbonates weathering would lead to the marine ?13Ccarb values returning to baseline value.2. Multiple S-isotopic records (?33S, ?34S, and A36S) from Honghuayuan section and its implications for environment changesHigh-resolution multiple S-isotopic compositions (?33S, ?34S, and ?36S) of pyrites from Honghuayuan section in Guizhou Province were analyzed, and identify preservations of S-isotopes mass-independent fractionation (S-MIF) signals (?33S values with a maximum of ?0.9‰) in Phanerozoic samples that fundamentally differ from those of rocks younger than ?2.45 billion years. Combined with our data and the principles of S-isotopic fractionation, we suggest that stratospheric photochemical reactions following massive volcanic eruptions can account for our anomalous S-isotopic records. These anomalous S-isotopic data identify stratospheric eruptions in South China during the Late Ordovician, link the massive volcanisms to the Late Ordovician mass extinction, extend the geologic record of S-MIF signals to Phanerozoic, and provide a possibility to reconstruct the environmental and ecological impacts of past volcanic eruptions. Moreover, this explanation is further bolstered by identifications of altered volcanic ash beds (termed as K-bentonites) globally, which indicate that the early end-Ordovician was a time interval of worldwide volcanisms.The deterioration of environments and ozone depletion induced by the volcanic eruptions may have contributed to the Late Ordovician mass extinction.3. Mercury (Hg) records from Xiushan and Vinini Creek section and their paleoenvironmental implicationsWe measured Hg concentrations, Hg/TOC ratios and Hg isotopic compositions of marine sediments from two sections in Xiushan, South China and Vinini Creek, Nevada,USA across the Late Ordovician mass extinction interval. The results from the both sections show anomalous Hg enrichments (up to 736.6 ppb in the Vinini Creek section)far more than that of modern normal marine sediments, suggesting enhanced external Hg loading. Combined with the Hg isotopic data, we argue that the Hg anomalies observed in sedimentary rocks from Xiushan and Vinini Creek most likely resulted from increased emissions of volcanic Hg during the Late Ordovician. The Hg enrichments before and during the Late Ordovician mass extinction provide evidence for a causal link between volcanic eruptions and the Late Ordovician mass extinction, this conclusion is in agreement well with that of the multiple S-isotopic data in this study.Moreover, the Hg concentrations and Hg/TOC from Vinini Creek and Xiushan suggest that the local conditions could strongly affect the Hg anomaly records in sediments. The sedimentary preservation of enhanced Hg loading induced by volcanisms or other processes is primarily controlled by the burial of organic matters.In environment where production and burial of organic matter outstrips the demand of organo-Hg scavenging, any real/potential Hg/TOC excursions will be overprinted by excessive deposition of TOC. Whereas, if the fluxes and organic carbon burial rates are negligible, the signal of enhanced Hg loading could not be recorded in the sediments.Therefore, the Hg/TOC in the geological record as a proxy of volcanisms should be used with comprehensive consideration of Hg concentrations, Hg/TOC, TOC contents and depositional settings and paleogeography.