The Ordovician climate : constraints from carbon and oxygen isotopes
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The Ordovician Period (483 -- 443 Ma) is an attractive interval to study for understanding the earth's climatic system because of the juxtaposition of climatic extremes. The Period is marked by major shifts in climatic conditions, increases in marine biodiversity (GOBE: Great Ordovician Biodiversification event), and the 2nd largest mass extinction event of the Phanerozoic. Uncertainty regarding the evolution of the Ordovician climate, however, complicates attempts to relate climatic and paleontological changes to various climatic forcing's (e.g. atmospheric CO2, tectonics, ocean circulation). This project was designed to test competing models for the evolution of Ordovician climate using stable oxygen and carbon isotopic measurements from North America and Australia. I measured carbon isotopic values from carbonates and trace organic matter to provide a record of fluctuations in the carbon cycle and used oxygen isotopic values measured from bioapatite (conodont elements and brachiopod shells) to infer trends in sea surface temperatures and test for climatic cooling during the Ordovician. Samples discussed come from seven sections in eastern North America and seven sections from New South Wales, Australia, coverage that allow for consideration of local, regional, and global influences on geochemical records. From these results I conclude that measured trends primarily reflect global to regional paleoenvironmental patterns. As such, these results improve knowledge of the evolution of the Ordovician climate. Carbon isotopic results are consistent with a drawdown of atmospheric CO2 during the early Late Ordovician; however, the lack of evidence for associated cooling at the same time suggests that any changes were not large enough to cause resolvable climatic cooling. Instead, oxygen isotopic results suggest that the climatic cooling and glaciation was confined to the last stage of the Ordovician (Hirnantian). Finally, inferred cooling in the Early to Middle Ordovician strengthens arguments that climate played an important role in the Great Ordovician Biodiversification event.