Constraining the duration and magnitude of Late Ordovician to Early Silurian glaciation

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Constraining the Duration and Magnitude of Late Ordovician to Early Silurian Glaciations Woodrow Fisher, Caltech EAR-1053523 ABSTRACT One of the most severe mass extinctions in the fossil record occurred near the end of the Ordovician Period (488 to 444 million years ago). Although this episode is clearly linked to global cooling and glaciation of the southern supercontinent of Gondwana, the precise nature of the relationship is obscured by fundamental uncertainties about the timing of glaciation, the magnitude of the Gondwanan ice sheet(s), and the impact of global cooling on low-latitude ocean temperatures. For this project PIs are applying the recently developed carbonate ?clumped? isotope paleothermometer to well-preserved Late Ordovician and Early Silurian fossils from Anticosti Island, Québec and the US midcontinent. This proxy offers a major advantage over conventional carbonate-water oxygen isotope paleothermometry in that it requires no assumptions about the isotopic composition of seawater. It can therefore be used, in combination with conventional approaches, to help resolve a problem that has engaged paleoclimatologists, geochemists, and paleontologists for half a century: what are the relative contributions of seawater temperature and global ice volume to temporal trends in the oxygen isotopic composition of marine carbonates? Application of this proxy thus has the potential to give a much more highly resolved picture of ancient climate changes, giving us a better understanding of past climate states and their effects on the global biota. The proposed work has two related objectives: 1) to conduct a detailed examination of clumped isotope systematics across a range of preserved carbonate phases, taxonomic groups, and depositional settings to evaluate the effects of diagenesis, potential vital effects, and highlight the ways in which fossils preserve signals of ancient climate, and 2) to use this knowledge to construct high-resolution time series of local temperature and seawater oxygen isotope composition through Late Ordovician and Early Silurian time. This approach will provide important new information on climate dynamics during an interval characterized by both a greater inventory of greenhouse gases (namely CO2) and substantial continental ice sheets. It will also help clarify the relationship between climate change and extinction patterns during the Late Ordovician-Early Silurian interval.