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.
The Late Ordovician mass extinction is the first of the great Phanerozoic mass extinctions, eliminating > 60% of marine genera globally. For a long time it has been hypothesized that this extinction was causally related to climate change and glaciation of the southern hemisphere supercontinent of Gondwana, but the general lack of quantitative paleoenvironmental data prevented us from speaking directly to observed paleobiological changes. With substantial fieldwork we collected > 2 tons of geological materials to address this problem. We measured the first clumped isotope paleotemperature record covering ~25 million years of Late Ordovician to early Silurian time and were able to make quantitative estimates of the amount of temperature change experienced in the tropics, and the size of the latest Ordovician ice sheets. We used these paleoenvironmental insights to reflect on causal mechanisms of the extinction by examining extinction selectivity. The main challenge was that the glaciation corresponds to both a major drop sea level, which led to both a reduction in available rock area for sampling and also real habitat loss associated with the loss of shallow seas during the retreat of sea level. This exemplifies a classic problem in paleobiology—are extinctions that commonly coincide with hiatuses in the sedimentary record merely a consequence of reduced fossil preservation and rock record bias or do they reflect the actions of a shared common environmental forcing? By integrating spatiotemporally explicit stratigraphic and paleontological data from North America, we were able to quantitatively assess how the waxing and waning of the marine sedimentary record affected marine biota on a taxon-by-taxon basis for the first time. Results showed that 1) the extinction is real and cannot be attributed to record failure, 2) habitat loss was a major correlate of extinction risk, but that this represents an intensification of background extinction patterns rather than imposition of a new selective regime, and 3) that after controlling for habitat loss and other potential correlates of extinction risk, the preferential extinction of taxa with exclusively tropical distributions emerges as the distinctive signature of this event. These results support the hypothesis that cooling of the tropical oceans contributed to the Late Ordovician mass extinction and indicate that it cannot be fully explained by falling sea level and reductions in shallow marine habitat area. Through our study we were also able to directly address a longstanding and controversial hypothesis in geochemistry that holds that the isotopic composition of seawater has increased gradually but substantially through Earth’s history. A by-product of our clumped work to constraint ancient Earth surface temperatures is that we demonstrated that seawater has been broadly stable over long timescales, which implies the operation of poorly understood buffering processes that hold Earth’s water budget and the rock cycle largely constant over large swaths of time. On this project we trained a postdoc who was subsequent hired into a tenure track position. We also supported two female graduate students (one of whom was hired into a tenure track position), and two high school students (one from an underrepresented background in Science). Finally, we created a critical reading exercise for middle and high school students in the Understanding Evolution Resource Library.
