A Late Middle Ordovician Carbon Isotope Excursion In Central And Eastern North America

"The Ordovician period is a significant chapter in Earth's history that included the great Mid-Ordovician biodiversification event, the Hirnantian glaciation, and long-term greenhouse conditions. Ordovician rocks are widespread on most continents and the recent finalization of a modern chronostratigraphic classification of the Ordovician system now facilitates high-resolution correlations that allow for integrated multidisciplinary research. The diverse papers comprising this volume address orogenesis, paleogeography, climate modeling, sedimentation, biodiversity, and isotopic excursions; together they promote an integrated view of the Ordovician earth system."--Publisher's description.

Abstract: The latest Ordovician (444 million years ago) was a critical period in Earth history. This was a time of significant climatic global change with large-scale continental glaciation. Moreover, the end-Ordovician mass extinction is recognized as the secondmost devastating mass extinction to have affected the Earth. The anomalous Late Ordovician icehouse period has perplexed many researchers because all previous model and proxy climate evidence suggest high levels of atmospheric CO2 during the Late Ordovician glaciation. Also associated with this period is a large positive carbon isotope ([delta]13C) excursion (up to +7%) that represents a global perturbation of the carbon cycle. Additionally, a large decrease (0.001) in seawater 87Sr/ 86Sr occurs several million years prior (~460 million years ago); this could reflect an increase in atmospheric CO2 uptake due to weathering of volcanic rocks involved in uplift of the early Appalachian Mountains. To address these Ordovician anomalies, well-studied, thick, and continuous Late Ordovician limestone sequences from eastern West Virginia, south-central Oklahoma, central Nevada, Quebec (Canada), Estonia, and China have been sampled. Carbon and strontium isotopic ratios have been measured on samples from these localities of which Estonian and Chinese sample sites represent separate paleocontinents (Baltica and South China) and are compared with other data sets from North America. These data test previous interpretations that the well-documented latest Ordovician carbon isotope excursion coincides with maximum glaciation. They support a hypothesis that the large positive carbonate carbon isotope excursion was coincident with a warm interglacial (high CO2 levels) period that separated two major glacial advances (with lowered CO2 levels). There are clear parallels between the Late Ordovician and the Late Cenozoic (the most recent) greenhouse to icehouse transitions, with silicate weathering providing the initiator and positive feedback on changes in atmospheric CO2 levels. The results lead to a more complete understanding of climatic and biotic events of this critical interval, which will certainly help the understanding of the period of global climatic and biotic change affecting Earth today.

"The Ordovician period is a significant chapter in Earth's history that included the great Mid-Ordovician biodiversification event, the Hirnantian glaciation, and long-term greenhouse conditions. Ordovician rocks are widespread on most continents and the recent finalization of a modern chronostratigraphic classification of the Ordovician system now facilitates high-resolution correlations that allow for integrated multidisciplinary research. The diverse papers comprising this volume address orogenesis, paleogeography, climate modeling, sedimentation, biodiversity, and isotopic excursions; together they promote an integrated view of the Ordovician earth system."--Publisher's description.

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.

Stratigraphy and Timescales covers current research across a wide range of stratigraphic disciplines, providing information on recent developments for the geoscientific research community. This fully commissioned review publication aims to foster and convey progress in stratigraphy, including geochronology, magnetostratigraphy, lithostratigraphy, event-stratigraphy, isotope stratigraphy, astrochronology, climatostratigraphy, seismic stratigraphy, biostratigraphy, ice core chronology, cyclostratigraphy, palaeoceanography, sequence stratigraphy, and more. Contains contributions from leading authorities in the field Informs and updates on all the latest developments in the field Aims to foster and convey progress in stratigraphy, including geochronology, magnetostratigraphy, lithostratigraphy, event-stratigraphy, and more