A chemostratigraphic investigation of the late Ordovician greenhouse to icehouse transition: Oceanographic, climatic, and tectonic implications

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 second-most 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 (delta13C) 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.