It is widely accepted that the end of the Ordovician period (445-443 Ma) was a time of large continental ice sheets in polar regions. However, the circumstances surrounding the initiation of cooling and the transition from a greenhouse climate remains controversial. It has been proposed that cooling was brought about by a lowering of atmospheric carbon dioxide levels. Ordovician carbon dioxide could be lowered either by burial of organic carbon or by silicate weathering (burial of carbonate carbon). Our research will use multiple isotopic proxy records with different causal factors and response times (C, Sr, Nd) to investigate links between Ordovician climate and processes affecting the long term carbon cycle. PIs will obtain these coupled isotopic records primarily in thick stratigraphic sections preserved on opposite sides of low-latitude Laurentia that are well-dated by conodont biostratigraphy. Previous isotopic studies have identified significant changes in both strontium and carbon isotopes of carbonates through the Ordovician climate transition. PIs research aims to address the cause and consequences of the large drop in Sr isotopes in the Middle-Late Ordovician using neodymium isotope patterns in conodont apatite and C isotopes in organic matter. The 87Sr/86Sr of seawater is commonly used as a proxy for paleotectonic events because it is controlled by a balance between riverine inputs and seafloor hydrothermal alteration. Like Sr, the Nd isotopic composition of seawater can record changes in continental weathering. However, a key difference is that the oceanic inventory of Nd is not balanced by seafloor hydrothermal inputs. Thus, 143Nd/144Nd can provide independent constraints on the sources of seawater 87Sr/86Sr variations. PIs will also construct paired records of C isotopes in carbonate and organic matter that may potentially provide information on relative changes in atmospheric carbon dioxide that resulted from changes in silicate weathering or previously identified organic carbon burial events. In addition, by coupling C with Nd which has a residence time considerably shorter than interocean mixing times, it is possible to obtain local information on weathering patterns and nutrient cycling.
