NEODYMIUM (ND) AND STRONTIUM (SR) ISOTOPE EVIDENCE FOR WEATHERING OF ARC VOLCANICS DURING THE ORDOVICIAN GREENHOUSE-ICEHOUSE TRANSITION

Our understanding of the origin and timing of the Ordovician greenhouse-icehouse transition is primarily based on comparing oxygen isotope paleotemperature curves with estimates of atmospheric carbon dioxide levels using global carbon cycle models. For carbon cycle models, both the degassing flux of carbon dioxide from volcanism/metamorphism and removal by silicate weathering must be estimated. Organic carbon burial and oxidation are also important but likely a secondary influence.

Previously one of us was involved in a modeling study (Young et al. 2009) that utilized the seawater Sr isotope curve to drive enhanced non-radiogenic basaltic weathering and a mid-Ordovician draw down of pCO₂. In order to match the paleotemperature curve (Trotter et al., 2008), however, it was necessary to assume the degassing flux balanced this enhanced weathering until just prior to the end-Ordovician glaciation. More recently, a study using detrital zircon ages (McKenzie et al., 2016) proposed that a lowering of the degassing flux was the principal cause of the Ordovician greenhouse-icehouse transition. Collectively, these studies raise questions about the importance of silicate weathering in driving Ordovician pCO₂and climate.

Our current effort examines the possibility that the decrease in mid-Ordovician Sr isotopes was actually caused by a flux of non-radiogenic Sr from mid-ocean ridge hydrothermal alteration. We measured paired Sr and Nd isotopes in marine carbonates in the Appalachian region because there is no appreciable hydrothermal flux of Nd and therefore changes in Nd isotopes should signify changes in weathering sources. Our results show a fall in Nd of 10 epsilon units that is coeval with the mid-Ordovician Sr drop of 0.0005, which is consistent with enhanced basaltic (juvenile) rock weathering in the Appalachian region that was large enough to affect the global Sr cycle. A modeling study using the global carbon cycle model COPSE is planned to address whether the observed change is really due to an increase in basalt weathering versus decreased weathering of old granitic crust (e.g., if sea level flooded cratonic interiors), and the implications for Ordovician pCO₂. This work is being conducted in parallel with efforts focused on modeling early land plant forcing of Ordovician weathering (Lenton et al., 2012).