Paper No. 8-4
Presentation Time: 8:55 AM
PAIRED MIDDLE–LATE ORDOVICIAN CONTINENTAL WEATHERING (87SR/86SR AND ΕND(T)) AND PALEOTEMPERATURE (δ18O) PROXY MEASUREMENTS LINK ENHANCED MAFIC WEATHERING TO GLOBAL COOLING
It remains unclear whether waning of the volcanic degassing CO2 source or enhancement of the mafic (Ca, Mg-silicate) weathering CO2 sink, or both, caused global cooling leading to the Ordovician greenhouse–icehouse transition. Here we use a uniquely age-constrained and integrated Middle–Late Ordovician (470–450 Ma) continental weathering (87Sr/86Sr and εNd(t)) and paleotemperature (δ18O) isotopic proxy dataset from bulk carbonate and conodonts of the Antelope Range, central Nevada to investigate the role of mafic weathering in Ordovician cooling. εNd(t) measurements of the acid-soluble portion of bulk carbonate samples demonstrate a steady pre-shift baseline at εNd(t) = −18 to −19 followed by a sharp inflection at 463 Ma leading to εNd(t) = −12 ± 4 at ~455.5 Ma. These seawater εNd(t) measurements align well with detrital (shale) εNd(t) measurements in Taconic foreland basins, indicating that the shift was caused by a change in weathering source lithology. 87Sr/86Sr of bulk carbonate and conodont apatite align well with conodont 87Sr/86Sr measurements across Laurentia, decreasing from 0.70881 at 471 Ma to 0.70797 at 453 Ma with a sharp inflection at 463 Ma marking one of the most prominent periods of seawater 87Sr/86Sr change in the Phanerozoic. Published δ18O measurements of conodont apatite from the Antelope Range section show a ~2.7‰ increase spanning 465–457 Ma, which corresponds to a ~11 °C cooling and aligns well with globally compiled conodont δ18O measurements. The coeval shifts in seawater εNd(t) and 87Sr/86Sr to more juvenile values (higher εNd(t), lower 87Sr/86Sr) at 463 Ma are roughly coincident with an inflection in global δ18O to higher (cooler) values captured in the Antelope Range section at 460 ± 1 Ma, suggesting that an increase in mafic weathering of the Taconic mountains forced a period of global cooling. We use a published weatherability-driven 87Sr/86Sr and pCO2 mass balance model to simulate global CO2 fluxes during the Ordovician. A 25% increase in global weatherability at 463 Ma causes a decrease in pCO2 from 11.75 PAL to 4.7 PAL (present atmospheric level; 1 PAL = 280 ppmv CO2) at the end of the Ordovician. Together, our geochemical and modeling results suggest that CO2 drawdown from mafic weathering was a primary driver of Middle–Late Ordovician cooling leading to end-Ordovician glaciation.