87Sr/86Sr evidence from the epeiric Martin Ridge Basin for enhanced carbonate weathering during the Hirnantian

A pronounced positive delta C-13 excursion in the Hirnantian Age has been documented globally, reflecting large perturbations of carbon cycling in the Late Ordovician oceans. Increased organic-carbon burial or enhanced carbonate weathering during glacioeustatic sea-level regression has been proposed to account for this anomalous C-isotope excursion. To test the two competing hypotheses, we measured Sr-87/Sr-86 and delta C-13 of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative (8)7Sr/Sr-86 shifts that coincide with two prominent positive delta C-13 excursions and glacial advances. Numerical model simulations suggest that enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level fall is required to produce the observed drops of Sr-87/Sr-86 and the coeval large positive delta C-13 excursions, possibly with or without increased organic carbon burial.