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

A pronounced positive δ(13)C 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 (87)Sr/(86)Sr and δ(13)C of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative (87)Sr/(86)Sr shifts that coincide with two prominent positive δ(13)C 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 (87)Sr/(86)Sr and the coeval large positive δ(13)C excursions, possibly with or without increased organic carbon burial.