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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.

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