Well-preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the Siljan area of central Sweden have the heaviest stable oxygen and carbon isotope values yet reported for Lower Palaeozoic marine sediments. The coquinas formed during the eustatic regression at the onset of the late Ordovician glaciation: the isotopic compositions reflect simultaneous shifts in both carbon and oxygen (more than 5‰ δ13C and up to 2º‰ δ18O) away from more normal Lower Palaeozoic values in similar carbonates from within the mounds. Oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature. The changes in carbon values suggest enhanced deposition of organic carbon, a process which would have decreased pCO2 in the ocean and the atmosphere and thus contributed to rapid global cooling.

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