The δ18O record of phanerozoic abiotic marine calcite cements

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Abstract
Monomineralic, abiotic marine cements formed in low‐latitude Phanerozoic reefs provide the direction and amplitude of secular variation of δ13C and δ18O in marine calcite and defines two end member compositions — 580 to 360 my (−7 to −5‰ δ18OPDB) and 360 to present (−3 to 0‰ δ18OPDB). Sampling of the Devono‐Carboniferous transition (375‐320 my) at several global sites reveals a rapid change in carbonate isotopic compositions. Bracketed within Fammenian to Early Visean‐aged strata, a 7 to 15 my time interval, this shift corresponds to a 2‰ offset in mean δ13C and 3‐4‰ offset in δ18O. The abruptness of such change, and its overall correlation with variations in 87Sr/86Sr, δ34S, δ13C, and Li/Al ratios in marine sediments suggests a primary offset in marine water composition.