The δ¹⁸O record of phanerozoic abiotic marine calcite cements

Monomineralic, abiotic marine cements formed in low‐latitude Phanerozoic reefs provide the direction and amplitude of secular variation of δ 13 C and δ 18 O in marine calcite and defines two end member compositions — 580 to 360 my (−7 to −5‰ δ 18 O PDB ) and 360 to present (−3 to 0‰ δ 18 O PDB ). 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 δ 13 C and 3‐4‰ offset in δ 18 O. The abruptness of such change, and its overall correlation with variations in 87 Sr/ 86 Sr, δ 34 S, δ 13 C, and Li/Al ratios in marine sediments suggests a primary offset in marine water composition.