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 87Sr86Sr, Δ34S, Δ13C, and Li/Al ratios in marine sediments suggests a primary offset in marine water composition. ¿ American Geophysical Union 1989