The Oligo-Miocene Gambier Limestone is a poorly lithified, fine-grained, bryozoan-rich grainstone/wackestone and marl. Allochems in these rocks, which are typical of many modern and ancient cool-water carbonates, were originally calcite (LMC), Mg-calcite (IMC and HMC) and minor aragonite, but most are now neomorphosed to LMC. With the exception of calcretes, most limestones have, 5% cement, delta 13 C and delta 18 O values from 0 to +1.5 per mil, and preserved 87 Sr/ 86 Sr ratios that indicate that neomorphism occurred in seawater rather than meteoric or evolved pore waters. Marine diagenesis of the limestones was facilitated by their slow accumulation rates, which were ca. 15 mm/ky based on Sr-isotope stratigraphy, relatively fine grain size, and high porosities which locally reach 50%. The Sr and Mg contents of grainstones are significantly lower than in the original sediments, whereas a less porous marl has higher Sr and Mg contents. The grainstones have about 300 ppm Sr and 6000 ppm Mg, consistent with slow abiotic precipitation of calcite under near-equilibrium conditions with seawater, which is the thermodynamic drive for this diagenesis. The higher Sr and Mg content of the marl results from variable preservation of original allochems of biotic calcite. The results from this study and others on modern cool-water carbonates indicate that the extent of diagenesis varies with lithology, but much neomorphism occurred in seawater, with minimal effects from meteoric waters. As such, the chemical characteristics of seawater may be effectively preserved in some ancient cool-water carbonates.