Palaeo-seawater temperatures based on carbonate oxygen isotope data: Why we might have a problem?

The stable oxygen-isotope ratios (delta18O) of marine carbonate materials are conventionally used to estimate – within geological and analytical error bars - the ambient palaeo-seawater temperature during the precipitation... [ view full abstract ]

The stable oxygen-isotope ratios (delta18O) of marine carbonate materials are conventionally used to estimate – within geological and analytical error bars - the ambient palaeo-seawater temperature during the precipitation of marine carbonate cements or the secretion of a skeletal hardpart. Known problems mainly include: (i) the various temperature equations used commonly demand an assumption of the (often unknown) palaeo-seawater oxygen isotopic composition and (ii) open-system geochemical behaviour may result in a post-depositional/post-mortem exchange of oxygen between a reactive diagenetic fluid and a diagenetically altered carbonate. Conversely, when oxygen isotope values of a given carbonate material sampled fall within the range of what are considered “normal” marine seawater values, a good or at least reasonable degree of preservation is argued for. Here we argue that in some cases, the oxygen isotope ratios of a given carbonate might fail to record a burial fluid environment (closed system behaviour) and data extracted from these materials might then not be representative of the burial pathways. We present data from experimental work using aragonitic bivalve shells (A. islandica) exposed to artificial diagenetic environments and compare these with such from natural calcitic samples (radiaxial fibrous calcites, Triassic of Hydra, Greece). Conventional oxygen isotope data are present in context to carbonate clumped isotope data in a process-oriented context. The kinetics of oxygen isotope geochemistry and clumped isotope bond reordering along diagenetic gradients is discussed.