Assessing delta oxygen-18 in the coral genus Isopora for reconstructing Indo-Pacific regional and seasonal climate

Paleoclimate reconstructions often utilize coral reefs with very long time spans such as the genus Porites and Diploastrea , because of their potential to provide centuries of continuous climate records via geochemical signatures. Smaller corals, such as the genus Isopora, have been essentially unexplored as climate archives because their small skeletons (<1 m) and short lifespans (years to decades) do not provide such continuous geochemical records. There has not been a practical application for such corals until recently. In early 2010, the Integrated Ocean Drilling Program Leg 325 (IODP-325) cored drowned fossil reefs off the Great Barrier Reef (GBR) with the objectives of reconstructing sea level and surface ocean conditions since the Last Glacial Maximum. Out of 213 massive fossil corals that were recovered, most were massive Isopora colonies. A 30-specimen subset of these fossils range in age from ∼32,000 to ∼11,500 years before present with even temporal spacing, based on preliminary U/Th dating of core catcher samples. This age distribution is excellent for meeting IODP-325 objectives, but the suitability of Isopora for paleoclimate analyses remains unknown.;This study presents an important preliminary step for the analysis of this fossil collection---a geochemical assessment of modern Isoporan specimens grown under known conditions. Millimeters-scale delta 18O analyses of eight Isoporan corals from three locations in the Indo-Pacific region (Heron Reef, Myrmidon Reef, and Madang, Papua New Guinea) were performed. delta18O analysis results reflect relative regional differences in seasonal sea surface temperature (SST) cycle amplitude and mean annual SST. A delta18O-SST calibration using seasonal amplitudes yields a relationship of -0.15‰/°C, while a calibration using mean annual values yields -0.1‰6/°C. These values suggest a low delta18O-SST sensitivity in comparison to what has been determined for many other coral genera. In addition to serving as a preliminary study for analysis of these fossil specimens, this assessment will help pave the way for utilizing similarly small coral types for paleoclimate applications.