A Modern Sr/Ca-δ18O-Sea Surface Temperature Calibration for Isopora Corals in the Great Barrier Reef

Abstract:

Most coral-based paleoceanographic studies have used massive colonies of Porites or Faviidae, due to their long, continuously accreted skeletal records and sub-annual resolution, but other sub-massive corals provide an untapped resource. The genus Isopora is a dominant reef builder in some high-energy environments in the tropical western Pacific, and was a major component of cores recovered on IODP Leg 325 off the Great Barrier Reef (GBR). Despite its abundance, Isopora remains largely unexplored and hence underutilized in paleoceanographic studies. We present a modern Sr/Ca-δ¹⁸O-Sea Surface Temperature (SST) calibration of modern Isopora corals (n=3) collected from inner and outer reef locations ranging from 1-13m depth by Heron Island in the southern GBR in 2012. Pairing the Isopora Sr/Ca record with monthly SST yielded an average relationship of SST=−11.48×(Sr/Ca)+131.1 (r² = 0.42-0.78). The Sr/Ca sensitivity of −0.087 mmol/mol/°C is similar to the sensitivity for Porites that was corrected for tissue layer smoothing effects determined by Gagan et al. (2012). The similarity between our Sr/Ca-SST sensitivity and the corrected sensitivity for Porites suggests tissue layer effects are minimal in Isopora. The mean annual SST amplitude recorded by the corals from 2008-2011 (full annual cycles) was 5.3°C and the average δ¹⁸O annual cycle of 1.1‰ approximates that expected if salinity had little effect on coral δ¹⁸O, assuming a previously established conversion of -0.23‰ (δ¹⁸O)/°C for biogenic aragonite. The average annual salinity amplitude of 0.3 in gridded data from around Heron Island supports our conclusion that δ¹⁸O variability is forced almost completely by SST. This modern Sr/Ca-SST calibration will expand the paleoceanographic utility of Isopora and, by assisting interpretation of Sr/Ca data from fossil corals collected during IODP 325, will better constrain the timing and magnitude of sea level changes and surface conditions since the Last Glacial Maximum.