A coral skeleton geochemical record of surface water properties in the Oman upwelling region

Geochemical records from scleractinian corals have the potential to reconstruct surface water temperature and salinity, expanding temporal and spatial coverage beyond that available from in situ and remotely sensed data and clarifying the ocean's role in climate change. Here, I present delta 18O and Sr/Ca records from a Porites coral near the shelf-break off southern Oman to evaluate the potential for reconstructing monsoon upwelling histories. This sample represents growth (19 mm y -1) during August 1989 to April 1996. Both delta18 Ocoral (-3.7 to -5.0 ‰PDB) and Sr/Ca (9.0 to 9.6 mmol mol-1) records anticorrelate strongly with weekly instrumental sea-surface temperature (SST) records from several sources (IGOSS, Pathfinder 5 (Pv5) AVHRR, and COADS). A systematic comparison shows that the Pv5 AVHRR quality 3 SST dataset is the most appropriate choice for calibrating the Sr/Ca proxy, largely because of its maximal temporal coverage of demonstrably accurate data. Using the optimal calculated SST-Sr/Ca calibration of -15.525 (+/- 1.83) °C mol mmol-1 and the assumed SST-delta18O calibration of -4.76 (+/-0.69) °C ‰PDB-1 [ Ren et al., 2002] to reconstruct the isotopic composition of the ambient seawater (delta18Osw), the mean annual delta 18Osw variation ranges were determined to be 0.5--0.9 ‰PDB. The timing and relative magnitude of these variations were consistent with known hydrography (isotopically lighter during SW monsoon, scaling with SST minima) but were greater than anticipated from observed annual salinity changes of ∼1.0 and the estimated delta18O sw-salinity relationship for regional surface waters. Simulations of reconstructed delta18Osw using minimum SST-Sr/Ca and maximum SST-delta18O calibration slopes from the literature showed that the resultant mean annual range of delta18O sw (0.4 ‰PDB) was closer to the expected magnitude. I conclude that: (1) Sr/Ca measurements provide a very good record of SST variations, particularly the interannual variation in the SW monsoon minimum, and (2) derived delta18Osw records indicate salinity variations that may be accurate in timing and relative magnitude but are unrealistically large in absolute magnitude. This coral holds strong promise for reconstructing multi-decade absolute SST and relative salinity records associated with climate-related variations in the Arabian Sea upwelling system.