Sea surface temperature and sea surface salinity calibration using porites coral geochemical proxies from Dongsha Atoll in the South China Sea

Abstract

Sea surface temperature (SST) and sea surface salinity (SSS) records provide significant insights into ocean thermohaline circulations and their interaction with the global climate system. Geochemical proxies (e.g., Sr/Ca, δ18O) in tropical reef corals, such as Porites, are commonly used to reconstruct SST and SSS with a seasonal resolution. In this study, we sampled two Porites coral cores from Dongsha Atoll, the South China Sea, and analyzed their trace element and stable isotope compositions to investigate the suitability of different calibration methods for SST and SSS reconstructions. We established monthly-resolution time series of 16 years and 20 years on corals cored from 2.5-meter and 8.6-meter depths, respectively. Both Sr/Ca and δ18O show strong seasonal variations over the past 20 years. The Sr/Ca time series in the two cores are in good agreement with the instrumental SST observations from both in-situ and satellites. The linear regression between coral Sr/Ca and instrumental SST yields a slope of -0.07 mmol/℃ and -0.05 mmol/℃ (r=-0.87, p<0.001), for two cores, respectively, within the published range of Porites corals. The regression remains nearly constant across various time intervals, and thus the slight difference in regression between the two cores could be attributed to their different oceanic settings and/or growth rate. Because the physiological vital effect of coral biomineralization can confound the interpretation of coral paleothermometer records, the Sr-U thermometry was recently proposed to reduce the influence of vital effects on Sr/Ca. Here we compared the Sr/Ca and Sr-U thermometry for the two Porites coral samples across different timescales. The newly developed Sr-U thermometry appears not as robust as the traditional Sr/Ca thermometer for our samples, especially for high temporal resolution. Because coral δ18O records both SST and SSS, the SSS addressed by seawater δ18O can be decoupled by removing the temperature component inferred from paired Sr/Ca ratio. We calculated SSS based on different methods. However, no significant correlations are observed between the calculated SSS through seawater δ18O and instrumental SSS regardless of coral cores or methods, which implies some limitations in the current methods on SSS reconstructions. This project emphasizes the utility and sensitivity of coral geochemical proxies from Dongsha Atoll in the South China Sea for reliable SST and SSS reconstructions. Reconstructed SST and SSS from longer geochemical records to the past may offer new insights into climate change, especially in tropical and subtropical oceans.