The Coral Porites lutea as a Regional and Local Proxy for Sea Surface Temperature in Hong Kong, and the Effects of Typhoons on Proxy Accuracy

Proxies related to the climate have been used as a method to observe long-term changes occurring underwater that span long before we had the capability to measure environmental characteristics. Porites corals have been used as sea surface temperature (SST) proxies due to annual cyclicity in their growth process and mineral composition. As the climate differs globally, it is important to ensure that proxies are accurate regardless of location to measure changes in climate. This study examines Porites lutea corals as an SST proxy in the region of Hong Kong. Potential local differences in SST proxy calibrations within the surrounding waters of Hong Kong are examined, as the inlets, tropical weather, and large human population create a highly active and volatile environment. Due to the intense wet seasons experienced in Hong Kong, typhoons are included as a covariate and interaction term in the proxy calibrations to test for significant influence in the correlational relationship. Age estimates for each coral sample are determined using a combination of mapped chronologies and data analysis. All age estimates are cross-referenced with estimated growth rates in past Porites Hong Kong studies to ensure accuracy. The SST proxy calibrations are estimated using linear, ANCOVA, and mixed effects models. The ratio of Strontium isotope 88 and Calcium isotope 43 (Sr/Ca) is a reliable SST proxy for the region of Hong Kong, at an overall slope of -0.606 mmol/mol per °C (p < 2.82e-06). Between the three sites sampled in this study, two have statistically significant different proxy slopes. As these sites are the farthest apart, this indicates that regions may need more localized SST proxy calibrations, especially if there are highly variable and locally isolated environments within the region. Additionally, all sites exhibit different intercepts, indicating that local conditions influence the baseline Strontium and Calcium quantities in corals. Typhoons do not significantly impact Sr/Ca-SST proxy calibrations in Hong Kong, but the explained variation in the correlational model is still relatively low (R2 = 0.606). This indicates that variables unrelated to weather--such as human impact, or proximity to land--should be included instead in future studies.