Coral skeletal isotopes (δ¹³C and δ¹¹B) as indicators of seawater light attenuation and pH chemistry in the Singapore Strait

Abstract

This study investigates the interaction between δ¹³C and δ¹¹B with terrigenous carbon dynamics in the Singapore Strait, a region characterized by distinct monsoon patterns and significant terrigenous input from surrounding peatlands. We hypothesized that elevated levels of colored dissolved organic matter (CDOM) during the Southwest Monsoon would decrease light penetration, leading to more negative δ¹³C values in coral skeletons. Additionally, we expected that remineralization of terrigenous dissolved organic matter (tDOM) would acidify seawater, resulting in more negative δ¹¹B values in corals. Analysis of Porites spp. corals from two plug cores (KUK and KUL) and seawater data from Kusu Island (2017-2020) revealed no significant correlation between CDOM and coral δ¹³C anomalies— deviations between coral skeletal δ¹³C values and the δ¹³C values of dissolved inorganic carbon (DIC) in seawater— contradicting our hypothesis. Instead, variations in coral δ¹³C appear to be related to a reservoir effect associated with negative δ¹³C in seawater DIC, influenced by tDOC remineralization. Although not statistically significant, the positive correlation pattern observed between δ¹¹B and seawater pH in the KUL core suggests that δ¹¹B might serve as a useful proxy for historical seawater pH and acidification. This finding also supports the idea that Porites corals may regulate their internal pH in response to changes in seawater acidity, potentially influenced by tDOC remineralization. Inconsistencies in the KUK core could be attributed to data offsets from our age-depth model. Further research with extended sampling is needed to confirm δ¹¹B’s sensitivity to pH changes and understand its impact on coral physiology. This study highlights the complex interplay between seasonal changes, carbon dynamics, and coral isotopic records.

Keywords: Coral Skeletal Isotopes, δ¹³C, δ¹¹B, Light Attenuation, Seawater pH, Colored Dissolved Organic Matter (CDOM), Southwest Monsoon, Singapore Strait