Exploring the Use of Mg/Ca Ratios Oyster Shells as Proxy for Water Temperature in the Anyanui Creek.

Abstract

The research focuses on exploring the potential of Mg/Ca ratios in the shells of Crassostrea tulipa, an oyster species found in the Anyanui Creek, as a proxy for water temperature. In the field of paleoclimatology, foraminifera and oyster shells have been widely used as proxies to infer historical water temperatures. Given the complex environmental conditions of estuaries, which are influenced by seasonal variations in freshwater influx and salinity, this research aimed to determine whether Mg/Ca ratios could reliably reflect temperature in a setting where additional factors, such as salinity and pH, may interfere. Monthly water and oyster shell samples were collected from three stations in the Creek between March 2023 and April 2024. Temperature, salinity, and pH were measured in situ, and the shells underwent hot plate digestion with hydrofluoric acid and aqua regia to release trace metals for analysis. Mg/Ca ratios in the shells were analysed and correlated with temperature using regression models, including Linear, Ridge, Lasso, and Random Forest regressions, to assess the strength of the Mg-temperature relationship across variable salinity conditions. Results showed moderate correlations between Mg/Ca ratios and temperature at the upstream station, suggesting some potential for using Mg/Ca ratios as temperature indicators in more stable environments. However, correlations were weaker in the midstream and ocean-fed stations, where salinity variability due to freshwater input disrupted the Mg-temperature relationship. The pH also influenced Mg incorporation, particularly in midstream zones, where fluctuating conditions complicated Mg/Ca ratios as standalone proxies for temperature. The findings indicate that while Mg/Ca ratios can offer insights into temperature in estuarine environments, their accuracy is limited in variable salinity and pH settings. A multi-proxy approach incorporating additional indicators is recommended for future studies to improve estuary temperature reconstruction. This research contributes to understanding the environmental applications of biomineralization and highlights the need for multi-faceted approaches in complex estuarine ecosystems as the variability in temperature in estuarine systems like Anyanui Creek can make the correlation between temperature and Mg/Ca ratios insignificant, especially when other environmental factors, such as salinity, pH, and biological regulation, exert stronger influences.