Phosphorus-to-calcium ratios in benthic foraminiferal shells as a proxy for coastal seawater phosphate concentrations

Establishing a reliable proxy for seawater phosphate concentrations is important for understanding phosphorus (P) biogeochemistry in the ocean and its change over geological time. P/Ca ratios in benthic foraminiferal shells were examined as a potential proxy for seawater dissolved inorganic phosphate (DIP) concentrations. We developed a method for analyzing benthic foraminiferal P/Ca ratios using Electron Probe Microanalysis (EPMA). We first examined the effect of two different cleaning protocols on P/Ca ratios in foraminiferal shells. Average P/Ca ratios only after physical were over 50 % higher than those after physical and oxidative cleaning, indicating that oxidative cleaning efficiently removed P associated with organic matter. Our results also showed that P concentrations were high in the pore-rich regions of foraminiferal shells, which may be related to higher concentration of organic matter. We collected P/Ca ratios in non-pore regions of oxidative cleaned shells of the benthic foraminifera species Florilus decorus (F. decorus) from the Changjiang Estuary. Results show a significant positive relationship between the average P/Ca ratios of these samples and DIP concentrations in bottom seawater. We therefore propose that P/Ca ratios in non-pore regions of oxidative cleaned foraminiferal shells hold potential as a proxy for DIP concentrations in marine environment. Further exploration into phosphorus phases and the role of organics within foraminiferal shells is needed to advance this proxy and enable its quantitative application in reconstructing ancient ocean DIP history.