The Proxies of Submarine Groundwater Discharge and Its Ecological Effects: foraminiferal shell chemistry perspective

Submarine Groundwater Discharge (SGD) plays a crucial role in coastal ecosystems by transporting nutrients and potentially inducing hypoxic conditions. Robust geochemical proxies are therefore essential for reconstructing SGD histories and quantifying its role in regulating coastal ecosystems, particularly for identifying critical tipping points in ecosystem state. This understanding would also benefit predictions of potential outcomes under future environmental change scenarios.

We use benthic foraminiferal shell chemistry to trace SGD and its associated ecological effects. Benthic foraminiferal Ba/Ca ratios in SGD hotspots are more than twice those in non-SGD areas of the Changjiang estuary, showing strong correlations with both bottom water ²²²Rn activities and Ba/Ca ratios. This indicates that elevated and highly variable Ba/Ca ratios in SGD hotspot areas are primarily driven by coastal groundwater discharge. We evaluated benthic foraminiferal Mn/Ca ratios as a proxy for coastal hypoxia and found strong correspondence with dissolved oxygen variations. Average Mn/Ca ratios in tests of living benthic foraminifera in the Changjiang estuary are 2.3 times higher during hypoxic periods compared to well-mixed conditions. Fluctuations of Mn/Ca and Ba/Ca ratios measured along successive chambers of individual foraminiferal shells correspond well with seasonal-scale variations in hypoxia and SGD, reflecting the co-occurrence of SGD and its associated hypoxic effects. We further demonstrate a significant linear relationship between average P/Ca ratios in foraminiferal shells and seawater dissolved inorganic phosphate (DIP) concentrations (r² = 0.7973, p < 0.05). These results demonstrate the promising potential of using P/Ca ratios in benthic foraminiferal shells as a proxy for recording SGD-induced DIP enrichment.

Our findings indicate that elemental-to-calcium ratios in benthic foraminiferal shells serve as powerful proxies for archiving environmental conditions induced by SGD over seasonal and potentially longer timescales. This multi-proxy approach provides a comprehensive toolbox for reconstructing past SGD dynamics and associated ecosystem impacts.