A novel salinity proxy based on Na incorporation into foraminiferal calcite

Abstract. Salinity and temperature determine seawater density, and differences in both thereby control global thermohaline circulation. Whereas numerous proxies have been calibrated and applied to reconstruct temperature, a direct and independent proxy for salinity is still missing. Ideally, a new proxy for salinity should target one of the direct constituents of dissolved salt, such as [Na⁺] or [Cl⁻]. This study investigates the impact of salinity on foraminiferal Na/Ca values by laser ablation ICP-MS analyses of specimens of the benthic foraminifer Ammonia tepida cultured at a range of salinities (30.0–38.6). Foraminifera at lower salinities (30.0 and 32.5) added more chambers (10–11) to their test over the course of the culturing experiment than those maintained at higher salinities (36.1, 7–8 chambers, and 38.6, 6–7 chambers), suggesting that growth rates in this species are promoted by lower salinities. The Na/Ca of cultured specimens correlates significantly with seawater salinity (Na/Ca = 0.22S–0.75, R² = 0.96, p < 0.01) and size. Values for Na/Ca and D_Na vary between 5.17 and 9.29 mmol mol⁻¹ and 0.12–0.16 × 10⁻³, which are similar to values from inorganic precipitation experiments. The significant correlation between test size and Na/Ca results from co-variation with salinity. This implies that foraminiferal Na/Ca could serve as a robust and independent proxy for salinity, enabling salinity reconstructions independent of calcitic δ¹⁸O.