Abstract
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&amp;plus;] 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 DNa 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.
Highlights
Temperature and salinity are among the most relevant parameters in studying past ocean circulation, since together they control ocean water density and thereby thermohaline circulation
The benthic, cosmopolitan foraminifer Ammonia tepida inhabits a wide range of environments, and is found in habitats with highly variable temperatures and salinities (Murray, 1968; Hayward et al, 2004)
Vitality of specimens was assessed by checking for algal content (Dunaliella salina) in the last three chambers, movement within Petri dish and presence of active pseudopodia emerging from the aperture
Summary
Temperature and salinity are among the most relevant parameters in studying past ocean circulation, since together they control ocean water density and thereby thermohaline circulation. Salinity is reconstructed using dinoflagellate and diatom species composition (Zonneveld et al, 2001), process length of dinoflagellates (Mertens et al, 2009), bivalve strontium isotope composition (Israelson and Buchardt, 1999), Ba / Ca of foraminiferal calcite (Weldeab et al, 2007) and the morphology and size of Emiliania huxleyi placoliths (Bollmann et al, 2009). These proxies, are either inferred from indirect changes in ecology or strongly dependent on regional oceanography (i.e., river water input). Foraminiferal δ 18 O (after correcting for the effect of temperature using Mg / Ca or Uk37 ) or hydrogen isotope composition (δD) of alkenones can be used to infer salinity
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