Impact of seawater sulfate concentration on sulfur concentration and isotopic composition in calcite of two cultured benthic foraminifera

Abstract. Marine sediments can be used to reconstruct the evolution of seawater [SO₄^2-] and δ³⁴S over time, two key parameters that contribute to refine our understanding of the sulfur cycle and thus of Earth’s redox state. δ³⁴S evolution can be measured from carbonates, barites and sulfate evaporites. [SO₄^2-] variations can be reconstructed using fluid inclusions in halites, a method that only allows a low-resolution record. Reconstruction of the past sulfur cycle could be improved if carbonates allowed to track both seawater δ³⁴S and [SO₄^2-] variations in a sole, continuous sedimentary repository. However, most primary carbonates formed in the ocean are biogenic, and organisms tend to overprint the geochemical signatures of their carbonates through a combination of processes often collectively referred to as vital effects. Hence, calibrations are needed to allow seawater δ³⁴S and [SO₄^2-] reconstructions based on biogenic carbonates. Because foraminifera are important marine calcifyers, we opted to focus on calcite synthesized by individuals of two benthic strains cultured in laboratory under controlled conditions, with varying seawater [SO₄^2-] (ranging from 0 mM to 180 mM). Our experimental design allowed us to obtain foraminiferal asexual reproduction over several generations. We measured bulk carbonate associated sulfate (CAS) content and sulfur isotopic composition (δ³⁴SCAS) on samples of tens to hundreds of specimens for each culture medium, where [SO₄^2-] varied from 5 to 60 mM. Increasing or decreasing [SO₄^2-] with respect to modern-day seawater concentration (28 mM) impacted foraminiferal population size dynamics and the total amount of bioprecipitated carbonate. Foraminiferal CAS concentration increased proportionally with [SO₄^2-] concentration from 5 mM up to a threshold value of 40 mM, highlighting the extent of control on the precipitation fluid chemistry that foraminifera exert on the carbonate precipitation loci. Yet, despite the significant effect of [SO₄^2-] on foraminiferal physiology and on CAS incorporation, the isotopic fractionation between CAS and seawater remains stable through varying seawater [SO₄^2-]. Altogether, these results illustrate that CAS in biogenic calcite could constitute a good proxy for both seawater [SO₄^2-] and δ³⁴S and contributes to emphasize the role played by sulfate on foraminiferal biomineralization and biological activity.