Abstract
Element concentrations of calcite precipitated by foraminifera reflect chemical and physical properties of seawater and can therefore be used to reconstruct (paleo-)environmental conditions. Foraminiferal carbonate associated manganese incorporation (expressed here as Mn/Ca) is a potential proxy for seawater oxygenation, although the impacts of other environmental parameters need to be quantified before Mn/Ca can be robustly applied. Here we report the isolated impact of seawater carbonate chemistry on manganese incorporation in the shells of two large symbiont-bearing benthic foraminiferal species. Moreover, we investigated the role of biomineralization on manganese incorporation by using species with contrasting calcification pathways: the hyaline species Amphistegina gibbosa and the porcelaneous species Sorites marginalis. Furthermore, analyzing shells from a wide range of species grown under identical conditions allowed assessment of species-specific Mn incorporation in other foraminiferal species. Our observations show that species specific differences in biomineralization strategies are the dominant factor determining Mn content. Shells from porcelaneous species, with relatively high Mg contents, are generally also enriched in Mn compared to low-Mg/Ca foraminifera. Superimposed on the effect of biomineralization, chemical speciation of elements in seawater as a function of pCO2 also affects their incorporation. Whereas the impact of the carbonate system is limited, the inter-specific differences call for species specific calibrations in order to use Mn uptake as a (paleo-)oxygenation proxy.
Highlights
The chemical composition of foraminiferal shells is widely used by paleoceanographers to reconstruct ocean conditions, because past fluctuations in physico-chemical conditions of seawater are recorded in the chemistry of the shell
In this study we analyzed specimens of two species of foraminifera, A. gibbosa and S. marginalis grown under a range of controlled pCO2, as well as larger benthic foraminifera collected from an Indo-Pacific reef aquarium
Standard deviation (SD) per treatment increases with increasing pCO2, and increasing Mn/Ca
Summary
The chemical composition of foraminiferal shells is widely used by paleoceanographers to reconstruct ocean conditions, because past fluctuations in physico-chemical conditions of seawater are recorded in the chemistry of the shell. The Mg–Ca ratio (Mg/Ca) of foraminiferal calcite reflects seawater temperature (Nürnberg et al, 1996) and can be applied to fossil foraminifera to reconstruct past bottom water (Lear et al, 2000) and sea surface (Barker et al, 2005) temperatures. The toolbox that foraminiferal calcite provides, is ever expanding by addition of new elements of interest, including e.g., the recently proposed Sr/Ca (Keul et al, 2017) and S/Ca (Van Dijk et al, 2017a) as recorders of the seawater carbonate system. Manganese (Mn) is a promising candidate for Frontiers in Earth Science | www.frontiersin.org van Dijk et al. Impact of pCO2 on Foraminiferal Mn/Ca such a proxy, since solubility and/or the oxidation state of Mn is a function of the redox status of the environment (Tribovillard et al, 2006) and pore water carbonate chemistry (Middelburg et al, 1987). Foraminifera incorporate dissolved Mn2+ (Reichart et al, 2003; Koho et al., 2015; Barras et al, 2018) and the Mn/Ca of their shell is suggested to change with oxygenation conditions
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