Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state

Abstract

Abstract.We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state ( ) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca 2+ ], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation D TE =(TE/Ca calcite )/(TE/Ca seawater ). The culturing study shows that D Mg of A. tepida significantly decreases with increasing at a gradient of −4.3×10 −5 per unit. The D Sr value of A. tepida does not change with , suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, D Mg of H. depressa shows only a minor decrease with increasing , while D Sr increases considerably with at a gradient of 0.009 per unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50-100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that deCorrespondence to: M. Raitzsch (raitzsch@uni-bremen.de) creases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca 2+ ] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca 2+ ] of seawater, which in turn would lead to a temperature underestimation of more than 2 • C.

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

“…However, given that coccolithophores have Mg isotope ratios similar or higher than inorganic calcite (i.e. lower than seawater, but significantly higher than foraminifera) (Wombacher et al, 2011;Ra et al, 2010), this combined model cannot explain why foraminifera have such low δ 26 Mg relative to coccolithophores.…”

“…Another possible process is active pumping of Ca, via Ca pumps, from vacuolisation of seawater into the Ca reservoir, rather than removal of Mg from the Ca reservoir (Raitzsch et al, 2010;ter Kuile, 1991), thereby changing the Mg / Ca ratio. Nehrke et al (2013) suggest that Ca for test calcification is primarily supplied by a combination of active transmembrane transport, which has been shown to strongly discriminate against other elements due to differences in surface charge density, and passive transport (i.e.…”

Variability in calcitic Mg/Ca and Sr/Ca ratios in clones of the benthic foraminifer Ammonia tepida

“…In a number of species, foraminiferal Sr/Ca is positively correlated with temperature (Rathburn and De Deckker, 1997;Reichart et al, 2003;Mortyn et al, 2005;Rosenthal et al, 2006;Kisakürek et al, 2011), salinity Kisakürek et al, 2011) and seawater Sr/Ca (Delaney et al, 1985;Raitzsch et al, 2010). Reported inter-and intra-individual variability in Sr/Ca is smaller than observed for Mg/Ca (e.g.…”

Geochemistry of surface sediments from the fjords of Northern Chilean Patagonia (44–47°S): Spatial variability and implications for paleoclimate reconstructions

“…However, since Mg is also associated to ocean carbonate productivity (e.g., Raitzsch et al, 2010), this relationship may not be valid where carbonate productivity is high. In addition, our data demonstrate that the litho-Si and quartz contents of the sediment can be used as proxies for grain size.…”