Effect of different seawater Mg^2 + concentrations on calcification in two benthic foraminifers

Magnesium, incorporated in foraminiferal calcite (Mg/Ca_CC), is used intensively to reconstruct past seawater temperatures but, in addition to temperature, the Mg/Ca_CC of foraminiferal tests also depends on the ratio of Mg and Ca in seawater (Mg/Ca_SW). The physiological mechanisms responsible for these proxy relationships are still unknown. This culture study investigates the impact of different seawater [Mg^2 +] on calcification in two benthic foraminiferal species precipitating contrasting Mg/Ca_CC: Ammonia aomoriensis, producing low-Mg calcite and Amphistegina lessonii, producing intermediate-Mg calcite. Foraminiferal growth and test thickness were determined and, Mg/Ca was analyzed using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Results show that at present-day seawater Mg/Ca_SW of ~ 5, both species have highest growth rates, reflecting their adaptation to modern seawater element concentrations. Test thickness is not significantly affected by different Mg/Ca_SW. The relationship between Mg/Ca_SW and Mg/Ca_CC shows a distinct positive y-axis intercept, possibly reflecting at least two processes involved in foraminiferal biomineralization. The associated Mg partition (D_Mg) changes non-linearly with increasing Mg/Ca_SW, hence suggesting that the D_Mg is best described by an exponential function approaching an asymptote.