This study presents a new methodology to obtain highly precise measurements (±0.1‰) of magnesium isotope ratios in very small samples of foraminiferal carbonate (40–50 μg). Here this technique is used to examine Mg isotopic variation among different species of core top foraminifera over a range of different ambient conditions. Despite the high degree of temperature control on the abundance of elemental Mg incorporated into foraminiferal tests, analyses of surface dwelling Globigerinoides ruber and G. sacculifer from five cores, with associated sea surface temperatures ranging from 20 to 31°C, show no significant temperature-dependent variations in their Mg isotope ratios. Analyses of different size fractions of G. sacculifer show an increase in Mg/Ca with test size but no variation of Mg isotope ratio. In all, nine planktic and benthic species were analyzed; all show identical Mg isotope ratios with a mean of δ26Mg = −4.72‰, apart from small differences in three species, namely O. universa, G. sacculifer (which are both ∼0.4‰ lighter than the average), and P. obliquiloculata (which is ∼0.4‰ heavier). These results highlight the constancy of foraminiferal Mg isotope ratios, despite changing environmental conditions which dominate Mg/Ca variation and arguably affect Ca isotope fractionation. This is an important observation which needs to be included in any model of foraminiferal calcification. The insusceptibility of δ26Mg values to external parameters makes Mg isotopes ideally suited to constraining past variations in the Mg isotope budget of the oceans and the information this carries about the history of oceanic dolomitization, continental weathering, and hydrothermal behavior.
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