Diagenetic impacts on calcium isotopes and Mg/Ca ratios of Globigerinoides ruber shells in the eastern Mediterranean

Abstract

The presence of Mg-rich calcite in foraminifera shells has impeded traditional calcite-based paleothermometry in the eastern Mediterranean, complicating our understanding of the region's past climate. This study analysed the calcium isotopic composition ( δ 44/40 Ca) in the shells of the planktonic foraminifera species Globigerinoides ruber (white), collected from 12 core top sediment samples spanning the Aegean and Levantine Seas in the eastern Mediterranean. The shells exhibited varying degrees of early diagenetic Mg-rich authigenic calcite coatings, which complicates traditional calcite-based paleothermometry in the region. Our findings demonstrate a significant correlation between the δ 44/40 Ca data and the Mg/Ca ratios of the shells and suggest that the inorganic calcite overgrowth originates from seawater. Moreover, the calcium isotopic composition increases with the amount of calcite overgrowth. This relationship allowed us to quantify the maximum potential inorganic endmember. The highest amount of post-depositional calcite precipitation was observed in the warm, shallow basins of the South Aegean Sea. The diagenetic alteration of the primary biogenic δ 44/40 Ca geochemical signal is linked to the quantity of secondary calcite present. By combining Ca isotope and Mg/Ca measurements on isolated foraminifera tests, we provide novel insights into the early diagenetic history of carbonate-bound geochemical proxies.