Sodium in foraminiferal calcite as a direct proxy for salinity

Salinity is one of the most important parameters in paleo-oceanography, controlling -together with temperature- changes in sea-water density and ocean currents. An integrated understanding of past ocean functioning hence critically relies on accurate past density values. A recently developed direct proxy for salinity, based on incorporation of sodium (Na) into benthic foraminiferal shells, may circumvent inaccuracies associated with indirect salinity reconstructions, when combining for instance foraminiferal stable oxygen isotope values with independent temperature reconstructions. Here we present new results of culture experiments with planktonic foraminifera (e.g. Globigerinoides ruber) and of a field calibration covering a broad salinity range. The foraminiferal Na/Ca was determined by laser ablation-ICP-MS, allowing geochemical analysis on a single chamber. The obtained planktonic Na/Ca values show a similar sensitivity, albeit with slightly higher values as previously reported for benthic foraminifera. This offset indicates that albeit values are in line with what is expected from previous inorganic precipitation experiments, biological control on calcium carbonate precipitation must play a role.