Abstract. The quantitative reconstruction of past seawater salinity has yet to be
achieved, and the search for a direct and independent salinity proxy is
ongoing. Recent culture and field studies show a significant positive
correlation of Na∕Ca with salinity in benthic and planktonic
foraminiferal calcite. For accurate paleoceanographic reconstructions,
consistent and reliable calibrations are necessary, which are still missing.
In order to assess the reliability of foraminiferal Na∕Ca as a direct
proxy for seawater salinity, this study presents electron microprobe
Na∕Ca data measured on cultured specimens of Trilobatus sacculifer. The culture experiments were conducted over a wide salinity
range of 26 to 45, while temperature was kept constant. To further understand
potential controlling factors of Na incorporation, measurements were also
performed on foraminifera cultured at various temperatures in the range of
19.5 to 29.5 ∘C under constant salinity conditions. Foraminiferal
Na∕Ca values positively correlate with seawater salinity
(Na/CaT. sacculifer=0.97+0.115⋅salinity, R=0.97, p<0.005). Temperature, on the other hand,
exhibits no statistically significant relationship with Na∕Ca values,
indicating salinity to be one of the dominant factors controlling Na
incorporation. The culturing results are corroborated by measurements on
T. sacculifer from Caribbean and Gulf of Guinea surface sediments,
indicating no dissolution effect on Na∕Ca in foraminiferal calcite
with increasing water depth up to >4 km. In conclusion, planktonic
foraminiferal Na∕Ca can be applied as a potential proxy for
reconstructing sea surface salinities, although species-specific calibrations
might be necessary.