Uncertainty in palaeosalinity estimates from foraminiferal geochemical records in the northern Indian Ocean
Abstract Palaeosalinity (S*) estimates rely on calculations stemming from the oxygen isotopic composition of foraminiferal shells (δ18OF) preserved in ocean sediments. δ18OF depends on palaeo in-situ temperature (T*) and the oxygen isotopic composition of seawater (δ18Osw), which in turn depends on various physical oceanographic conditions including evaporation, precipitation, advection, and freshwater fluxes. δ18Osw and salinity covary linearly owing to similar controls, as they both increase with increases in evaporation and decrease with higher precipitation and continental runoff. Typically, palaeosalinity is estimated assuming a constant spatial and temporal relationship between δ18Osw and salinity. However, evaporative and freshwater fluxes exhibit spatiotemporal variability and thus, can change the slope and intercept of the linear δ18O-salinity relationship. Hence, the use of a constant δ18Osw-salinity relationship may produce (systematic) errors in palaeosalinity estimates. Here, we revisit palaeosalinity estimates and provide a reassessment of errors arising from these calculations based on recently reported δ18Osw-salinity relationships in the northern Indian Ocean. Our calculations point to errors in palaeosalinity that can be as large as 55% - significant enough to change our understanding of past ocean circulation and palaeomonsoon variability. These results have important implications for reconstructing the palaeoceanographic record and interpreting past ocean-atmosphere variability.