NOAA/WDS Paleoclimatology - La Reunion Coral d18O and Sr/Ca Data during the 20th Century

Sea surface salinity (SSS) is an important variable in the global ocean circulation. However, decadal to interdecadal changes in SSS are not well understood due to the lack of instrumental data. Here, we reconstruct SSS from a paired, bimonthly resolved coral d18O and Sr/Ca record from La Reunion Island that extends from 1995-1913. Coral Sr/Ca correlates with regional sea surface temperature (SST) until 1966, when instrumental coverage is good, while coral d18O does not. The slope of the monthly (annual mean) coral Sr/Ca-SST regression is -0.040 mmol/mol per 1°C (-0.068 mmol/mol per 1°C) consistent with published estimates of the Sr/Ca-SST relationship. Coral Sr/Ca suggest a warming of 0.39°C since 1913. d18O seawater is calculated by subtracting the temperature component from measured coral d18O, using coral Sr/Ca as well as historical SST products. The derived d18O seawater reconstructions are correlated (r>0.6), and all show a significant shift in the mid-20th century (-0.17 to -0.19 permil), indicating a freshening of SSS by 0.7 psu. However, the timing of this shift depends on the temperature component and varies from 1947 (d18O seawater calculated with historical SST) to the late 1950s (d18O seawater calculated with coral Sr/Ca). Coral Sr/Ca shows warm temperature anomalies in the mid-1950s, while historical SST products show warm anomalies from 1940-45 followed by cooling in the 1950s, a pattern typical for the World War II bias. This suggests that historical SST may bias reconstructions of d18O seawater and SSS from corals.