Coral δ18O Reconstructs Sea Surface Salinity in the Singapore Straits Revealing Impacts from ENSO

Coral proxies provide us with valuable climatic and environmental information especially where instrumental data is very sparse and limited. Although reanalysis salinity products are available, they are less accurate in highly turbid, non-homogenous coastal environments, such as the Singapore Straits. Moreover, in-situ salinity data measurements are very limited (7 years) in Singapore. Thus, finding a suitable proxy is vital for us to understand past environmental changes in this region. To investigate this, multiple coral cores were sampled from massive Porites sp. corals collected from Kusu Island, Singapore (1.225^°N, 103.86^°E), where water circulation is greatly impacted by monsoonal changes. Cores were sub-sampled at a high resolution (0.05-0.1 cm) along the maximum growth axis to examine the seasonality of δ¹⁸O and to test its robustness as a proxy for sea surface salinity (SSS). Based on a 7-year calibration period, coral δ¹⁸O and in-situ SSS have a strong and significant relationship (p<0.0001), potentially allowing for salinity reconstructions. The δ¹⁸O of coral shows no significant correlation to SST (p>0.05), similar to other corals from the region in which SST variability is less than 3.0°C. Annual SSS anomalies from our 43-year (1973-2015) record reveals a significant relationship with the Southern Oscillation Index (SOI) (p<0.001). However, there is no significant correlation between SSS anomalies and local precipitation (p=0.15). These findings indicate that water from the Western Pacific Warm Pool is possibly being advected to the Singapore Straits and that the El Niño Southern Oscillation strongly influences interannual salinity variability in this region. A freshening trend of 0.04 psu yr^-1 was noted, consistent with other studies found in the western Pacific.