Reconstructing sea surface salinity variability from Porites corals in the Indonesian Berau delta system
Summary
Sea surface salinity is a crucial parameter for global ocean circulation and climate reconstruction. However, until now there is a lack of proxies that can be directly related to salinity. Na/Ca in foraminiferal carbonate is potentially such a proxy (Wit et al., 2013), which has never been applied to coral carbonate. Corals are known to be excellent recorders of environmental variability with strong age control with sub-seasonal resolution. This study explores the potential of the Na/Ca proxy from coral carbonate. A transect with a strong salinity gradient through the Berau delta is used as a natural laboratory to test several conventional sea surface salinity proxies (seawater δ18O, Ba/Ca, G/B ratio) and the Na/Ca proxy from Porites coral carbonate. The data of the Na/Ca proxy showed clear seasonal cycles corresponding to the expected salinity fluctuations due to changes in river runoff. However, no correlation was found between the Na/Ca proxy and the other conventional runoff and salinity proxies, nor the climate indices. The conventional proxies were also used to assess the most important climatological drivers in eastern Kalimantan up to 1970. The most important climatologic factor influencing the Berau delta system seems to be the El Niño-Southern Oscillation. This climatic variability has a large impact on the precipitation over the Berau catchment area, and thus the amount of runoff by the Berau River. Changes in the precipitation over the area do not necessarily lead to an increase in erosion and sediment supply for the Berau River, although the decrease in forest cover in the catchment area, in combination with the increase in precipitation will potentially lead to an increase in sediment supply for the Berau delta. This will have a major effect on the corals that are located close to the Berau delta shoreline.