Unit Affiliation: Biology and Paleo Environment, Lamont-Doherty Earth Observatory (LDEO)
The Indonesian Throughflow (ITF) is the main conduit for transporting surface and upper thermocline water from the western equatorial Pacific into the Indian Ocean. The volume and temperature of the ITF varies seasonally due to the seasonal influx of low salinity surface water from the South China and Java Seas into the Makassar Strait during the northern winter monsoon. This low salinity "plug" generates a northward pressure gradient in the Makassar Strait that seasonally inhibits the flow of warm surface water in the far western Pacific Ocean from freely flowing southward into the Indian Ocean. The net result is a cooling and freshening of the Indian Ocean thermocline. Despite the oceanographic significance of this seasonal freshening of the Makassar Strait on the ITF, little is known about century to decadal-scale variability of salinity in the Makassar Strait. Previous low-resolution studies based on sediment cores in the Makassar Strait indicate that during the Little Ice Age (~1600-1850 AD), salinity and sea surface temperature in the strait were both reduced. This research utilizes Porites sp. coral cores from Kapoposang in the southern Makassar Strait near S.W. Sulawesi and from Gili Meno in the Lombok Strait near Bali to develop partially replicated, near-monthly resolution, trace element (Sr/Ca) and stable isotope (d18Osw) records spanning the last 250 years. Results show that coral Sr/Ca variability accurately tracks the annual bi-modal sea surface temperature cycle. The research will enhance understanding of salinity and SST variability in this critical strait in order to constrain ITF variability over the last 250 years. Changes in the freshwater budget of the southern Makassar Strait are expected to alter the intensity of the "freshwater plug", with lower salinity in the Makassar Strait surface layer leading to a cooler ITF. Funding supports a postdoctoral associate and research experiences for undergraduates.
OUTCOMES: Demonstrated that the annual winter monsoon salinity reduction of 2-3 SP is consistent, while during El Nino freshening of the strait ranges from 50-90%. Demonstrated that interannual coral variability is driven by sea surface salinity, not temperature.