Coral oxygen isotope reconstruction of sea surface salinity variability in the southern Makassar Strait since 1938 C.E. and its influence on the Indonesian Throughflow Braddock K. Linsley, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA Henry C. Wu, MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany Jessie Mathews, Boston University, Boston, MA, USA Tim Rixen, Leibniz Center for Tropical Marine Ecology, University of Bremen, Bremen, Germany Chris Charles, SCRIPPS Institution of Oceanography, University of California, San Diego, CA, USA PP21B-2014 Introduction and Summary: The Indonesian Throughflow (ITF) annually transports surface and thermocline depth water from the Pacific Ocean north of the equator to 12°S in the Indian Ocean. Today, the net result of the ITF is a cooling and freshening of the Indian Ocean thermocline. One mechanism to explain this relatively cool ITF comes from limited instrumental data suggesting that the seasonal influx of low salinity surface water from the South China and Java Seas into the southern Makassar Strait lowers surface salinity by 2 to 3 S P and generates a northward pressure gradient during the winter monsoon (Dec-Mar). The low salinity “plug” seasonally inhibits the flow of warm surface water in the far western Pacific Ocean from freely flowing southward into the Indian Ocean. We present a new sub-seasonally resolved δ 18 O records from a 0.8 m Porites sp. coral core collected from Kapoposang near southwest Sulawesi and from the top of a 2.8m Porites sp. core from Gili Meno near Bali (see map) that record seasonal and lower frequency variations in salinity in the Makassar Strait. The Kapoposang coral series spans 2004 to 1938 (66 years) and the Gili Meno series so far extends to Annual δ 18 O variations of 1-1.5‰, density banding, and thin but distinct fluorescent banding coincident with δ 18 O minima were used to develop the chronologies. Preliminary Findings: ①-At Kapoposang, sub-seasonal δ 18 O is highly correlated (r 2 = 0.74, ) to the gridded instrumental sea surface salinity (SSS) dataset (Simple Ocean Data Assimilation; SODA). The Gili Meno δ 18 O is less well correlated to SODA SSS (r 2 = 0.38, ) in part due to greater influence of seasonal SST change at this site. Both coral δ 18 O records demonstrate that the annual 2-3 S P salinity reduction during the winter monsoon (Dec- Mar) is a consistent pattern. ② -At Kapoposang, during El Niño conditions the freshening in the strait is reduced by ~50% for moderate events, and by up to 90% for “very strong” El Niño’s such as the 1982/83 and 1997/98 events. The 1982/83 El Niño event is clear at Gili Meno, but other El Niño events are less clearly recorded, perhaps due to the influence of the Indian Ocean Dipole (IOD). ③-Interannual coral δ 18 O variability in the central Makassar Strait closely tracks the Niño3.4 SST, the Southern Oscillation Index, and additional equatorial Pacific coral δ 18 O records. ④ -The fact that interannual Sr/Ca variability in the Kapoposang coral is correlated with interannual coral δ 18 O in this colony, but not interannual Extended Reconstructed Sea Surface Temperature (ERSST) supports our conclusion that interannual coral δ 18 O variability in this setting is primarily driven by SSS changes. This also suggests that interannual Sr/Ca at this location is in part due to SSS variability. Figures above: Coral  18 O verus a “pseudo-coral”  18 O constructed from instrumental SST (IGOSS and ERSST) and SODA SSS. Regression of  18 O against SSS at both sites revealed slopes of 0.47 (at Kapoposang) and 0.27 (at Gili Meno) per mil per 1 SSS unit change. Clearly, the Kapoposang (central Makassar) pseudo-coral is more closely correlated with coral  18 O than at the Gili Meno site. It is possible that SODA SSS in the central Makassar more accurately reflects actual conditions than SODA SSS in the Bali region of the southern Makassar. Fluorescent banding for chronology Comparison to equatorial ENSO Kapoposang (top) and Gili Meno (bottom) coral  18 O vs SODA Salinity; The dramatic reduction in salinity freshening during very strong El Niño events is clear. Coral  18 O vs. “pseudo-coral”  18 O (from SST and SSS) Maiana coral  18 O (ENSO-band) vs. Nino3.4 SST Maiana coral  18 O vs. Kapoposang Coral  18 O (ENSO-band) Kapoposang Coral  18 O vs. Kapoposang Coral Sr/Ca (ENSO-band) Salinity Coral  18 O vs. Salinity Acknowledgment: Research supported by NSF Award OCE