coral oxygen isotope reconstruction of sea surface salinity variability in the southern makassar...

Coral oxygen isotope reconstruction of sea surface salinity variability in the southern Makassar Strait since 1938 C.E. and its influence on the Indonesian ThroughflowBraddock K. Linsley, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA [blinsley@ldeo.columbia.edu]Henry C. Wu, MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany [hwu@marum.de]Jessie Mathews, Boston University, Boston, MA, USA [jmathews@bu.edu]Tim Rixen, Leibniz Center for Tropical Marine Ecology, University of Bremen, Bremen, Germany [tim.rixen@zmt.uni-bremen.de]Chris Charles, SCRIPPS Institution of Oceanography, University of California, San Diego, CA, USA [ccharles@ucsd.edu]

PP21B-2014Introduction 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 SP 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 δ18O 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 1919. Annual δ18O variations of 1-1.5‰, density banding, and thin but distinct fluorescent banding coincident with δ18O minima were used to develop the chronologies.Preliminary Findings:①-At Kapoposang, sub-seasonal δ18O is highly correlated (r2 = 0.74, 1980-2004) to the gridded instrumental

sea surface salinity (SSS) dataset (Simple Ocean Data Assimilation; SODA). The Gili Meno δ18O is less well correlated to SODA SSS (r2 = 0.38, 1970-1990) in part due to greater influence of seasonal SST change at this site. Both coral δ18O records demonstrate that the annual 2-3 SP 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 δ18O variability in the central Makassar Strait closely tracks the Niño3.4 SST, the Southern Oscillation Index, and additional equatorial Pacific coral δ18O records.

④ -The fact that interannual Sr/Ca variability in the Kapoposang coral is correlated with interannual coral δ18O in this colony, but not interannual Extended Reconstructed Sea Surface Temperature (ERSST) supports our conclusion that interannual coral δ18O 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 d18O verus a “pseudo-coral” d18O constructed from instrumental SST (IGOSS and ERSST) and SODA SSS. Regression of d18O 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 d18O 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 d18O vs SODA Salinity; The dramatic reduction in salinity freshening during very strong El Niño events is clear.

Coral d18O vs. “pseudo-coral” d18O (from SST and SSS)

Maiana coral d18O(ENSO-band)

vs. Nino3.4 SST

Maiana coral d18Ovs. Kapoposang

Coral d18O(ENSO-band)

KapoposangCoral d18O

vs. KapoposangCoral Sr/Ca

(ENSO-band)

Salinity Salinity

Coral d18O vs. Salinity

Acknowledgment: Research supported by NSF Award OCE-1211110