The magnitude of a mid-Holocene sea-level highstand in the Strait of Makassar

Knowledge on the timing andmagnitude of past sea-level changes is essential to understandmodern and futuresea-level variability.Holocene sea-level data fromliterature on thewest coast of Sulawesi, central Indonesia, suggestthat this region experienced two relative sea-level highstands over the last 6000 years, with magnitudes exceedingtwo meters. However, recent datasets from the Indo-Pacific region do not support high-magnitude sealeveloscillations during the Holocene in tectonically stable far-field locations. Here we present a new, highprecision,mid-Holocene sea-level dataset from the Spermonde Shelf off southwest Sulawesi. We surveyed 21fossil microatolls on the reef flats of two coral islands (Pulau Panambungan and Pulau Barrang Lompo) and referredtheir elevations to local mean sea level and to the height of living coral. Radiometrically calibrated agesfrom emergent fossil microatolls on Pulau Panambungan indicate a relative sea-level highstand not exceeding0.5 m above present at ca. 5600 cal. yr BP. The highstand is followed by a relatively rapid sea-level fall towardspresent sea level that was reached at around 4000 cal. yr BP. Fossil microatolls from nearby Pulau BarrangLompo show the same trend, however with a coherent negative vertical offset of about 0.8 m compared totheir equivalents on Pulau Panambungan. The largely consistent gradients of both trends (~-0.14 mm yr-1),the consistent elevation of living microatolls in the Spermonde, and a number of instructive geomorphic featuresindicate a localized, post-formational and probably recent drop of the fossilmicroatolls on the densely populatedisland Pulau Barrang Lompo. The relative sea-level trend inferred from Pulau Panambungan is well within therange of geophysical predictions based on ANICE-SELEN ice sheetmodel,which predict a highstand that is significantlylower than those predicted by other GIA models for this area. Although a complete interpretation of theHolocene sea-level history will require additional high-resolution datasets from this and surrounding territoriesin SE Asia, our results suggest that there was merely a single Holocene highstand in central Indonesia, the magnitudeof which was substantially lower than hitherto assumed