DETAILS OF THE MID-HOLOCENE RELATIVE SEA-LEVEL HIGHSTAND AT BELITUNG ISLAND, INDONESIA, ON THE SUNDA SHELF, FROM CORAL MICROATOLLS

Details of relative sea level (RSL) since the mid-Holocene in Southeast Asia are poorly determined. We have developed mid-Holocene RSL records at two sites on Belitung Island, Indonesia, on the Sunda Shelf, based primarily on coral microatolls. Microatolls’ upward growth is limited by aerial exposure, and hence they track low water (e.g., mean low water springs) to within a few cm, providing a high resolution record of former low water level. On northwestern Belitung, we slabbed 14 fossil and 2 modern microatolls (each yielding a series of sea-level index points) and slabbed or cored 14 additional fossil corals (each yielding a minimum sea-level marker). Preliminary U-Th and ¹⁴C dates suggest that low water levels reached a peak elevation of +1.9 m (compared to equivalent tidal datum levels at the site today) at ~6.7 ka; low water level fell to +1.3 m quickly thereafter, but was back up at around +1.7 m by ~6.5 ka. If the tidal range at the site in the mid-Holocene was similar to that today, these differences in elevation would reflect differences in mean RSL. At a site on southeastern Belitung, 80 km away, coral microatoll morphologies reflect similar fluctuations in RSL at similar times, but with the entire relative water level history shifted half a meter lower. We are investigating whether the differences between the two sites can be explained by changes over time in the tidal range at one or both sites. Nonetheless, the independent evidence at sites 80 km apart for a double-peaked RSL highstand at ~6.7 and ~6.5 ka suggest that these fluctuations reflect real changes in regional RSL, and are not an artifact of local effects.