Holocene Sea-Level Changes in Palau, West Caroline Islands

Abstract

Abstract14C ages supplemented by 230Th/234U determinations have been obtained for calcareous deposits on Koror, Babelthuap, Auluptagel, and Adorius Islands in the Palau Group, West Caroline Islands. Test borings for a bridge between Koror and Babelthuap reveal shoreface terraces consisting largely of bioclastic sand, but resembling fringing reefs. The base of the shoreface terraces dates to between 7000 and 8000 14C yr B.P. and the upper surfaces are slightly younger than 4000 yr B.P. Spacing of the subsurface isochrons indicates that the rate of sedimentation increased up-section on Koror terrace and decreased up-section on Babelthuap terrace. The average rate of deposition in the terraces was 0.5 cm/yr. Isochrons in the Babelthuap shoreface terrace are 7 m higher than those in the Koror terrace. If deposition was sufficient to keep the surfaces of the terraces at low tide level, then the Babelthuap side was essentially stable and the Koror side was uplifted between about 8000 and 6300 yr ago, and then subsided until 4000 yr ago; since then there has been about 2 m of uplift. “Top hat” microatolls on Koror terrace indicate that relative sea level has dropped about 30 cm in the past 75 yr. In the absence of good evidence for changes of level in water ponded in a moat, it is likely that the microatolls indicate uplift of the terrace. On the other hand, if tectonic activity was minimal, then differences in the two terraces are due to differences in sedimentation with the Koror side of the channel being substantially subtidal between about 7000 and 5000 yr B.P. Taking the composite eustatic sea-level curve of Hawaii and elsewhere as a reference standard, it is deduced that Auluptagel Island has risen 0.8 m in the last 2900 yr, and Adorius Island has risen approximately 8 m in the last 5000–6000 yr.

Holocene Paleoshoreline and Paleoenvironmental History of Palau: Implications for Human Settlement

“…Contrary to previous interpretations for late Holocene uplift of Palau (Easton and Ku 1980;Masse 1990), we argue that results from recent offshore and onshore coring demonstrate that Palau has been subsiding since at least mid-Holocene time. Continuing subsidence of Palau has implications for the preservation of early archaeological sites that are opposite to those drawn previously from the supposition of island uplift.…”

Stratified Marine Lakes of Palau (Western Caroline Islands)

“…The Rock Islands now exhibit a typical karst topography of sharp solution surfaces, dripstone 1 80 HAMNER AND HAMNER caverns, caves, and fissures, some of which extend hundreds of meters below the current water table owing to dissolution of carbonates during the Pleistocene. Dur ing the later Holocene, there have been modest changes in sea level (Easton and Ku, 1980). All of the marine lakes have retained hydraulic connections to the surround ing lagoon via the brackish lens of water beneath the islands and/or directly through percolation channels, fissures, and tunnels.…”

Cryptic diversity, molecular phylogeny and biogeography of the rock- and leaf litter-dwelling land snails of Belau (Republic of Palau, Oceania)

“…During the Pleistocene and into the Holocene, the Belau islands have experienced substantial sea-level fluctuations (Easton & Ku 1980). Because sea levels were probably depressed by 120 m during the last glacial maximum (18 000 years ago), many islands were presumably connected at times, and, conversely, fragmented when sea level was higher than now (Crombie & Pregill 1999).…”