ATOLL AND MOTU HISTORY: EUSTASY AND HYDRO-ISOSTASY

Annular atoll reefs perched atop carbonate platforms capping buried volcanic edifices of the Pacific Ocean basin are underlain by from 8 m to 28 m of Holocene limestone disconformably overlying a substratum of last-interglacial limestone. Comparable thicknesses (from 9 m to 23 m) of Holocene sediment are present beneath atoll lagoons that are uniformly <85 m deep. During glacio-eustatic drawdowns in Pleistocene sea level by as much as 120 m, carbonate platforms of modern atoll provinces rose abruptly from the sea surface as clusters of subaerial limestone plateaus with steep flanks (modern analogues are provided by atolls uplifted on the bathymetric arches of trench forebulges). Karstic erosion during the last glaciation lowered the surfaces of last-interglacial reef edifices by amounts sufficient to provide accommodation space for eventual Holocene reef growth, and produced solution ramparts at platform margins to guide growth of annular Holocene reefs. During early Holocene eustatic rise in global sea level, atoll reefs began to grow by ~8 ka when rising sea level overtopped degraded remnants of interglacial reefs, but remained submerged until carbonate buildups approached sea level in mid-Holocene time. The classic morphology of modern atolls, with circlets of multiple islets dotting annular reefs, is a transient state formed in response to regional late Holocene hydro-isostatic drawdown in tropical Pacific sea level by a regionally variable 1.6-2.6 m (atoll emergence stemmed from equatorial ocean siphoning, a facet of global isostatic adjustment to the postglacial transfer of mass from circumpolar ice sheets to the world ocean). Cemented mid-Holocene paleoreef flats now stranded above modern sea level serve as resistant underpinnings for stable motu islets that were not present along atoll rims until late in Holocene time. Although shifting sand cays may have nucleated earlier, the more stable motus could not form until after ambient high-tide levels fell below mid-Holocene low-tide levels. Present atoll landforms thus have a time depth consistently <2 ka, and as short as ~1 ka.