Impacts of eustasy and hydro-isostasy on the evolution and landforms of Pacific atolls

Consideration of eustatic and hydro-isostatic effects on late Quaternary sea levels in the tropical Pacific Ocean indicates that the configuration of modern atolls with emergent annular reef flats is a transient morphology not developed until post-mid-Holocene time. Annular atoll reefs, perched atop carbonate platforms which cap buried volcanic edifices, are underlain by 8–28 m of Holocene limestone disconformably overlying a substratum of last-interglacial or older limestone. Comparable thicknesses (9–23 m) of Holocene sediment are present beneath atoll lagoons that are uniformly <85 m deep. During glacio-eustatic drawdowns in global sea level by 120–125 m, carbonate platforms of modern atoll provinces rose abruptly from the sea as clusters of subaerial limestone plateaus flanked by steep cliffs. Modern analogues are provided by emergent atolls uplifted on the flexural arches of trench forebulges. Slow subsidence coupled with karstic erosion of emergent atolls during the last glaciation lowered the surfaces of last-interglacial reef edifices by the amounts needed to provide accommodation space for Holocene reef growth during the postglacial eustatic rise in sea level. Modern atoll reef caps began to grow after ∼9 ka when rising Holocene sea level overtopped degraded remnants of interglacial reefs, but remained submerged until carbonate buildups approached sea level in mid-Holocene time (6–4 ka). Classic atoll morphology, with circlets of multiple islets dotting annular reefs, formed in combination with late Holocene hydro-isostatic drawdown in tropical Pacific sea level in response to equatorial ocean siphoning, a facet of global isostatic adjustment (GIA) to deglaciation. Early Holocene eustatic rise in sea level and late Holocene hydro-isostatic decline in sea level combined to produce a regionally variable mid-Holocene highstand in tropical Pacific sea level that stood 1.0–2.6 m above modern sea level. Cemented mid-Holocene paleoreef flats now stranded well above sea level serve as resistant foundations for non-migratory (pinned) islets that were not present along atoll rims until after the local crossover date, when ambient high-tide level first fell below mid-Holocene low-tide level. Existing atoll landforms have a time depth generally <1–2 ka.