THE EVOLUTION OF MAJURO ATOLL IN RESPONSE TO SEA-LEVEL CHANGE DURING THE MID-HOLOCENE

Pacific Island sedimentology provides an invaluable record of geomorphic and environmental consequences of coastal evolution in response to changes in sea-level. On low lying islands dominated by unconsolidated reef derived sediment, large benthic foraminifera can be used to reconstruct sedimentary facies. Here we propose a three phase model of island formation for Majuro atoll, Marshall Islands based upon the age and abrasion of the foraminifera species Calcarina derived from island trenches and drill cores excavated along a tidal reef flat. A 1.1 m mid Holocene highstand approximately 2000-4000 years BP enabled accretion of the reef flat. Following the highstand, sea-level drawdown coupled with reduction in wave energy enhanced foraminiferal supply along the ocean-side reef flat and triggered island emergence. Elevation of the central body of the island exceeded high water levels by approximately 1200 cal yr BP and basal island sediment transitioned from coral boulders to Calcarina rich sand (62-83% total sediment composition). After 800 cal yr BP island accretion extended longitudinally from the central core and was completed by 500 cal yr BP. High ratios of Calcarina foraminifera with intact spines document rapid transport of foraminifera from the reef flat and brief exposure to physical erosion prior to burial. Comparisons amongst a windward reef island to previously published data collected at a leeward reef island show that despite the highest production of Calcarina occurring along modern windward reef flats, island building in these environments lagged leeward reef islands by nearly 1000 years. This indicates the spatial and temporal complexity of reef island evolution, and the importance of managing sediment systems as future sea-level rise threatens coastal societies, ecosystems and infrastructure.