Multi-decadal planform changes on coral reef islands from atolls and mid-ocean reef platforms of the equatorial Pacific Ocean: Gilbert Islands, Republic of Kiribati

Abstract

Constructed of poorly lithified carbonate sediments with elevations of <3–5 m above MSL, coral reef islands are considered extremely vulnerable to the effects of anthropogenic climate change, particularly sea-level rise. While some studies have shown islands are dynamic landforms with the ability to physically adjust their shape and position on the reef platform, large geographical gaps still exist in reef island change records. This study presents a multi-decadal analysis of shoreline changes and planform geomorphic adjustments across coral reef islands in the equatorial Pacific from the Gilbert archipelago, Republic of Kiribati. Shoreline positions of 71 islands from 3 atolls and 4 mid-ocean reef platforms were analysed by comparing historical aerial photographs (from 1940s and 1960s) and recent satellite imagery covering a period of local sea-level rise rate of ~2.2 mm/year. Results show ~47% of the shorelines were characterised by statistically significant accretion leading to a net increase of 274.07 ha (2.45%) of planform land area. A comparative examination of changes on platform-islands and atoll-islands revealed platform-islands were relatively more stable (mean rate of 0.01 m/year) with significantly less variability in shoreline change rates, while atoll-islands were characterised by a higher mean rate of accretion (0.09 m/year). Further analysis of change rates across various shoreline exposure categories, demonstrate high dynamism across atoll-island shorelines is a function of distinctive local-scale processes occurring within atoll settings; lagoon-facing shorelines were characterised by the highest rate of accretion (0.14 m/year), spits were found to be highly dynamic and embayment infilling accounted for ~9% of the net increase in land area across the study islands. Results are instructive in defining styles of morphological change across various geomorphic settings to inform projections of future island adjustments, assessments of attribution of island change and formulation of adaptation responses across atoll nations.