Geophysical Visualization of Reef Island Accretion and Stratigraphic Architecture using Ground Penetrating Radar

Coral reef islands are small, low-lying landforms that provide the only habitable land for atoll nations and support a range of endemic flora and fauna. Thus, the vulnerability of these islands to projected sea-level rise is a global concern. The internal structure and modes of island accretion are integral in understanding how islands have formed and subsequently, how they will change in the future. This study employs extensive ground penetrating radar (GPR) imaging on a reef island on Huvadhoo Atoll, Maldives, combined with island coring and stratigraphic analyses, in order to infer styles of island accretion. The GPR images imply central core accretion, with vertical accretion occurring on the initial core and progradation occurring outwards in both directions. However, some transects display progradation occurring predominately in one direction, which may also indicate erosional and rollover or overwash processes that could have led to island migration. Furthermore, using inferences of major radar units and boundary changes in signal reflectors combined with detailed subsurface sedimentology, the subsurface stratigraphy of the island sediments was divided into four discrete facies: velu (lagoon infill), finolhu (initial island core), athiri (beach and paleobeach), and muraka (surface layer) facies. The GPR records on KAN provide visual evidence of the mode of island evolution to complement previous theoretical interpretations, and have demonstrated the capability of GPR to discern sedimentary facies, subsurface features, and changes in sediment characteristics on reef islands. Results from this study show past processes of island building and could infer island response to various stages of sea-level change, which has implications for the stability of reef islands in the face of projected sea-level rise.