Seismic evidence of glacial-age river incision into the Tahaa barrier reef, French Polynesia

Rivers have long been recognized to shape reef-bound volcanic islands. On the time-scale of glacial-interglacial sea-level cycles, fluvial incision of exposed barrier reef lagoons may compete with constructional reef processes to shape the coastline of ocean islands. However, overprinting by Holocene lagoon sedimentation has largely obscured the role lowstand river incision has played in forming the deep lagoons typical of modern barrier reefs. Here we use high-resolution seismic imagery and core stratigraphy from the island of Tahaa, a barrier reef bound island located along the Society Islands hotspot chain, French Polynesia, to understand how rivers shape reef morphology. On Tahaa, we find that many channels, incised into the lagoon floor during Pleistocene sea level lowstands, are located opposite of upstream terrestrial drainages. Steeper antecedent topography appears to have enhanced lowstand fluvial erosion along Tahaa’s southwestern coast and maintained deep passes. During subsequent highstands upland drainages contribute little sediment to refill accommodation space in the lagoon. Rather, the flushing of fine sediment out deep passes by storms and currents appears to have limited lagoonal infilling and reinforced channel development during periods of highstand submersion. Without highstand deposition to balance lowstand fluvial erosion, Tahaa's deep barrier reef lagoon, seen today, has developed.