Multiple interglacial sequences in a Darwin-type barrier-reef lagoon: Implications for paleoclimate, sea-level changes and subsidence since the Late-Pleistocene

Charles Darwin described in 1842 the island of Bora Bora (Society Islands, Central South Pacific) as key example for a subsiding basaltic oceanic island with related reef development. He recognized that the Bora Bora lagoon developed between an outer barrier reef with a sand apron and an inner fringing reef attached to the shore of the volcanic island. In order to quantify past sea-level and paleoenvironmental changes and island subsidence, the lagoonal sediments were cored in 2024 in the context of the Bora²coring project. Previous shorter cores indicated that the Holocene lagoonal sediments are of a mixed-carbonate-siliciclastic nature: the carbonate fraction is formed in-situ in the shallow-water depositional environment, whereas the siliciclastic fraction originates from the volcanic island. A recent seismic survey documented that below the Holocene sequence, several stacked depositional sequences occur, which must reflect the combined effect of sea-level fluctuations and ongoing island subsidence. To unravel the complex depositional history, a full suite of sedimentological, paleontological, petrophysical and geochemical analysis of the cores are conducted. In addition, samples of recent soil and lagoonal sediment will provide a data set to calibrate the measured proxies from the cores.

A total of 33 m of sediment cores were recovered and spliced into a composite section with a length of 18.2 m. The composite section reveals variable lithologies. A 4.5 m thick Holocene carbonate mud overlays a stiff, red to grey-bluish mottled, carbonate-free clay, forming the next underlying sequence. Coarse-grained carbonate sediments reappear at a depth of 9 m. Below this second carbonate unit, carbonate-free, grey-to-brown clay occurs, with occasional interbeds of white carbonate. Downcore, the clay becomes reddish again.

These sediments will be interpreted in the context of the interplay between sea-level, island subsidence and resulting accommodation space. A permanent connection to the open ocean seems to have existed only during the Holocene. In contrast, the deposition of siliciclastic fines during glacial phases suggests a distal alluvial or even shallow lacustrine depositional environment with no carbonate production within the lagoon. The occurrence of carbonates in 9 m depth indicate an older marine transgression and regression cycle presumably during an interglacial period overlying another glacial sequence.

All the different analyses will eventually merge toward an improved knowledge of island subsidence and the chronology and amplitudes of sea-level changes. The siliciclastic fines will additionally serve as an excellent proxy for hydroclimate-dependent weathering and erosion processes on the island.