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

This project aims to drill and recover the sedimentary archive of the Bora Bora Lagoon in the South Pacific that records paleoenvironmental changes over several glacial-interglacial cycles back to the Mid-Pleistocene. Through combined geophysical, sedimentological and geochemical approaches, this drilling project will provide new insights into central pacific paleoclimate at high temporal resolution while also elucidating the subsidence and sea-level history of the Bora Bora lagoon. As described by Darwin in 1842 in his theory of subsiding basaltic ocean islands with related reef development, the Bora Bora lagoon formed between an outer barrier reef with a sand apron and an inner fringing reef attached to the edge of the volcanic island. Sedimentation in the lagoon is of mixed-carbonate-siliciclastic nature: the carbonate fraction is formed in the in-situ shallow-water depositional environment, whereas the siliciclastic fraction originates from the volcanic island and respective soil formation that is weathered and brought in by runoff processes. Reflection seismic data indicate that the lagoonal succession is composed of at least six depositional sequences separated by high-amplitude reflections. The uppermost sequence has been cored all the way to an exposure surface and peat horizon and could be related to the Holocene marine isotope stage (MIS) 1 that was deposited following transgression after termination 1. Using this sequence as a proof of concept, the underlying aggrading and oceanward prograding lagoonal sequences are interpreted to contain interglacial sea-level highstand deposits from periods when the lagoon was flooded, overlying sequence boundaries with erosional surfaces and soils that formed during multiple sea-level lowstands. The stack of sequences is interpreted to reach back to MIS 13 or beyond. Accommodation space and flooding thresholds in the lagoon are controlled by the combined effect of these sea-level fluctuations and ongoing island subsidence. Three sites are proposed to be drilled along a lagoonal transects that each will recover this 30-40 m thick sedimentary succession. The island-proximal drillhole is intended to reach the volcanic basement, providing age and nature of the initial transgression at that site. A full suite of sedimentological and petrophysical analysis of the drill cores will reconstruct the depositional history (incl. paleo-water-depth) of the lagoon that will be chronologically dated using the U-series. A multiproxy study using inorganic geochemistry (inorganic/organic carbon, elemental composition, stable isotopes) and organic geochemistry (GDGTs, alkenones) will provide critical paleoenvironmental targets including temperature and salinity. A modern sea-floor sampling campaign, water-column samples and soil samples from the volcanic island will furthermore provide a dataset to calibrate the measured proxies from the drill cores. All the analyses will eventually merge towards an improved knowledge of island subsidence, chronology and amplitudes of sea-level changes, hydroclimate-dependent weathering and erosion processes on the island, and on paleoceanographic data such as sea-surface temperatures and salinities.