Assessing subsidence rates and paleo water-depths for Tahiti reefs using U–Th chronology of altered corals

We present uranium–thorium chronology for a 102m core through a Pleistocene reef at Tahiti (French Polynesia) sampled during IODP Expedition 310 “Tahiti Sea Level”. We employ total and partial dissolution procedures on the older coral samples to investigate the diagenetic overprint of the uranium–thorium system. Although alteration of the U–Th system cannot be robustly corrected, diagenetic trends in the U–Th data, combined with sea level and subsidence constraints for the growth of the corals enables the age of critical samples to be constrained to marine isotope stage 9. We use the ages of the corals, together with δ18O based sea-level histories, to provide maximum constraints on possible paleo water-depths. These depth constraints are then compared to independent depth estimates based on algal and foraminiferal assemblages, microbioerosion patterns, and sedimentary facies, confirming the accuracy of these paleo water-depth estimates. We also use the fact that corals could not have grown above sea level to place a maximum constraint on the subsidence rate of Tahiti to be 0.39mka−1, with the most likely rate being close to the existing minimum estimate of 0.25mka−1. ► Open system U–Th data is used to constrain coral ages to marine isotope stages. ► Age control allows water-depth, and island subsidence rate, estimates to be tested. ► Paleo depth estimates from foraminifera and coralline algae are found to be robust. ► The maximum rate of subsidence for Tahiti is limited to 0.39mka−1.