High-precision TIMS U-series and AMS 14C dating of a coral reef lagoon sediment core from southern South China Sea

Abstract

Accurate dating of lagoon sediments has been a difficult problem, although lagoon profiles, usually with high deposition rates, have a great potential for high-resolution climate reconstruction. We report 26 high-precision TIMS U-series dates (on 25 coral branches) and five AMS 14C dates (on foraminifera) for a 15.4-m long lagoon core from Yongshu Reef, Nansha area, southern South China Sea. All the dates are in the correct stratigraphical sequence, providing the best chronology so far reported for lagoon deposits. The results reveal a ∼4000-a continuous depositional history, with sedimentation rates varying from 0.8 to 24.6 mm a −1, with an average of 3.85 mm a −1, which corresponds to an average net carbonate accumulation rate of ∼2700 g CaCO 3 m −2 a −1, significantly higher than the mean value (800±400 g CaCO3 m −2 a −1) used for lagoons in general in previous studies of global carbonate budget. Episodes of accelerated depositions within the last 1000 years correlate well with strong storm events identified by U-series dates of storm-transported coral blocks in the area. However, in the longer term, the sedimentation rates during the past 1000 years were much higher than earlier on, probably due to more vigorous wave–reef interaction as a result of relative sea-level fall since 500 AD and expansion of reef flat area, supplying more sediments. The coral TIMS U-series ages and foraminifera AMS 14C dates reveal intriguing apparent “radiocarbon reservoir ages” (R) from 572 to 1052 years, which are much higher than global mean values of ∼400 years.