Open system U-series ages of corals from a subsiding reef in New Caledonia: Implications for sea level changes, and subsidence rate

Abstract

On the Amédée islet, 4 drill cores were recovered from the barrier reef of Western New Caledonia. The coral reef is slowly subsiding and is thus percolated by sea water during sea level highstands. The cores sample a ∼ 10 m thick Holocene reef overlying a 24 m thick reef of marine isotope stage (MIS) 5.5, which in turn overlies older reef material from MIS 7.5 and beyond. ( 234U/ 238U) and ( 230Th/ 238U) ratios and 232Th were determined by thermal ionization mass spectrometry on aragonitic coral samples that were carefully investigated using X-ray diffraction and scanning electron microscopy. The petrographic study shows an increasing coral weathering with growing coral age that causes different degree of U-series open system behavior and 232Th accumulation. Holocene corals exhibit a small degree of early diagenesis and yield 230Th/ 238U ages according to the Holocene sea level rise from ∼ 8200 years to 5000 years BP. Corals from the last Interglacial section have experienced more frequent replacement of aragonite fibers and minor dissolution, and U-series open system behavior is evident. To estimate the impact of recoil processes and alteration on the U-series system two models by Villemant and Feuillet [B. Villemant, N. Feuillet, Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces, Earth and Planetary Science Letters 210(2003) 105–118.] and Thompson et al. [W. G. Thompson, M. W. Spiegelman, S. L. Goldstein, R. C. Speed, An open-system model for U-series age determinations of fossil corals, Earth and Planetary Science Letters 210(2003) 365–381.] have been tested. These models yield identical ages within uncertainty, which are in agreement to the sea level history of the past 250,000 years, as long as physico-chemical alteration and re-crystallization is small. Consequently, we were able to estimate the subsidence rate from the subsidence observed between the end of MIS 5.5 and the early Holocene, which is ∼ 0.16 ± 0.04 m per 1000 years. In addition, by using this subsidence rate and the sea level highstand of + 6 ± 3 m during MIS 5.5 we can estimate the sea level highstand at 244,000 (MIS 7.5) to be between + 2 m to − 2 m, respectively. Corals from underneath the marine isotope stage 7.5 sequence show more significant dissolution, increasing re-crystallization, elevated 232Th, and excess 234U and 230Th. Here the Villemant and Feuillet [B. Villemant, N. Feuillet, Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces, Earth and Planetary Science Letters 210(2003) 105–118.] model fails to predict ages, but the Thompson et al. [W.G. Thompson, M. W. Spiegelman, S. L. Goldstein, R. C. Speed, An open-system model for U-series age determinations of fossil corals, Earth and Planetary Science Letters 210(2003) 365–381.] model was applicable predicting ages between 355,000 to > 430,000 years, i.e. coral growth potentially during MIS 9 or 11.