U-series dating of diagenetically altered fossil reef corals

Abstract

Initial ( 234U/ 238U) activity ratios higher than those expected from closed system evolution of seawater are an outstanding problem in U-series dating of fossil reef corals. The increased activity ratios are ascribed to post-depositional diagenesis. We analysed six Last Interglacial and one Holocene coral of the genus Porites collected near the northern end of the Gulf of Aqaba (northern Red Sea) together with three recent corals from this location as a modern reference. While the values measured on the recent samples show no deviation from expected values, the Holocene and Last Interglacial corals display highly elevated ( 234U/ 238U) activity ratios as well as increased 238U and 232Th concentrations. ( 234U/ 238U) activity ratios are strongly correlated with ( 230Th/ 238U) and total uranium content. A model assuming different degrees of uranium addition and subsequent loss in different sub-samples of one coral produces straight lines (isochrons) on a ( 234U/ 238U)–( 230Th/ 238U) plot and predicts that the true age of the coral can be calculated by intersecting this isochron with the seawater evolution curve. We used the strong correlation detected in the Aqaba corals to calculate isochron ages. The isochron age of the Holocene coral (3116 +167 −156 yr) is concordant with its calibrated 14C age (2963 +92 −103 yr). The ages of the Last Interglacial corals are between 121 (+6.7 −5.3) and 121.9 (+7.0 −6.3) kyr for a higher terrace (7–10 m asl) and 106.4 (+8.9 −8.1) and 117.1 (+19.7 −15.3) kyr for a lower terrace (4–5 m asl). One Last Interglacial coral with an aragonite content of ∼85% could not be dated. The elevation and ages of the fossil terraces are consistent with existing sea level reconstructions from the Red Sea.