ASSESSING SEALEVEL CHANGE WITH COUPLED 230TH/234U-ESR ANALYSES IN HERMATYPIC CORALS

Although coupled ²³⁰Th/²³⁴U-ESR dating has become routine for dating teeth, it has never been used to date corals. Both ESR and ²³⁰Th/²³⁴U can date coralline aragonite ranging in age from 1 to 400 ka. The ESR age depends on the time-averaged cosmic dose rate, D_cos(t), assumed, but the ²³⁰Th/²³⁴U dates do not depend on D_cos(t). Because D_cos(t) experienced by corals depends on the amount of water and sedimentary cover overlying the coral, D_cos(t) response reflects changing water depths due to subsidence, uplift, and sealevel change. An accurate ESR age, therefore, requires assumptions about D_cos(t). By coupling the two methods, one can determine the age and a unique D_cos(t) simultaneously.

For reefs on stable platforms, D_cos(t) recorded by the coral depends only on the water depth and sedimentary cover thickness overlying the dated coral sample. From a coral's water depth and sedimentary history predicted with a given sealevel curve, one can estimate a D_{cos, sealevel}(t). If D_{cos, coupled}(t) from the coupled analysis (i.e., D_cos(t) actually experienced by the coral) agrees well with D_{cos, sealevel}(t), this provides independent validation for the sealevel curve used to build D_{cos, sealevel}(t). For six corals ranging in age from 75 to 135 ka collected from reef crests on the Florida platform, their D_{cos, coupled}(t) values and ages agreed well with D_{cos, sealevel}(t)predicted from the sealevel curve by Waelbroeck et al. (2002). For three, however, their coupled D_{cos, coupled}(t) values and ages did not agree well with the D_{cos, sealevel}(t) estimated from the SPECMAP sealevel curve.

This study demonstrated that couple ESR-²³⁰Th/²³⁴U dating can independently test the validity of sealevel curves. More precise measurements for water depth and coral cover thickness would improve the precision. Where a large coral can be sampled at several points, or a reef can be sampled over a transect, a precise test for sealevel curves could be developed.

WAELBROECK, C., L. LABEYRIE, E. MICHEL, J.C. DUPLESSY, J.F. MCMANUS, K. LAMBECK, E. BALBON, M. LABRACHERIE, 2002. Sea-level and deep water temperature changes derived from benthonic foraminifera isotopic records. Quaternary Science Reviews 21: 295–305.