OPTICAL DATING OF MARINE SEDIMENT FROM ODP CORE 658B - AN INTERCOMPARISON WITH PRE-EXISTING AMS 14C CHRONOLOGY

This study presents the results of an intercomparison of ages produced for marine sediments from Ocean Drilling Program (ODP) Site 658, using optical dating and previously published AMS ¹⁴C (deMenocal et al., 2000). This is the first large-scale, systematic application of optical dating to marine sediment, and demonstrates that the technique has considerable potential for the age estimation of otherwise undateable deep-sea material.

ODP Site 658 is located off Cap Blanc, Mauritania. It has a high sedimentation rate (18 cm/ka), due to high regional surface productivity and large inputs of windblown Africa dust (the material dated in this study). The Pleistocene-Holocene sections of core 658C are well dated (18 AMS ¹⁴C ages to 23 Cal. ka BP). As Core 658C has been extensively studied and core material exhausted, samples for optical dating were obtained from the neighbouring core 658B. Age/depth correlation between the two cores was made using oxygen isotope stratigraphy and carbonate content.

Optical dating techniques measure the total ionising radiation dose that a mineral grain has received since its last exposure to sunlight (i.e. during burial). This figure is divided by the environmental dose rate, giving the burial period for that sample. The environmental dose rate is mainly derived from the decay of radioisotopes in the sediment (K, U and Th). In terrestrial sediments, the uranium and thorium decay chains can be assumed to be in equilibrium. In the marine realm this assumption is not valid, and unsupported Uranium decay series exist on deposition. Consequently, the environmental dose rate changes with sample age. This problem was circumvented by quantifying U and Th decay series disequilibrium, using an MC-ICP-MS. The evolution of the environmental dose rate during burial was modelled, and ages for each sample calculated.

The ¹⁴C and optical dating ages are generally in good agreement, indicating that the latter technique can provide robust chronologies for marine sediments. Optical dating has been successfully applied to terrestrial sediments ranging in age from 0-200 ka, and hence should provide a valuable geochronological tool for marine sediments extending well beyond the time range of ¹⁴C dating, and dating marine sediments that contain no dateable carbon.