ABSTRACT

Two sediment cores longer than 20 meters, obtained from two abyssal plains of the North Atlantic during the ESOPE cruise were analyzed for U and Th isotopes. In the Madeira Abyssal Plain sediment (ESOPE core 10), excess ²³⁰Th data do not show a simple exponential decrease with depth, being reflected by complex depositional history of the sediments, and hence the Th isotope data can only weakly constrain the rate of sediment accumulation. Relatively low ²³⁰Th concentration in the top 5 meters may be caused by rapid accumulation of turbiditic materials. That a significant excess ²³⁰Th exists even at depths of ∼20m in the core suggests that the average sedimentation rate is rapid, perhaps ∼10 of centimeters per thousand years. This high rate of sediment accumulation has resulted largely from frequent turbidites, and not from pelagic clay sedimentation. In the Nares Abyssal Plain sediment (ESOPE core 48), excess ²³⁰Th relative to ²³⁴U has been found down to ∼5m, showing its almost exponential decrease with depth. Based on the excess ²³⁰Th distribution, an average sedimention rate for the core is estimated to be 1.2 ± 0.3 cm/10³ y.