Variations in Holocene sedimentation in the North American Basin determined from 230Th measurements

Abstract

Excess 230Th was measured in 20 sediment samples from the upper 251 cm of KNR31 GPC5, a giant piston core taken at a depth of 4583 m from a drift deposit on the northeastern Bermuda Rise, in an attempt to determine the cause of fluctuations in calcium carbonate percentage within the Holocene section. The excess 230Th activity, corrected for radioactive decay since the time of deposition, is strongly correlated with CaCO 3 percentage, suggesting that both are controlled by the same factor(s). We conclude that the fluctuations in CaCO 3 percentage are due primarily to variations in the supply of the terrigenous (non-carbonate) components. From a simple model that assumes a constant supply of excess 230Th, we estimate that during the past 11,000 years the CaCO 3 flux in this region has remained within the narrow range 0.58– 0.79 g cm −2 ky −1. Non-carbonate flux, on the other hand, has varied during the same period by as much as a factor of 2.3 (1.25–2.84 g cm −2 ky −1). Non-carbonate flux maxima occur at ∼1580 and ∼3360 years BP and coincide with enrichments of high latitude clay minerals. These features may reflect variations in the rate at which terrigenous sediment has been resuspended from the continental slope and transported to the interior of the basin. From the measured accumulation of excess 230Th in radiocarbon-dated intervals, it can be shown that the degree of sediment focusing at the core site has increased by more than a factor of three since the beginning of the Holocene.