High Resolution ¹⁰Be and ²³⁰Th Stratigraphy of Late Quaternary Sediments from the Fram Strait (Core 23235)

Abstract

We obtained a high resolution stratigraphy of sediment Core 23235 (Fram Strait, 78°51,55′N, 01°18,59′E, 2456 m water depth) from the measurement of the concentration of ¹⁰Be and ²³⁰Th in depth profiles. The average sedimentation rate is approximately 3±0.3 cm/kyr. On a length of 806 cm the Core 23235 covers a time span of approximately 270 kyr. The profiles of the concentration of ¹⁰Be and ²³⁰Th have maxima during interglacials and minima during glacial periods. During the interglacials the fluxes of ¹⁰Be (up to 4.35±0.17 · 10⁶ at cm^-2 yr^-1 and of ²³⁰Th (up to 26.70±6.37 dpm cm^-2 yr^-1) greatly exceed the depositional fluxes expected from the production of of ¹⁰Be in the atmosphere and ²³⁰Th in the water column. In contrast, during glacial stages 2 and 6 the ¹⁰Be fluxes are similar to or slightly lower than production fluxes, while the ²³Th fluxes are significantly lower than the predicted flux. The only exception to this trend occurred during a short period of stage 6, at about 135 ka, which has the highest ²³⁰Th flux in the core record. At this ‘²³⁰Th flux anomaly’ the ratio of the production exceeding isotope fluxes of ¹⁰Be and ²³⁰Th has a value of 0.3:1, whereas during interglacial conditions this ratio approximates 4 : 1. We presume that the ‘²³⁰Th anomaly’ reflects a very short period of time during glacial stage 6, when renewal of deep water took place (it is generally assumed that no deep water was formed in the North Atlantic during stage 6). As a consequence of sudden oxygen supply to the water column at 135 ka, ²³⁰Th was scavenged from the water column with Mn-oxides more efficiently than ¹⁰Be.