Late Quaternary sapropel on the Mediterranean Ridge: U-budget and evidence for low sedimentation rates

Abstract

Alpha-spectrometric determinations of U- and Th-isotopes and petrographical examination were performed on Late Quaternary sapropels of cores from the Mediterranean Ridge (22M48, 22M50, PC 6, 22M45 and 17M21) and of two cores taken in the eastern part of the Levantine Basin (PC 20 and PC 18). The average U-content of 20 samples is 21.4 ppm (range 4.3–47 ppm). If compared to the normal oxidized sediments on the Mediterranean Ridge this means an U-enrichment by a factor of 30. Uranium in sapropels was deposited from seawater (authigenic) at an average deposition rate of 11 μg/cm 2 per 1000 years. To explain the low activity ratios of 234U/ 238U near unity as found in several sapropel sections we propose a half-open system, where in situ produced 234U is able to diffuse out of the sapropel (diffusion constant: 10 −11 to 10 −12 cm 2/sec). By means of a 230Th budget and taking into account the known ages of the sapropels, sedimentation rates as low as 0.75 cm/1000 y, were evaluated. These rates are up to two times lower than the average rate for the last 350,000 y. These low rates give evidence for the climatically controlled variations of the sedimentary regime in the Eastern Mediterranean, as the majority of sapropels occur in the warmer intervals of the climatic cycles.