Recycling of manganese from anoxic sediments in stagnant basins by seawater inflow:: a study of surface sediments from the Gotland Basin, Baltic Sea

The occurrence of Mn carbonates in sapropelic sediments has been proposed to indicate that the host sediment originally accumulated under oxygenated bottomwaters. In the central Baltic Sea, Ca-rich rhodochrosite layers in sapropelic sediments have been related to inflows of oxic seawater from the North Sea into the brackish, predominantly anoxic, deeps of the central Baltic. This study attempts to verify the model of authigenic Mn carbonate formation by comparing oceanographic records directly with the composition of surficial sediments of the Gotland Basin. Surface sediments of the Gotland Deep sampled in 1997 displayed significant Ca-rich rhodochrosite enrichments at 6.5, 11 and 15 cm sediment depth which reflect periods of intense seawater inflows in 1969-76, 1948-56, and 1931-39, respectively. However, the latest major seawater inflow detected in 1993 was not reflected in thr surface sediment. The topmost 6 cm of the sediment was totally depleted in Mn and (210)b data implied a disturbed top layer down to 5 cm. Calculations of saturation indices indicate that the porewaters were undersaturated with respect to Mn and Ca carbonate phases in the uppermost 8 cm in 1997. At greater depths, the porewaters were close to equilibrium with respect to calcite and rhodochrosite. The Ca-rhodochrosite layers corresponded well to a sediment core sampled at the same location in 1994, but that core displayed a further enrichment of Ca-rich rhodochrosite close to the sediment surface, which can be related to a major inflow of North Sea watts in 1993. The significant decrease of about 2.3 mol/m(2) Mn in the uppermost 6 cm of a disturbed sediment surface from 1994 to 1997 corresponds to an enrichment of about 20 mu mol/l of dissolved Mn within the 90 m thick anoxic watts column in 1997. We conclude that several inflows of dense seawater into the Gotland Deep, monitored between 1994 and 1997, led to a resuspension of the unstable, fluffy surface sediment layer. Authigenic Ca-sich rhodochrosite: which had formed immediately after the major inflow of oxic seawater in 1993 was therefore redissolved in undersaturated bottomwaters and Mn was recycled from surficial sediments into the water column. This in turn means that, in contrast to the established model of Ca-rhodochrosite formation as a result of seawater inflow, the occurrence of mixed, Mn-depleted layers does not prove that seawater inflow has not occurred during the deposition of these sediments. (C) 2001 Elsevier Science Ltd All rights reserved.