Recycling of manganese from anoxic sediments in stagnant basins by seawater inflow: a study of surface sediments from the Gotland Basin, Baltic Sea
✍ Scribed by Uwe Heiser; Thomas Neumann; Jan Scholten; Doris Stüben
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 648 KB
- Volume
- 177
- Category
- Article
- ISSN
- 0025-3227
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✦ Synopsis
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 in¯ows 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 sur®cial sediments of the Gotland Basin.
Surface sediments of the Gotland Deep sampled in 1997 displayed signi®cant Ca-rich rhodochrosite enrichments at 6.5, 11 and 15 cm sediment depth which re¯ect periods of intense seawater in¯ows in 1969±76, 1948±56, and 1931±39, respectively. However, the latest major seawater in¯ow detected in 1993 was not re¯ected in the surface sediment. The topmost 6 cm of the sediment was totally depleted in Mn and 210 Pb 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 in¯ow of North Sea water in 1993. The signi®cant 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 mmol/l of dissolved Mn within the 90 m thick anoxic water column in 1997.
We conclude that several in¯ows of dense seawater into the Gotland Deep, monitored between 1994 and 1997, led to a resuspension of the unstable, ¯uffy surface sediment layer. Authigenic Ca-rich rhodochrosite which had formed immediately after the major in¯ow of oxic seawater in 1993 was therefore redissolved in undersaturated bottomwaters and Mn was recycled from sur®cial 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 in¯ow, the occurrence of mixed, Mn-depleted layers does not prove that seawater in¯ow has not occurred during the deposition of these sediments.