Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

Abstract

Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of well-dated sediment cores from 8 locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic and hypoxic sites are in line with an active release of Mn from these areas. However, this flux of Mn is only small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers form independent of inflow events, with pore water Mn produced in deeper layers of the sediment acting as a key Mn source. While formation of Mn enrichments in the Landsort Deep continues to the present, this does not hold for the Gotland Deep area. Here, increased euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), goes hand in hand with a decline in sediment Mn and recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. We postulate that the reduction of Mn oxides by hydrogen sulfide following inflows has become so rapid that Mn2+ is released to the water column before Mn carbonates can form. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

Ocean Circulation in the Toarcian (Early Jurassic): A Key Control on Deoxygenation and Carbon Burial on the European Shelf

“…Ratios of iron over aluminum (Fe/Al) above crustal averages (0.5 wt%/wt%) are typically associated with anoxic and sulfidic basins, where Fe becomes enriched through effective trapping of any incoming Fe from shallower areas (Lyons & Severmann, 2006;Owens et al, 2012). Because input from these shallower areas can be enhanced under anoxic but not sulfidic conditions, Fe/Al can also indicate how widespread anoxia is in a region (e.g., Eckert et al, 2013;Lenz et al, 2015;Raiswell & Anderson, 2005). Molybdenum (Mo) is conservative in oxic waters but is fixed in sediments in the presence of free hydrogen sulfide through conversion of molybdate to particle reactive thiomolybdate (Helz et al, 1996).…”

Mid‐ to late Holocene environmental separation of the northern and central Baltic Sea basins in response to differential land uplift

“…For the Landsort Deep, however, Lepland & Stevens (1998) and Lenz et al (2015a) suggested Mn(Ca)CO 3 formation to occur irrespective of oxygenation events. Lepland & Stevens (1998) related a pronounced Mn (Ca)CO 3 precipitation to the expansion of euxinic bottom waters under increased organic matter input resulting in elevated alkalinity and a Mn surplus due to the reduction of Mn oxides at the basin slope.…”

“…Subsequently, Mn oxides accumulate at the sea floor and are converted into Mn(Ca)CO 3 under conditions of increased alkalinity and re-established euxinia. However, recent studies by Lenz et al (2015a) argued that modern oxygenation events may not result in the formation of Mn(Ca)CO 3 layers in the eastern Gotland Basin. High sulphide concentrations in the sediments and in the bottom water following inflow events probably cause the fast dissolution of Mn oxides and the escape of dissolved Mn to the open water column.…”

“…High sulphide concentrations in the sediments and in the bottom water following inflow events probably cause the fast dissolution of Mn oxides and the escape of dissolved Mn to the open water column. Ongoing eutrophication and reduced input of Fe oxides representing an important sink for sulphide may be the reason for the rapid expansion of hypoxia (Lenz et al 2015a).…”

“…Huckriede & Meischner 1996) and Mn(Ca)CO 3 precipitation under pronounced euxinic bottom-water conditions (e.g. Lepland & Stevens 1998;Lenz et al 2015a).…”