Control of manganese and iron in Skagerrak sediments (northeastern North Sea)

Abstract

Three cores with variable physicochemical characteristics from different areas of the Skagerrak, northeastern North Sea, have been studied to understand the processes such as redox conditions, CaCO₃, bioturbation, etc., controlling the Fe and Mn abundance in sediments. Sequential extraction was performed to identify the partitioning patterns of these elements into sorbed and carbonate, (Mn,Fe)-oxyhydroxide, Fe-oxyhydroxide, residual leachable and residual non-leachable phases. Pore-water Fe and Mn contents in the central Skagerrak increase up to 100-mm depth while they reach maximum concentrations around 50-mm depth in the southern Skagerrak sediments. The Fe redox boundary lies below the Mn redox boundary whereas the latter coincides with the level of oxygen penetration. Extractable Mn in the upper layers of the central basin sediments is 10 times higher than that present in sediments of the southern part (30 mmol kg^-1). Fe is present in all fractions extracted but Mn occurs mainly in carbonate and (Fe,Mn)-oxyhydroxide phases. Total extractable Fe is about three times less than that of total Fe in all cores whereas total Mn is nearly equivalent to the total extractable Mn. Mn in sediments showed two peaks, the surficial one due to precipitation of oxyhydroxides from the diagenetically upwelled and sedimented Mn from overlying waters and the deeper one associated with diagenetically formed carbonate phase. The substitution approach has been used to explain the presence of significant quantities of Mn and Fe in the carbonate phase. The computed observed partition coefficients decrease with depth, in general, but increase with corresponding decrease in Ca substantiating the ionic replacement reactions. Mn in the carbonate phase is negatively related to CaCO₃, suggesting possible rhodochrosite formation. Evaluated observed and thermodynamic partition coefficients revealed the likelihood of siderite formation. Solutions to diagenetic equations showed contrasting behaviour of Mn and Fe in Skagerrak sediments. Increased reduction of Mn-oxyhydroxides in the central parts compared to that in the southern Skagerrak results in the significant enrichment rate of Mn in pore water (1365 mol m^-3 yr^-1) in sediments of the former area. Large amounts of Fe-oxyhydroxides are reduced in southern parts compared to the Skagerrak basin itself. Mn and Fe fluxes from reducing to oxidizing zones are larger in the southern Skagerrak (8.8 and 9.0 m mol m^-2 yr^-1, respectively) than in central parts (5.3 and 4.6 m mol m^-2 yr^-1, respectively). Mn and Fe also exhibit contradicting sedimentation patterns where the former has high deposition fluxes in the central Skagerrak while the latter is deposited more in the southern Skagerrak. Mn seems to be internally cycled within the Skagerrak.