ABSTRACT
In this study, we investigate how salinity, oxygen concentration and catchment area characteristics impact the dynamics of Fe-P-S cycling in the continental shelf sea sediments. Samples were collected from three sites representing different environmental conditions: Gdansk Deep (southern Baltic Sea), Gotland Deep (central Baltic Sea) and Bothnian Sea (northern Baltic Sea). Sediments were analysed for their mineral composition and speciation of iron and phosphorus. The main groups of Prokaryota involved in Fe-P-S cycling in sediments were indicated. Concentrations of sulphate, hydrogen sulphide, alkalinity, chloride, calcium, phosphate and iron were measured in pore waters. We demonstrated that in the eutrophicated southern region with moderate salinity and oxygen deficit in bottom water, sediments had high potential for retaining Fe and releasing P as indicated by high concentrations of pyrite and labile forms of phosphorus, respectively. Strong salinity stratification and intermittent pelagic redoxcline in the central Baltic Sea resulted in a clearly higher rate of pyrite deposition. Sediment was enriched with Mn due to the formation of Ca-Mn carbonates driven by intensive Mn redox cycling and sulphate reduction. Because of high availability of Mn oxides connected with episodic inflows of oxic seawater from the North Sea, sulphate was present in the entire profile of the studied sediments in the Gotland Deep. Sediments in the well-oxygenated, virtually fresh and rich in land-derived iron northern Baltic Sea retained significant amounts of P in authigenic minerals. Organic matter mineralisation in the surface sediment of this area was dominated by iron reduction. The variability of environmental conditions and consequent availability of electron acceptors were the cause of regional differences in the composition of Prokaryota communities - the number of sulphate reducers in the Gdansk and Gotland Deeps was greater than in the Bothnian Sea, where there were more Fe reducers and bacteria that oxidise Fe and S.
Subject(s)
SalinityABSTRACT
High-resolution bathymetric data were collected with a multi-beam echosounder in the southern part of the Baltic Sea (region MET1, Gulf of Gdansk) revealing the presence of a 10 m deep and 50 m in diameter pockmark (MET1-BH) on the sea bottom (78.7 m). To date, no such structures have been observed to reach this size in the Baltic Sea. The salinity of the near-bottom water in the pockmark was about 2 PSU (about 31.22 mmol/l Cl-), which clearly indicated the presence of a submarine groundwater discharge (SGD). Water column, sediments and the seabed structure were investigated in the MET1-BH area using various hydroacoustic devices: multi-beam and splitbeam echosounders and a sub-bottom profiler. Geochemical analyses of sediment pore waters (CH4, Cl-, Br-, F-, SO42-, Ca2+, Mg2+, K+, Na+, ∑H2S, dP, dSi, NH4+, DIC, DOC) and microbiological analysis of sediments (16S rRNA) were performed. The content of CH4 and CO2 in the outflowing gas and its origin (δ13C-CH4 and δ2D-CH4) were determined. Hydroacoustic data showed that gas was emitted intensively from the inside of the structure. The height and intensity of the gas flares varied depending on the hydrostatic pressure. The gas contained 76.1% of CH4, 17.6% of CO2 and 0.39% of He. Methane source was microbial. Geophysical investigation revealed the presence of dislocations in sub-surface sediment layers in the MET1 region, which could have created a passage for groundwater and gas. Geochemical analyses pointed to intensive processes of organic matter decomposition in this area, active methanogenesis in the surface sediment layer, lack of the sulphate-methane transition, and freshwater seepage at a depth of ~88 m (bottom of the pockmark), probably from Upper Cretaceous deposits. The Prokaryota composition, atypical for marine surface sediments, resulted from the combination of freshwater and high organic matter content, and reflected active in situ methanogensis.
Subject(s)
Groundwater , Geologic Sediments , Methane/analysis , RNA, Ribosomal, 16S , ShipsABSTRACT
Due to changing climate conditions, such inland seas as the Baltic are expected to become more eutrophicated and less saline (causing lower availability of sulphates). This may promote methanogenesis as the main process of organic matter (OM) degradation in marine sediments. Presence of methane, in turn, may affect biogeochemical cycling of many elements, including iron. Thus, in the present study we attempted to investigate the influence of CH4 on the Fe forms in marine sediments. Sediment cores were collected from three physico-chemically different stations within the Southern Baltic, taking into account such parameters as water depth, OM content, bottom zone oxygenation and distance from the Vistula River (main source of anthropogenic material). At two sampling stations methane was present in shallow sediments (P1, MET2) and at one station (W6) any traces of this gas were not determined. Iron species (FeCARB, FeOX1, FeOX2, FeMAG, FePRS, FeT, FeFeS2, FeFeS) in sediments were investigated using the sequential extraction. Pore water was analysed to obtain vertical profiles of hydrogen sulphide, sulphate and dissolved inorganic carbon (DIC). Additionally, such parameters as water and OM content, CH4 and Eh in sediments were determined. Results showed that methanogenesis affects the biogeochemical iron cycling in sediments of the Southern Baltic, leading to the increase of carbonates containing ferrous iron (as a result of DIC production during intensive OSR, AOM and methanogenesis) and decrease of ferric iron compounds used in the AOM. Moreover, in the areas of lower salinity, pyrite formation is limited by an insufficient amount of hydrogen sulphide, leading to the situation when a significant part of Fe is accumulated in the form of monosulphides. In turn, in the areas of higher salinity, where oxygen deficiencies occur more often, more hydrogen sulphide is present in pore water. Pyrite formation is then limited by iron, not by sulphur.