A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated the depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygenation since the beginning of the 1900s was fueled by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s. We find no evidence of similar anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.</p

Data for: Dissimilar behaviors of the geochemical twins W and Mo in hypoxic/euxinic basins

Raw water column, suspended particulate matter, pore water, and sediment data (Fe, Mn, Mo, W) from the Black and Baltic Seas

Data for: Dissimilar behaviors of the geochemical twins W and Mo in hypoxic/euxinic basins

Raw water column, suspended particulate matter, pore water, and sediment data (Fe, Mn, Mo, W) from the Black and Baltic Seas.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany)

Three drill cores from the marshlands of NW Germany, which cover the entire Holocene, were analyzed at high-resolution for bulk composition, Al, Fe, selected trace metals, and stable sulfur isotopes. The drill cores contain two lithological types of peat: (i) basal pears overlying Pleistocene sands and (ii) intercalated peats situated between clastic sediments of predominantly marine origin. The peat layers are characterized by distinct enrichments in pyrite due to microbial sulfate reduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning. The main Fe source seems to be the freshwater environment. The determination of dissolved and particulate Fe of channels and small rivers close to the study area revealed a 50-fold higher Fe content of the freshwater environment when compared with North Sea water. Pyrite enrichments are explained by two scenarios: (i) pyrite formation coincides with fen reed peat growth (basal and intercalated) under the influence of a brackish water zone (salinity app. 5-15) and (ii) pyrite was formed after peat growth in the lowest limnic basal peat intervals. Maximum pyrite accumulation (TS 28%) occurs in latter peats that contain thin clastic layers as a result of tidal channel activities after peat formation. The occurrence of clastic layers may have favoured the inflow of saline groundwater. The peat layers are also characterized by enrichments in redox-sensitive trace metals (As, Mo, Re, U) and Cd, whereas Co, Cr, Cu, Mn, Ni, Pb, Tl, and Zn reflect the geogenic background. Leaching experiments have shown that As, Co, Cu, Mo, Re, and Tl are predominantly fixed as sulfides and/or incorporated into pyrite. The remaining trace metals show no distinct trends, only Cr reveals a strong relation to the lithogenic detritus. Seawater is the dominating source for As, Cd, Mo, Re, and U. The remaining trace elements seem to have a freshwater source similar to Fe, In contrast to the distribution of pyrite, highest amounts of redox-sensitive trace metals are seen in fen reed pears (basal and intercalated) that were formed under a direct influence of seawater and brackish water, respectively. Therefore, we suggest that saline groundwater entering the basal peats was probably depleted in redox-sensitive trace metals, e.g. owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organic matter. (C) 2002 Elsevier Science B.V. All rights reserved