Description: Enhanced nutrient input and warming have led to the development of low oxygen (hypoxia) in coastal waters globally. For many coastal areas, insight into redox conditions prior to human impact is lacking. Here, we reconstructed bottom water redox conditions and sea surface temperatures (SSTs) for the coastal Stockholm Archipelago over the past 3000 yr. Elevated sedimentary concentrations of molybdenum indicate (seasonal) hypoxia between 1000b.c.e.and 1500c.e. Biomarker-based (TEX86) SST reconstructions indicate that the recovery from hypoxia after 1500c.e.coincided with a period of significant cooling (similar to 2 degrees C), while human activity in the study area, deduced from trends in sedimentary lead and existing paleobotanical and archeological records, had significantly increased. A strong increase in sedimentary lead and zinc, related to more intense human activity in the 18(th)and 19(th)century, and the onset of modern warming precede the return of hypoxia in the Stockholm Archipelago. We conclude that climatic cooling played an important role in the recovery from natural hypoxia after 1500c.e., but that eutrophication and warming, related to modern human activity, led to the return of hypoxia in the 20(th)century. Our findings imply that ongoing global warming may exacerbate hypoxia in the coastal zone of the Baltic Sea.

Description: The data set includes Toarcian sedimentary records of total organic carbon contents and elemental concentrations of phosphorus, aluminium, iron, molybdenum, manganese, sulphur, vanadium and copper at 3 sites in the northern European Epicontinental Shelf (Yorkshire, Schandelah and Dotternhausen). It span the Toarcian Ocean Anoxic Event (T-OAE), including the onset and termination.

Description: The Cenomanian–Turonian oceanic anoxic event (OAE 2) took place 94 million years ago and featured the development of widespread marine anoxia throughout a large part of the global ocean. In addition to the anoxic conditions other environmental disturbances also occurred, including extremely high global temperatures (with a superimposed cooling pulse dubbed the Plenus Cold Event), an intensified hydrological cycle, enhanced silicate weathering, and wildfires. OAE 2 and the climate/environmental perturbations associated with this event are thought to have been ultimately triggered by large-scale submarine volcanism during the emplacement of one or more large igneous provinces into/onto the oceanic crust. This link with volcanism is chiefly evidenced by geochronology studies of the volcanic rocks and stratigraphic archives of the event, and sedimentary proxies of volcanism utilized on these same records of OAE 2, with osmium-isotopes apparently the most reliable of these volcanic markers (at least for OAE 2). This dataset presents a new osmium-isotope dataset for a record of OAE 2 that was deposited on the New Jersey Shelf at the northeastern margin of the proto-North Atlantic, highlighting a signal of volcanic activity that is comparable with other sites. The data are in review for publication at the journal Paleoceanography and Paleoclimatology, where they are stratigraphically correlated with previously published evidence for temperature changes during OAE 2, highlighting the complex temporal and causal relationships between volcanism and climate during that event.

Description: Estuarine sediments are key sites for removal of phosphorus (P) from rivers and the open sea. Vivianite, an Fe(II)-P mineral, can act as a major sink for P in Fe-rich coastal sediments. In this study, we investigate the burial of P in the Öre Estuary in the northern Baltic Sea. We find much higher rates of P burial at our five study sites (up to ∼ 0.145mol m−2 yr−1) when compared to more southern coastal areas in the Baltic Sea with similar rates of sedimentation. Detailed study of the sediment P forms at our site with the highest rate of sedimentation reveals a major role for P associated with Fe and the presence of vivianite crystals below the sulfate methane transition zone. By applying a reactive transport model to sediment and porewater profiles for this site, we show that vivianite may account for up to ∼ 40% of total P burial. With the model, we demonstrate that vivianite formation is promoted in sediments with a low bottom water salinity and high rates of sedimentation and Fe oxide input. While high rates of organic matter input are also required, there is an optimum rate above which vivianite formation declines. Distinct enrichments in sediment Fe and sulfur at depth in the sediment are attributed to short periods of enhanced input of riverine Fe and organic matter. These periods of enhanced input are linked to variations in rainfall on land and follow dry periods. Most of the P associated with the Fe in the sediment is likely imported from the adjacent eutrophic Baltic Proper. Our work demonstrates that variations in land-to-sea transfer of Fe may act as a key control on burial of P in coastal sediments. Ongoing climate change is expected to lead to a decrease in bottom water salinity and contribute to continued high inputs of Fe oxides from land, further promoting P burial as vivianite in the coastal zone of the northern Baltic Sea. This may enhance the role of this oligotrophic area as a sink for P imported from eutrophic parts of the Baltic Sea.

Description: Continental shelf sediments are a major source of iron (Fe) for phytoplankton in surface waters. In this study, we investigate the mechanisms that control release of Fe from shelf sediments and its lateral transport (shuttling) in oxic and hypoxic waters on the northwestern Black Sea shelf. We find that at two coastal stations near the outflow of the Danube river high input of organic matterdrives strong reductive dissolution of Fe(oxyhydr)oxides (henceforth termed Fe oxides) in surface sediments, supporting high rates of Fe release to oxygenated bottom waters (∼0.36 mmol m-2d-1).We suggest that bioirrigation plays a key role in the release of Fe from these sediments. At four stations further offshore organic matter deposition is lower resulting in limited mobilization of Fe2+in the sediment and low benthic fluxes of Fe (〈0.07 mmol m-2d-1). Lateral transport of Fe from the coastal zone towards the deep basin mostly takes place in colloidal and/or particulate form (〉0.2μm) in the lower part of the water column, likely through repeated deposition and resuspension of Fe oxides from surface sediments. Using synchrotron-based X-ray spectroscopy and sequential chemical extractions, we demonstrate that the suspended matter and surface sediments are enriched in easily reducible Fe oxides (mostly ferrihydrite) and Fe associated with clay. The mobilization of Fe in the coastal zone and subsequent lateral transport of these Fe-bearing particles results in higher ratios of Fe/Al in surface sediments at outer shelf stations (ca. 1.2 to 2 wt% wt%-1) than at coastal stations (ca. 0.5 to 0.9 wt% wt%-1). However, below the sediment surface layer Fe/Al ratios are similar at all stations indicating limited burial of the laterally transported Fe. Our results highlight the critical role of organic matter input, associated biological activity and riverine Fe input as drivers of Fe shuttling on continental shelves. We also show that in shelf areas where sediments receive low inputs of organic matter, physical transport controls the ultimate fate of the shuttled Fe.

Description: The Baltic Sea is characterized by the largest area of hypoxic (oxygen 〈2 mg/L) bottom waters in the world's ocean induced by human activities. Natural ventilation of these oxygen depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in oxygen and decline in phosphate in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese, iron and phosphorus in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total manganese and to a lesser extent iron oxides and phosphorus were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of phosphorus-bearing rhodochrosite and manganese(II) phosphates. In contrast to what is typically assumed, the formation of iron oxides in the surface sediment was limited. We attribute this lack of iron oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows iron(II) in the form of iron monosulphide to be preserved and restricts the penetration of oxygen into the sediment. We estimate that enhanced phosphorus sequestration in surface sediments accounts for only ∼5% of water column phosphate removal in the Eastern Gotland Basin linked to the recent inflows. The remaining phosphate was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic oxygen demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, phosphorus sequestration in the sediment in association with iron oxides is limited. This implies that artificial ventilation projects that aim at removing water column phosphate and thereby improving water quality in the Baltic Sea will likely not have the desired effect.