Recovery from multi‐millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

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

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for ten sites along a bottom water oxygen (2 to 83 μmol L−1 O2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past

Recovery from multi-millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

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 1000 b.c.e. and 1500 c.e. Biomarker-based (TEX86) SST reconstructions indicate that the recovery from hypoxia after 1500 c.e. coincided with a period of significant cooling (∼ 2°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 18th and 19th 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 1500 c.e., but that eutrophication and warming, related to modern human activity, led to the return of hypoxia in the 20th century. Our findings imply that ongoing global warming may exacerbate hypoxia in the coastal zone of the Baltic Sea