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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 13 | Copyright
Biogeosciences, 15, 3975-4001, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Jul 2018

Research article | 05 Jul 2018

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

Sami A. Jokinen1, Joonas J. Virtasalo2, Tom Jilbert3, Jérôme Kaiser4, Olaf Dellwig4, Helge W. Arz4, Jari Hänninen5, Laura Arppe6, Miia Collander7, and Timo Saarinen1 Sami A. Jokinen et al.
  • 1Department of Geography and Geology, University of Turku, 20014 Turku, Finland
  • 2Marine Geology, Geological Survey of Finland (GTK), P.O. Box 96, 02151 Espoo, Finland
  • 3Department of Environmental Sciences, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
  • 4Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Seestrasse 15, 18119 Rostock, Germany
  • 5Archipelago Research Institute, University of Turku, 20014 Turku, Finland
  • 6Finnish Museum of Natural History, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 7Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 66, 00014 Helsinki, Finland

Abstract. 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.

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Oxygen deficiency is a major environmental problem deteriorating seafloor habitats especially in the coastal ocean with large human impact. Here we apply a wide set of chemical and physical analyses to a 1500-year long sediment record and show that, although long-term climate variability has modulated seafloor oxygenation in the coastal northern Baltic Sea, the oxygen loss over the 20th century is unprecedentedly severe, emphasizing the need to reduce anthropogenic nutrient input in the future.
Oxygen deficiency is a major environmental problem deteriorating seafloor habitats especially in...