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Seasonal hypoxia was a natural feature of the coastal zone in the Little Belt, Denmark, during the past 8 ka
Univ Utrecht, Fac Geosci, Dept Earth Sci, Geochem, Princetonpl 9, NL-3584 CC Utrecht, Netherlands..
Lund Univ, Dept Geol, SE-22362 Lund, Sweden..
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Natural Resources and Sustainable Development.
Akita Univ, Grad Sch Int Resource Sci, 1-1 Tegata Gakuin Cho, Akita 0108502, Japan..
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2017 (English)In: Marine Geology, ISSN 0025-3227, E-ISSN 1872-6151, Vol. 387, 45-57 p.Article in journal (Refereed) Published
Abstract [en]

The extent of the hypoxic area in the Baltic Sea has rapidly expanded over the past century. Two previous phases of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTM; 8-4 ka before present; BP) and the Medieval Climate Anomaly (MCA; 2-0.8 ka BP), have been identified. Relatively little is known about bottom water redox conditions in the coastal zone of the Baltic Sea during the Holocene, however. Here we studied the geochemical composition of a sediment sequence from a currently seasonally hypoxic site in the Danish coastal zone, the Little Belt, retrieved during Integrated Ocean Drilling Program Expedition 347 (Site M0059). The base of the studied sediment sequence consists of clays low in organic carbon (C-org), molybdenum (Mo) and iron sulfides (Fe-sulfides), and rich in iron oxides (Fe-oxides), indicative of a well-oxygenated, oligotrophic (glacial) meltwater lake. An erosional unconformity separates the glacial lake sediments from sediments that are rich in C-org. The absence of Mo, in combination with high C-org/S values, indicates that these sediments were deposited in a highly productive, well-oxygenated freshwater lake. The transition to modern brackish/marine conditions was very rapid, and subsequent continuous sequestration of Mo in the sediment and high ratios of reactive iron (Fe-HR) over total Fe (Fe-TOT) suggest (seasonal) hypoxia occurred over the last similar to 8 ka. Maxima in sediment Core, Mo and Fe-HR/Fe-TOT ratios during the HTM and MCA suggest that the hypoxia intensified. Our results demonstrate that the Little Belt is naturally susceptible to the development of seasonal hypoxia. While periods of climatic warming led to increased deoxygenation of bottom waters, high nutrient availability in combination with density stratification were likely the main drivers of hypoxia in this part of the coastal zone of the Baltic Sea during the Holocene.

Place, publisher, year, edition, pages
2017. Vol. 387, 45-57 p.
National Category
Geology
Identifiers
URN: urn:nbn:se:uu:diva-329121DOI: 10.1016/j.margeo.2017.03.008ISI: 000403526400004OAI: oai:DiVA.org:uu-329121DiVA: diva2:1150443
Funder
EU, FP7, Seventh Framework Programme, 278364Swedish Research Council FormasSwedish Research Council, 637-2014-499
Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2017-10-19Bibliographically approved

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Lougheed, Bryan C.

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