Department of Biology

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Bo Barker Jørgensen

A Holocene history of dynamic water column redox conditions in the Landsort Deep, Baltic Sea

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DOI

  • Dalton S. Hardisty, Univ Calif Riverside, University of California Riverside, University of California System, Dept Earth Sci
  • ,
  • Natascha Riedinger, Oklahoma State Univ, Oklahoma State University - Stillwater, Oklahoma State University System, Boone Pickens Sch Geol
  • ,
  • Noah J. Planavsky, Yale Univ, Yale University, Dept Geol & Geophys
  • ,
  • Dan Asael, Yale Univ, Yale University, Dept Geol & Geophys
  • ,
  • Thomas Andren, Sodertorn Univ, Sodertorn University, Sch Nat Sci Technol & Environm Studies
  • ,
  • Bo B. Jorgensen
  • Timothy W. Lyons, Univ Calif Riverside, University of California Riverside, University of California System, Dept Earth Sci

The modern Baltic Sea is the world's largest anthropogenically forced anoxic basin. Using integrated geochemical records collected during Integrated Ocean Drilling Program (IODP) Expedition 347 from the deepest and one of the most reducing sub-basins in the Baltic Sea, Landsort Deep, we explore the degree and frequency of natural anoxia through the Baltic Holocene. A marked decrease in carbon-to-sulfur ratios (C/S) from the cores indicate the transition from the Baltic Ice Lake to the current brackish sea, which occurred about 8.5 kyrs B.P. Following this, laminations throughout sediments recording brackish deposition suggest sustained anoxia or extreme low oxygen, while high molybdenum (Mo) concentrations of >100 ppm and iron (Fe) geochemistry suggest water column sulfide accumulation, or euxinia, that persisted beyond seasonal timescales during deposition of two distinct sapropel units. Sedimentary Mo isotope values range from +1.11 to -0.50 permil, which are distinctly fractionated from modern Baltic seawater (+2.26 to -2.67 parts per thousand) and thus indicate that each of the sapropels experienced only weak and/or oscillatory euxinia-in contrast to the more stable euxinic conditions of more restricted basins. A shift in delta Mo-98 starting above the lower sapropel to a distinctly more negative range suggests particularly weak and oscillatory euxinia, with an enhanced contribution of manganese (Mn) redox cycling to Mo deposition relative to the lower portion of the profile. This conclusion is supported by extreme sedimentary Mn enrichments of up to 15 weight percent. We interpret the combined data to indicate episodic but major Baltic inflow events of saline and oxygenated North Sea water into the anoxic Landsort Deep that limited the concentrations and residence time of water column sulfide and caused episodic oxide deposition. Considering the temporal overlap between the most reducing conditions and periods of redox instability, we hypothesize that major Baltic inflows, as is observed today, lead to short-term instability while simultaneously supporting longer-term Baltic anoxia by strengthening the halocline. Ultimately, our results indicate that periods more reducing than the modern Baltic Sea have occurred naturally over the Holocene, but the characteristic dynamic saline inputs have historically prevented the relatively more widespread and stable anoxia observed in other classic restricted basins and will likely continue to do so.

Original languageEnglish
JournalAmerican Journal of Science
Volume316
Issue8
Pages (from-to)713-745
Number of pages33
ISSN0002-9599
DOIs
Publication statusPublished - Oct 2016

    Research areas

  • IODP Expedition 347, Baltic Sea, Landsort Deep, paleoredox, molybdenum isotopes, MOLYBDENUM ISOTOPE FRACTIONATION, BLACK-SEA, EUXINIC SEDIMENTS, MARINE-SEDIMENTS, PYRITE FORMATION, ORGANIC-CARBON, FERRUGINOUS CONDITIONS, ENVIRONMENTAL-CHANGE, PALEOREDOX PROXIES, MANGANESE OXIDES

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