Seasonal hypoxia was a natural feature of the coastal zone in the Little Belt, Denmark, during the past 8 ka

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doi: 10.1016/j.margeo.2017.03.008
Author(s): van Helmond, Niels A. G. M.; Quintana Krupinski, Nadine B.; Lougheed, Bryan C.; Obrochta, Stephen P.; Andrén, Thomas; Slomp, Caroline P.
Author Affiliation(s): Primary:
Utrecht University, Department of Earth Sciences, Utrecht, Netherlands
Lund University, Sweden
Uppsala University, Sweden
Akita University, Japan
Sodertorn University, Sweden
Volume Title: Marine Geology
Source: Marine Geology, Vol.387, p.45-57. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0025-3227 CODEN: MAGEA6
Note: In English. 93 refs.; illus., incl. 3 tables, sketch map
Summary: 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 (Corg), 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 Corg. The absence of Mo, in combination with high Corg/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 (FeHR) over total Fe (FeTOT) suggest (seasonal) hypoxia occurred over the last ∼ 8 ka. Maxima in sediment Corg, Mo and FeHR/FeTOT 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.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 24 Surficial Geology, Quaternary Geology; Absolute age; Aerobic environment; Anaerobic environment; Ancylus Lake; Atlantic Ocean; Baltic Sea; Brackish-water environment; C-14; Calcium carbonate; Carbon; Cenozoic; Chemical ratios; Chemostratigraphy; Chronostratigraphy; Cores; Dates; Denmark; Depositional environment; Europe; Expedition 347; Fresh-water environment; Holocene; IODP Site M0059; Integrated Ocean Drilling Program; Iron; Isotopes; Lacustrine environment; Lithostratigraphy; Little Belt; Littorina Sea; Lower Holocene; Marine environment; Marine sediments; Medieval Warm Period; Metals; North Atlantic; Organic carbon; Quaternary; Radioactive isotopes; Scandinavia; Sea water; Sediments; Sulfur; Western Europe
Coordinates: N550017 N550018 E0100631 E0100628
Record ID: 2017058682
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands