HR: 0800h
AN: OS41A-0165    [Abstracts]
TI: Past Occurrences of Hypoxia in the Baltic Sea
AU: * Zillen, L
EM: lovisa.zillen@geol.lu.se
AF: Lund University, GeoBiosphere Science Centre, Quaternary Sciences, Solvegatan 12, Lund, 22362, Sweden
AU: Conley, D J
EM: daniel.conley@geol.lu.se
AF: Lund University, GeoBiosphere Science Centre, Quaternary Sciences, Solvegatan 12, Lund, 22362, Sweden
AU: Bjorck, S
EM: svante.bjorck@geol.lu.se
AF: Lund University, GeoBiosphere Science Centre, Quaternary Sciences, Solvegatan 12, Lund, 22362, Sweden
AB: The hypoxic zone in the Baltic Sea has increased in area by about four times since 1950. Widespread oxygen deficiency below the halocline has severely reduced macro benthic communities in the Baltic Proper and the Gulf of Finland over the past decades and negatively effected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. In addition, hypoxia alters nutrient biogeochemical cycles. The cause of the increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as phosphorous and nitrogen. Conditions prior to the 1950s are considered as the benchmark and some authors suggest that the earlier Baltic Sea was an oligothrophic, clear-water body with oxygenated deep waters. By contrast, studies of short sediment cores reveal that hypoxia has been present in some of the deepest basins for at least the last 100-200 years. In addition, long sediment cores suggest that hypoxia in the Baltic Sea has occurred intermittently in deep basins over the last c. 8500 years. Thus, the occurrence of present day hypoxia in the deeper basins need not necessarily be attributed to human activity but rather to natural oceanographic, geologic and climate conditions. We present a compilation of previous publications that reported the occurrence of laminated sediments (i.e. a palaeo-proxy for hypoxia) in the Baltic Sea. This review shows that the deeper parts of the Baltic Sea have experienced either intermittent or more regular hypoxia during most of the Holocene and that more continuous laminations started to form c. 7800-8500 cal. yr BP ago, in association with the establishment of a permanent halocline during the transition from the Ancylus Lake to the Littorina Sea. Laminated sediments were more common during the early and late Holocene and coincided with intervals of high organic productivity (high TOC content) and high salinity during the Holocene Thermal Maximum and the Medieval Climate Optimum. This study implies that there may be a correlation between past climate variability and the state of the marine environment, where milder periods correspond to increased primary production and higher salinities resulting in amplified hypoxia and enlarged distribution of benthic mortality and laminated sediments.
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4834 Hypoxic environments (0404, 4802)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting