Geochemical response on redox fluctuations in Holocene lake sediments, Lake Steisslingen, Southern Germany

Abstract

Holocene sediments of Lake Steisslingen record sedimentation under different redox conditions. This is proven by the changing depth of the chemocline and the resulting varying extension of laminated sediments from the deepest part of the lake to shallower water depths. Redox fluctuations are additionally reflected by colour changes between red and green and the quality of the nonglacial varves. Major and trace elements were analysed on bulk sediment samples (period between 8200–6200 and 2650–2000 cal a BP) to test their response to the observed changes in past depositional environments. Mn, Mo, U, Fe and S were found to be redox-sensitive elements in Lake Steisslingen. Fe and S are almost entirely fixed in pyrite, because pyrite formation is limited by Fe availability. A parallel distribution of Mo, U, Fe and S between 8200–7200 cal a BP is interpreted as a signal for syngenetic pyrite formation from anoxic and sulfidic hypolimnetic waters. Since bioproductivity was low this must have been caused by conditions favouring a longterm stable stratification of the lake (i.e. high water level, high summer temperatures, ice cover, wind protection and/or low storm activity). Between 6000–2000 cal a BP frequently changing redox conditions from anoxic to dysoxic are assumed. Mo and U curves are mirror images of the Fe, S and Mn variations. This is interpreted as showing diagenetic pyrite formation from pore waters, which is enhanced during times of high Fe availability under rather oxygenated conditions, i.e. when high input of Fe oxy-hydroxides is possible, but when U and Mo cannot necessarily be fixed in the sediments. Changes from slightly disturbed lamination (dysoxic, restricted mixis) to homogenous sediments (oxic conditions, holomixis) cannot be traced within the geochemical record. The observed redox fluctuations between 6000 and 2000 cal a BP are not correlated to changes in bioproductivity, detrital input or cultural influences and must therefore be of a natural, probably climatic origin. The North Atlantic Oscillation is discussed as a possible cause.