Sedimentary lipid geochemistry of Framvaren: impacts of a changing environment

Abstract

Abstract Framvaren, a highly anoxic fjord in southern Norway, has a well documented history of environmental change from fjord to lake, and then back to a fjord once more. The basin is permanently stratified, dominated by a c. 160 m deep mass of highly sulphidic water overlain by 20 m of oxic water. Sediment cores from both oxic and anoxic environments contain organic carbon contents of 6–18% and reveal complex distributions of lipid compounds, the dominant classes being n -alkanes, n -alcohols, sterols and long-chain alkenones, although the fatty acids were not examined. The distributions and relative contributions of these compound classes vary extensively. The alkenones, which are predominantly produced by marine prymnesiophyte algae, are most abundant in the fjord facies of both cores, but are also detectable in the lacustrine sediments, albeit in much lower concentrations. The dramatic increase in abundance of these compounds is interpreted to record the change in environment from lake to fjord. No other lipid classes, except the “spike” in the n -alkanes, show a marked change at this particular level. Reconstruction of surface-water palaeotemperatures using the alkenone unsaturation index ( U 37 k) gave reasonable values for the anoxic fjord sediments, but could not be performed for the sediments deposited under lacustrine environments where the greater relative abundance of the tetra-unsaturated component gave spurious values for the index. There is also some evidence that selective degradation processes under oxic depositional conditions has modified the ( U 37 k) index, compromising its use. The highest organic carbon contents were measured in the shallow-water (oxic site) core during lacustrine times (i.e. pre 1850), suggesting that the chemocline (and thus anoxia) may have been above 15 m water depth at that time, thus enhancing preservation. An offset (of c. 2 cm) between the change in TOC values and the alkenone concentrations has been interpreted as a result of “burn down” caused by the sudden input of oxygenated water over anoxic sediments.