The palaeoproductivity of ancient Lake Superior

We examine here variations in the organic carbon and nitrogen contents and isotopic compositions of Lake Superior sediments over the last 10,600 years, using bulk organic matter from four cores distributed across the Lake Superior Basin. Very low Organic Carbon (OC) and Total Nitrogen (TN) contents but high C-org Mass Accumulation Rates (MARs) characterize these sediments until glacial meltwater supply to the Basin ended at similar to 9000-8700 cal BR The C/N ratios for organic matter from the glacial sediments span the range known both for organic matter adsorbed on soil clays and lacustrine algae grown under conditions of nitrogen deficiency. Organic matter in the glacial sediments has more-or-less uniform carbon-isotope compositions (similar to-27 parts per thousand), perhaps associated with a steady flux of soil clays. Coexisting ostracode valves vary by in carbon isotopic composition by as much as 5 parts per thousand, particularly between 10,600 and 10,400 cal BP, reflecting changing DIC sources at this time. In the postglacial sediments, the C-org MARs decreased sharply from glacial levels. The OC and TN concentrations increased steadily until similar to 6000 cal BP, after which OC and TN concentrations either stabilized or increased at a slower rate. The C/N ratios of the postglacial organic matter (mostly < 10) are typical of lacustrine algae. The carbon-isotope compositions of bulk organic matter in the postglacial sediments immediately after the end of glacial meltwater supply are lower (-29 parts per thousand) than those of underlying glacial sediments. An upward increase in the carbon-isotope compositions within the postglacial sediments serves to characterize increasing primary productivity in the lake. This observation is supported by the small but steady rise in the nitrogen-isotope compositions of bulk organic matter in the postglacial sediments towards present time. (C) 2011 Elsevier Ltd. All rights reserved.