Anthropogenic influences on the sedimentary geochemical record in western Lake Superior (1800-present)

The sediments of western Lake Superior hold a record of environmental changes that have accompanied the settlement and urbanization of the surrounding watershed. Organic carbon concentrations are low (1.5%) with little variation in stable isotope composition (-26.5±0.5‰) prior to 1900. Organic carbon and nitrogen concentrations begin to rise after 1900, as increased anthropogenic disturbance led to enhanced inputs of terrigenous matter as well as nutrients (i.e., nitrogen and phosphorus) from the watershed. An episode of enhanced aquatic productivity from 1900 to 1970 is recorded in the sediments by the ¹³C-enrichment of bulk organic carbon as well as the observed correlation between the bulk and aquatic molecular δ¹³C records, coinciding with the major developmental period of the Duluth-Superior harbor region. Decreasing organic carbon accumulation after 1925, prior to regulatory implementation of municipal discharges to the lake, is likely due to the construction of hydropower dams along the St. Louis River and a decrease in forest harvest within the immediate watershed. Recentshort-lived decreases in the accumulation rate of organic matter can be attributed to the implementation and operation of water treatment plants, but the ¹³C-enrichment observed in the last~decade remains enigmatic, though we hypothesize that it may be attributable to climate change impacts on primary production.