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.
Bibliographical noteFunding Information:
Support for this research was provided by Minnesota Sea Grant to RLH, TCJ and JPW. The 210 Pb analyses were carried out by alpha spectrometry in the Department of Soil Science, University of Manitoba, under the direction of Dr. Paul Wilkinson. The authors give thanks to the following: the crew of the R/V Blue Heron for core collection, Sarah Grosshuesch and Koushik Dutta for their laboratory expertise, Julia Halbur for help with sample preparation and Jillian Votava. The authors also thank the associate editor and two anonymous reviewers whose valuable comments considerably improved this manuscript.
© 2014 Elsevier B.V.
Copyright 2015 Elsevier B.V., All rights reserved.
- Anthropogenic impacts
- Carbon isotopes
- Lake Superior
- N-Alkane biomarkers
- Sedimentary organic carbon