My research centers on the use of lake sediment records to understand long-term environmental change, particularly the effects of human activities on water quality, atmospheric chemistry, and biogeochemical processes. I’m particularly interested in approaches that quantify the magnitude and rates of change and establish mechanistic linkages to modern-day systems.
Atmospheric mercury deposition and cycling
Mercury (Hg) is a highly toxic pollutant that is carried atmospherically to remote lakes and landscapes where it is biologically concentrated through the food chain to levels in game fish that are potentially harmful to humans and fish-eating wildlife. Concentrations in aquatic systems have increased greatly over the last two centuries due to human-related emissions from industrial uses and fossil fuel combustion. My research on mercury has focused on (1) the use of lake sediments to quantify global changes in Hg deposition, and (2) the experimental manipulation of precipitation chemistry to explore synergistic effects of sulfate on mercury methylation and cycling.
Contaminant and sediment loading in the upper Mississippi River
Many of the world’s great rivers have been highly impacted by excess nutrients, suspended sediments, and toxic pollutants from human activities in their watersheds. Understanding the magnitude of these changes, their timing, and likely causes is critical to developing sound management strategies for their remediation. My work on the upper Mississippi River has focused on quantifying the historical flux of these contaminants from sediment cores, tracing sediment sources through geochemical fingerprinting, and evaluating the dual effects of climate and land-use change on increased river flows and watershed erosion
Boreal lakes and climate change
Multiple lines of evidence suggest that boreal lake ecosystems are changing rapidly, with unprecedented appearances of potentially toxic cyanobacterial blooms, significant shifts in algal communities, increased watershed inputs of dissolved solids, and increased carbon burial in lake sediments. Given the lack of local human impact on most boreal lakes, climate change is the most likely driver of these ecological shifts. I have worked on this problem in several national parks in the western Great Lakes region, principally Isle Royale and Voyageurs, using a combination of lake-sediment records, long-term monitoring, and watershed and lake-thermal models to explain the varied response of lakes to a warming climate and the drivers and processes involved.
Radiometric dating
The short-lived radioisotopes, lead-210 and cesium-137, are powerful tools for dating lake, estuarine, and marine sediments covering the last 200 years, a period of intense human impact on global ecosystems. I established the radiometric dating laboratory of the St. Croix Watershed Research Station in 1995 (and the Limnological Research Center, University of Minnesota, prior to that). Over that time period these facilities have dated some 1500 sediment cores from all continents and for hundreds of researchers around the globe.