An 8km2 area representative of deep offshore basins in Lake Superior was surveyed with multi-beam sonar and a high-frequency seismic-reflection system to create a high-resolution bathymetric map of the lake floor morphology, which is dominated by ring-shaped depressions attributed to the dewatering of glacial-lacustrine clays. Ten multi-cores were recovered from the survey area. The cores were scanned for magnetic susceptibility (MS), dated by 210Pb and analyzed for water content, total organic carbon (TOC) and nitrogen (TON), biogenic silica (BSi), and total (THg) and methyl (MeHg) mercury. MS profiles varied considerably, inferring substantial centennial-scale differences in sedimentation history among the core sites. Concentration profiles of the analyzed constituents displayed differences of about ±15% TOC, ±40% BSi, ±50% THg and ±50% MeHg. Total mercury and methylmercury concentrations were typical of past measurements, and the mean THg accumulation rate (12μg/m2year) was similar in magnitude to that of atmospheric Hg deposition. Sediment mass accumulation rates (MAR) ranged among the cores between average values of about 50g/m2year in the ring centers to as high as 180g/m2year between rings. Temporal variation in MAR within cores varied considerably on a decadal scale as well. Sediment redistribution by bottom currents over the complex morphology of the Lake Superior basin is not uniform in space and time, and indicates that a single core from any given area in the lake may not reflect the true history of environmental conditions that extend even a few hundred meters beyond the core site.
Bibliographical noteFunding Information:
We thank Capt. Mike King and the crew of the R/V Blue Heron for their able assistance on Lake Superior. Anders Noren, Amy Myrbo, and Kristina Brady at LacCore, the national lake core repository on the Minneapolis campus of the University of Minnesota provided excellent facilities for splitting sediment cores, digital imaging, and whole core analyses for magnetic susceptibility and wet bulk density. Elizabeth Schmidt and Katie Bluske conducted THg and MeHg analyses on core sections at the University of Wisconsin — La Crosse. Financial support was provided through small grants from UMD's Large Lakes Observatory and Department of Geological Sciences , from the Geological Society of America , and from the Minnesota Sea Grant Program .
- Biogenic sediment
- Great Lakes
- Lake Superior sediment
- Pb geochronology
- Ring depressions