A high resolution study of spatial and temporal variability of natural and anthropogenic compounds in offshore Lake Superior sediments

An 8 km² 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 (12 μg/m² year) was similar in magnitude to that of atmospheric Hg deposition. Sediment mass accumulation rates (MAR) ranged among the cores between average values of about 50 g/m² year in the ring centers to as high as 180 g/m² year 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.