A High Resolution Study of the Spatial and Temporal Variability of Natural and Anthropogenic Compounds in Offshore Lake Superior Sediments
2006-05
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A High Resolution Study of the Spatial and Temporal Variability of Natural and Anthropogenic Compounds in Offshore Lake Superior Sediments
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2006-05
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Nine multi-cores were recovered during the summer of 2005 from an eight square kilometer area that is typical of the deep depositional environments found in the central and western parts of Lake Superior. Core sites were located in the troughs, centers, edges, and ambient regions around ring structures, which are believed to be the surface expression a polygonal fault system generated from the dewatering of underlying glaciolacustrine sediments (Cartwright et al. 2004). One sub-core at each site was extruded at 0.5 cm intervals to 10 or 12 cm depth, and analyzed for 210Pb, biogenic silica (BSi), total organic carbon (TOC), total organic nitrogen (TON), and methyl mercury (MeHg). The accumulation of bulk sediment, BSi, TOC, and MeHg cumulative inventories display large temporal and spatial variation among core sites throughout the study area. The total inventories of all three parameters vary by nearly a factor of two among the core sites. A comparison of BSi and TOC inventories to bulk sediment inventory reveals a direct relationship between the total compound accumulation at a particular core site and its bulk sedimentation rate. MeHg analyses show that spatial and temporal variability in MeHg appears to be due primarily to proximity to ring structures rather than variable bulk sediment accumulation. Our 8 km2 study area exhibits a larger range of MeHg values than shown in previous lake-wide studies of MeHg. Lake Superior is subject to a similar mass balance problem found in the oceans; a previous study determined that up to 80% of the river input of dissolved silica to the lake is not accounted for by either outflow of dissolved silica or by deposition of biogenic silica. However, new estimates based on the mass accumulation of BSi in this study suggests that there is only little imbalance, if any, and that silicate mineral authigenesis is not required to explain the fate of dissolved silica flowing into the lake. Single core analysis is not valid in regions of complex lake-floor terrain found throughout the central and western basins of Lake Superior. To accurately asses the accumulation of natural and anthropogenic compounds future investigations need to recover cores in the regions around complex lake-floor terrain that have exhibited 'normal' sedimentation, or attain duplicate cores to establish the degree of sediment and compound variability.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jon D. Van Alstine in partial fulfillment of the requirements for the degree of Master of Science, May 2006.
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