Sediments Of Southern Lake Huron: Elemental Composition And Accumulation Rates

Robbins, John A.
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It is widely recognized that most metal contaminants in lakes are primarily associated with particulate matter and are conveyed to underlying deposits in association with fine-grained materials such as organic debris, hydroxides of iron, and manganese or clay minerals. In the Great Lakes the fine-grained sediments and associated contaminants are not deposited uniformly over the bottom but are confined to "pockets" or depositional basins which are of more limited extent and generally found in deeper areas of each lake. This report is the first in a series of three comprehensive reports which describes the composition and rates of accumulation of metal contaminants in the depositional basins of Lake Huron. This first report deals with the two principal depositional basins in southern Lake Huron: the Port Huron basin and the Goderich basin. Over a period of a year (1974-1975) nearly 100 sediment cores were taken within these two basins. Cores were carefully sectioned aboard ship and subsequently analyzed for large number of elements using state-of-the-art methods. Elements determined include Al, As, Ba, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Eu, Fe, K, La, Lu, Hg, Hf, Mg, Mo, Mn, Na, Ni, P, Pb, Rb, Sb, Sc, Si, Sm,Sn, Sr, Ti, Th, U, V, Yb and Zn. Most of the known or potential metal contaminants are included in this list. Many of the cores were dated by radiometric methods using lead-210 and cesium-137. In addition, for a limited set of cores the distribution of dissolved substances was determined to estimate possible exchanges with overlying water. Results of this study include: (1) greatly improved estimates of sediment accumulation rates, (2) recognition of the role of sediment mixing by benthic organisms in modifying metal contaminant and radioactivity profiles, (3) estimates of the rates of accumulation of metal contaminants in these depositional basins, (4) identification of which metals are contaminants and their degree of surface enrichment (in particular, the discovery of tin as a contaminant metal), and (5) limited comparisons of accumulation rates with lake loadings. The extent of contamination of sediments relative to natural levels is greatest for Hg and then, in decreasing order Sn, Pb, Sb, As, Cd, Zn, Ni, U, Cu, and Br. Data presented include: (1) contour maps of surface concentrations of major and trace constituents, enrichment factors, total anthropogenic metal accumulation since about 1800, vertically integrated cesium-137, depth of sediment mixing, mass and mean linear sedimentation rates, the intrinsic time resolution, rates of accumulation of major constituents and contaminant metals, (2) vertical distributions of major, trace, and dissolved constituents, (3) models for the behavior of cesium-137 in water, sediment mixing, and silicon dissolution in sediments, (4) area-wide loadings of rnajor and trace constituents plus contaminant metals.
Sediments , Southern Lake Huron , Elemental Composition , Accumulation Rates