Multibeam sonar backscatter lineaments and anthropogenic organic components in lacustrine silty clay, evidence of shipping in western Lake Ontario

A multibeam sonar survey (95 kHz) covering more than 500 km² of western Lake Ontario revealed anomalous lineaments of relatively high backscatter. The lineaments did not align with or parallel the most prominent structural zones beneath the lake as expected. Instead, the principal lineaments lay on lines between ports on opposite sides of the lake, especially between Toronto and Welland Canal, and Toronto and Niagara River mouth. As the lineaments underlie current and historical shipping routes used during the steamship era, they are interpreted as an acoustic response to shipping debris cumulated in the near-surface bottom sediment. An exploratory study of the organic components in the silty clay surface sediment, using geochemical and petrological techniques, shows that the upper 10 cm commonly contains silt-sized particles of anthropogenic origin, especially combustion residues. Combustion residues are more abundant on or near the lineaments, consistent with an origin related to shipping. Enhanced acoustic backscatter is evident where silt-sized combustion particles are hosted in dominantly clay-sized sediment. The coarser-grained anthropogenic particles increase the acoustic impedance of the lakebed relative to the bottom water as well as the roughness and volume scattering contributions to lakebed backscatter. (C) 2000 Elsevier Science B.V. All rights reserved.