Sedimentary Characteristics and Acoustic Detectability of Ship-derived Deposits in Western Lake Ontario

Dual-frequency side-scan sonar surveys (100 and 500 kHz) were conducted in western Lake Ontario to locate and characterize ship-derived deposits. Sedimentary characteristics of these deposits were investigated with the aid of a Remote Operated Vehicle (ROV) and sediment samples. Distinct Acoustic Backscatter Anomalies (ABAs) identified with the sonar, when sampled, were found to be surficial and subsurficial deposits of ship-derived debris including coal, taconite, and cement clinker. These deposits, which are estimated to range in age from less than 10 years to over 100 years, are located in fine-grained lakebed sediment, which has relatively low acoustic attenuation even at high frequency. As a result, the coarse ship-derived debris can be distinguished on side-scan sonar records even though it is buried at depths of up to 15 cm. Calculations suggest that ship-derived debris as deep as 2.5 m should be observable in 100 kHz side-scan sonar records from this environment. Since sediment accumulation rates are less than 0.29 cm/y and shipping has occurred on Lake Ontario for approximately 140 years, most coarse ship-derived debris ever deposited by ships into fine-grained sediments should be detectable with a 100 kHz sonar signal. Layers of granular diagenetic vivianite were identified at about 15 cm depth in several cores. This material may be a good acoustic reflector and may be responsible for some of the broad but diffuse reflective patterns observed in the side-scan sonographs.