ABSTRACT: Rapid in-situ measurement of magnetic susceptibility in unconsolidated lake sediments.

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Abstract

Magnetic susceptibility measurements can provide a useful indicator of anthropogenic effects in lake basins, including the onset of land clearance, forest fires, soil erosion and as a proxy for estimating contaminant levels in sediment. Susceptibility is commonly measured on whole or split cores, or on core sub-samples, but coring can be expensive and time consuming where a large number of profiles are required to correlate and map sediment volumes. Post- sampling mineralogic changes in cores are also a potential concern. An alternate approach investigated in this study is to obtain near continuous in-situ measurements of sediment volume susceptibility (k) using a probe driven into the lake bottom. An inexpensive proto-type probe was constructed using a Bartington MS2-F sensor mounted in waterproof housing with an extendable 10 m handle. Several calibration runs were made in a laboratory test column to determine the probe response characteristics and repeatability. Testing showed that the effective sensing volume is a 0.8 cm radius around the probe tip and that edge effects from sensor shoulders are negligible. The probe was then used to measure the thickness and distribution of a post-colonial sediment layer in a shallow coastal lagoon (Frenchman's Bay) in western Lake Ontario. Volume susceptibility profiles were collected at 40 locations by driving the probe up to 2.5 m into the lagoon bottom sediments at 2 cm measurement intervals. The in-situ volume susceptibility profiles were then compared with volume and bulk susceptibility measurements obtained on 10 vibracores extracted from the lagoon. The probe measurements showed comparable resolution to the core-derived data and closely paralleled the core susceptibility curves. The base of the post-colonial sediment layer was identified by an abrupt increase in magnetic susceptibility at 0.5-1.5 m depth. The marker horizon was correlated across the lagoon and the thickness and volume of the anthropogenic layer was estimated. The results demonstrate that in-situ susceptibility measurements using a sediment probe can provide a rapid and highly repeatable method for correlating shallow stratigraphic boundaries within unconsolidated lake bottom sediments.

Bibliography of Canadian Geomorphology