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The distribution of 210Pb, 137Cs, and Ambrosia (ragweed) pollen in two sediment cores from Lake Ontario and in three cores from Lake Erie provides independent estimates of sediment accumulation rates. Geochronology with 210Pb is based on radioactive decay of the isotope following burial in sediments. The method can reveal with precision changes in sedimentation occurring over the past 100 yr or so. Geochronologies with 137Cs and Ambrosia are based on the occurrence of a horizon corresponding, respectively, to the onset of nuclear testing 25 yr ago and to regional forest clearance in the middle 1800s. These methods provide estimates of long-term average sediment accumulation rates. In all but one core, the distributions of 137Cs and 210Pb indicate no physical mixing of near-surface sediments. In two cores, including one from central Lake Erie collected by diver, all three estimates of sedimentation rates are in excellent agreement. In two other cores, rates based on 210Pb are significantly higher than those inferred from Ambrosia pollen profiles. Lower average rates appear to result from occasional massive losses of sediments. Such events, apparent in the distribution of 210Pb but not in pollen records, correlate with the occurrence of major storm surges on the lakes during this century. In one core from western Lake Erie, exponential distributions of both 210Pb and Ambrosia appear to be artifacts which may result from extensive biological or physical reworking of sediments in shallow water (11 m). Previous indications of increased sedimentation in Lake Erie since about 1935 based on Castanea (chestnut) pollen data are not substantiated.

On the basis of extensive sampling and echo sounding, three major lithological units are recognized in the main basin of Lake Huron: (1) glacial till and bedrock; (2) glaciolacustrine clay; and (3) postglacial mud. Sand is a lesser unit in the Huron surficial sediments. Owing to the wide range in bathymetric complexity, postglacial muds occur in basins of three distinct types:Type A. Regular basins in which mud forms a continuous cover.Type B. Irregular basins with undulating bottom topography. Glaciolacustrine clays outcrop in the crests and mud fill occurs in the hollows. Mud cover is greater than 50%.Type C. As for type B but with mud cover less than 50%.The sediment distribution pattern is essentially simple with a natural superposition of sediment units reflecting the glacial and postglacial history of the lake. A bedrock escarpment from Point Clark to Thunder Bay divides Lake Huron into two distinct morphological regions. To the south of the escarpment, the lake has a gentle topography. A second low amplitude escarpment, trending northeast from Ipperwash, divides the southern region of the lake into two large depositional basins. To the north of the major escarpment, the lake is much deeper and has a complex bottom topography. The northern region is separated into two large basins of discontinuous sediment deposition by a rise of glacial sediments trending north from Thunder Bay. The inshore region of Lake Huron and the two escarpments are composed of glacial till and bedrock. The till is overstepped in the deeper water by glaciolacustrine clays which are themselves overstepped by postglacial muds. Postglacial mud accumulation is continuous in the southern basins due to the gentle relief of the lake bottom. In the northern region of the lake, mud accumulation is discontinuous due to the undulating nature of the lake bottom. Mud fills the hollows leaving glaciolacustrine clay exposed at the top of the undulations in this region.The surficial sediments contain variable amounts of quartz, clay minerals, organic carbon, and carbonates. Quartz content is greatest in the coarser inshore sediments while clay minerals and organic carbon are greatest in the liner offshore sediments. Carbonate is low throughout the lake, except along the eastern edge. Blite is the dominant clay mineral with lesser amounts of chlorite and kaolinite.Amphipods, oligochaetes, and chironomids are the major benthic organisms in the Huron sediments. Amphipods are most numerous at a water depth of 70 m, oligochaetes at 140 m, and chironomids in the shallowest depths.The textural characteristics of the sediments, defined by moment measures, can be interpreted as resulting from variable mixing of a clay and a sand end member population.Both end member populations are leptokurtic and asymmetrically skewed due to the introduction of a silt size mode predominantly in the form of a carbonate. The sand end member population is positively skewed and occurs in the inshore zone comprising the coastal nearshore region and the shallow mid-lake regions. The clay end member is negatively skewed and occurs in the depositional basins. Between these two extremes there is a gradual prograding from sand to clay related to a progressive mixing of the two populations. This mixing is believed to be a direct function of declining energy with increasing water depth.Sediment composition reflects both the source materials and the textural properties. The sediments of the southern basin are derived predominantly from glacial tills whereas the composition of the sediments of the northern basin has been substantially modified by dilution with reworked glaciolacustrine clays.

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