Palynological studies in the western arm of Lake Superior

Lake Superior sediments contain pollen whose changes through time can be corelated with dated pollen diagrams from small lakes in the region. A core collected in 1972 from the deep trough (265 m) off Silver Bay (47° 09′N, 91°20′W) penetrated 6.25 cm of taconite tailings, 55 cm of postglacial silty clay, and 93.5 cm of late-glacial varves. Seven levels in the core were dated by stratigraphic and palynological techniques. The varves stopped forming about 9000 years ago, probably when glacial ice retreated to the north shore. The last 100 varves accumulated at about 10 mm/yr, but the average net sedimentation rate subsequently slowed to 0.05 mm/yr until the time of settlement about 1890. The sedimentation rate then increased by a factor of 10, to 0.5 mm/yr until 1956 when taconite processing began. The postsettlement interval can be recognized by marked increases of ragweed and chenopod pollen that result from land disturbance caused by forest clearance and agriculture. The postsettlement interval is also present at the top of six cores from four other sites in western Lake Superior, collected in water from 25–140 m deep. The postsettlement sedimentation rate varies from 0.1–0.8 mm/yr, suggesting that man has greatly increased sediment yield to the lake in the last 80 years.