Sedimentation in Lake Mendota, a hard-water, eutrophic lake in southern Wisconsin, has changed abruptly in the recent past. This change is recorded in cores by a buff marl overlain by 1–14 inches of black sludge. The interface between the two sediments is very sharp. The marl and sludge differ in being high-carbonate low-clastics and low-carbonate high-clastics sediments, respectively, but their organic content is essentially the same. The change in sedimentation is ascribed to increased deposition of clastic material in the lake as a consequence of farm and domestic practice. Contemporaneous with this increased clastic deposition there has been an increase in the biologic productivity of the lake with its accompanying hypolimnia oxygen deficiency. The increase in productivity may also have resulted from domestic drainage. The black color of the sludge results from the presence of ferrous sulfides deposited under conditions of oxygen deficiency and not from organic content as previously supposed.
The sediments of Trout Lake, a medium-soft-water, oligotrophic lake in northern Wisconsin, have high organic content and are dark green. Where complete sections of the sediments are available, the distribution of organic content shows increased organic deposition with the advance of time. The most recent sediments show a slight decrease in organic content which is ascribed to recent increased clastic deposition. Lake Wingra is a shallow, hard-water, advanced eutrophic lake, near Lake Mendota. Its recent bottom deposits consist of a gray marl, which in shallow water becomes shell marl.
No evidence was found to support the theory that important diagentic changes are taking place in any of the sediments studied. Changes in the sediments which have taken place after deposition, exclusive of compaction and water loss, appear to be restricted to the upper few inches of the most recent record.