Holocene and recent sediment accumulation rates in Southern Lake Michigan

Rates of sediment accumulation in Lake Michigan are a key component of its geologic history and provide important data related to societal concerns su ch as shoreline erosion and the fate of anthropogenic pollutants. Previous attempts to reconstruct Holocene rates of sediment accumulation in Lake Mic higan, as well as in the other Laurentian Great Lakes, have been bedeviled by the effect of refractory terrestrial material on radiocarbon ages from t otal organic carbon samples of lake sediments. AMS radiocarbon ages on smal l samples of biogenic carbonate (ostracodes and mollusks) in Lake Michigan provide accurate Holocene ages. The present bicarbonate reservoir effect is estimated from shells of mollusks collected live before atmospheric nuclea r testing to be 250 yr. From paired samples of biogenic carbonate and terre strial macrofossils, the past reservoir effect is thought to be less than 5 00 yr. The radiocarbon ages indicate a distinct decrease in sediment accumu lation rates throughout the southern basin of Lake Michigan at about 5 ka, about the time when lake level stabilized at the Nipissing level after risi ng rapidly for several thousand years. Average rates of sediment accumulati on for the historic period (the last 150 yr) can be estimated from radioiso topes (Pb-210 and Cs-137), pollen stratigraphy, and changes in sediment pro perties associated with human activity. Multiple methods are necessary beca use at any given site, problems arise in the assumptions or applicability o f one or more methods. In general, the mass accumulation calculations sugge st that sediments were deposited 4 to 11 times faster in the historic perio d than before human settlement. The character of the sediment did not chang e in a dramatic way, but sediment magnetic properties suggest shifts in the sources of sediment. The data suggest that some of the changes in sources and (or) character of the sediment occurred just before human settlement an d were probably related to climatic changes associated with the Little Ice Age. Published by Elsevier Science Ltd.