Spatial variations and chronologies of aliphatic hydrocarbons in Lake Michigan sediments

Four sediment cores were collected in fine-grained depositional areas of the southern basin of Lake Michigan. Spatial variations of aliphatic hydrocarbons in surficial sediments were consistent with a lakeward movement of riverine sediments in a series of resuspension-settling cycles in which an unresolved complex mixture (UCM) of hydrocarbons associated with dense sediments is deposited in nearshore areas, fine-grained sediments of terrestrial origin accumulate in the deep basin, and planktonic hydrocarbons are depleted by microbial degradation during transport to the deep basin. The rate of accumulation of the UCM (a marker of petroleum residues) in deep basin sediments has increased by more than an order of magnitude since 1880, from 60 microg m(-2) x a(-1) to approximately 960 microg m(-2) x a(-1) in 1980. Crude estimates of the atmospheric loading of the UCM (1100 microg m(-2) x a(-1)) indicate that accumulations in deep-basin sediments might be supported by atmospheric deposition. Agreement was poor between the atmospheric flux of the terrestrial n-alkanes (sigmaC25, C27, C29, C31) to the deep basin (3200 microg m(-2) x a(-1)) and the sediment accumulation rate (660 microg m(-2) x a(-1)). Understanding of atmospheric fluxes, estimated from the very few available data, would be improved by more frequent measurement of the levels of aliphatic hydrocarbons in air and precipitation and a better knowledge of the particle deposition velocities and precipitation scavenging coefficients.