Spatial variations and chronologies of aliphatic hydrocarbons in Lake Michigan sediments

Four sediment cores were collected in fine-grained depositional areas of th e southern basin of Lake Michigan. Spatial variations of aliphatic hydrocar bons in surficial sediments were consistent with a lakeward movement of riv erine sediments in a series of resuspension-settling cycles in which an unr esolved complex mixture (UCM) of hydrocarbons associated with dense sedimen ts is deposited in nearshore areas, fine-grained sediments of terrestrial o rigin accumulate in the deep basin, and planktonic hydrocarbons are deplete d by microbial degradation during transport to the deep basin;The rate of a ccumulation of the,UCM (a marker of petroleum residues) in deep. basin sedi ments has increased by more than an order of magnitude since 1880, from 60 mug m(-2).a(-1) to approximately 960 mug m(-2).a(-1) in 1980. Crude estimat es of the atmospheric loading of the UCM (1100 mug m(-2).a(-1)) indicate th at accumulations in deep-basin sediments might be supported by atmospheric deposition. Agreement was poor between the atmospheric flux of the terrestr ial n-alkanes (SigmaC(25), C-27, C-29, C-31) to the deep basin (3200 mug m( -2).a(-1)) and the sediment accumulation rate (660 mug m(-2).a(-1)). Unders tanding of atmospheric fluxes, estimated from the Very few available data, would be improved by more frequent measurement of the levels of aliphatic h ydrocarbons in air and precipitation and a better knowledge of the particle deposition velocities and precipitation scavenging coefficients.