Use of Lipids as Indicators of Diagenetic and Source-Related Changes in Holocene Sediments.

Abstract

Lipids in sediments can provide information about both organic mattter source changes and post-depositional alteration processes. This study concentrates on alcohol, sterol and alkane distributions present in Great Lakes and Northwest Atlantic sediments to define these processes. Potential lacustrine source samples analyzed include plankton tows and bluff samples. Lipid distributions of these samples provided distinctive indicators of lake inputs from aquatic and terrigenous sources. Distributions of lipids settling through the lacustrine water column were investigated by analysis of sediment trap samples and suspended particulates. Preferential degradation of shorter chain alcohols and the aquatic sterols occurs during settling. Two sediment cores representing the last 5000 years deposition in Lake Michigan and the last 600 years of deposition in Lake Huron were investigated. Trends in their lipid components indicate that decreases downcore result from microbial degradation and not source change. Stenol to stanol conversion proceeds faster for the C(,27) (aquatic) sterols than for the C(,29) sterols. First order rate constant approximations support faster decreases for the aquatic relative to terrigenous components. The different rates of degradation of lipid classes (alkanes<<sterols<alcohols) suggest that downcore changes represent degradation and not source changes. Linear programming analyses indicate that the unbound extract has a primarily terrigenous origin while the bound extract has a more aquatic character. Marine sediments from the Gulf of Maine, the North Atlantic continental rise, and the Bermuda Rise were analyzed. While the Gulf of Maine and Great Lakes samples contained similar types of input, different processes such as extensive bioturbation and conversion from an unbound to a bound form are active in the Gulf of Maine core. With increasing distance from shore, the marine sedimentary lipids have a more aquatic character. The Bermuda Rise sediment lipids reflect significant downcore source changes which mask any diagenesis which has occurred. Microbial degradation of lipids occurs in Great Lakes Holocene sediments. Study of such degradation requires relatively fast and uniform sedimentation rates which were not provided by the marine sediments. However, differences between postdepositional processes occurring in the marine and lacustrine sediments were made evident by combinations of several lipid classes as source indicators.