LIPID BIOMARKERS IN DEEP ALLUVIAL SEDIMENTS OF THE MIDWESTERN U.S

Investigations of deep alluvial sediments range from modeling climate and geomorphologic change to understanding groundwater flow and contaminant fate, and typically include the analysis of soil chemical and physical properties. Soil textural and bulk organic matter (OM) analysis (as determined by total carbon and nitrogen abundance) can indicate the presence of organic-rich horizons, however, complementary analyses are needed for characterizing the chemical composition and origin of the organic matter (OM) deposited (or preserved) in the sediments. Lipids are primordial components of any living organism. Due to their chemical nature, certain classes of lipids (molecular fossils) are selectively preserved in sediments and thus may be used to trace the origins of the OM and understand its dynamics. Other lipids, like the intact polar lipids, are rapidly degraded after cell death and are indicators of living microbial communities. We identified lipid molecules in deep alluvial sediments and explored their utility as OM tracers and microbial biomarkers in two mid-continent river terrace sequences. Sediments were collected from four depths in Quaternary deposits in the Kansas and Neosho River valleys. A modified Bligh and Dyer method was used to extract the free lipids (alkanoic acids, alkanols, alkanes, steroids, terpenoids, etc.) from the sediments. The residual sediments were alkaline hydrolyzed to release bound lipids (alkanoic acids as monomers of cutin and suberin, plant complex biopolyesters). Free and bound lipids, as OM tracers, were analyzed by gas chromatography-mass spectrometry. Intact polar lipids, as microbial biomarkers, were profiled by electrospray tandem mass spectrometry. Our ongoing analysis includes detailed vertical lipid profiling in continuous soil cores obtained from complete terrace sequences. OM deposited in the sediments is expected to originate from various terrestrial and aquatic sources (plant, microbial, faunal), and the detailed profiling may reveal variations coincident with periods of landscape stability and climatic change. Lipid analysis is expected to allow for a better understanding of the possible mechanisms of OM preservation and microbial dynamics in deep alluvial sediments.

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