Abslract Falty acid and hydrocarbon distributions in surficial sediments from ten locations in southern Lake Huron indicate both aquatic and terrigenous recent biological sources of these geolipid materials. Normal alkanoic acids appear to be derived primarily from aquatic sources whereas normal alkanes are from multiple origins. Geolipid distributions are determined by the nature of the biogenic source and are modified during transport and deposition. Sediments receiving urban runoff in Saginaw Bay contain large amounts of non-biogenic hydrocarbons of a petroleum or industrial nature.

Compositions of aliphatic hydrocarbons and of fatty acids were analyzed in two half-meter cores of postglacial Lake Huron sediment. One core represents a continuous record of the past 450 yr of sediment accumulation; the other consists of a surficial layer of modern sediment overlying 40cm of 1 l,ooO-12,000 yr-old sediment. Concentrations of hydrocarbons are higher in the younger core than in the older one. Based upon n-alkane distributions, this reflects a smaller input of terrigenous material to Lake Huron 11,000 yr ago rather than diagenetic losses. Most of the hydrocarbons present in the 450yr-old core are allochthonous while half are autochthonous in the older core. Fatty acids are primarily of aquatic character in both cores, and their concentrations decrease rapidly with depth. Unsaturated acids disappear more quickly than do their saturated analogs. Fatty acid degradation occurs mostly in the biologically active zone of these sediments, and little further alteration of fatty acids appears to happen over times as long as 12,000 yr.

/ Changes in organic materials preserved within sediments of Saginaw Bay deposited over the past two centuries record corresponding periods in the environmental history of this part of Lake Huron and its watershed. Sediments deposited since 1940 show an increasingly greater input of aquatic organic matter in response to accelerating cultural eutrophication of Saginaw Bay. Concentrations of fatty acids, sterols, fatty alcohols, and aliphatic hydrocarbons are higher in these modern sediments than in deeper ones. Molecular distributions of these geolipids reflect less aquatic material deeper in the sediments. Prior to 1875, sediment organic matter appears to be diluted by mineral matter from enhanced erosion caused by clearing of the watershed for farming and settlement. During this period there is better preservation of carbonate minerals due to quicker burial. Since 1875, petroleum components comprise over 90% of the total aliphatic hydrocarbon content of these sediments, reflecting the advent and continued existence of chronic, low-level petroleum contamination of this part of the Great Lakes.

Flight instrumentation for the analyses of extraterrestrial environments must often perform under severely restricted conditions. Often, the detection and identification of a multitude of chemical species is required to fulfill the scientific objectives of the mission. It is therefore important that the analytical instrumentation have universal response. The gas chromatograph-ion mobility spectrometer (GC-IMS) has the potential to provide the sample separation, identification and sensitivity necessary for a successful analysis. However, the IMS has poor sensitivity for the C1-C4 alkanes. The abundance of these molecules at various extraterrestrial sites is often of great importance to exobiologists. This study focuses on optimizing IMS sample ionization mechanisms for the C1-C4 alkanes and other hydrocarbons of interest to exobiology.

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