ALIPHATIC BIOMARKERS IN RECENT SEDIMENTS AND ORGANIC SOURCE MATERIAL FROM LAKE BOSUMTWI, GHANA: IMPLICATIONS FOR ORGANIC MATTER PALEOCLIMATE PROXIES

Although total organic carbon, total nitrogen, and bulk organic matter stable isotopic composition are widely used in paleoclimate reconstructions to identify changes in sources and levels of organic productivity, there are large variances in the chemistry of organic source material that complicate the interpretation of these proxies particularly in lacustrine settings. More advanced organic geochemical methods such as biomarker concentrations and compound-specific stable carbon isotopic composition show great promise of providing less equivocal proxies for lacustrine productivity. However, before these types of data can be interpreted meaningfully in the sedimentary record a thorough understanding of how modern processes affect these parameters is required. This study focuses on a series of aliphatic hydrocarbon biomarkers and how they occur in organic source material and sediments within a productive tropical lacustrine basin.

Twenty-four surface sediment, twelve land plant, and six plankton samples collected from the Lake Bosumtwi basin in central Ghana, West Africa have been extracted and analyzed by gas chromatography mass spectrometry. A series of n-alkanes (C10-C35) have been quantitated, and the concentrations range from below detection limit to 157 ppm with the highest concentrations at C29. All organic source and sediment samples show similar distributions of C10-C35 n-alkanes, odd-even ratios, and average chain lengths. Principal component analysis (PCA) of the n-alkane concentrations produced a tight cluster of all sediment and organic source material samples consistent with the qualitative observations. In contrast, PCA of the entire mass spectral data set clearly separates plant and plankton samples into distinct clusters with sediments dispersed but more closely associated with the plant data. This result is consistent with the more refractory nature of lipids derived from terrestrial plants compared to aquatic sources, and suggests that the minor aliphatic hydrocarbons may be more indicative of the organic source material than the major n-alkanes (C10-C30). The stable carbon isotopic composition of the C10-C35 n-alkanes is also being investigated to determine the utility of these parameters for estimating source-specific organic productivity.