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Part I: Concepts and Methods |
Organic Geochemistry Unit, University of Bristol, School of Chemistry, Cantock’s Close, Bristol BS8 1TS
Masspec Analytical, Bath Road, Wallbridge, Stroud, Gloucestershire GL5 3JA
The solvent extracts of contemporary aquatic sediments contain a complex assemblage of compounds derived from living organisms. Among such biological debris many lipid components occur which possess structural and stereochemical characteristics that attest their origin from terrigenous higher plants, unicellular algae such as diatoms and dinoflagellates, or bacteria, including methanogens. Such compounds are markers of the organisms contributing to a given environment, and can provide information not available from micropalaeontological studies, namely evidence of bacterial activity, contributions from calcareous algae to sediments deposited below the calcite compensation depth and possibly sediment inputs from coccolithophorids in their non-coccolith bearing growth stage. In certain circumstances, especially in sedimentary sequences not subjected to elevated temperatures, many source-specific compounds can survive unaltered and thus are found in Mesozoic marine sediments deposited over 165 million years ago. In such immature ancient sediments, unaltered lipids occur together with their early stage diagenetic products, many of which retain molecular features that reflect their biological origins and are therefore also of value as source indicators. As diagenesis proceeds to catagenesis and kerogen breakdown increases, part of the molecular information is gradually lost as defunctionalization occurs and aliphatic and aromatic hydrocarbons come to dominate the sedimentary lipid distributions. Hence, attempts to identify the detailed sources of sedimentary organic matter from geolipid distributions become more difficult and rely more heavily on the occurrence of particular biologically-specific skeletons (e.g. head-to-head isoprenoids). The recognition of the diagenetic products of biolipids does, however, provide the basis for understanding lipid diagenetic pathways and offers the possibility of extending the scope of molecular assessment of sediment inputs to older and more thermally mature sediments. Despite these limitations imposed by the processes of sediment diagenesis and catagenesis, molecular organic geochemistry provides a unique record of past and present environments, with the potential of evaluating both biological inputs and subsequent sedimentary processes.
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