Geology Reference
In-Depth Information
Chapter 5
Molecular Transformation of Plant
Biopolymers in High P-T Conditions
Abstract
Experimental heating of plant tissues (350 °C, 700 bars) generated a
resistant non-hydrolysable aliphatic macromolecule similar to that comprising
organic matter in ancient sediments and fossil leaves. Comparison of the products
derived from such heating of different pre-treated plant tissues clearly demonstrates
that solvent-extractable and hydrolysable lipids are precursors of the generated
macromolecular material. Thus, these experiments indicate that labile alkyl com-
pounds can be a source of the insoluble aliphatic component of fossil organic matter
in the absence of a resistant aliphatic precursor (e.g. cutan) in the living organism.
Keywords
Maturation Aliphatic macromolecule Fossil organic matter Kerogen
Pyrolysis
Introduction
Ancient sedimentary organic matter is formed by diagenetic and catagenetic
alteration of biological material, typically yielding a non-hydrolysable, organic
solvent insoluble macropolymer called kerogen (Tissot and Welte
1984
). The com-
position and type of kerogen is heavily dependent on the nature of the biological
input, the environment of deposition and the diagenetic pathway (de Leeuw and
Largeau
1993
) but many kerogens are highly aliphatic (especially Type I/II) and
serve as a source of petroleum products during thermal maturation. Thus, the path-
ways and mechanism of kerogen formation and preservation are critical to the for-
mation of fossil fuel deposits, have important implications for the global carbon
cycle and are essential processes in the preservation of macroscopic and morpho-
logically intact organic materials in the fossil record.
Kerogen formation is generally attributed to neogenesis (Tissot and Welte
1984
)
in which sedimentary organic matter is preserved by random intermolecular poly-
merisation and polycondensation of biological residues, natural vulcanisation which
involves the reaction between reduced sulfur and various functional groups in organic
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