Plant Biopolymer–Geopolymer: Organic Diagenesis and Kerogen Formation

Sedimentary organic matter is formed by diagenesis (reactions in sediments up to 60°C, Tegelaar et al., 1989) and catagenesis (those >100°C induced by thermal cracking, Tissot and Welte, 1984) of biological material introduced during deposition from primary producers. Over 90% of this sedimentary organic matter is a non-hydrolyzable (i.e., immune to acid–base hydrolysis) macropolymer called kerogen (de Leeuw and Largeau, 1993) that does not dissolve in organic solvents and produces petroleumupon catagenesis (Tissot andWelte, 1984). The composition and type of kerogen is heavily dependent on the nature of the biological input (de Leeuw et al., 2006), the environment of deposition (Goth et al., 1998), and the preservation pathway (diagenesis, Briggs, 1999). Both diagenesis and catagenesis can be simulated in the laboratory using P-t apparatus (Stankiewicz et al., 2000) and environmental decay experiments (Briggs, 1999; Gupta et al., 2009). Kerogen formation is generally attributed to neogenesis (Tissot and Welte, 1984), in which sedimentary organic matter is formed by random intermolecular polymerization and polycondensation of biological residues (e.g., amino acids, sugars, and lipids) including melanoidins or the Selective Preservation of resistant biosynthesized macromolecules that undergo limited chemical change during diagenesis (i.e., they remainmorphologically and chemically recognizable as organic remains in the sedimentary rock; Goth et al., 1998). Selective preservation which has gained widespread acceptance as a counter thesis to neogenesis since the mid-1980s posits that aliphatics in fossil organic matter are derived from highly aliphatic and resistant (insoluble and non-hydrolyzable) biopolymers in living organisms, such as algaenan (present in algae; Goth et al., 1998), cutan (present in plant leaves;Mösle et al., 1998), and suberan (present in suberinized vascular tissue, de Leeuw and Largeau, 1993). These survive decay more readily than labile biopolymers such as polysaccharides, proteins, and nucleic acids (Tegelaar et al., 1989). A thorough geochemical analysis of organic fossil matter using high-resolution spectroscopy in conjunction with mass spectrometry [such as in Goth et al. (1998)] and laboratory analytical chemistry methods (de Leeuw and Largeau, 1993) establishes the diagenetic pathway degree of biological preservation (Briggs, 1999) and insights into geochemical transformation of biomolecules to geomolecules (Gupta et al., 2009). Organic sulfurization (Kok et al., 2000) and oxidative reticulation of unsaturated cross-linkages in reacting molecules (Riboulleau et al., 2001) offer mechanistic insights through analysis and case study of modern (extant) and fossil material (discrete) or those disseminated in sediment as kerogen. In light of this, this synthesis opinion article focuses on complimentary and conflicting arguments from plant fossil analysis, their widespread relevance, the biochemical description of contributors, insights from controlled laboratory experiments (decay), and simulated autoclave experiments in pressure–temperature regimes from 270 to 350°C (Stankiewicz et al., 2000). Morphology and chemical structure of fossil leaves from the Ardèche diatomite (Late Miocene, from southeast France) were detected using pyrolysis–gas chromatography–mass spectrometry, tetramethylammoniumhydroxide (TMAH)-assisted pyrolysis, micro-FTIR, and solid-state 13C NMR spectroscopy to improve understanding of these questions (Gupta et al., 2007a).