Submarine and Continental Hydrothermal Systems—A Review of Organic Matter Alteration and Migration Processes, and Comparison with Conventional Sedimentary Basins
Bernd R.T. Simoneit, 1997. "Submarine and Continental Hydrothermal Systems—A Review of Organic Matter Alteration and Migration Processes, and Comparison with Conventional Sedimentary Basins", Ore Genesis and Exploration: The Roles of Organic Matter
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Organic matter provenance
Organic matter in sedimentary basins, usually marine and either of Recent or geologically old origin, is derived from the syngenetic residues of posthumus biogenic debris (Simoneit, 1982a; Tissot and Welte, 1984; Hunt, 1996). This material is composed of both autochthonous detritus and allochthonous residues derived from continental sources (Simoneit, 1982a). Aquatic sediments receive allochthonous organic detritus primarily by river wash-in and eolian fallout particles, with ice-rafting and sediment recycling as minor contributing processes (Simoneit, 1975, 1978). Organic matter that accumulates in contemporary sediments represents the residues from primary biological carbon fixation and its degradation (remineralization; Table 1). The nature of this immature organic material is described below, followed by a general description of more mature organic matter formed as a result of maturation in subsiding basins.
Nature of immature organic matter in sedimentary basins
Gas: Interstitial gas in recent sedimentary environments consists primarily of methane, carbon dioxide, and sometimes hydrogen sulfide (Claypool and Kaplan, 1974; Claypool and Kvenvolden, 1983). The biogenic hydrocarbon gases usually have CH4/(C2H6 + C3H8) ratios greater than 1,000, while those of a thermogenic origin have ratios less than 50 (Bernard et al., 1976). For example, the C1/(C2 + C3) ratios for shallow sediment gases from Guaymas basin, Gulf of California range from 41 to 150 and thus indicate a mixed origin of biogenic (CH4) and thermogenic (C1-C8) hydrocarbons (Simoneit et al., 1979). The depth range where biogenic gas can be found is variable but generally shallow (∼100 m) and depends on microbial production and environmental conditions in the sediments.