Steroids diagenesis

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

There are several natural non-combustion sources of PAHs. A study in 1980 by Wakeham [43] concluded that phenanthrene could be created by the dehydrogenation of steroids, retene could be produced by the diagenesis of abietic acid, and alkyl chrysenes could form from the degradation of the pentacyclic triterpenes alpha- and beta-amyrin, which are components of higher plant waxes. In this section we will look at the natural non-combustion sources of retene and perylene and how these sources might impact the Great Lakes. 

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

Figure 6 Diagenetic conversion of sterol (I) to various aromatic hydrocarbons during diagenesis. Abbreviation M implies that the reaction involved multiple steps, R represents aromatization of the A ring, and L represents the aromatization of the B ring. Compounds II, V, and VI are intermediates for the formation of triaromatic steroids...

Fig. 5.15 Summary of reaction pathways to major steroidal products of anoxic diagenesis and catagenesis (large arrows indicate biogenic input after Brassell et al. 1984 Mackenzie 1984 Peakman Maxwell 1988 de Leeuw et al. 1989 van Kaam-Peters et al. 1998).

As for steroids, defunctionalization of pentacyclic triterpenoids occurs during early diagenesis and reduction or aromatization later. Some bacterial degradation of triterpenols has even been observed in the leaves of... 

The scheme inFigure 4.12 is an example of extensive and variable biomarker reactions after sedimentation. It illustrates the fate of sterols particularly during diagenesis. Although the scheme looks complex, it shows oidy a few selected stmctures out of more than 300 biogenic steroids and geochemical conversion... 

Fig. 4.12 Diagenetic and catagenetic transformation of steroids. The precursor sterols are gradually transformed during diagenesis into saturated hydrocarbons by dehydration (elimination of water) and hydrogenation of the double bonds. At higher temperatures, during catagenesis, the thermodynamically most stable stereoisomers are formed. Alternatively, dehydration leads to aromatic steroid hydrocarbons which are stable enough to occur in crude oils (after Rullkotter 2001). See text for detailed description of the reaction sequences.

Thus, it appears that there are highly unique environments in the upwelling area off southwest Africa providing the microbiological and chemical conditions for rapid diagenesis of steroids within a time scale of 100 years or less. Although these conditions exist in the sediments, they do not appear... 

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