Transformation diagenetic

Brault M, Simoneit BRT, Steroid and triterpenoid distributions in Bransfield Strait sediments Hydrothermally-enhanced diagenetic transformations, Org Geochem 13 697—705, 1988. 

A high variety of isoprenoids is also observed for fossil molecules (Fig. 11.1, pink ribbon, first to the left). This suggests that the first cnidarians at Cambrian times have extensively exploited what the terpene chemistry allows (Pietra 1995). Although overshadowed by diagenetic transformations, and not comprising volatiles for obvious reasons, Chart 16.1 is suggestive of the structure of these early isoprenoids. 

These are processes where a certain set of biodiversity disappears, leaving space for another set. Abundant fossil remains speak for the turnover of species. What happet to natural products is more difficult to appreciate because fossil molecules are only rarely found, or have suffered from diagenetic transformations. 

Representative examples of fossil molecules from the geological eras, from Precambrian to recent times, are shown in Chart 16.1. The molecular skeletons are displayed according to the strategy set forth in Part III this is a minor change with respect to the actual fossil molecules, since most functionalization was lost in the diagenetic transformations. Blank spaces reflect our limited knowledge of fossil molecules. 

Sequential extraction experiments have been shown to provide a convenient means to determine the metals associated with the principal accumulative phases in sedimentary deposits, to elucidate the mechanisms of their diagenetic transformation, and to elucidate release processes upon changing environmental conditions (Kersten and Forstner, 1989). A general goal of all studies involving selective chemical extraction is the accurate determination of partitioning of elements of environmental concern among different discrete phases of a sample. 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

The main objective of this research effort was to characterize diagenetic transformations among the various sulfide phases and analyze pore fluid chemistry with respect to depth. In addition, it also examined if the overlying organic-rich Spartina marsh sediments affected the formation of iron sulfides in the underlying sediments. 

Figure 5.5. Steps in the development of a limestone. A myriad of processes are involved in the conversion of skeletons of organisms into sediments and the transformation of sediments into rocks. The diagenetic fence represents that complex of processes that lead to the conversion of a sediment into a rock. (After Chave, 1960.)...

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