Odd-over-even predominance

A number of selected molecular parameters obtained from analysis of immature crude oils and sediment extracts are evaluated as indicators of palaeosalinity. The nature of these parameters is discussed taking into account the role of intermolecular and intramolecular incorporation of sulfur into specific functionalized lipids. Specific distribution patterns of methylated chromans and C20 isoprenoid thiophenes and the relative abundance of gammacerane are excellent indicators for palaeosalinity, whilst other parameters such as 14< (H),17a(H)/140(H),170(H) -sterane ratios, the pristane/phytane ratio, the even-over-odd carbon number predominance of n-alkanes and the relative abundance of C35 hopanes and/or hopenes may indicate palaeohypersalinity but are affected by environmental factors other than hypersalinity and by diagenesis. 

The importance of these depositional environments makes it desirable that studies concerned with the reconstruction of palaeoenvironments from sediments or source rocks of oils also establish molecular parameters for palaeohypersalinity. Recently, ten Haven et al. (7-9) have summarized a number of "organic geochemical phenomena" related to hypersaline depositional environments. In addition to previously known parameters, such as an even-over-odd carbon number predominance of n-alkanes and a low pristane/ phytane ratio (<0.5), several new parameters were suggested. These parameters, however, are mainly based on empirical relations. 

Even-Over-Odd Carbon Number Predominance of /i-Alkanes... 

The phenomenon of even-over-odd carbon number predominance of n-alkanes has long been recognised in a number of Recent and ancient sediment... 

Desulfurisation of polar fractions of relative immature oils and sediment from hypersaline palaeoenvironments yields n-alkanes with strong even-over-odd carbon number predominances (20,42 e.g., Figure 11). This phenomenon is, however, not restricted to samples from hypersaline palaeoenvironments (e.g., Jurf ed Darawish Oil Shale, Monterey Shale). 

Odd-over-even carbon number predominance. ° Even-over-odd carbon number predominance. 

In certain ranges of some crude oils, odd-numbered n-alkanes predominate appreciably over the even-numbered homologs. Especially striking are odd predominances from Cn to C19 that accompany declines in abundance from Ci6 to Ci8 and from Ci8 to C2o (i2). Odd-even predominances were also observed in ozokerites, but not as prominently and at higher carbon numbers they will be presented and discussed in detail elsewhere. 

The saturated hydrocarbon distributions of the marl samples are dominated by long-chain n-alkanes of higher land plant origin (21) with a strong odd-over-even carbon number predominance. Hopanoid hydrocarbons are the next most abundant constituents, but other hydrocarbons particularly abundant in the laminite samples described hereafter are also clearly recognizable. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

Previously known as cyclopolyphosphates, these rings may contain up to 12 tetrahedra, but those with three, four, and six units are most common (see Table 1). The cyclotri- and cyclotetraphosphate rings adopt puckered geometries typical of saturated six and eight atom rings. The predominance of even membered cyciophosphates reflects their ability to pack efficiently in the solid state, rather than any inherent stability over odd membered ones. This is often reflected by a high internal symmetry in the crystalline state an analysis of thirty reliably determined cyclohexaphosphate structures shows that 18 have inversion symmetry and a further seven have threefold (Dsd) internal symmetry. ... 

Example Propyl and pentyl loss from 2-ethyl-cyclohexylamine molecular ion, = 127 (odd m/z) are competitive (Fig. 6.12). Pentyl loss, m/z 56 (even m/z), is favored over propyl loss, m/z 84 (even m/z), in accordance with Stevenson s Rule. The peak at m/z 98 may be rationalized in terms of ethyl loss due to a minor contribution of 1,4-H shift, i.e., from position 3 instead of the predominant 1,5-H shift from position 2. The [M-CHs]" peak, m/z 112, is accompanied by a [M-NHs]" signal, m/z 110, which is typical of primary - and to a lower extend also secondary - amines. 

The carbon preference index (CPI) is a numerical means of representing the odd-over-even predominance in -alkanes in a particular carbon-number range. It is often used as a maturity measurement where... 

Confirming the data of Blumer (1970), the isolation of n-alkanes ranging from about C,4 to C36 without predominance of odd over even carbon number compounds was noticed by several authors for example, Parker et al. (1972) for unfiltered surface samples from the Gulf of Mexico and the Carribean sea, Barbier et al. (1973) for filtered samples of coastal waters off the French coast and open waters from the eastern Atlantic Ocean, Iliffe and Calder, (1974) for filtered samples from the eastern Gulf of Mexico loop current and the Carribean Sea, Hardy et al. (1977b) for waters collected around the United Kingdom, and Saliot and Tissier (1977) for the Norwegian Sea. 

We have studied the synthesis of fatty acids by the closed Fischer-Tropsch process, using various carbonates as promoters and meteoritic iron as catalyst. The conditions used were D2/CO mole ratio = 1 1, temperature == 400°C, and time = 24-48 hr. Sodium, calcium, magnesium, potassium, and rubidium carbonates were tested as promoters but only potassium carbonate and rubidium carbonate produced fatty acids. These compounds are normal saturated fatty adds ranging from C5 to Cis, showing a unimodal Gaussian distribution without predominance of odd over even carbon-numbered aliphatic chains. The yields in general exceed the yields of aliphatic hydrocarbons obtained under the same conditions. The fatty acids may be derived from aldehydes and alcohols produced under the influence of the promoter and subsequently oxidized to the acids. 

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