Salinity Biomarkers- Uncommon Steranes from Neoproterozoic Sediments and Oils

We are using molecular fossils to evaluate paleoenvironmental conditions in the Ediacaran by analyzing Neoproterozoic sedimentary rocks and oils from the Centralian Superbasin in Australia, Eastern Siberia, and the South Oman Salt Basin (SOSB). The temporal and facies controls on, and the origins of, C₁₉ norsteranes and 21-norsteranes are the current focus.

In recent studies of Ediacaran source rocks and oils from Oman, a series of C₁₉ norsteranes with three isomers, referred to as A, B, and C, were identified (Grosjean et al., 2005). In SOSB samples, the relative abundance of compound C increases with the ratio of gammacerane to hopane, an indicator of stratification and thus possibly the salinity of the depositional environment. We will show that it is an indicator of a hypersaline, possibly carbonate-rich, depositional environment for rocks and oils of Neoproterozoic to Cambrian age (Kelly et al., 2007).

21-norsteranes exist as 21-norcholestane, and recently their 24-methyl and 24-ethyl homologues have been identified in saline depositional environments (Bao & Li, 2001; Grosjean et al., 2005). These have been proposed to be enhanced in hypersaline environments in immature rocks and oils, but for samples in the mid oil window or of higher maturity it may be difficult to resolve diagenetic versus original sources (Moldowan et al., 1991). Using C29 sterane aaa 20S/ 20S+20R ratios as maturity indicators, the broad range in abundances of 21-norsteranes among samples that have reached racemization confirms that there is depositional control in samples of low to moderate thermal maturity. Their presence in kerogen pyrolysates suggests that they are most likely formed during the earliest stages of diagenesis.

References

Bao & Li. (2001). Org. Geochem. 32: 1031-1036.

Grosjean et al. (2005). Abstr. 22nd IMOG. Seville, OOS-1.

Kelly et al. (2007). Abstr. 23rd IMOG. Torquay, P203.

Moldowan et al. (1991). Geochim. Cosmochim. Acta 55: 1065-1081.