Abundance and geochemical significance of C 2 n dialkylalkanes and highly branched C 3 n alkanes in diverse Meso- and Neoproterozoic sediments.

Abstract

Several series of branched alkanes with quaternary carbon centers have been detected by GC–MS in the saturated hydrocarbon fractions of Neoproterozoic sedimentary rocks associated with fossilised microbial mats (Officer Basin, South Australia) and of shallow water carbonates (Spitsbergen, East Greenland and Baffin Island, Canada). A series of 5,5-diethylalkanes was predominant in most of these sediments and was typically accompanied by related series in lower abundance which, based on mass spectral characteristics, are tentatively assigned as 6,6-diethylalkanes, 5-butyl, 5-ethylalkanes and 6-butyl, 6-ethylalkanes. Each series displays strong odd or even carbon number preferences. Several of the sediments contained additional series of structurally undefined C 3 n alkanes, notable for very negative retention time factors which become more negative with increasing molecular weight—indicating that branching increases with each C 3 addition. The strongly sequential carbon preference of the respective C 2 n and C 3 n branched alkane series, and a predominantly light 13C signature of the 5,5-diethylalkanes (cf. co-occurring n-alkanes), are consistent with an origin from organisms with a distinct physiology and/or carbon source. The relatively wide occurrence and high abundance of these branched alkanes in Proterozoic sediments containing benthic microbial mats suggests a relationship with specific paleoenvironmental conditions, for example strong water column redox gradients, that may have been more prevalent than in the Phanerozoic.