Abundance and geochemical significance of C2n dialkylalkanes and highly branched C3n alkanes in diverse Meso- and Neoproterozoic sediments

• Published in: Organic Geochemistry
• Main Authors: Greenwood, Paul F., Arouri, Khaled R., Logan, Graham A., Summons, Roger E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2004
• Subjects: Baffin Island; Baffin; East Greenland; Greenland; Spitsbergen
• Online Access: https://researchers.mq.edu.au/en/publications/d79b0421-233e-4ffa-90bf-4fea05f9a168; https://doi.org/10.1016/j.orggeochem.2003.10.013; http://www.scopus.com/inward/record.url?scp=1442340231&partnerID=8YFLogxK

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 3n 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 2n and C 3n branched alkane series, and a predominantly light 13 C 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.