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

摘要

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 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-3 branched alkane series, and a predominantly light C-13 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. (C) 2003 Elsevier Ltd. All rights reserved. [References: 54]