Organic geochemistry of a Chilean paraffin dirt

Abstract

A “paraffin dirt” collected from the surface in the salar environment of Siglia on the inter-Andean Altiplano, Antofagasta Province, was analyzed. The organic matter (about 19% organic C) consisted of predominantly kerogen, humates and fulvates, with lesser amounts of carbohydrates, proteinaceous material and lipids. The 14C age of this organic matter is old (> 40,000 yr. B.P.) and the stable isotope ratios of C (δ13C ⋍ −29‰) and N (δ15N ⋍ −14‰) do not fall in the range of terrigenous or marine organic matter. These isotope values are, however, consistent with a source of ancient C, such as a natural gas seep.The lipids were separated by thin-layer chromatography into hydrocarbon, fatty acid (as methyl esters), ketone and alcohol fractions. The hydrocarbons consisted predominantly of a large hump of branched and cyclic compounds maximizing at n-C17 retention and C27 and C29 hopane-type triterpanoids. No phytane or pristane was found. The fatty acids consisted mainly of triterpanoidal acids (C31 to C33 hopane skeleton) and minor amounts of n-C12 to n-C20 fatty acids (even/odd predominance, maximum n-C16) and branched and cyclic acids. No isoprenoid or resin diterpenoid acids were detected. The ketones were comprised mainly of triterpanoids (C27 and C29 hopane skeleton) and minor amounts of n- or iso-C16 to C20 ketones. No isoprenoid ketone or steroidal ketones were present. The alcohol fraction contained no triterpenoids, steroids or phytol (detectable by gas chromatography), only minor amounts of n-, and branched-C25 alcohol. No porphyrin or chlorin pigments were detected in the total lipids.These geochemical data, coupled with the geologic observations (e.g. natural gas seep in the area), appear to indicate that this organic matter represents degraded bacterial (and possibly fungal) residues which thrived on a paleo-seep of natural gas. It does not represent a Recent sediment, a petroleum seep or a non-volatile condensate from a gas seep. “Paraffin dirts” may be good surface indicators of natural gas seeps (present or past).