Extractable organic matter in Precambrian cherts

Abstract

The concentrations of hydrocarbons and fatty acids, and the ratios of the stable isotopes of carbon and sulfur were determined for Precambrian cherts from the Gunflint Iron-Formation, the Paradise Creek Formation, and the Bitter Springs Formation. All three cherts are known to contain organically preserved microfossils. For comparison, studies were conducted on two fossiliferous Phanerozoic cherts (from the Rhynie Chert Beds and Serian Volcanic Formation) of comparable origin and geologic history. The highest concentrations of n-alkanes, pristane and phytane, and saturated and unsaturated fatty acids were generally recovered from the untreated surfaces of the samples; these compounds are primarily, and probably entirely, of recent origin. Extremely small concentrations (a few ppb) of similar compounds were extracted from interior portions of the Precambrian samples; although in part apparently indigenous to the sediments, these compounds are not demonstrably syngenetic with original sedimentation, and the major portion of these extracts also appears to be of relatively recent origin. Permeability and porosity measurements conducted on separate rock samples from the collection on which the organic studies were made, showed the presence of microfractures that could allow the passage of ground water under a pressure gradient. In the absence of chemical criteria firmly establishing the syngenetic nature of extracted organic constituents, such studies of Precambrian sediments may only provide ambiguous evidence of early biochemical processes.