The Combined Petrographic and Chemical Analysis of end-Permian Kerogens

Abstract.

The end of the Permian was marked by one of the greatest mass extinctions of all time. A valuable record of life and death during this event is contained within sedimentary organic matter. The stable isotopic, molecular and morphological information contained within remains of end-Permian organisms represent an important resource for scientists attempting to produce paleoenvironment reconstructions. Most meaningful data derive from multidisciplinary analyses of the same samples. In these circumstances it is desirable that sample preparation for one approach does not hinder subsequent analysis by another. To ensure compatibility of sample processing procedures the petrographic and chemical consequences of two common kerogen preparation steps, demineralization and screening (sieving), were simultaneously monitored using transmitted light microscopy and flash pyrolysis. Two end-Permian sediments, whose organic content was predominated by land-plant debris, were chosen for this purpose. A limestone was used to assess the problem of fluoride production when dematerializing carbonates and a marl was used to investigate the possibility of introducing a sampling bias following kerogen screening. Flash pyrolysis results of demineralization residues indicate that neoformed fluorides can be effectively removed by repeated treatments with excess concentrated HCl. Flash pyrolysis of screened size fractions (<10 μm, 10–18 μm, 18–30 μm, 30–125 μm, 125–250 μm, >250 μm) suggest that, for the end-Permian kerogen used, the various fractions are qualitatively representative of the unscreened kerogen. In a paleoenvironmental context, the homogeneity of the land plant derived kerogen reflects a period of organic accumulation on land followed by rapid deposition and burial in a marine setting. These findings constitute a step forward in the quest for parity between petrographic and chemical analyses of the same kerogen samples.