Alkylpyrroles in a kerogen pyrolysate: Evidence for abundant tetrapyrrole pigments

Abstract

C1-C6 alkylated pyrroles were identified as major constituents of the flash pyrolysate of a kerogen from the Miocene Monterey Formation (California, USA) using gas chromatography with an N-selective detector and gas chromatography-mass spectrometry. The major alkylpyrroles identified are 2,3,4-trimethylpyrrole; 3-ethyl-4-methylpyrrole; 2,3-dimethyl-4-ethylpyrrole; 2,4-dimethyl-3-ethylpyrrole; and 3-ethyl-2,4,5-trimethylpyrrole. The alkyl substitution patterns of the alkylpyrroles strongly suggest an origin from tetrapyrrole pigments. Evidence for this hypothesis was provided by flash pyrolysis of the tetrapyrrole pigments chlorophyll-a, protoporphyrin-IX dimethyl ester, and bilirubin, which yielded alkylpyrroles with a similar isomer distribution. Quantitative pyrolysis using a polymer internal standard of both the kerogen and the tetrapyrrole pigments revealed that ca. 5% of the kerogen consists of macromolecularly bound tetrapyrrole pigments or that this fraction contains ca. 5% insoluble tetrapyrrole salts. These results show that in specific cases tetrapyrrole pigments can contribute significantly to the sedimentary organic matter.