Abstract

Direct molecular evidence for ancient photosynthetic pigment inputs in early Proterozoic sedimentary rocks remains scarce, partly because of long-standing concerns about contamination in Precambrian biomarker studies. Here, we report maleimide and phthalimide compounds released by chromic-acid oxidation of bitumen-free kerogen from ca. 2.1 Ga black shales of the Franceville Basin, Gabon. The detected assemblage includes 2-methylmaleimide, 2,3-dimethylmaleimide, 2-methyl-3-ethylmaleimide, and phthalimide, whereas higher-alkyl maleimides commonly associated with bacteriochlorophyll c-, d-, and e-derived inputs were not detected. Paired analyses of kerogen isolated from exterior and interior rock portions show near-unity exterior/interior ratios and broadly similar relative compositions, supporting a kerogen-bound origin rather than substantial surficial contamination. In the context of organic-rich marine shales containing evidence for microbial ecosystems, the detected compounds are most plausibly interpreted as reflecting photosynthetic pigment inputs, especially (bacterio)chlorophyll-related tetrapyrrole precursors. The absence of higher-alkyl maleimides may reflect either limited original inputs from anoxygenic phototrophs or the preferential degradation of higher-alkyl maleimides during thermal alteration. Overall, these results show that kerogen-bound maleimide-type compounds provide a promising approach for tracing ancient tetrapyrrole and photosynthetic pigment inputs in Precambrian sedimentary rocks.

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