Kerogen-Bound Maleimides Associated with Photosynthetic Pigment Inputs in 2.1 Ga Sedimentary Rocks

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.