Exceptional preservation of expandable clay minerals in the ca. 2.1Â Ga black shales of the Francevillian basin, Gabon and its implication for atmospheric oxygen accumulation

- Sedimentary smectite-rich particles present in the ca. 2.1 Ga Francevillian Group, Gabon
- Exceptional preservation of primary minerals due to moderate degree of diagenesis
- Smectite clay minerals indicate intense weathering during the Lomagundi event.
- Oldest example of sedimentary clay-organic complexes
- Organic burial and atmospheric O2 rise aided by clay minerals in the Paleoproterozoic

Clay minerals are exceptionally well preserved in marine black shale of the ca. 2.1 Ga Francevillian Group in southeastern Gabon. The FB Formation of the Francevillian Group is characterized by smectite-rich clay minerals including randomly ordered (R0-type) and ordered (R1-type) mixed layer illite/smectite (I/S). The preservation of R0-type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis, which is unique, considering the Paleoproterozoic age of the sedimentary rocks. R0- and R1-type, smectite-rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures, suggesting formation of clay-organic matter complexes. Our data suggests that clay minerals may have enhanced organic matter preservation, providing the oldest example where a link between clay minerals and organic matter sequestration can be established. Our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time.