Organic matter enrichment in black shales: How important are clay minerals?

Clay minerals possess large surface areas and have long been thought to contribute to organic matter (OM) enrichment in black shales through the adsorption of OM in clay mineral interlayers. In this study, we review OM types and their modes of occurrence in fresh muddy sediments and their lithified counterparts, black shales, as well as the potential role of clay-OM interactions during black shale deposition. Solid OM (tens of nanometer- to hundreds of micrometer-scale particles) is the dominant form of OM in fresh muddy sediments and black shales and is too large to be accommodated in the interlayer region (<5 nm) of smectite and interstratified illite/smectite. For this reason alone, it is implausible that an interlayer adsorption mechanism would be responsible for bulk organic carbon preservation in black shales. OM and clay minerals in nature are instead physically associated via the formation of floccules and aggregates, which serve to retard the microbial degradation of OM. The association of OM and clay minerals argues for a reevaluation of the impact of clay minerals on organic carbon preservation in the sedimentary systems and the global carbon cycle.