INTERLAYER MINERAL SURFACE AREA OF SMECTITIC CLAY MINERALS AS AN IMPORTANT CONTROL OF TOC IN BLACK SHALE

The depositional controls of black shale deposits are controversial, but are widely viewed as a complex interplay between surface marine productivity, rates of oxidation of organic material, and/or dilution by non-organic material. We show a strong positive correlation between mineral surface area and total organic carbon (R2=0.89) in representative black shale deposits spanning Tertiary to Mesozoic ages. This relation suggests that, as in modern marine sediments, adsorption of dissolved carbon compounds onto clay mineral surfaces played a fundamental role in the burial and preservation of organic carbon. This model of adsorption is different from these previous studies however, in that it focuses on the role of interlayer sites of 2:1 smectite family of minerals rather than the relatively limited external surface area. Our results imply that both polar and non-polar organic compounds are capable of gaining entry to the interlayer sites of smectite clay minerals, and can account for commercially important concentrations of TOC (>15%) in a smectite rich sediment. To test the role of interlayer adsorption on TOC in black shales, we determined the percent of smectite, versus non-interlayer illite, and kaolinite using quantitative XRD techniques in a variety of black shales of varying thermal maturity and stages of diagenesis. We also conducted laboratory experiments emplacing polyvinylpyrrolidone (PVP) into the smectite interlayer space, as a proxy for naturally adsorbed organic matter (OM). This is shown to disrupt the coherency of X-ray diffraction along (00l), but not (hkl). The scale of the X-ray peak disruption is directly related to the amount of interlayer OM, and can be used to independently estimate the interlayer OM in natural black shale sediments such as our study material, the Cretaceous Pierre Shale from the western Interior Seaway. Preliminary results show the highest amounts of interlayer OM (per mole of smectite) are in the more distal samples. The MSA-TOC (mineral surface area) association implies organic carbon sequestration in an important class of black shales and petroleum source rocks may be more closely related to patterns of continental weathering and secular clay mineral trends than to the traditional models that consider only ocean water chemistry or marine productivity.