MINERAL SURFACE AREA CONTROL OF ORGANIC CARBON: A NEW MODEL FOR LATE CRETACEOUS BLACK SHALE

Mineral surface area (MSA) is shown to have a strong positive relationship with total organic carbon (TOC) (R2=0.90, N=45) in the late Cretaceous Pierre Shale from the western interior seaway, USA. The Organic-carbon richness (0.5-13 %) of these sediments is commonly attributed to oceanographically enhanced marine productivity and/or preservation of particulate organic material. Here we suggest the strong relationship between MSA and TOC indicates organic matter (OM) accumulation and preservation are controlled by adsorption of molecular scale compounds to mineral surfaces. This study extends to the ancient rock record observations made in recent continental margin sediments in which MSA is shown to play an important role in OM accumulation and preservation. Our data also provide evidence for OM preservation within interlayer sites of smectitic clays, because our MSA values include both the interlayer and external surface area measured with the EGME method. This method contrasts with more conventional BET measurements which include only external surface area, and include the assumption that interlayer sites are inactive. Interlayer inclusion of OM is widely accepted within the soil science community and is consistent with the observation of a TOC-smectite correlation within modern marine sediments as well as recent geochemical studies. This work implies that the spatial and temporal distribution of some black shales and petroleum source rocks appears to be more closely related to the genesis and sedimentology of specific detrital clay minerals than to variations in ocean water chemistry and marine productivity. Our model provides a simple mechanistic addition to paleoceanographic models that relate high frequency changes in terrigenous deposition and continental weathering to organic carbon rich intervals in the rock record.