Mechanistic Control of Organic Matter Preservation Governed by Mineral Surface Area in Laboratory and Natural Samples from the Eocene Green River Formation

The close association between mineral surfaces and organic carbon preserved in carbon rich sediments suggest a mechanistic link capable of enhancing preservation. The nature of this mechanism is enigmatic however. Here we use laboratory mixtures and natural samples from the Eocene Green River Formation to investigate the potential role interlayer sites of smectite (the Total Mineral Surface Area Hypothesis) play in organic matter preservation in highly enriched sediments. The preservation of organic matter (OM) in sediments has presented a challenging and contentious problem. Evidence indicates that greater than 90% of sedimentary organic matter can not be physically separated from its mineral medium. Clay minerals comprise the bulk of the surface area in marine sediments protecting OM from oxidation. Smectite clays comprise the majority of the surface area with values of 750m2 g-1 measured by water adsorption. These smectite clays contain expandable interlayer sites that can facilitate the adsorption of cations, water and OM. Organic matter can enter the interlayer sites of expandable clays blocking it from biological oxidation. Verification of the close association between amino acids and fatty acids in smectite interlayer spaces was directly measured using XRD. The adsorption of OM in the interlayer space is coupled to the reduction of Fe3+ in the octahedral sites of the clay minerals facilitating the formation of organic polymers that are not as readily desorbed form the clays. The flocculation of clay minerals during increasing salinity was measured with a particle size analyzer. These larger floccules of clay are deposited at greater rates and have the ability to adsorb OM and deposit it through the highly oxidizing water column. Analysis of smectite clays provides evidence that OM is entombed in coalescing aggregates and adsorbed into interlayer sites where it can be altered by reduction of the Fe3+ in the octahedral sites.