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38466-AC2
Incorporation of Sedimentary Organic Matter Into Mesopores
This project set out to test if organic matter in sediments was protected against biological attack due to encapsulation within small pores – called mesopores – that are associated with mineral grains. Pore size distributions of a wide variety of sediments from around the world were determined by nitrogen gas sorption. We developed a technique using pore size measurements before and after oxidation to remove organic matter, which provides information on the occurrence of organic matter in pores of various sizes. To enable this technique we also developed and applied a high-resolution pycnometric method to directly measure the density of organic matter; our work represents the first such measurement of this density in situ. Densities of sedimentary organic matter ranged from 1.14 to 1.82 g/cc.
Small fractions of the total sedimentary organic matter, ranging from 1 to 20%, were found in smaller mesopores of less than 8 nm in diameter in these samples. This lack of pore filling, combined with the slit-like nature of the pores that implies their nature as spaces between flat faces of clay grains, is consistent with a lack of sorption on these siliceous clay faces. Organic matter that is contained within these small pores can be expected to be protected against attack by hydrolytic enzymes from sedimentary organisms, because such enzymes are too large to enter these small pores. The majority of sedimentary organic matter, however, lies outside of these small pores, which allows rejection of the mesopore encapsulation hypothesis as an explanation for bulk organic matter preservation. Larger mesopores and small macropores of up to 200 nm held significantly greater fractions of total organic matter in some samples – as much as a majority of the total organic matter. Our findings are consistent, however, with a hypothesis by which a network of small mesopore throats prevents access by enzymes to organic matter that is held in larger pores. This work with nitrogen sorption was extended to soil samples, which serve as source materials for coastal ocean sediments and similar results were found.
Small angle X-ray scattering (SAXS) data were obtained on these sediments, in collaboration with Dr. J. McCarthy (University of Tennessee). These SAXS data corroborate the nitrogen gas sorption data, in that minor differences in scattering intensity between unmuffled and muffled samples indicate minimal filling of smaller pores by organic matter. They also reduce the possibility that changes in nitrogen sorption isotherms between muffled and unmuffled samples are due to pore blockage, as opposed to pore filling. This work with SAXS was also extended to soil samples, and similar relationships between pores and organic matter were again found. This work was further extended to examine water retention by soils, and we found that the hysteresis between wetting and drying of soils could be largely explained by organic matter contained in pores.
These inferences from gas sorption and SAXS techniques were then tested in laboratory experiments on model systems, using transmission electron microscopy (TEM). This work, carried out with Dr. K. Curry (University of Southern Mississippi) and Dr. R. Bennett (SeaProbe), involved mixing polysaccharides with clay minerals and then incubating them with hydrolytic enzymes. We found that polysaccharides that survived enzyme attack were strongly associated with clay grains, especially in interparticle mode. This finding corroborates our nitrogen sorption work that indicated that organic matter-clay associations more likely occur at clay edges than faces. Surviving polysaccharide could be found in relatively large pores, indicating that either pore throats in connective networks protected the polysaccharides from diffusing enzymes or that simple adsorption confers a protective advantage.
We applied our findings of clay minerals' role in organic matter protection to the geological record, in collaboration with Dr. M. Kennedy (University of California at Riverside). We sampled fine-grained rocks from the late Precambrian to examine the possibility that clay abundance increased in response to increased terrestrial weathering during that time. We found such a secular increase, suggesting that increased organic matter burial at the end of NeoProterozoic resulted from higher clay mineral supply to the oceans, which would have enhanced oxygen accumulation in the atmosphere and oceans and thus facilitated the evolution of animals.
We examined the possibility that sorption onto iron oxyhydroxides could also lead to major preservation of organic matter. We developed a method to examine sorptively associated organic matter on these oxide phases and, in a series of soils from around the world, found that simple sorption did not account for major fractions of stabilized organic matter. This work was then extended to coastal sediments where similar results were found.
