Density Fractionation and X-Ray Photoelectron Spectroscopy as Methods to Study Organic-Mineral Interactions in Marine Sediments

Introduction: Interactions between organic matter and minerals appear to be very important for the preservation of organic materials in marine sediments. Recent evidence suggests that these interactions may not be due to protection of the organic matter due to simple sorption to minerals, but rather that the organic matter is acting as a glue between mineral particles resulting in aggregate formation [1,2]. Density fractionation into several different density fractions is a powerful approach for looking at this kind of patchy organic matter distributions [1] because it can isolate organic-mineral aggregates that have varying organic matter loadings due to the density difference between organic matter (1 gcm) and minerals (~2.5 gcm). Because the organic matter appears to surface associated, some way of studying only the surfaces of the organo-mineral particles is needed. X-ray Photoelectron Spectroscopy (XPS) is a technique that is surface specific ( 2.5 gcm density fractions. The sample was exposed to each density solution multiple times, until no more light material could be isolated. For XPS analyses samples were mounted by double-sided adhesive tape to a glass circle which was then fastened to the sample holder. Analyses were performed in a Surface Science Instruments (SSI) S-Probe ESCA instrument. Results and discussion: Density fractionation of this organic rich (10% bulk OC) sediment shows that 62% and 80% of the sedimentary mass and organic matter, respectively, is in mesodensity fractions between 1.6 and 2.2 gcm. It appears that even in a sample of high organic matter content such as our sample, mineral-organic aggregates are still important, there is not just a great increase in the relative contribution of the lightest fraction, as might have been expected. C:N decreases slightly (from 11 to 9.8) with increased density, which may be due to increased contribution of microbial biomass to higher density fractions, i.e. increased degradation. This is consistent with the amino acid data, which indicates that the decomposition state of the organic matter increases with increased density. However, this decomposition is occurring without changing the oxidation state of the organic matter, as the XPS-determined fCOx (fraction of carbon bonded to oxygen) is relatively stable at around 0.35 for all of the density fractions. %C by XPS and %OC by CHN for the lightest fractions agree, while for the heavier fractions the organic matter is concentrated on the surface. The lightest fractions are almost pure organic matter so they can be thought of as being organic with some minerals glued to them, and therefore, must have the same surface and bulk composition. However, the heavier fractions are mostly mineral aggregates with some organic matter glued to them, resulting in an increased carbon concentration of the surface relative to the bulk. Surfacebound carbon, measured by XPS, levels off at around 10 wt%C while OC:SA decreases (SA=surface area; see figure), suggesting that the organic matter is somehow maintaining a steady contribution to the surface composition, while from OC:SA it would be expected to be decreasing. The reason for this could be that the organic matter is stuck to the surface in discrete places, e.g. on edges of clay plates, and in those places the organic matter is >10 nm thick.