A simple, sorption-driven coating of organic matter over mineral surfaces is not the sole cause of the consistent OC:SA relationship observed in continental margin sediments. While sorption clearly influences the domain, reactivity and diagenesis of organic matter in sediments (Henrichs, 1993; Henrichs, 1995; Henrichs and Sugai, 1993; Sugai and Henrichs, 1992), the physical domain of most organic matter is as organo-clay aggregates that contain not only sorbed components but also distinct amorphous organic matter. Ransom et al. (1997; 1998a; see also Bennett et al., 1991) suggested that organic matter essentially acts as a glue holding aggregated minerals together. Organo-clay aggregates are thus the primary physical domain of organic matter in sediment systems.
Here in the UW-AOG group, we are investigating organo-mineral clusters using a combined approach of density fractionation and X-ray photoelectron spectroscopy (XPS) analysis. The basic idea behind XPS is to illuminate a material with a beam of x-rays (1-2 keV) and observe the energy distribution of the ejected photo electrons. Shifts in the spectra of emitted electrons are specific to the atom ejecting the electron and the chemical conformation of that atom. XPS is surface-specific because electrons generated deeper than a few nanometers under the surface of a substance do not make it out of the material. Panel C in the figure below shows an organo-clay cluster. Rather than the organic matter coating the clay mineral surfaces, it is now clear that the clay minerals act as a layered coating that protects the organic matter within. These clusters are the dominant form of organic matter found in marine sediments. We are currently relating cluster formation and character to the organic diagenetic state of the organic matter within aggregates using amino acid and protein analyses.
Panel A shows the potential interactions between organic molecules and surfaces. Panel B is a conceptual diagram of the organic-mineral onion in which organic matter is protected within a mineral sheath, largely composed of clay minerals, especially the expandable clays. Panel C shows a ESEM image of one such organo-clay onion isolated from the Mexican Margin. The aggregate has a density between 2.05 – 2.2 g cm-3 and is roughly 25% organic matter by weight. The size of the cluster is ~50 x 100 microns.
Some of our recent published work on this topic can be downloaded from the Publications page.
- Arnarson, T.A. and Keil, R.G., 2000. Mechanisms of pore water organic matter adsorption to montmorillonite. Marine Chemistry, 71: 309-320.
- Arnarson, T.S. and Keil, R.G., 2001. Organic-mineral interactions in marine sediments studied using denisty fractionation and X-ray photoelectron spectroscopy. Organic Geochemistry, 32: 1401-1415.
- Arnarson, T.S. and Keil, R.G., submitted. Effects of mineral grain interactions on the bacterial degradation of the phytoplankton Scrippsiella trochodea. Organic Geochemistry..
