Only within the last two decades have scientists come to appreciate that most organic matter in aquatic sediments is strongly associated with mineral surfaces. Much of our knowledge is due to the steady efforts of Larry Mayer from the Darling Marine Center, University of Maine.
Very little organic matter can be separated from a mineral matrix via density fractionation, indicating that the organic matter and mineral matrix are tightly associated. The study of this relationship between mineral surfaces and organic matter, and the influence it may have in preserving organic matter in sediments, is sometimes given the term Sorptive Preservation. This term refers to the potential role sorption plays in protecting organic matter from degradation.
Questions under the heading of Sorptive Preservation generally take one of five directions:
- How does the amount of mineral surface relate to the amount of preserved organic matter?
- Does the sorption of organic matter to surfaces influence the reactivity of the organic matter?
- Exactly where on the mineral is the organic matter? Does it coat the whole surface? Is it mainly located in small cracks and pores? Is it glommed on as a blob, leaving most of the mineral surface uncoated?
- What have we learned about the marine environment to date?
- Why care about this stuff anyway?
How does the amount of mineral surface relate to
the amount of preserved organic matter?
Typically, relationships between mineral surface and organic matter content are measured by taking the ratio of organic to surface. Mineral surface is measured by first cleaning the mineral of its organic matter, and then adsorbing a gas (usually nitrogen) to the surface and quantifying the amount of gas adsorbed. In our lab, we use the BET (Brunauer-Emmett-Teller) 1-point method for routine analyses and more elaborate schemes when we want to characterize more about the surface than its total area. Measurements are in units of square meters of surface per gram of sediment. Organic matter is measured as organic carbon. We use a CHN analyzer to combust the organic matter to carbon dioxide and measure that using a thermal conductivity detector. The units are milligrams of organic carbon per gram of sediment. When the organic carbon and surface area measurements are ratioed, the resulting ratio is called OC:SA and has units of milligrams carbon per meter of surface.
This figure illustrates what is known thus far about the OC:SA ratio of marine sediments in the worlds oceans. Data from Mayer (1994a and b), Keil et al., (1994, 1997, 1999), and unpublished data from Keil, Devol, Cowie and Mayer.
Does the sorption of organic matter to surfaces influence
the reactivity of the organic matter?
The answer to this question is not clear, but the evidence is mounting that the answer is ‘yes’. In many instances sorption prevents or significantly slows the remineralization of organic matter (see Sugai and Henrichs, 1992, Mar. Ecol. Prog. Ser. vol88 -or- Ding and Henrichs 2002 Marine Chemistry vol77). Susan Henrichs had conducted some wonderful research in this area.
However, no question ever seems to have a simple answer, and in some cases, sorption may actually lead to enhanced degradation (Talyor, 1995, Limnol. Oceanogr. 40: 875-885). An assessment of literature data suggests that the effect of surfaces on organic matter bioavailability may be related to the exact nature of the organic-mineral interaction. Thick coatings in which only a portion of the organic molecule is associated with the surface generally results in more rapid remineralization. Thin coatings in which the organic matter is intimately associated with the mineral surface tends to result in slower rates of degradation.
Previous work conducted here in the UW-MOG group on this topic been focussed in two areas:
- measuring the effect of removing organic matter from a mineral matrix on its bioavailability
- Keil R. G., Montluçon D. B., Prahl F. G., and Hedges J. I. (1994) Sorptive preservation of labile organic matter in marine sediments. Nature 370; 549-551.
- evaluating the surface chemistry of the interactions between natural organic matter and clay minerals
- Arnarson T. A. and Keil R. G. (2000) Mechanisms of pore water organic matter adsorption to montmorillonite. Marine Chemistry 71, 309-320.
- Thimsen C. A. and Keil R. G. (1998) Potential interactions between sedimentary dissolved organic matter and mineral surfaces. Marine Chemistry 62, 65-76.
- Satterberg, J., Arnarson, T.S., Lessard, E.J. and Keil, R.G., 2003. Sorption of organic matter from four phytoplankton species to montmorillonite, chlorite and kaolinite in seawater. Marine Chemistry, 81: 11-18.
- (Some of these articles can be downloaded from our Publications page.)
Exactly where on the mineral is the organic matter?
Does it coat the whole surface? Is it mainly located in small cracks and pores?
Is it glommed on as a blob, leaving most of the mineral surface uncoated?
The answer to these questions is slowly becoming clear. There are several ways in which the location of organic matter in the sediment fabric can be determined. Again, Larry Mayer from the University of Maine, has lead the pack. He developed a technique based on the fact that the physisorption of nitrogen or argon gas involves higher enthalpies onto naked oxide surfaces relative to those that are coated with organic matter. Larry conducted studies on model adsorbate-adsorbent systems, produced an algorithm relating gas adsorption energetics to the fraction of surface coated with organic matter. Application of this algorithm to marine aluminosilicate sediments shows that those with low to moderate loadings of organic matter (<3 mg organic carbon m-2) have generally less than 15% of their surfaces coated. Most minerals in these sediments, which account for most sediments in the ocean, therefore present a largely naked aluminosilicate surface to aqueous solutions. (Mayer, L.M., 1999. Extent of coverage of mineral surface by organic matter in marine sediments. Geochimica et Cosmochimica Acta, 63(2): 207-215.)
Larry has also developed a method to evaluate whether organic matter is present within the mesopores of mineral grains. This work suggests that while some messopores contain appreciable organic matter, the majority of the organic matter in sediments is not conmtained within mesopores. (Mayer, L.M., Schick, L.L., Hardy, K.R., Wagai, R. and McCarthy, J., in press 2004. Fraction of organic matter residing in small mesopores in sediments and soils. Geochimica et Cosmochimica Acta.)
Our efforts in this regard have lead us to evaluate the importance of organic-mineral aggregates. Click hear to read about this aspect of our research.
