Mesodensity organo-clay associations in a near-shore sediment

The well-known correlation between mineral grain size and organic matter concentrations in sediments has recently been parameterized in terms of organic matter per unit surface area. Normalized thus, organic matter concentrations in many ocean margin sediments are on the order of 1 mg organic carbon m(-2) (Mayer, L.M., 1994a. Relationships between mineral surfaces and organic carbon concentrations in soils and sediments. Chem. Geol. 114, 347-363; Mayer, L.M., 1994b. Surface area control of organic carbon accumulation in continental shelf sediments. Geochim. Cosmochim. Acta 58, 1271-1284.). If this organic matter is physically associated with fine clay grains, the resultant organo-clay aggregates should have a density of ca. 2.2 g cm(-3), but if associated with larger grains the densities should be much closer to those of pure minerals. We found organo-mineral aggregates of density 2.0-2.3 g cm(-3) ("mesodensity") in a shelf sediment to contain the majority of the sedimentary organic matter and surface area. Their mineralogy was composed almost exclusively of fine clays. The correlation of organic matter concentration with sediment surface area may thus be derivative of associations with the fine clay minerals that largely control sediment surface area. Pore size distributions of these aggregates indicate that most surface area is contained within small mesopores of < 10 nm width. Thus, the hypothesis of organic matter protection by enclosure within small mesopores must be framed within the context of inter- rather than intragranular pores. (C) 2000 Elsevier Science B.V. All rights reserved.