Relationships between mineral surfaces and organic carbon concentrations in soils and sediments

Abstract

Relationships between mineral specific surface area and organic carbon (OC) concentration are examined for sediments and soil A-horizons from throughout the world. I found (published elsewhere) that continental shelf sediments from many different regions exhibit downcore loss of OC to a refractory background level which shows a consistent relationship with mineral surface area (slope = 0.86 mg m −2 OC). This trend is equivalent to a monolayer of organic matter over all surfaces, and is termed the monolayer-equivalent (ME) level. Sediments and soils from other environments are compared to this empirically derived relationship. Several continental slope areas show extension of this relationship to considerable depth. Marked excesses of OC above this trend, which persist downcore, were found in sediments with high carbonate mineral content or slope sediments with low dissolved oxygen concentrations in the overlying water column. About half of the soils examined also adhered to this relationship, while soils with high carbonate content, low pH, or poor drainage showed OC concentrations higher than the ME level. OC concentrations below the ME level are found in deltaic regions and areas with low organic matter delivery such as the deep sea or arid soils. The nature of mineral surfaces was examined using N 2 adsorption-desorption isotherms, and most surface area was found to be present as pores with < 8-nm widths. A hypothesis is developed that explains the observed OC concentrations as a saturation of adsorption sites within small pores, which are small enough to exclude hydrolytic enzymes and hence protect organic matter against biological attack. The relationship between this hypothesis and other hypothesized protection mechanisms is discussed. Adsorption is shown to provide a mechanistic and quantitative explanation for spatial and temporal relationships between sedimentation rate and OC burial.