Mechanisms of organic carbon burial trapping in clay mineral interlayers

There is a third proposed controlling mechanism for the preservation and burial of organic matter in sedimentary rocks - one involving a certain type of clay mineral. Dissolved organic molecules are ubiquitous in the ocean and in the pore spaces of accumulating sediments, and are attracted to the charged surfaces of clay minerals. Therefore, by burying clay minerals in sedimentary formations one would a priori expect a sink of carbon (Hedges and Keil, 1995). Not all clay minerals are created equally, and a mineral called smectite turns out to be 1-2 orders of magnitude more effective than other mineral grains. This is because unlike other clay minerals such as illite, kaolinite or chlorite, which have external surface areas of approximately 20-30 m2/g, smectite also has an 'internal' (interlayer) surface, giving it a combined surface area of ~800 m2/g (Kennedy et al., 2002). These internal surfaces arise because of the particular way in which the individual crystalline sheets of smectite are stacked with 'large' interlayer gaps (Fig. 6.10). Polar (and even non-polar) organic molecules such as humic acids and proteins become sorbed into the interlayer sites from the surrounding pore water environment, thereby effectively protecting them from degradation by bacteria (Keil et al., 1994).

Clay mineral formation is the result of the weathering of (mainly feldspar) minerals, such as the hydrolysis of orthoclase feldspar to form kaolinite:

Al4Si4O10(OH)8 + 8SiO2

Chemical weathering of feldspars to form clays is complex, and is dependent on weathering intensity, rate and starting composition

Tetrahedral and octahedral sheets share oxygen atoms at their apices

Silica octahedra (one silicon atom surrounded by six oxygen atoms)

Tetrahedral and octahedral sheets share oxygen atoms at their apices

Silica octahedra (one silicon atom surrounded by six oxygen atoms)

1:1 stacking

Kaolinite is a 1:1 clay mineral consisting of tetrahedral-octahedral packages, and thus has little charge imbalance and functionally no interlayer space

1:1 stacking

2:1 stacking

Tetrahedral layer

Octahedral layer

Tetrahedral layer

Silica tetrahedra (one silicon atom surrounded by four oxygen atoms)

Smectite is a 2:1 clay mineral, consisting of tetrahedral-octahedral-tetrahedral packages. Substitution of silicon atoms by other ions (such as iron and aluminium) causes charge imbalance within the sheets. This allows adsorption of charged or polar molecules within the abundant interlayer space, which makes up most of the surface area. Note that the interlayer space arises between the tetrahedral-octahedral-tetrahedral packages, not the individual tetrahedral and octahedral layers.

Fig. 6.10. Illustration of the structural units and stacking of two different common clay minerals - kaolinite and smectite. (Unlike the schema, real clays have vastly greater lateral dimension than shown: typically 3-8 layers thick (3-8 nm) and 1 mm in lateral extent. Therefore, for smectite, most surface is internal.) Because clays are formed at surface temperatures and pressures, there is abundant substitution of ions such as aluminium and iron for silica. This results in a net charge imbalance, which is why clay minerals adsorb charged and polar molecules (organic molecules and water most commonly), and exchangeable cations such as calcium and potassium. However, simple substitution is not the only control on the degree to which exchangeable cations and polar molecules are attracted to, and sorbed onto, interlayer surfaces: the location of the substitution (tetrahedral versus octahedral sheet) also has a large influence on the effectiveness of interlayer adsorption. Thus, 2:1 clays such as smectite have more substitution than 1:1 clays such as kaolinite, in addition to having greater interlayer space.

2:1 stacking

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of the silicate minerals undergoing reaction. However, it is interesting to note that if the feldspar in question is calcium-rich plagio-clase (CaAl2Si2O8), the weathering reaction produces calcium ions in solution, which could then react with bicarbonate ions, forming inorganic carbonate.

An increase in the rate of formation and supply of smectite clays would strengthen the geologic organic carbon sink and help to sequester fossil fuel CO2. However, smectites are preferentially formed at high weathering intensity, which does not necessarily mean a high overall weathering rate. For instance, while a change in weathering regime in the Himalayas during the late Miocene resulted in significantly more smectite deposition, it is thought to have resulted in reduced physical erosion at the same time (Derry and France-Lanord, 1996). Thus, decreased

CO2 consumption because of reduced silicate weathering might offset some (or all) of the 'gains' made by greater carbon burial associated with smectites.

tion cannot be ruled out, particularly since peatlands are increasingly coming under protection, and in some cases formerly drained peatland forests are being reflooded to promote long-term carbon sequestration and other beneficial reasons.

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