Surficial carbonate and land surfaces of the Pamir Plateau, Tajikistan

The Pamir plateau is a prominent physiographic feature in Central Asia (Figure 1). The geomorphology of the plateau interior is a result of its geologic setting and its climate. The Pamir plateau is surrounded by high elevation mountain ranges and extensive low elevation deserts. The climate is continental; marine sources of heat and water vapor are thousands of kilometers removed. The long distance to water vapor sources combined with high elevation result in an extremely cold and arid climate in the plateau interior. One of the interesting geomorphic features of the Pamir plateau is thick horizons of carbonate cement (CaCO₃) on land surfaces. In fact some fluvial terraces exhibit 3 m thick horizons of carbonate cement (Figure 2). The circumstances under which these carbonates have formed are investigated. A general equation for the precipitation of carbonate at the earth surface is given below:

CaCO₃(s) + CO₂ (g) + H₂O = Ca⁺⁺ (aq) + 2HCO₃^- (aq)

Carbonate minerals often precipitate in soil environments (pedogenic carbonate; left hand side of the equation). Soils are leached of carbonate minerals in the upper soil horizons (right hand side of the equation). Although this suggests that weathering of carbonate is necessary, in fact silicate minerals can just as easily provide the Ca for this reaction. The CO₂ for the reaction may come from the atmosphere, from the soil itself, or some combination of the two. Carbon dioxide (CO₂) concentration in soil is generally greater than 3,000–5,000 parts per million and is dominated by the CO₂ respired from plants and from breakdown of soil organic matter. This concentration is an order of magnitude greater than the atmosphere and results in diffusion of CO₂ out of soils into the atmosphere - pedogenic carbonate is often precipitated in equilibrium with this CO₂ (soil respired CO₂). As a consequence, the geochemistry of carbonate minerals precipitated in soil forming environments has been used to great effect for the study of climate and ecology (cf. Cerling and Quade, 1993).

The study of surficial carbonate has also been commonly exploited as a tool for dating landforms in arid regions. The accumulation of pedogenic carbonate can be classified in stages, each relating to the degree of development (Machette, 1985). The degree of development is controlled by the rate of carbonate accumulation and the age of the soil. In study areas where accumulation rates are well constrained, assigning a specific stage to a soil permits quantitative estimates of the soil's age. Often, the accumulation rate of pedogenic carbonate is not known and the stage of development can be used only for dating of soils relative to each other. It is also common for the age of landforms to be estimated by the degree of soil development.

Several initial observations hint at the conditions associated with precipitation of surficial carbonate on the Pamir plateau. Carbonate accumulation is observed throughout the thickness of soil profiles and the upper horizons are not leached of carbonate (Figure 2a, b). In fact it appears that the upper soil horizons are indurated with carbonate to a degree exceeding that of lower soil horizons (Figure 2c, d). These observations suggest that atmospheric CO₂ may be an important component of carbonate precipitation. The strongly cemented nature of these surfaces makes them resistant to erosion. The carbonate armoring of land surfaces also controls how they erode. Erosion of extensive carbonate cemented land surfaces (i.e., Figure 2a, b) results in a collapse of the surface and armoring of adjacent hillslopes (i.e. Figure 2d).

Observations of surficial carbonate from the Kara Köl basin in the northern Pamir Plateau further refine our model of carbonate accumulation. The Kara Köl basin is ringed by peaks in excess of 6,000 m in elevation and the center of the basin is occupied by a large lake at an elevation of 4,000 m (Figure 3). Surficial carbonate is present in the mountains surrounding the Kara Köl basin to elevations of at least 5,500 m. Currently, plants are uncommon or totally absent above ~4,500 m elevation, again suggesting that atmospheric CO₂ plays a major role in the accumulation of surficial carbonate. The bedrock geology of the Kara Köl basin is predominantly granite. Importantly, surficial carbonate coatings are found on the underside of granitic clasts on long narrow granitic summit ridges. This implies that surficial carbonate is not derived locally from carbonate bedrock. Carbonate bedrock, which is common on much of the Pamir Plateau, may partially contribute to surficial carbonate deposits but it must be transported and deposited by eolian processes.

Taken together, geologic observations indicate that carbonate precipitation on the Pamir plateau is concentrated near the land surface and that soil respired CO₂ plays a minimal role in the process. This is consistent with the extremely arid climate of the region. It is known that both eolian input of carbonate dust and atmospheric CO₂ are possible sources of CO₂ to surficial carbonate in desert soils (McFadden et al., 1998; Quade et al., 2007). The contribution of these potential sources can be further assessed by analyzing the ratio of stable carbon isotopes in surficial carbonate from the Pamir plateau. Carbonate precipitated in equilibrium with soil respired CO₂ and atmospheric CO₂ has different ratios of ¹³C to ¹²C and contribution of carbonate dust from eolian sources also has a different carbon isotope ratio. The carbon isotope composition of surficial carbonate from the Kara Köl basin is consistent with contributions from eolian material and atmospheric CO₂ (Figure 4a). These data also indicate that some of the carbonate was precipitated from atmospheric CO₂ under non-equilibrium conditions and may be a result of freezing. Surficial carbonate from the rest of the Pamir plateau appears to be precipitated predominantly from atmospheric CO₂ under equilibrium conditions (Figure 4b). These carbonates are from much lower elevations that most of the Kara Köl samples and are all from localities with carbonate bedrock.

Field observations coupled with stable isotope analysis indicate that surficial carbonate on the Pamir Plateau is precipitated from atmospheric CO₂ and incorporates some carbonate bedrock. Freezing and strong evaporative gradients likely contribute to the process. Precipitation of surficial carbonate is important to the geomorphology of some landforms and provides clues to climate and environment, but is poorly suited for standard applications such as dating landforms or paleoclimatic and paleoecologic reconstructions.

• Cerling, T.E., and Quade, J., 1993. Stable carbon and oxygen isotopes in soil carbonates in Swart, P.K., Lohman, K.C., McKenzie, J., and Savin, S., eds., Climate Change in Continental Isotopic Records. Monograph 78, American Geophysical Union, p. 217-231.
• Machette, M.N., 1985. Calcic soils of the southwestern United States in Weide, D.L., ed., Soils and Quaternary geology of the southwestern United Stated: Geological Society of America Special Paper 203., p. 1-22.
• McFadden, L.D., McDonald, E.V., Wells, S.G., Anderson, K., Quade, J., and Forman, S.L., 1998. Influences of eolian and pedogenic processes of the origin and evolution of desert pavements. The vesicular layer and carbonate collars of desert soils and pavements: formation, age, and relation to climate change. Geomorphology 24, 101-145.
• Quade, J., Rech, J.A., Latorre, C., Betancourt, J.L., Gleeson, E., and Kalin, M.T.K., 2007. Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile. Geochimica et Cosmochimica Acta 71, 3372-3795.