What factors influence the development of carbonate bands around gravel clasts at the soil surface? These pedogenic features, informally known as carbonate collars, are enigmatic due to their unusual formation at the soil surface. Typical pedogenic carbonate accumulations occur in drylands few to many centimeters below the surface on all rock types. Understanding how these collars amass in drylands at the surface horizon can elucidate how near surface hydrologic and geochemical processes differ from those that favor pedogenic carbonate accumulation in virtually all calcic soils.

McFadden et al., 1998 hypothesized that collars could form through the intrusion of rain in soil of high temperatures, followed by prompt evaporation. Dissolved carbonate following an upward pCO2 gradient re-precipitates where meteoric water becomes oversaturated at the collar or soil rock interface. Precipitation is constrained by kinetic fractionation. Numeric modelling has expressed collars may form in diverse settings, if calcic surface soil, rapid evaporation at high temperatures, and upward diffusion that precludes basal accumulation occurs.

A limestone clast from the Sandia Foothills exhibiting both carbonate pendants on its base and a collar was scraped and sampled. This clast was not found at its original position. It was proposed that the pendants formed ~25cm subsurface in the B horizon, surface erosion or bioturbation led this clast to the surface where collar formation occurred. Existing d¹⁸O and d¹³C isotope analysis of collar carbonates indicate precipitation during intense evaporation of soil water, under non-equilibrium DIC conditions. Further d¹³C analysis will reveal trends involving the establishment and evolution carbonate. Combined, these results will help instantiate the microenvironment needed to form these pure carbonate collars to better record climatic and hydrologic conditions in arid desert soils.

Citation

McFadden, L.D., McDonald, E.V., Wells, S.G., Anderson, K., Quade, J. & Forman, S.L. (1998) The vesicular layer and carbonate collars of desert soils and pavements: formation, age and relation to climate change. Geomorphology, 24, 101–145.