Influence of parent material and grain size on carbonate coatings in gravelly soils, Palo Duro Wash, New Mexico

Close examination of carbonate coatings found on the bottoms of pebbles from gravelly soils indicates that soil hydraulic conductivity and respiration rate have a strong influence on the carbonate morphology of aridic soils. Clast size and lithology strongly correlate with coating thickness. Because of the nature of unsaturated flow within macropores of gravelly soils, thicker coatings are found at the bottom of larger clasts. Coatings are thickest on limestone pebbles that commonly display dissolution features at their tops. Model values for soil CO2 based on gas diffusion predict soil atmospheric condition where soil water can remain undersaturated with respect to calcite and dissolve calcareous pebbles throughout the depth of the soil profile. This, coupled with soil hydrology creates a microenvironmental condition where the largest limestone clasts have the thickest coating. A second phenomenon that is apparent from close observation of coatings is that soils containing abundant fines and translocated dust in the matrix have continuous coatings. Continuous coatings are absent, however, in soils that do not neighbor a dust source or do not have sufficient fines in the matrix. In these soils, coatings are almost completely absent from sand and granule size particles suggesting that zones of preferential flow in the soil inhibit carbonate precipitation in parts of the soil profile. These inconstancies in the continuity and thickness of coatings suggest that stage III morphology may not develop at the same rate even locally.