Is the Dust Paradigm for Pedogenic Carbonate Formation Overblown?.

During the first half of the 20^th Century calcium carbonate in soil profiles was generally considered to demark the boundary between the pedogenic “A” and “B” horizons and the underlying calcareous parent material, which was designated by the “Cca” symbol. It was within this framework that Jenny illustrated a progressively shallower depth to soil carbonate with progressively dryer climates. Subsequent studies of carbonate horizons, but in igneous parent materials, recognized that the carbonate must be pedogenic and the designation in these soils changed from “Cca” to “K” or “Bk” horizons. Concurrent with this recognition, the paradigm for the source of calcium in desert soils became dust and Ca in rain. Without question, there is abundant evidence in the form of dust traps, rain chemistry, and Sr isotopes that atmospheric additions are important sources of Ca. Yet, a few issues confound the atmospheric-additions explanation. If Ca is from dust, and the dust is greater than 90% silicates, where is the loess cap on soils with Stage V and VI petrocalcic horizons? If Ca is from rain, why do soils formed in limestone or basalt parent materials contain greater amounts of carbonate than neighboring soils formed in granite or sandstone parent materials? If the explanation for Ca in rain is calcareous dust from desert soils, then a chicken-and-egg issue arises: Is the rain high in Ca because of calcareous dust from desert soils, or are desert soils calcareous because of Ca in rain? We need to re-examine the dust paradigm using Ca-isotopes, mass balance, and especially the involvement of biogeochemical weathering and microbial-vegetative harvest and uplift of Ca.