Effect of temperature and rhizosphere processes on pedogenic carbonate recrystallization: Relevance for paleoenvironmental applications

In soils of arid and semiarid climates, dissolution of primary (lithogenic) carbonate and recrystallization with CO2 from soil air leads to precipitation of pedogenic carbonates and formation of calcic horizons. Thus, their carbon isotope composition represents the conditions prevailing during their formation. However, the widespread use of the isotopic signature (δ13C, δ18O, Δ14C) of pedogenic carbonates for reconstruction of local paleovegetation, paleoprecipitation and other environmental conditions lacks knowledge of the time frame of pedogenic carbonate formation, which depends on climatic factors. We hypothesized that temperature-dependent biotic processes like plant growth and root and rhizomicrobial respiration have stronger influence on soil CaCO3 recrystallization than abiotic temperature-dependent solubility of CO2 and CaCO3. To assess the effect of temperature on initial CaCO3 recrystallization rates, loess with primary CaCO3 was exposed to 14CO2 from root and rhizomicrobial respiration of plants labeled in 14CO2 atmosphere at 10, 20 or 30°C. 14C recovered in recrystallized CaCO3 was quantified to calculate amounts of secondary CaCO3 and corresponding recrystallization rates, which were in the range of 10−6–10−4 day−1, meaning that 10−4–10−2% of total loess CaCO3 were recrystallized per day. Increasing rates with increasing temperature showed the major role of biological activities like enhanced water uptake by roots and respiration. The abiotic effect of lower solubility of CO2 in water by increasing temperature was completely overcompensated by biotic processes. Based on initial recrystallization rates, periods necessary for complete recrystallization were estimated for different temperatures, presuming that CaCO3 recrystallization in soil takes place mainly during the growing season. Taking into account the shortening effect of increasing temperature on the length of growing season, the contrast between low and high temperature was diminished, yielding recrystallization periods of 5740years, 4330years and 1060years at 10, 20 and 30°C, respectively. In summary, increasing CaCO3 recrystallization rates with increasing temperature demonstrated the important role of vegetation for pedogenic CaCO3 formation and the predominantly biotic effects of growing season temperature. Considering the long periods of pedogenic carbonate formation lasting to some millennia, we conclude that methodological resolution of paleoenvironmental studies based on isotope composition of pedogenic carbonates is limited not by instrumental precision but by the time frame of pedogenic carbonate formation and hence cannot be better than thousands of years.