Isotopic approach to calcite dissolution and precipitation in soils under semi-arid conditions

Abstract

Two phases of crystallization of secondary calcite are identified in shallow soil horizons from northeastern Brazil where present-day climatic conditions are semi-arid. The 14C and 13C contents of the total dissolved inorganic carbon (TDIC) from soil solutions are compatible with open-system conditions. In the deeper part of the profile (1.5–3-m depth) the precipitation of secondary calcite is attributed to the degassing of solutions under wet climatic conditions. Interpretation of the radiocarbon activities leads to ages between 22,000 and 17,000 yr. for the pure secondary calcite. In the upper part (0.3–1.5-m depth) of the profile, active formation of calcrete through dissolution-precipitation cycles is occurring. In this zone the precipitation is controlled mainly by the evaporation of soil solution as shown by the progressive enrichment in 18O of secondary carbonates. The accretion of calcrete occurs at the surface and at 1.5-m depth due to changes in the position of the evaporation front.