The stable isotope chemistry of pedogenic carbonate in an alluvial soil from the punjab, pakistan

In the Punjab territory of the Indo-Gangetic plains, calcite nodules commonly are found in soils on older stream terraces composed of calcareous silt. For this paper, we studied the morphology, mineralogy, and isotope chemistry of pedogenic calcite at Harappa, Pakistan, to determine the process by which they form and the climatic conditions under which they developed. A well-developed soil (fine-silty, mixed, hyperthermic Aridic Haplustalf) showed greatest nodule complexity in the calcic horizon by x-ray diffraction and thin section analysis; inner portions of these nodules had a radiocarbon age of 7080 +/− 120 yr BP Inner portions of nodules, which are presumed to have formed in equilibrium with soil CO₂, reflect a large influence of atmospheric CO₂and a very low soil respiration rate. Delta¹³C values indicated that a maximum of 71% of the disseminated calcite was pedogenic in origin. Oxygen isotope ratios of disseminated and nodular calcite indicate calcite precipitation in equilibrium with soil water, with δ¹⁸O values similar to that of modern precipitation. A shallower water table at some time in the past may have influenced calcite deposition deep in the profile.