Pedogenic carbonates on Precambrian silicate rocks in South India: Origin and paleoclimatic significance

Abstract

Calcrete in Alfisols on Precambrian silicate rocks is studied here in an attempt to reconstruct paleorainfall trends in semiarid South India. Facies of observed carbonate accumulation are pseudomycelium, calcrete nodules, nodular horizons, hardpan, and laminar horizons. Based on geochemical and Sr isotope tracing we relate the presence of calcrete to local geologic conditions within the Precambrian basement, notably discrete Ca-rich metamorphic outcrops, and rule out eolian input. Micromorphologic analysis reveals a multi-stage origin of the calcretes involving soil, colluvial, geochemical and biologic processes mostly in vadose environments. Preliminary U–Th ages of 190–200 ka obtained on calcrete nodules suggest a major phase of Ca release (rock weathering) and precipitation during marine oxygen isotope stage 7. The calcrete thus indicates fairly stable semiarid conditions at the ⩾200 ky time scale despite fluctuations around an annual rainfall average close to current values (∼700 mm). The stability of the Western Ghats barrier as a shield to the southwest monsoon explains the long-term average stability of the pedoclimatic environment in the rainshadow zone. The radiometric ages suggest that weathering may occur massively at discrete intervals of geologic history, during spikes of humidity in the paleoclimatic record. The rest of the time is spent recycling and redistributing the products of weathering incrementally at landscape scale, within seasonal cycles of a relatively stable semi-arid climate. Dating the calcrete therefore does not equate with dating the soil but, instead, the residence times of nodules in a dynamic soil system in which different constituents have different turnover rates.