Paleoclimatic implications of pedogenic carbonates in Holocene soils of the Gangetic Plains, India

This study attempts to reconstruct the Holocene climatic history of the Indo-Gangetic Plains based on micromorphological characteristics and stable isotope composition of calcretes in a soil-chronoassociation between the Ramganga and Rapti rivers. The calcrete from the B2 horizon of older soils (QGH4-QGH5: 6500-13 500 year BP) consists of septaric and irregularly shaped nodules of dense micrite and diffused needles associated with the illuvial clay pedofeatures. Fabric of the calcretes of older soils show inclusion of soil constituents and precipitation of carbonates in between weathered layers of micas. Formation of this calcrete took place during the pedogenesis in an and to semi-arid climate that also influenced vegetation and C4 type biomass such as Chenopodium and Typha angustata spread in the area. A change to warm and humid phase at 6500 year BP led to dissolution and reprecipitation of the calcretes. This led to the formation of blocky crystals and needles of calcite in voids and coarsening of the fabric in the lower horizons. The vegetation pattern also changed to woodland with Anogeissus and Tecomella spp. The younger soils (< 2500 year BP) show secondary carbonate accumulation associated mainly along the voids in lower parts of the profiles. These are relatively coarse textured with little or no inclusion of soil constituents. This coarse grained calcrete results from capillary rise from a fluctuating water table in a sub-humid climate similar to present. Carbon and oxygen isotope data suggest that the formation of pedogenic carbonate in old soils (QGH4 and QGH5) by evapotranspiration during and to semi-arid climate. In the upper horizons, the carbonates are depleted in C-13 as their formation was influenced by the sparse vegetation and low soil respiration rate. In the lower horizons, the carbonates show enrichment of C-13 due its extensive dissolution-reprecipitation during the humid climate. The younger soils showed the enrichment of 13C in their calcretes and support their formation from fluctuating shallow groundwater table. The oxygen isotope values similar to that of meteoric water of the area indicate calcrete precipitation in equilibrium with the soil water. (C) 2001 Elsevier Science B.V. All rights reserved.