Geochemistry and stable isotope (δ13C & δ18O) signatures of Calcrete in the Ganga Plains: Implications for paleoclimate and Paleovegetation patterns

Calcretes, also known as caliche or calcium carbonate-rich soils, are valuable paleoclimate archives. Their development is influenced by local hydrology and vegetation, making them sensitive indicators of past climate, especially in regions where other paleoclimate records are scarce. Calcretes from two sections in the Ganga-Yamuna Floodplains (Hamirpur and Lucknow) were analysed for major, trace, rare earth elements, and stable carbon and oxygen isotopes (δ¹³C and δ¹⁸O). The aim was to reconstruct paleo-hydrology and paleo-vegetation conditions. The study measured soil carbonate nodules from an abandoned morrum quarry in Hamirpur and an open pit in Lucknow, located approximately 150 km apart at the Birbal Sahni Institute of Palaeosciences (BSIP) campus.

The δ¹³C and δ¹⁸O values in nodules from Hamirpur ranged from −5.26 to −3.18‰ and −6.42 to −5.56‰, respectively. In nodules from Lucknow, δ¹³C and δ¹⁸O values ranged from −1.64 to −1.28‰ and −7.47 to −5.14‰, respectively. Mollusc specimens from the same horizon in Lucknow showed δ¹³C and δ¹⁸O values of −3.2‰ and −5.7‰, respectively. The co-variation of δ¹³C and δ¹⁸O values suggests a significant influence of paleo-climate and paleo-vegetation. FTIR and XRD analyses of soil carbonate nodules indicate that the majority composition is calcite, with minor occurrences of quartz. Geostatistical evaluations, including multiple correlation, cluster analysis, and principal component analysis (PCA) of major, trace and rare earth elements geochemistry, reveal interrelationships and affinities among the elements within the calcretes. The reconstructed water isotopes from Hamirpur and Lucknow align with isotopic signatures from Betwa River water and rainwater, respectively. Similarly, paleo-water isotopes from co-occurring molluscs at Lucknow support these findings. The carbon isotope data suggest that C3 (e.g. Alstonia scholaris, Butea monosperma) and C4 (e.g. Amaranthaceae, Chenopodiaceae) plants coexisted during calcrete deposition at Hamirpur, while C4 plants dominated in Lucknow during the same period. Overall, the proxies from major, trace and rare earth elements, along with stable isotope signatures in calcrete deposits, indicate arid to semi-arid climatic conditions.