Effect of pedogenesis on the stable isotopic composition of calcretes and n-alkanes: Implications for palaeoenvironmental reconstruction

In a fluvial system, depending on sub-aerial exposure, non-pedogenic pond calcretes can be modified into pedogenic calcretes. The present study attempts to understand the effect of sub-aerial exposure and pedogenesis on calcretes using carbon and oxygen isotopic composition. For this purpose, two profiles (profile-A and profile-B) from the same stratigraphic level in Rayka from the western part of India were selected. The profiles are separated by a distance of 500 m and showed differences in calcrete characteristics. In profile-A, the calcretes showed pedogenic features (root traces and void filling spar) whereas calcretes in profile-B showed non-pedogenic characteristics (fine laminations). However, some of the calcretes in profile-A exhibited remnants of fine laminations suggesting that initially the calcretes had a non-pedogenic origin but were modified due to pedogenesis. In profile-A, the carbon and oxygen isotope values of pedogenic calcrete (delta C-13(PC) and delta O-18(PC)) showed more variation compared with non-pedogenic pond calcretes (delta C-13(SPC) and delta O-18(SPC)) in profile-B. The delta C-13(PC) and delta C-13(SPC) values exhibited a spread of 3 center dot 0 parts per thousand and 1 center dot 3 parts per thousand, respectively, and delta O-18(PC) and delta O-18(SPC) values showed a spread of 2 center dot 3 parts per thousand and 1 center dot 3 parts per thousand, respectively. The differences in the isotopic composition between the two profiles suggest that pedogenesis controlled the isotopic inheritance in calcretes. In addition, the carbon isotopic composition of organic matter (delta C-13(OM)) and n-alkanes (delta C-13(n-alk)) that forms the basis of palaeovegetational reconstruction have also been measured to understand the effect of pedogenesis on organic matter in both of the profiles. The average delta C-13(OM) values in profile-A and profile-B are -23 center dot 4 parts per thousand and -21 center dot 1 parts per thousand, respectively. The disparity in delta C-13(OM) values is a result of the difference in the sources and preservation of organic matter. However, the delta C-13(n-alk) values show a similar trend in profile-A and profile-B, indicating that sources of n-alkanes are the same in both of the profiles and delta C-13(n-alk) values are unaffected by the pedogenic modifications.