Stable isotopic results from paleosol carbonate in South Asia: Paleoenvironmental reconstructions and selective alteration

Abstract

We measured δ 18O and δ 13C values of paleosol carbonate nodules from a variety of sedimentary units in southern Asia in order to reconstruct paleoenvironmental conditions and to evaluate for possible diagenetic alteration. Paleosol carbonate nodules were collected from Lower to Upper Cretaceous strata exposed in two locations in southern Tibet, and from Lower to Middle Miocene strata in northern India and Nepal. Additional samples were collected from stratigraphically adjacent marine carbonate units to test for possible resetting of stable isotope values. Based on the implausibly low δ 18O values (∼ − 13‰) of most of the marine carbonate samples, we interpret the δ 18O values of the associated paleosol carbonate nodules as being reset, and therefore unreliable for paleoenvironmental or paleoelevation reconstruction. Importantly, the δ 18O values of both marine and nonmarine carbonate samples have been altered irrespective of texture, including micritic fabrics, suggesting textural criteria alone are insufficient to determine whether primary stable isotope values have been altered. In contrast, δ 13C values of marine carbonate rocks fall within the expected primary isotopic range, and so by association we interpret the δ 13C values of paleosol carbonate from the same sections to have been unaffected by diagenesis. Using the δ 13C values of paleosol carbonate nodules from Cretaceous strata in southern Tibet, atmospheric pCO 2 is estimated to have been 1400–2000 ppmV between 130–120 Ma, and 2600–3200 ppmV between 100–90 Ma, consistent with previous estimates of pCO 2 for these time periods. Paleosol carbonate nodules within early to middle Miocene strata in northern India and Nepal have average δ 13C values of − 10.4‰, indicating virtually pure C 3 vegetation and lower pCO 2.