Oxygen isotope composition of modern pedogenic carbonate from the southern margin of the Tibetan Plateau

Oxygen-isotope paleoelevation estimates of large plateaus provide important geodynamic constraints on the teconic evolution of orogenic systems as well as offering insight into the dynamic feedbacks between surface uplift and regional- to global-scale climate systems. If the isotopic lapse rate (δ18O vs elevation) is known, then the oxygen isotope composition of ancient meteoric water can be used to estimate paleoelevation. The oxygen isotope composition of pedogenic carbonate preserved in paleosols has been used as a proxy for the oxygen isotope composition of soil water in order to reconstruct paleoelevation in a number of settings. Isotopic equilibrium between carbonate and water is assumed in order to calculate the δ18O value of soil water from measured δ18O values of pedogenic carbonate (δ18Opc). Uncertainties surrounding the temperature of isotopic equilibrium and the degree of evaporation of soil water limit the precision of elevation estimates from pedogenic carbonate. In this study, measurements of the oxygen isotope composition of pedogenic carbonate forming in modern soils from the Mt. Everest Region of Tibet are compared with modern meteoric water δ18O values (δ18Omw) to calibrate δ18Opc as a proxy for elevation. Pedogenic carbonate samples coating the underside of clasts were collected along depth profiles in soils at different elevations ranging from 3750 - 5200m on the southern margin of the Tibetan Plateau. Incipient soils developing in the lowest and presumably youngest river terraces were chosen for δ18Opc measurements because these are the most likely to have formed under the influence of modern precipitation. The oxygen isotope composition of modern spring and stream waters along the Bhote Kosi and Arun River were also measured in this study and agree well with previously published elevation- δ18Omw relationships for the Himalayas. Average δ18Opc values below 50 cm in the modern soils were used to calculate equilibrium δ18Omw values which were in turn used to calculate elevation from this relationship. Calculated elevations were compared with measured elevations for each location. Using estimates of mean annual temperature for isotopic equilibrium between pedogenic carbonate and soil water, the calculated elevations average ~800m higher than actual elevations. If evaporation measurably increased the δ18O value of soil water in these soils, predicted elevations would be lower, not higher, than actual elevations. It is therefore unlikely, despite the sparse vegetation cover on these terraces, that evaporation affects the oxygen isotope composition of soil water below 50 cm. This conclusion is supported by depth profiles which suggest δ18Opc does not change below ~50 cm. Overestimation of elevations is explained by pedogenic carbonate formation near maximum soil temperature, a conclusion also drawn from previous carbon and oxygen isotope studies of pedogenic carbonate. If estimates of maximum soil temperature are used to calculate ancient meteoric water δ18O values from δ18Opc in Nepalese and Tibetan paleosols, the resulting paleoelevations are at the low end of the ranges previously reported and agree with the range of modern plateau elevations.