THE EFFECT OF ALTITUDE ON THE ISOTOPIC COMPOSITION OF PALEOSOLS: EXAMPLES FROM SOUTHERN TIBET AND THE BOLIVIAN ALTIPLANO

Altitude has a significant effect on the oxygen and carbon isotopic composition of paleosols. The δ¹⁸O value of meteoric water decreases with increasing altitude, with an average isotopic lapse rate of ~2.5 to 3‰/km. Soil carbonates precipitate from meteoric water that infiltrates the soil and therefore reflect the elevation of rainfall. The δ¹³C value of soil carbonate records the C isotopic composition of soil respired CO₂, which experiences a different degree of fractionation depending on the photosynthetic pathway of plants growing in the soil (C₃ or C₄). C₄ plants, which fractionate carbon isotopes less than C₃ plants, are favored in climates with warmer growing seasons, and therefore decrease in abundance with increasing altitude. This leads to a decrease in the δ¹³C value of paleosol carbonates and soil organic matter with increasing altitude. The oxygen and carbon isotopic composition of paleosol carbonate and organic matter from the Thakkhola graben in southern Tibet (current elev.=3000 to 4200 m) are compared to paleosols deposited in the Siwalik foreland basin, south of the Himalaya. Paleosols that are less than 7 Myr old in both the Siwaliks and Thakkhola graben display significant differences in δ¹⁸O values of ~12 to 17‰, which suggests that deposition in the Thakkhola graben occurred at a paleoelevation similar to modern elevations in the basin. The δ¹³C values of paleosol carbonates in the Thakkhola graben, which average ~2‰ more negative than the Siwalik paleosols, suggest a smaller proportion of C₄ grasses in the Thakkhola graben, consistent with the interpretation of high elevation. The natural variability in meteoric water and additional uncertainties associated with determining the oxygen isotopic composition of paleo-meteoric water from paleosol carbonates generate large uncertainties when attempting to estimate paleotemperature. However, propagating the uncertainties in paleoelevation estimates from paleosol carbonate in the Tibetan plateau and Bolivian Altiplano suggests an overall 1σ uncertainty ~500 to 800 m on the calculated paleoelevation. Because oxygen and carbon isotope fractionation associated with changes in elevation is significant, paleoclimate studies should consider potential changes in paleoelevation and the difference in elevation between study sites.