Stable isotopes of surface water across the Longmenshan margin of the eastern Tibetan Plateau

Characterization of the stable isotope compositions (δ¹⁸O and δD) of modern-day surface waters traversing mountain ranges and bordering continental plateaus is important for refining climate models and establishing modern isotope-elevation gradients along mountain ranges. The Longmenshan margin of the Tibetan Plateau is a steep, 4 km topographic front situated near the boundaries between westerlies and Asian monsoon moisture sources, and is previously unexplored with respect to the variation in water isotopic composition with elevation. This study reports stable isotope data from 101 water samples collected from streams, springs, and ponds along the Min River (Minjiang) watershed, which traverses the Longmenshan margin. Local meteoric water lines, d-excess values, and surrounding precipitation and river water data sets suggest that precipitation across the Longmenshan margin is dominated by the East Asian summer monsoon. The increase in d-excess values with increasing catchment elevation breaks down as local moisture recycling becomes important at elevations > 3 km a.s.l. Along the Min River, however, the δ¹⁸O_w and δD_w values decrease with increasing catchment elevation, which fit second-order polynomial curves and are well-approximated by a simple Rayleigh fractionation processes. The temperature-corrected oxygen values in authigenic carbonates from the Pleistocene Zoige Basin, north of the Min River watershed, yield elevations equivalent to present by the oxygen isotope-elevation relationship of the Longmenshan margin.