A Process‐Based Model of Precipitation Isotopes, Orographic Lifting and Leaf Wax δ ² H Across the Tibetan Plateau and South Asia

Abstract Effective fractionation factors ( ε ) relate the δ 2 H of leaf waxes to ambient water and are essential for paleoclimate and paleotopography reconstructions using biomarker isotopes. The δ 2 H of the water used by plants is often poorly constrained and may contribute to producing highly variable estimates of ε . Herein, we compile ∼700 modern water δ 2 H and δ 18 O values from across the Himalaya and Tibetan plateau (TP) to calibrate a new orographic precipitation isotope model that produces precise estimates of long‐term average primary precipitation isotopes. We couple data and precipitation isotope model predictions with ∼200 leaf wax δ 2 H values to calculate ε between leaf wax n ‐alkanes and meteoric water. Our results demonstrate that: (a) modern primary (non‐evaporated) surface water isotopes across the TP and surrounding region can be effectively estimated with less error than existing spatial interpolation models, (b) the precipitation isotope field across this region is dominated by orographic lifting and condensation superimposed on synoptic precipitation patterns, and (c) the effective fractionation between hydrogen isotopes of leaf waxes in soils and meteoric water shows no correlation with elevation, temperature, relative humidity, or vapor pressure deficit and may be constrained by a single ε value of −120.2‰, despite large variations in ecosystem and climate. This new precipitation isotope modeling approach enables reliable prediction of modern primary (non‐evaporated) surface water isotopes (δ 2 H precip and δ 18 O precip ) as well as reconstruction of ancient δ 2 H precip using δ 2 H leaf wax .