Inverse altitude effect disputes the theoretical foundation of stable isotope paleoaltimetry

Stable isotope paleoaltimetry that reconstructs paleoelevation requires stable isotope (δD or δ¹⁸O) values to follow the altitude effect. Some studies found that the δD or δ¹⁸O values of surface isotopic carriers in some regions increase with increasing altitude, which is defined as an "inverse altitude effect" (IAE). The IAE directly contradicts the basic theory of stable isotope paleoaltimetry. However, the causes of the IAE remain unclear. Here, we explore the mechanisms of the IAE from an atmospheric circulation perspective using δD in water vapor on a global scale. We find that two processes cause the IAE: (1) the supply of moisture with higher isotopic values from distant source regions, and (2) intense lateral mixing between the lower and mid-troposphere along the moisture transport pathway. Therefore, we caution that the influences of those two processes need careful consideration for different mountain uplift stages before using stable isotope palaeoaltimetry.