Interpreting Precession‐Driven δ18O Variability in the South Asian Monsoon Region

Abstract

AbstractSpeleothem records from the South Asian summer monsoon (SASM) region display variability in the ratio of 18O and 16O (δ18O) in calcium carbonate at orbital frequencies. The dominant mode of variability in many of these records reflects cycles of precession. There are several potential explanations for why SASM speleothem records show a strong precession signal, including changes in temperature, precipitation, and circulation. Here we use an Earth system model with water isotope tracers and water‐tagging capability to deconstruct the precession signal found in SASM speleothem records. Our results show that cycles of precession‐eccentricity produce changes in SASM intensity that correlate with local temperature, precipitation, and δ18O. However, neither the amount effect nor temperature differences are responsible for the majority of the SASM δ18O variability. Instead, changes in the relative moisture contributions from different source regions drive much of the SASM δ18O signal, with more nearby moisture sources during Northern Hemisphere summer at aphelion and more distant moisture sources during Northern Hemisphere summer at perihelion. Further, we find that evaporation amplifies the δ18O signal of soil water relative to that of precipitation, providing a better match with the SASM speleothem records. This work helps explain a significant portion of the long‐term variability found in SASM speleothem records.