Interpreting Precession‐Driven δ18O Variability in the South Asian Monsoon Region. Issue 11 (12th June 2018) - Tabor, Clay R.
Abstract: Speleothem records from the South Asian summer monsoon (SASM) region display variability in the ratio of 18 O and 16 O (δ 18 O) 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 δ 18 O. However, neither the amount effect nor temperature differences are responsible for the majority of the SASM δ 18 O variability. Instead, changes in the relative moisture contributions from different source regions drive much of the SASM δ 18 O 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 δ 18 O 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. Plain Language Summary: Cave records suggest that there has been significant long‐term climate variability in India related to changes in Earth's orbit. However, these records are difficult to interpret because the signals can represent several different climate responses. Here we use a climate model that directly simulates the isotopic data captured in the cave records to better interpret their physical meaning. From these model simulations, we show that a large portion of the orbital signals found in the cave records are due to changes in the amount of water vapor coming from different sources. Changes in the amount of local evaporation compared to precipitation also have a large effect on the signals found in the cave records. Key Points: An Earth system model with stable water isotope tracers is used to examine precession‐driven variability of the South Asian monsoon South Asian monsoon variability in δ 18 O of precipitation is due to changes in the amount of moisture sourced from different regions Using simulated δ 18 O of soil water improves model‐speleothem signal agreement