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

Abstract

Speleothem 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.

The Connected Isotopic Water Cycle in the Community Earth System Model Version 1

“…Interpretation of isotopic measurements from the calcium carbonate of speleothems is one useful application of the coupled iCESM1 (Liu, Lu, et al, ; Tabor et al, ). While iCESM1 cannot directly simulate the cave drip water that ultimately produces the speleothem records, the simulated transport of water isotopes through the land and atmosphere provides several signals for comparison with the proxy records.…”

“…Furthermore, outputs from iCESM1 can be added into forward proxy models to provide valuable insights into how seasonal and annual isotopic variability translate into the speleothem records (e.g., Dee et al, ). Previous work has already shown the utility of this approach in constraining the properties of Heinrich events (Zhu et al, ), glacial ENSO variability (Zhu et al, ), and orbital‐scale monsoon variability (Tabor et al, ), and we anticipate that future investigations with iCESM over a wide variety of time periods will yield similar advances.…”

“…For instance, the scheme has already been used to examine global (Singh et al, ) and regional (Dyer et al, ) variations in moisture source, as well as to determine the average moisture sources for specific weather phenomena (Nusbaumer & Noone, ). This water tagging can be applied to water isotopologues as well, allowing one to determine the impact of moisture source and pathway changes on the isotope ratios for a particular region (Tabor et al, ; Zhu et al, ). We note that water tagging currently requires custom coding within the coupler to enable tag information to pass between components, and in the default configuration iCESM has tags within each of CAM5, CLM4, and POP2 as independent components (i.e., tags are not coupled).…”

“…This new model version (hereafter “iCESM1”) thus represents a significant technical advance, as the isotopic simulation capabilities will be retained through successive model generations. iCESM1 is already being used in a variety of contexts, including the water isotope‐enabled simulation of the Last Glacial Maximum (Zhu et al, ), freshwater hosing experiments to mimic the Heinrich‐like events (Zhu et al, ), orbital‐driven monsoon variability (Tabor et al, ), and the creation of an isotope‐enabled Last Millennium community ensemble analogous to the CESM Last Millennium Ensemble (Otto‐Bliesner et al, ).…”

Deciphering Oxygen Isotope Records From Chinese Speleothems With an Isotope‐Enabled Climate Model

“…Based on the local “amount effect” (i.e., the negative relationship between and rainfall amount at the same site) (Dansgaard, ; Ishizaki et al, ; Lee & Fung, ; Risi et al, ), the precipitation δ 18 O in the Asian monsoon region should represent the regional monsoon precipitation amount (Cheng et al, , ; Hu et al, ; Yuan et al, ; Wang et al, ). This assumes that the local amount effect is dominant, yet other studies have shown other processes to be important: upstream rainout processes (Pausata et al, ), changes in moisture sources or atmospheric circulation pathways (Breitenbach et al, ; Liu et al, ; Maher & Thompson, ; Tabor et al, ; Tan, ), convective activity (Aggarwal et al, ; Kurita et al, ), karst system processes (Baker et al, ), and sea level changes (Xue et al, ). Thus, a more refined interpretation of oxygen isotope excursions in Chinese speleothems requires that all contributions be quantified.…”