A Brief Commentary on the Interpretation of Chinese Speleothem 18O Records as Summer Monsoon Intensity Tracers

Daniel Gebregiorgis 1,* , Steven C. Clemens 2 , Ed C. Hathorne 3, Liviu Giosan 4, Kaustubh Thirumalai 5 and Martin Frank 3 1 Department of Geosciences, Georgia State University, Atlanta, GA 30302, USA 2 Department of Geological Sciences, Brown University, Providence, RI 02912, USA; Steven_Clemens@brown.edu 3 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany; ehathorne@geomar.de (E.C.H.); mfrank@geomar.de (M.F.) 4 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA; lgiosan@whoi.edu 5 Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA; kaustubh@email.arizona.edu * Correspondence: dgebregiorgis@gsu.edu


diagram in
illustrates the precession phasing of marine and speleothem based EASM and ISM reconstructions. and energy export [16][17][18]. The phase wheel diagram in Figure 2 illustrates the precession phasing of marine and speleothem based EASM and ISM reconstructions. Figure 1. June-August mean precipitation (mm/day) and 850-hPa winds in the Asian monsoon domain for the period 1979-2015 (GPCC precipitation data provided by the NOAA/OAR/ESRL PSD and can be accessed at https://www.esrl.noaa.gov/psd/). Orange and green filled circle shows the location of marine core sites in the East China Sea [14] and the Andaman Sea [15]. Purple filled circles show the locations of the three main Chinese caves (Dongge cave-Southern China; Sanbao cave-Central China; and Hulu cave-Eastern China) [4]. Also shown are some of the Arabian Sea marine core sites (all in blue) from [17] and [18].  [14] and the Andaman Sea [15]. Purple filled circles show the locations of the three main Chinese caves (Dongge cave-Southern China; Sanbao cave-Central China; and Hulu cave-Eastern China) [4]. Also shown are some of the Arabian Sea marine core sites (all in blue) from [17,18].
The conclusion of Zhang et al. [1] that EASM and ISM variability on orbital timescales has been driven directly by changes in Northern Hemisphere summer insolation without significant temporal lags is entirely inconsistent with the EASM δ 18 Osw reconstruction from the East China Sea [14]. This record shows no concentration of variance in the precession band once the temperature and ice-volume influence on foraminiferal-calcite δ 18 O are removed and, hence, cannot be plotted on Figure 2. It is also inconsistent with the Andaman Sea δ 18 Osw record, which closely follows the timing of the Arabian Sea precession band phase [15]. Zhang et al. do not present a discussion of these strongly divergent results and interpretations, which were both derived from the same isotopic system and archive (δ 18 O of CaCO 3 ) in the same monsoon regions. Given that large-scale atmospheric circulation controls the transport of moisture between sources and sinks, the resultant speleothem δ 18 O signal likely reflects changes in moisture sources and pathways [9,21] or land-surface temperature or other processes [22], whereas the seawater δ 18 O signature in marginal basins reflects the amount and timing of regional monsoon rainfall. In the phase wheel representation, the 12 o'clock position denotes minimum precession (maximum NH insolation during the boreal summer). Phase lags increase in a clockwise direction; the 3 o'clock position represents a 90° or 5.75 kyrs phase lag. Maximum latent heat export from the southern subtropical Indian Ocean is at −180° [17,18]. Green filled circles show the timing of potential summer monsoon forcing mechanisms and include the absolute maximum insolation over Asia [23], minimum ice volume [23], and maximum export of latent heat from the southern subtropical Indian Ocean [17,18].
The conclusion of Zhang et al. [1] that EASM and ISM variability on orbital timescales has been driven directly by changes in Northern Hemisphere summer insolation without significant temporal lags is entirely inconsistent with the EASM δ 18 Osw reconstruction from the East China Sea [14]. This record shows no concentration of variance in the precession band once the temperature and ice-volume influence on foraminiferal-calcite δ 18 O are removed and, hence, cannot be plotted on Figure 2. It is also inconsistent with the Andaman Sea δ 18 Osw record, which closely follows the timing of the Arabian Sea precession band phase [15]. Zhang et al. do not present a discussion of these strongly divergent results and interpretations, which were both derived from the same isotopic system and archive (δ 18 O of CaCO3) in the same monsoon regions. Given that large-scale atmospheric circulation controls the transport of moisture between sources and sinks, the resultant speleothem δ 18 O signal likely reflects changes in moisture sources and pathways [9,21] or land-surface temperature or other processes [22], whereas the seawater δ 18 O signature in marginal basins reflects the amount and timing of regional monsoon rainfall.
Author Contributions: All authors contributed equally to this work. In the phase wheel representation, the 12 o'clock position denotes minimum precession (maximum NH insolation during the boreal summer). Phase lags increase in a clockwise direction; the 3 o'clock position represents a 90 • or 5.75 kyrs phase lag. Maximum latent heat export from the southern subtropical Indian Ocean is at −180 • [17,18]. Green filled circles show the timing of potential summer monsoon forcing mechanisms and include the absolute maximum insolation over Asia [23], minimum ice volume [23], and maximum export of latent heat from the southern subtropical Indian Ocean [17,18].
Author Contributions: All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript.