Deciphering local and regional hydroclimate resolves contradicting evidence on the Asian monsoon evolution

Abstract

The winter and summer monsoons in Southeast Asia are important but highly variable sources of rainfall. Current understanding of the winter monsoon is limited by conflicting proxy observations, resulting from the decoupling of regional atmospheric circulation patterns and local rainfall dynamics. These signals are difficult to decipher in paleoclimate reconstructions. Here, we present a winter monsoon speleothem record from Southeast Asia covering the Holocene and find that winter and summer rainfall changed synchronously, forced by changes in the Pacific and Indian Oceans. In contrast, regional atmospheric circulation shows an inverse relation between winter and summer controlled by seasonal insolation over the Northern Hemisphere. We show that disentangling the local and regional signal in paleoclimate reconstructions is crucial in understanding and projecting winter and summer monsoon variability in Southeast Asia.

Ma/Ca Ratio in Stalagmites as a Potential Palaeo‐Temperature Indicator

“…In contrast, Mg/Ca ratio in speleothems is regularly interpreted as a proxy for past hydrologic conditions rather than temperature changes. This is primarily because the substantial variability in drip‐water Mg/Ca ratio tends to overshadow the influence of temperature on Mg partitioning (Cruz et al., 2007; Duan et al., 2012; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Sinclair et al., 2012; Treble et al., 2003; Tremaine & Froelich, 2013; Wolf et al., 2023). The qualitative relationship between Mg/Ca ratio and hydrologic conditions can be summarized as follows: When recharge diminishes, extended water residence time can enhance prior calcite precipitation (PCP) within the epikarst and/or on the cave ceiling, leading to higher Mg/Ca ratio in both the solution and speleothems (Cruz et al., 2007; Duan et al., 2012; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Treble et al., 2003; Tremaine & Froelich, 2013; Wolf et al., 2023).…”

“…This is primarily because the substantial variability in drip‐water Mg/Ca ratio tends to overshadow the influence of temperature on Mg partitioning (Cruz et al., 2007; Duan et al., 2012; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Sinclair et al., 2012; Treble et al., 2003; Tremaine & Froelich, 2013; Wolf et al., 2023). The qualitative relationship between Mg/Ca ratio and hydrologic conditions can be summarized as follows: When recharge diminishes, extended water residence time can enhance prior calcite precipitation (PCP) within the epikarst and/or on the cave ceiling, leading to higher Mg/Ca ratio in both the solution and speleothems (Cruz et al., 2007; Duan et al., 2012; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Treble et al., 2003; Tremaine & Froelich, 2013; Wolf et al., 2023). Similarly, enhanced PCP can also increase Sr/Ca ratio, establishing a positive correlation between Sr/Ca and Mg/Ca ratios (Cruz et al., 2007; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Sinclair et al., 2012; Treble et al., 2003; Tremaine & Froelich, 2013).…”

“…As Mg/Ca ratio of seawater is commonly constant over timescales shorter than 1,000 ka, Mg/Ca ratio in marine biogenic carbonates, such as foraminifera and corals, has been widely used for estimating past seawater temperature (Barker et al., 2005; Elderfield & Ganssen, 2000; Elderfield et al., 2012; Evans & Müller, 2012; Mitsuguchi et al., 1996). However, Mg/Ca ratio in speleothems primarily serves as an indicator of past hydrological conditions (Cruz et al., 2007; Duan et al., 2012; Fairchild & Treble, 2009; Fairchild et al., 2000; Huang & Fairchild, 2001; Huang et al., 2001; Sinclair et al., 2012; Treble et al., 2003; Tremaine & Froelich, 2013; Wolf et al., 2023). That is because the significant variations in drip‐water Mg/Ca ratio typically surpass the temperature‐dependence of Mg partitioning into speleothems (Fairchild & Treble, 2009).…”

“…• Stable drip water hydrogeochemistry and high temperature sensitivity of Mg partition coefficient may explain this temperature dependence Müller, 2012;Mitsuguchi et al, 1996). However, Mg/Ca ratio in speleothems primarily serves as an indicator of past hydrological conditions (Cruz et al, 2007;Duan et al, 2012;Fairchild & Treble, 2009;Fairchild et al, 2000;Sinclair et al, 2012;Treble et al, 2003;Tremaine & Froelich, 2013;Wolf et al, 2023). That is because the significant variations in drip-water Mg/Ca ratio typically surpass the temperature-dependence of Mg partitioning into speleothems (Fairchild & Treble, 2009).…”

Reply to Lu et al.: Robust Intertropical Convergence Zone variability reconstructed from Maritime Continent speleothem δ18O records

“…EOF analysis has been successfully applied in previous regional and global paleoclimate studies ( 9 , 10 ) and here in reconstructing ITCZ variability with the well-defined compiled records in ref. 1 .…”