Hydroclimate variability in southwest China during Marine Isotope Stage 9: Insights from multi–proxy stalagmite records

As a potential analogue of the present interglacial, the Marine Isotope Stage 9 (MIS 9) can help predict anthropogenically induced warm climates. However, limited attention and scarce paleoclimate archives have constrained our comprehension of large climate systems like the Asian summer monsoon (ASM) during this period. To better understand the ASM variability in MIS 9, we examined a precisely dated and highly resolved multi–proxy stalagmite record (SMY–1) (spanning from 333.4 to 268.1 kyr B.P.) from Shuiming Cave in southwest China. Its δ¹⁸O reveals three warm substages (MIS 9a, 9c, and 9e) and two cold substages (MIS 9b and 9d), following the high–northern–latitude insolation and interhemispheric insolation gradients, underscoring a key role of orbital forcing and low–latitude hydrological changes on ASM variabilities. Regional hydroclimate conditions recorded by its δ¹³C and trace element ratios (Mg/Ca, Sr/Ca, and Ba/Ca) respond to the ASM intensity on millennial scales. Increased ASM precipitation led to higher soil microbial activity and vegetation density, reducing the prior calcite precipitation effect and the water–rock interaction duration. Furthermore, we identified two–step increases in δ¹⁸O and trace element ratios during the MIS 9/8 transition, coinciding with two pulses of ice–rafted debris events and the gradual decline of the Atlantic meridional overturning circulation. Similar change patterns of δ¹⁸O between MIS 9 and MIS 3, alongside identified Chinese Interstadials 18–25, demonstrate that universal millennial–scale climate oscillations in the North Atlantic during Late Pleistocene climate cycles influenced variations in ASM via ocean–atmospheric reorganizations. Interestingly, the comparatively lower amplitude of SMY–1 δ¹⁸O variation between the antepenultimate deglaciation and the MIS 9e interglacial contradicts findings from other Chinese stalagmites. This spatial heterogeneity reflects the combined influence of regional–scale moisture sources and isotopic fractionation process along pathways on Chinese stalagmite δ¹⁸O records.