Environmental evolution inferred from multi-proxy speleothem records in central-southern Tibetan Plateau during Marine Isotope Stage 5

Trace-element concentrations and stable carbon isotope compositions of speleothem are considered effective proxies for characterizing hydroclimatic changes. Two stalagmites (TM-2 and TM-5) from Tianmen Cave (30° 55′ N, 90° 04′ E; ca.4800 m a.s.l.) on the south-central Tibetan Plateau were selected for this study. Mg/Ca, Sr/Ca, and Ba/Ca ratios were measured using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), and these data were integrated with previously published speleothem δ13C records to investigate regional hydroclimatic changes spanning three substages of Marine Isotope Stage 5 (MIS 5a, 5c, and 5e). Principal component analysis of the trace elements reveals a strong correlation between Mg/Ca and Sr/Ca ratios, indicating that their variations are likely influenced by common factors, while Ba/Ca experienced separate geochemical processes. The regression slope of ln (Sr/Ca) versus ln (Mg/Ca) approaching or slightly below the lower bound of slopes expected from prior calcite precipitation (PCP) further suggests that Mg/Ca and Sr/Ca variations are mainly driven by water-rock interaction (WRI) processes and are hydrologically significant. Although the internal fluctuations of Ba/Ca ratios in TM-2 and TM-5 are similar, the absolute values of Ba/Ca differ significantly between the two stalagmites over the same time interval, suggesting that Ba/Ca primarily reflects local factors, potentially related to changes in Ba sources or fluctuations in the background Ba concentration of infiltrating waters. Multiple proxy sequences show that Mg/Ca, Sr/Ca, and δ13C records reach their lowest values during MIS 5e, indicating the most humid conditions and consistent with the significant strengthening of the Indian summer monsoon during this period. In contrast, hydroclimate conditions during MIS 5a and 5c exhibit some decoupling from monsoon intensity changes. This may be caused by increased westerly moisture supply and reduced evapotranspiration at lower temperatures during MIS 5a, leading to a relatively humid regional hydroclimate. Furthermore, the overall trends in stalagmite δ13C, Mg/Ca ratio, and Sr/Ca ratio vary within each interglacial substage of MIS 5a, 5c, and 5e, indicating that the karst system's response to hydroclimate changes occurs over multiple timescales, thus contributing to the different structures of various proxy records.