Reconstruction of ultra-high-resolution paleoclimate records based on stalagmites: Theory, practice, and challenge

Stalagmites, characterized by high-precision 230Th dating, continuous deposition, and multi-proxy records, have become the " fourth pillar" of Quaternary paleoclimate research. Series of centennial-to millennial-scale abrupt climate events have been identified from stalagmite paleoclimatic records, such as Dansgaard-Oeschger (D-O) cycles, Heinrich events, the 8. 2 ka event, and 4. 2 ka event. However, it is worthy thinking about whether stalagmites can be used to reconstruct ultra-high-resolution records at the timescales of " annual-seasonal-monthly" or even " daily ". Based on cave monitoring conducted globally, this study analyzed the transmission and interference mechanisms of climate-environmental signals from atmospheric precipitation to the deposition of stalagmite. After that, the feasibility of using stalagmite to reconstruct ultra-high-temporal-resolution paleoclimatic records was preliminarily evaluated. The study suggests that key factors determining the temporal resolution of stalagmite paleoclimate reconstructions include: the infiltration rate of atmospheric precipitation, thickness of overlying bedrock, type of karst water, mixing effects, deposition rate, and sampling/analysis methods/techniques. In caves with thick overlying bedrock, because of the strong mixing effects, only the low-frequency signals at interannual and longer timescales can be saved in drip water. While, in caves with thin overlying bedrock, drip water can record variation of signals on daily to monthly timescales. Furthermore, the deposition of stalagmite from drip water is still determined by the pCO2 of cave air and the saturation state of drip water, which also influencing on the depositional rate of stalagmite. Techniques such as micro-drilling, laser ablation, and ion probe, enable sampling at resolution of 1 00~200 μm, 30~40 μm, and 1~1 0 μm, respectively. Considering the natural depositional rate of stalagmite (e. g. as high as 300~500 μm/a), the theoretical temporal resolution can reach monthly to daily scales. However, the epikarst zone is a complex system, because of the high porosity and strong permeability of heterogeneous structures, secondary karst water circulation, and the coupled interactions among the atmosphere-lithosphere-biosphere-hydrosphere. Therefore, the temporal resolution derived from experimental analysis cannot be simply equated with the true resolution of the signal. This research systematically constructed a three-level coupled model of " signal transmission-sedimentary record-analysis extraction", clarifying the source of the deviation between the time resolution of experimental analysis and the resolution of real climate signals, thereby providing a key theoretical basis for evaluating the feasibility of ultra-high resolution paleoclimate reconstruction from stalagmites. On this basis, in light of the current trend of the increasing popularity of rapid scanning analysis technology, future research should focus on developing an automatic identification and chronology model of stalagmite micro-layers based on artificial intelligence, and carry out targeted designs according to the specific hydrogeological conditions, climate background and research goals of the cave. At the same time, by strengthening modern cave monitoring and simulation studies of the stalagmite deposition process, the framework for interpreting high-resolution paleoclimate records can be further optimized. Only by the systematic integration of the above- mentioned theoretical, methodological and empirical researches, the important leap of speleothem paleoclimatology from the " orbital-millennial" timescale to the " seasonal-monthly-daily" timescale can be effectively promoted.