The Holocene stalagmite trace element and Sr isotope records from Central China and the inferred hydroclimate changes

Five stalagmites from Magou Cave in Henan Province, China, spanning most of the Holocene, were examined for trace elements (Mg/Ca and Sr/Ca) and⁸⁷Sr/⁸⁶Sr ratios. Two analytical methods, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Laser-Induced Breakdown Spectroscopy (LIBS), were employed to determine the Mg/Ca and Sr/Ca ratios. Consistent variations observed in the profiles obtained using these two methods suggest that LIBS is a reliable, cost-effective, and rapid qualitative analytical technique. The findings point to water-rock interactions as the dominant factor influencing the variations in Mg/Ca and Sr/Ca ratios in these stalagmites. The records from the stalagmites reveal regional hydroclimate changes during their growth periods: a period of dryness followed by increasing humidity during the Early Holocene, a “climatic optimum” period from 8.3 to 5.0 thousand years before the present (ka BP, before 1950 AD), oscillating to dry conditions from around 5.0 ka BP to ∼3.5 ka BP, and a return to moderate humidity with significant fluctuations after ∼3.5 ka BP. The⁸⁷Sr/⁸⁶Sr ratios of the Magou stalagmites are influenced not only by the proportions of limestone bedrock and soil leachates but also by the⁸⁷Sr/⁸⁶Sr ratios of soil leachates, which are determined by the degree of soil development in the surface loess deposits. The long-term increasing trend in stalagmite⁸⁷Sr/⁸⁶Sr ratios during the Holocene is attributed to the cumulative pedogenic processes in the overlying loess deposits, while the superimposed fluctuations are largely due to variations in the proportions of soil and limestone host rock.