Implications for soil environment from uranium isotopes of stalagmites

By analyzing U and Th isotopic compositions of 41 samples in two stalagmites from Hulu Cave, Nanjing, we first discovered that variations of ²³⁸U and δ²³⁴U₀ along the stalagmite growth-sequence (covering a period from 75 to 18 kaBP) are in high similarity to summer insolation curve at 33°N and δ¹⁸O-based climate record of the studied stalagmites. The concentration of ²³⁸U is mainly controlled by content of organic matter in the soil above the cave. This mechanism can be used to explain our result that ²³⁸U curve of stalagmites is in phase with fluctuation of the δ¹⁸O record of the same stalagmites and summer insolation at cave locality. However, ²³⁸U concentration curve vs. age is, in amplitude, inconsistent with the climatic curves, possibly due to complex processes of soil-water-rock interaction. δ²³⁴U₀ indicates pedogenic intensity of soil profile above the cave and sensitively reflects alternations of pedogenesis and aeolian accumulation processes of Xiashu loess in Nanjing. Consequently, uranium concentration and its isotope ratio in stalagmites may provide a new proxy for climate change.