Implications for soil environment from uranium isotopes of stalagmites

Abstract

By analyzing U and Th isotopic compositions of 41 samples in two stalagmites from Hulu Cave, Nanjing, we first discovered that variations of238U 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 U is mainly controlled by content of organic matter in the soil above the cave. This mechanism can be used to explain our result that238U 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 curvevs. 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.