Controls on Speleothem Initial U-234/U-238 Ratios in a Monsoon Climate

Speleothem initial uranium isotope ratios ((U-234/U-238)(i)) can be influenced by processes along the seepage water flow-path including alpha-recoil into porewater during U-238 decay and hostrock weathering, the balance of which can reflect the infiltration rate. Thus, speleothem (U-234/U-238)(i) may provide information about past changes in rainfall amounts. However, the utility of (U-234/U-238)(i) as a paleo-infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem (U-234/U-238)(i) record from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964-2012 CE. Speleothem (U-234/U-238)(i) was relatively constant from 1964 to 1984 but then linearly increased by 0.05 over similar to 15 years, a trend that does not correspond with observed rainfall changes. To evaluate potential drivers of (U-234/U-238)(i) variability, we model the evolution of water (U-234/U-238) in a simple karst system using an advection-reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water (U-234/U-238), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and (U-234/U-238) of the weathering hostrock. This suggests that larger shifts in speleothem (U-234/U-238)(i) may result from flow path changes, bringing waters in contact with hostrock of variable U characteristics. Consideration of published Mawmluh Cave records suggests that these mechanisms may also explain variability in stalagmite ( (U-234/U-238)(i) on precessional timescales. Further examination of speleothems (U-234/U-238)(i) from climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on (U-234/U-238)(i) in these dynamic environments. Plain Language Summary Chemical indicators from stalagmites are useful in reconstructing changes in monsoon systems. However, the chemical measurements typically made on stalagmites do not provide direct reconstructions of rainfall amount. We developed a record of uranium isotope variability over the last 60 years in a stalagmite from Northeast India to evaluate this potential metric of past monsoon rainfall. We use a geochemical model to evaluate the climatic and environmental controls on uranium isotopes in cave drip waters and stalagmites. Comparison of our record with measured rainfall indicates that the relationship between rainfall amount and uranium isotope ratios is opposite to that observed in arid environments. Further, a large linear increase in uranium isotopes between similar to 1985 and 2000 CE does not correspond to a change in rainfall. Rather, our model suggests that large changes in uranium isotopes likely result from the opening up of new water flow paths and the dissolution of fresh hostrocks. Investigation of previously analyzed stalagmites from the cave reveals a correspondence between uranium isotopes and insolation, suggesting that sustained changes in rainfall amount, coupled with opening of new flow paths, may be recorded in stalagmite uranium isotopes over the past similar to 100,000 years.