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

Speleothem initial uranium isotope ratios (( 234 U/ 238 U) i ) can be influenced by processes along the seepage water flow‐path including alpha‐recoil into porewater during 238 U decay and hostrock weathering, the balance of which can reflect the infiltration rate. Thus, speleothem ( 234 U/ 238 U) i may provide information about past changes in rainfall amounts. However, the utility of ( 234 U/ 238 U) i as a paleo‐infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem ( 234 U/ 238 U) i record from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964–2012 CE. Speleothem ( 234 U/ 238 U) i was relatively constant from 1964 to 1984 but then linearly increased by 0.05 over ∼15 years, a trend that does not correspond with observed rainfall changes. To evaluate potential drivers of ( 234 U/ 238 U) i variability, we model the evolution of water ( 234 U/ 238 U) in a simple karst system using an advection‐reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water ( 234 U/ 238 U), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and ( 234 U/ 238 U) of the weathering hostrock. This suggests that larger shifts in speleothem ( 234 U/ 238 U) 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 ( 234 U/ 238 U) i on precessional timescales. Further examination of speleothems ( 234 U/ 238 U) i from climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on ( 234 U/ 238 U) i in these dynamic environments.