NOAA/WDS Paleoclimatology - Mawmluh Cave, India Speleothem Uranium Isotope Data from 1964-2013 CE

Speleothem initial uranium isotope ratios ((234U/238U)i) can be influenced by processes along the seepage water flow-path including alpha-recoil into porewater during 238U decay and hostrock weathering, the balance of which can reflect infiltration rate. Thus, speleothem (234U/238U)i may provide information about past changes in rainfall amounts. However, the utility of (234U/238U)i as a paleo-infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem (234U/238U)i record from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964-2012 CE. Speleothem (234U/238U)i was relatively constant from 1964-1984 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 (234U/238U)i variability, we model the evolution of water (234U/238U) in a simple karst system using an advection-reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water (234U/238U), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and (234U/238U) of the weathering hostrock. This suggests that larger shifts in speleothem (234U/238U)i may result from flow path changes, bringing waters into contact with hostrock of variable U characteristics. Consideration of published Mawmluh Cave records suggests these mechanisms may also explain variability in stalagmite (234U/238U)i on precessional timescales. Further examination of speleothem (234U/238U)i from climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on (234U/238U)i in these dynamic environments.