Potential decoupling between the regional monsoon intensity and local moisture conditions during the last millennium in central Brazil

Reconstructing moisture variability throughout the tropics is essential to assessing recurrent droughts and the response of hydroclimate to global climate change. An increasing number of δ18O records in Brazil provide insight into late Holocene variation in the intensity of the South American Monsoon. δ18O values, although widely recognized as a proxy of regional climate, may not reflect local moisture conditions, making additional proxies necessary for reconstructing the impact of large-scale monsoon shifts on local hydrological cycles. We develop speleothem 87Sr/86Sr values as a local paleo-moisture proxy from Tamboril Cave in central Brazil to complement existing δ18O-based reconstructions of regional monsoon intensity. Analysis of leachates of soils and bedrock demonstrates that soils overlying the cave have distinctly higher 87Sr/86Sr values ( 0.732) than cave bedrock ( 0.712). Speleothem 87Sr/86Sr values (0.721-0.724) lie between soil and bedrock values suggesting that 87Sr/86Sr variations reflect varying extents of water-rock interaction. Greater extents of water-rock interaction results in evolution of water Sr isotope values away from those initially acquired from the soil and toward those of the bedrock. Increasing speleothem 87Sr/86Sr values over the past 1,500 years are consistent with decreasing initial δ234U values, a complimentary proxy for water-rock interaction, over the same period. Both isotopic proxies suggest progressively less water-rock interaction through time. A reduction of water-rock interaction could reflect faster water transit from the surface to the cave due to changes in the amount or intensity of precipitation. Notably, development of speleothem Sr isotopes as a proxy of infiltration rates may, unlike trace elements and carbon isotopes, work in settings where the influence of prior calcite precipitation is minimal. Furthermore, δ18O records from the region do not document progressive changes in monsoon intensity over this interval, suggesting a possible decoupling between local moisture conditions and regional monsoon intensity in the late Holocene. Confirmation of such a decoupling would have important ecological and societal implications.