Paleoclimate at the last glacial maximum revealed by a global database of fossil groundwaters

Natural climate warming since the last ice age provides an analogue to current warming likely due to human greenhouse gas and black carbon releases. Assessing the impacts of warming since the last glacial period has been aided by site-specific studies of the chemistry of groundwaters that recharged aquifers >20ka. Here we present the first comprehensive and global compilation of δ18O and δ2H values, corrected radiocarbon ages and noble gas concentrations for more than 100 major aquifers that contain water that recharged during the last glacial period. We estimate the global δ18O value of groundwater recharge during the last glacial maximum to be ~1 ‰ lower than modern. Recharge at individual aquifers during the last glacial period have δ18O values ranging from -6 to +2 relative to modern, with positive excursions limited to coastal aquifers influenced by proximity to the ~1 ‰ higher oceanic δ18O value that were present during Quaternary glaciations. Spatial patterns of δ18O values for last glacial period waters show similar patterns to modern recharge δ18O values over the continental USA, suggesting that the major continental-scale moisture sources of today were also important sources during the last glacial period. This dataset can be used in conjunction with other isotopic archives (e.g., lake sediments, speleothems) to help decouple confounding effects of shifting temperatures and isotope compositions. Further, this dataset provides a valuable constraint for isotope-enabled global climate model reconstructions of the last glacial maximum and can be used to delimit the global extent of fossil groundwater resources which are unsustainably extracted in a number of regions.