81Kr reveals one-million-year-old groundwater at the Atlantic coast of Argentina as a record of Mid-Pleistocene climate

Abstract

Sedimentary deposits in both Colorado Basin and Salado Basin, two geological basins originated after the Atlantic rifting in the coastal area of Argentina, contain deep confined aquifers bearing thermal waters. Previous studies on the paleogroundwater of this region have been limited by the radiocarbon dating range. In this study 81Kr dating is applied to reveal groundwater ages up to 1.1 Ma, so far the oldest found in South America. Groundwater in the Colorado Basin, previously assigned to the Last Glacial Maximum (LGM), is reassigned to a period from Marine Isotope Stage 1 to 5 (MIS 1 to MIS 5). The age information combined with water stable isotopes and noble gases suggests the possible existence of the South-American-Low-Level-Jet (SALLJ) in this period, that had carried the recycled water vapor from the tropical Amazonia to locations further south compared to the present time. Also, the inferred recharge conditions in both basins during Mid-Pleistocene agree with the very dry climate in this period suggested by previous studies.