Emergence of wet conditions in the Mono Basin of the Western USA coincident with inception of the Last Glaciation

At present, the Basin and Range of the western USA is arid, but geologic studies show evidence of past wetness. The timing of these wetter conditions reveals a close as-sociation with glacial conditions. This asso-ciation has led to the hypothesis of a causal link between glacial climate and regional wetness, but poor age control on the onset of regional wetness thwarts a test of this hy-pothesis. Here we determine the start of the most recent interval of persistent wetness in the Mono Basin, which is a hydrologically closed depression that sits at the west-central edge of the Basin and Range. The most recent emergence of persistent wetness in the Mono Basin is stratigraphically correlated with the depositional age of Ash 19-a rhyolitic ash bed that represents the oldest tephra of the Wilson Creek Formation and one of the earliest-known products of explosive volca-nic activity from the Mono Craters. We con-strain the depositional age of Ash 19 by using the U/Th disequilibrium dating method to date carbonates that are younger and older than Ash 19. Our U/Th dating results show that Ash 19 was deposited before the forma-tion of a cross-cutting carbonate bed dated to 69.2 +/- 0.3 ka but after an underlying carbon-ate tufa dated to 67.4 +/- 3.5 ka, which suggests that the start of wetness in the Mono Basin was contemporary with the inception of the Last Glaciation-the beginning of Marine Isotope Stage 4-at ca. 70 ka. This finding corroborates the hypothesis of a link between glacial climate and regional wetness.

Automated summary

Geologic studies in the western USA have revealed evidence of past wetness in the arid Basin and Range region. The timing of these wetter conditions suggests a link with glacial climate. By determining the start of persistent wetness in the Mono Basin, researchers have found that it aligns with the beginning of the Last Glaciation, supporting the hypothesis of a causal relationship between glacial climate and regional wetness.