Carbonate U/Th ages confirm the Mono Basin excursion as the Laschamps geomagnetic excursion

A long-standing problem in paleomagnetism has been the obtainment of accurate and precise age control on geomagnetic excursions, because they are critical reference markers for establishing the geologic time scale. More than 50 years of research has focused on documenting and dating an interval spanning a short-lived geomagnetic excursion in the Wilson Creek Formation of the Mono Basin, in the western United States. This excursion is bisected by a tephra named Ash 15, making the tephra’s depositional age crucial for constraining the timing of the associated excursion. Early radiocarbon dating of lacustrine carbonates placed Ash 15 at ca. 32−30 ka, supporting its correlation with a geomagnetic excursion of similar age—variously termed the Mono Lake or Auckland excursion—later observed in terrestrial and marine sedimentary records and dated to ca. 34.5 ka. However, subsequent studies demonstrated that lacustrine carbonate 14C ages of the Wilson Creek Formation were systematically biased due to post-depositional carbon contamination. Despite multiple geochronologic efforts corroborating this finding, uncertainty persists regarding the depositional age of Ash 15, highlighting the challenge of revising the age of a widely recognized geological marker. We present carbonate U/Th age data that show Ash 15 must be older than 39.1 ± 0.6 ka (2σ). This finding rejects the correlation of the Ash 15 excursion to the ca. 34.5 ka Mono Lake/Auckland excursion event and instead supports a growing body of evidence that favors an alternative interpretation: The geomagnetic event recorded at Ash 15 is a local expression of the ca. 41 ka Laschamps excursion.