AGE OF THE LOS CHOCOYOS TUFF AND THE LARGEST VOLCANIC ERUPTION IN CENTRAL AMERICA

The eruption that produced the Atitlán caldera in Guatemala has a Volcanic Explosivity Index (VEI) > 8, making it the largest of the Quaternary in the Central American Volcanic Arc. It produced ~1220 km³ of dispersed ash and more than 320 km³ dense rock equivalent of rhyolitic deposits known as the Los Chocoyos tuff (LCT). Despite its scale, the age of the LCT has been difficult to constrain. Its position in marine sediment off the coast of Ecuador suggests an astronomical age of 84 ± 5 thousand years (ka). However, recent U-Th/He and U-Th disequilibrium dates from zircon, though highly dispersed, yield a pooled age of 75 ± 4 ka. The U-Th/He age overlaps the ⁴⁰Ar/³⁹Ar age of 73.9 ± 0.6 ka from sanidine crystals in the Youngest Toba Tuff. It has been hypothesized that this pair of sulfate-rich super-eruptions may be responsible for a global cooling event at ~74 ka. We present new ⁴⁰Ar/³⁹Ar and U-Th disequilibrium data from plagioclase and zircon, respectively, to shed light on the timing of the LCT eruption and in turn to consider the climatic and stratigraphic implications of this super-eruption. Applying Bayesian estimation to the highly dispersed sets of ⁴⁰Ar/³⁹Ar plagioclase and U-Th disequilibrium zircon dates we estimate find an LCT age of 98 ± 6 ka. The latter is statistically older than prior astronomical and radioisotopic ages but are indistinguishable from the 92.4 ± 4.2 ka age of sulphate-rich rhyolitic glass shards in the North Greenland Ice Coring Project (NGRIP) core for which a source has remained elusive. These findings preclude a link between LCT and global cooling ~74 ka and rule out the super-eruption doublet trigger.