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Glass shard K-Ar dating of the Chalupas caldera major eruption: Main Pleistocene stratigraphic marker of the Ecuadorian volcanic arc

Abstract:

New K-Ar ages obtained on juvenile pumice glass shards indicate that the Chalupas ignimbrite, one of the main Pleistocene tephra markers of the Ecuadorian arc, was emplaced at 216 ± 5 ka. Morphology and major and trace element contents of the glass shards are similar to those of ash layers from deep-sea cores and allow correlation between continental deposits and marine tephra layers. Based on biostratigraphy and δ18O data, the age models of these cores support our K-Ar age. Fine ashes from the Chalupas eruption column have been found about 1000 km away from the source caldera, and the estimated deposit volume of both ignimbrite and co-ignimbrite deposits ranges from 200 to 265 km3. This suggests that the Chalupas event could have been strong enough to reach the stratosphere and inject large amount of SO2 in both hemispheres, possibly impacting global temperatures. In addition, the age of the Chalupas ignimbrite obtained here could provide a new radiometric constraint for the age of isotope stage 7 recorded in orbitally-tuned δ18O deep-sea cores. This study highlights the relevance of K-Ar dating applied to small glass shards from massive ignimbrite deposits, and the potential that it represents to improve risk assessments in volcanic zones where dating crystals is not possible. Finally, detailed tephrochronology of deep-sea cores and correlation between marine ash-layers and continental volcanic deposits constitute a strong tool to investigate the eruptive history of an active volcanic arc whose proximal products have been eroded or deeply buried by younger deposit sequences.