Holocene marine tephra offshore Ecuador and Southern Colombia: First trench‐to‐arc correlations and implication for magnitude of major eruptions

Tephra layers preserved in marine sediments are strong tools to study the frequency, magnitude and source of past major explosive eruptions. Thirty-seven volcanoes from the Ecuadorian and Colombian arc, in the northern Andes, experienced at least one eruption during the Holocene. The volcanic hazard is therefore particularly high for the populated areas of the Andes and in particular cases for the coastal region, and it is crucial to document such events to improve hazard assessment. The age and distribution of deposits from major Holocene eruptions have been studied in the Cordillera, but no descriptions of distal fallouts have been published. In this study, we focused on 28 Holocene tephra layers recorded in marine sediment cores collected along the northern Ecuador-Southern Colombia margin. New lithological, geochemical and isotope data together with C-14 datings on foraminifers allow us to determine the age and volcanic source of marine tephra, and to propose a first land-sea correlation of distal tephra fallouts. We show that at least seven explosive eruptions from Guagua Pichincha, Atacazo-Ninahuilca, Cotopaxi, and Cerro Machin volcanoes left tephra deposits recorded in marine cores over 250 km away from their source. Volume estimates of emitted tephra range between 1.3 and 6.0 km(3) for the tenth century Guagua Pichincha, similar to 5 ka Atacazo-Ninahuilca, similar to 6.7 and similar to 7.9 ka Cotopaxi events, suggesting that they were eruptions of Volcanic Explosivity Index of 5. The distribution of these deposits also brings new constraints for a better evaluation of the volcanic hazard in Ecuador.