Marine tephra offshore Ecuador and Southern Colombia: first trench-to-arc correlations and implication for the magnitude of major eruptions

International audienceMajor eruptions in the Andes are mainly characterized by the emission of large volumes of gas and volcanic ash. The plume may reach the stratosphere and be transported by winds. In Ecuador, the prevailing winds are westward, and the volcanic ash, also called tephra, is transported towards the Pacific Ocean. Vallejo (2011) studied tephra layers present in marine terraces and proposed a first draft of the identification of their source in the Cordillera. However tephras deposited onshore may have been remobilized, eroded or covered by younger deposits. There are therefore several interests in studying tephras recorded in marine sediments : it allows to (1) know the age and source of major eruptions whose products reached the coast, (2) know the distribution of fallout from each major eruption, (3) estimate the volume of largest events, (4) participate to better assess the current volcanic hazard, and (5) provide temporal constraints on continental or marine sediments that cannot be dated by radiocarbon. To study the tephra layers recorded in marine sediments I have described the glass morphology and the mineralogy of tephra beds. To determine the source of the eruptions, I have used data published onshore (e.g., Hidalgo et al., 2008; Hall et Mothes, 2008; Robin et al., 2010). As each volcano has its own geochemical signature when we combine major, trace and isotope data, I compared them with geochemical analyses performed on distal tephra recorded in marine sediments. To determine the age of distal tephra beds, we have performed 14C ages on foraminifera present above and below the bed for tephra emitted during the past 50 ka, and d18O and biostratigraphy for the older deposits. In this presentation, I first presented our results obtained on cores collected during Amadeus (2005) and Atacames (2012) oceanic campaigns along the Ecuadorian margin, and recently published (Bablon et al., 2022). We have sampled 28 tephra layers, from coring sites that cover 5 degrees of latitude, from the southern half of Ecuador to southern Colombia. We observed four main lithofacies: isolated lenses that typical of bioturbation, layers with sharp and sometimes diffuse contacts in the upper part that correspond to primary deposits of tephra fallout, and successions of thin laminated layers that correspond to tephra layers reworked by turbidity currents, and that have not been sampled. Volcanic glass shards have various morphologies depending on the density of vesicles and their deformation, such as block-shaped glass without bubbles, pumice-shaped glass, or glass with completely elongated bubbles. The glass morphology of each tephra allows us to propose a first correlation of layers between each marine core. Concerning the geochemistry, glasses are mostly rhyolitic and belong to the low potassic series typical of the volcanic front, and to high potassic series typical of the eastern cordillera. This distinction between the eastern and western cordillera is also found in their trace element signature. To identify the source volcano, we used Sr and Pb isotopes. On The compositional fields of the volcanoes products overlap little and thus allows us to refine the correlations. We show that distal marine tephra come from the Cerro Machin in Colombia, and from Pichincha, Atacazo, and Cotopaxi volcanoes in Ecuador. Together with the determination of the sources, the radiocarbon dating of sediments allowed us to show that the oldest tephras belonged to the about 8 ka eruption of Cotopaxi, and the youngest correspond to the 10th century eruption of Pichincha. Using the spatial distribution of tephra, we made isopach maps of the fourth major eruptions of Pichincha, Atacazo and Cotopaxi, and we estimated their volumes. They vary between 1.3 and 6 km3, which corresponds to volcanic explosivity indexes of 5, thus eruptions which would be particularly destructive today. A perspective for our work is to study of turbidite beds present in the Holocene cores along the coast. As such deposits are emplaced during major earthquakes, and we can therefore use their correlation to identify past seiscally active areas.Unfortunately, some major Holocene eruptions described in the Cordillera and constitute stratigraphic markers have not been recorded in marine sediments. Tephra layers may have been destructed during drilling such as the very young 700 BP eruption of Quilotoa, they may have been dispersed by ocean currents, or tephra were not present in the cores du to a restricted distribution of deposits, for example for the 3000 BP eruption of Cuicocha (Vallejo, 2011).In the second part of my presentation, I focused on another case study, performed at ODP site 1239, above the Carnegie Ridge. This core is much deeper as it reaches 500 m, and sediments deposited about 10 Myr ago. It contains 24 tephra layers, and we focused on the thickest, 18 cm-thick at 7 m deep (Schipboard Scientific Party, 2003; Bablon et al., 2020). The main volcanic structure that could be the source of such a thick deposit is the Chalupas caldera, located in the Eastern Cordillera, near Cotopaxi volcano. About 50 km southwest of the caldera, we sampled the ignimbrite and dated the glass shards at 216 +/- 5 ka using the K-Ar dating method applied on glass shards. In order to verify if the ignimbrite and marine tephra of ODP Site 1239 belong to the same eruption, we have compared their geochemistry, and we have shown that their major and trace element contents are very close. To check the reliability of this land-sea correlation, we have also compared their ages. Variations of d18O are related to climate changes linked to the Earth's orbital forcing. We then used d18O data available to know the age of sediments as a function of depth. The stratigraphic position of the tephra layer corresponds to the 7d isotopic stage that occured at 220 ka, in agreement with our K-Ar age obtained onland. We then have shown that the 216 +/- 5 ka eruption of Chalupas is the largest of the Quaternary in northern Andes, with products that reached more than 1000 km from their source. Our land-sea correlation also allow to provide an independant temporal constraint to the regional d18O records