Fire and Ice: Volcanic and Glacial Interactions CODEX project

The FKt240902 research cruise took place on the R/V Falkor (too) from 2nd to 23rd September 2024, from Puerto Montt to Talcahuano, Chile. The project aimed to generate a comprehensive understanding of the marine environmental processes that follow a large- magnitude explosive eruption of a coastal volcano, combining sedimentological, volcanological, geomorphological and biological approaches. The sedimentary archive adjacent to active volcanoes has the potential to hold a rich eruption record, which can be used to analyse the tempo of volcanism and how this reflects environmental and climatic factors. Unravelling such archives requires a holistic understanding of sedimentary processes following large eruptions, spanning terrestrial to marine settings. Building towards this long- term goal, we used the Chaitén 2008 eruption to exemplify the complex chain of processes initiated by an explosive eruption on a continental margin, producing the most complete reconstruction available of how such events impact, and are recorded within, the adjacent marine environment. To address this problem, the project investigated sediment pathways and sources through the Chiloe sea. Although the particular focus was on the submarine basins and channels adjacent to Chaitén, particularly those associated with the Rayas and Blanco river deltas, the cruise also targeted the Reloncaví Fjord, as a region affected by the Calbuco 2015 eruption, and more distal regions south of Chaitén, through to the continental margin at the Simpson canyon. Most of the area had no available high-resolution bathymetry or sub-bottom data, and an important part of the project was exploratory mapping and oceanographic observations, to characterise seafloor morphologies, patterns of sedimentation and erosion, and ocean currents throughout the survey area. To identify sediment routes following recent volcanic eruptions, especially Chaitén 2008, to assess the potential of the region for longer term volcanic eruption reconstructions, and to investigate the biological and environmental impacts and recovery from volcanic eruptions, a major target of the cruise was the recovery of seafloor sediment cores. A vibracorer mounted on the ROV SuBastian was used for the first time during this expedition, and successfully recovered 44 vibracores from 24 sampling stations. These samples were supplemented by push cores, sediment sampled from scoop bags, and from separate van-Veen grab and gravity core sampling deployed from the ship. Hydroacoustic data from throughout the region showed a complex seafloor morphology reflecting the influence of glaciation and postglacial sedimentation. Strong currents throughout much of the area exert a strong control on present-day sediment routes, with evidence from erosional and depositional relationships that these have varied through the postglacial period. Several cores preserved visible volcanic deposits, likely both from airfall and submarine gravity flow processes, further modified by ocean currents. Offshore the Blanco river system, sediment is transported southwards to the mouth of Chaitén bay, with several cores preserving multiple volcaniclastic units. Further north in the bay, evidence of volcaniclastic input is much more limited. Sedimentation patterns offshore the Rayas delta are very different, with strong currents producing a dune system dominated by mixed coarse sediment. Dune bedforms extend along coastal areas and NE along channels into the Ancud Gulf. Evidence of volcanic deposits likely to be from Chaitén was found at least 40 km away, although the age and origin of this remains to be verified by post-cruise geochemical and sedimentological analyses. This, alongside chronological data, will be key to understanding the volcanic event history of the region and the interplay of volcanic processes with environmental change. The cruise involved an international and cross disciplinary team, including expertise in geophysics, marine and glacial geology, volcanology, physical geography, marine geomorphology, micropalaeontology, marine biology and physical oceanography. Over half the science team were from Chile, alongside scientists based in the UK, USA, Italy, New Zealand and Malta, and with ten student participants. The team included two participants from SERNAGEOMIN, the Chilean geological survey, with expertise in regional volcanism. The team also included a schoolteacher from Isla Chuit, in the Chaitén region. This helped facilitate the outreach, education and community engagement programme, which formed a major part of the project. Over 21 ship-to-shores sessions took place during the cruise, 8 of which were to schools in the Chaitén area (each involving multiple schools). On 12th September, over 100 schoolchildren from 16 regional schools visited the ship, supported by the Chilean Navy and followed by visits from local authorities and government representatives. Continued community engagement will be an important element of post-cruise work.