International Ocean Discovery Program
Expedition 398 Preliminary Report
Hellenic Arc Volcanic Field1
11 December 2022–10 February 2023
Timothy H. Druitt, Steffen Kutterolf, Thomas A. Ronge, and the Expedition 398 Scientists
1 Druitt, T.H., Kutterolf, S., Ronge, T.A., and the Expedition 398 Scientists, 2024. Expedition 398 Preliminary Report: Hellenic Arc Volcanic Field. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.398.2024
Abstract
The objectives of International Ocean Discovery Program (IODP) Expedition 398, Hellenic Arc Volcanic Field (11 December 2022 to 10 February 2023), were to study the volcanic record of the central Hellenic island arc; document the links and feedbacks between volcanism/magmatism, crustal tectonics, and sea level; investigate the processes and products of shallow submarine eruptions of silicic magma; and groundtruth the seismic stratigraphy of Santorini caldera. Reconstructing the subsidence history of the southern Aegean Sea and searching for deep life inside and outside of Santorini caldera were additional objectives.
The expedition drilled 10 primary and alternate sites that were originally proposed, in addition to 2 extra sites that were requested during the expedition. Outside of Santorini caldera, drilling penetrated the thick basin fills of the crustal rift system hosting the Christiana-Santorini-Kolumbo volcanic field, identifying numerous pumice and ash layers, some known from on land and others hitherto unknown, pushing back the onset of volcanism in the area into the Early Pleistocene or even Pliocene. Significant events of mass wasting into the basins, accompanied by very high sedimentation rates, were also documented. These basin sites served to groundtruth the seismic stratigraphy of the basins and to open the way to unraveling relationships between volcanic activity and crustal rift pulses. Two sites of condensed sequences on the basin margins served to sample many volcanic layers within the detailed age-depth constraints provided mainly by biostratigraphy, as diagenetic effects complicated the magnetic reversal record significantly. Drilling penetrated the Alpine basement at three basin sites northeast of Santorini, whereas in the Christiana Basin to the southwest it penetrated a thick sequence of Messinian evaporites. Drilling inside Santorini caldera penetrated to ~120 meters below seafloor (mbsf), less than planned due to hole instability issues but deep enough to groundtruth the seismic stratigraphy and to sample the different layers. One intracaldera hole yielded a detailed tephra record of the history of the Kameni Islands, as well as possible evidence for deep bacterial colonies within the caldera. Despite variable recovery in the unstable pumice and ash deposits, the expedition was a significant success that may address almost all the science objectives once the laboratory work has been done.
A dense program of preexpedition and shipboard outreach during the expedition gave rise to 59 live ship-to-shore tours, reaching 6,400 people in 7 countries including many school children. A total of 51 journalists were contacted and 9 stories were written about the expedition, with a readership of almost 200,000 people. While in Santorini caldera, the ship hosted 12 documentarians and journalists, the future products of whom should include a 1.5 h documentary and a four-part TV series about Expedition 398. The expedition social media pages were active. Prior to the expedition, an exhibition, “In Search of Earth’s Secrets,” ran for a week on Santorini and was visited by more than 1,800 school children.
Plain language summary
About 800 million people are threatened by volcanic eruptions around the globe: high plumes of ash, ground-hugging flows of hot ash and rock, earthquakes, and associated tsunamis. The Christiana, Santorini, and Kolumbo volcanic group in the Aegean Sea of Greece is particularly hazardous because the volcanoes have produced many eruptions in the past, and some of them were highly explosive. Santorini is an iconic volcano because of its well-known eruption in the Late Bronze Age, and it is a major tourist destination. Although much has been learned about the eruption history of the Aegean volcanoes on land, most of their volcanic products lie on the seafloor, requiring research to move offshore.
During International Ocean Discovery Program (IODP) Expedition 398 we drilled the submarine sediment sequences, including muds and volcanic products, that fill the marine basins around the volcanoes and inside of Santorini caldera at twelve sites. Outside of the caldera, we penetrated many layers of pumice and ash, representing eruptive products settled at the seafloor. Some of these layers represent hitherto unknown explosive eruptions whose products do not occur on land. The ages of the different layers were determined mainly from the assemblages of microfossils preserved in them. We also identified times in the past when large fault movements and earthquakes appear to have caused remobilization of previously deposited ash layers, causing them to pour into the actively deepening basins. In the Christiana Basin south of Santorini, we unexpectedly penetrated a thick sequences of evaporates, formed 5.3 million years ago during the so called “Messinian Salinity Crisis.” Inside Santorini caldera, we drilled through sediments and volcanic products that have accumulated there since the Late Bronze Age eruption, offering a detailed record of the previously inaccessible prehistoric evolution of Kameni Volcano, inside the caldera. At two sites inside Santorini caldera and two others outside, we collected samples for microbiological and genetic analysis to identify and characterize any microbial life living in the muds below the sediment surface.
Despite variable recovery in the unstable pumice and ash deposits, the expedition was a great success that will enable us to address almost all of the original science objectives once detailed laboratory work has been done. Some particularly important aims will be to link the volcanic activity to ancient crustal tectonic processes, as well as to variations in sea level through time due to changes in Earth’s climate, and see any governing feedbacks between these processes.
A dense program of preexpedition and shipboard outreach during the expedition gave rise to 59 live ship-to-shore tours, reaching 2.5 million (6,400 not including China) people in 7 countries including many school children. Nine news stories were written about the expedition, with a readership of almost 200,000 people. While in Santorini caldera, the ship hosted 12 documentarians and journalists, the future products of whom should include a 1.5 hour documentary and a four-part TV series about Expedition 398. The expedition social media pages were very active. Prior to the expedition, an exhibition, “In Search of Earth’s Secrets,” ran for a week on Santorini and was visited by over 1,800 school children.