Iona McIntosh

Hydrothermal systems in volcanic calderas are critical in signalling volcanic unrest, forming ore deposits, and sustaining chemosynthetic microorganisms. Analysis of a ~3500-year sequence of sediments collected from the Santorini caldera, Greece, during International Ocean Discovery Program (IODP) Expedition 398 reveals the behaviour of a prolonged paleo-hydrothermal system. Sediment geochemical and metagenomic data record vigorous hydrothermal activity and metal fluxes for ~1100 years, within a 2270-year window between two major eruptions. Sediment hydrothermally-derived trace metals are significantly enriched over background (~200-fold for As and Hg, and 10–50-fold for Mn, Sb, Mo, and V), with long-term metal fluxes (9 t yr ⁻¹ As, 2.5 t yr ⁻¹ Cu, 7 kg yr ⁻¹ Ag) comparable to fluxes from present-day geothermal fields in the Taupo Volcanic Zone. Metagenomic analysis identifies elevated metal resistance genes—signals of microbial adaptation to heightened hydrothermal stressors. Here we integrate geological and genomic evidence to decipher the paleoenvironmental and biogeochemical history of the past hydrothermal system at Santorini caldera.

In early October 2023, earthquake swarms occurred near Izu-Torishima in the southern Izu arc. On October 9, tsunamis reaching up to 60 cm were observed at Hachijojima and along the Pacific coast of the Japanese archipelago where the height of the tsunamis were disproportionate to the earthquake magnitudes (Sandanbata et al., 2023). T-wave analysis identified the epicenter near Sofu Seamount, bathymetric high west of Sofugan volcano, and subsequent bathymetric surveys by JAMSTEC and Japan Coast Guard independently revealed significant topographical changes, including a concave depression near the western summit of Sofu Seamount (Fujiwara et al., 2024; Minami & Tani, 2024). This significant bathymetric change suggests an intense deep-sea eruption even though the water depth of the pre-eruption summit was ~800 m.We conducted seafloor geological surveys using ROV and dredges during cruises KM25-02Leg2 (March 2025) and KM25-09 (September 2025) by the R/V Kaimei. Sofu Seamount is an east-west trending bathymetric, which was not previously recognized as an active volcano. Its western edifice features a caldera-like structure approximately 5 km in diameter, with a central cone in the northern section now containing a 1.5 km wide crater formed during the 2023 activity. The crater floor lies at a depth of ~1200 m, with the rim at ~900 m. Recent shallowing within the outer caldera was detected, suggesting significant tephra deposition in the proximal area. Rock samples were collected from the inside of the central cone, inside of the outer caldera, and northern slope of the seamount. Around the central cone crater, dark-colored volcanic rocks were collected, some of which featured brown-colored coatings identified as iron oxyhydroxides. Bacterial mats were observed on both sides of the northeastern crater rim, suggesting an active hydrothermal venting. White pumice (clasts to boulders) was found deposited on the upper most surface of the caldera interior, likely originating from the October 2023 event. Large dark-colored volcanic rocks were observed outside the caldera, notably lacking iron oxide coatings.The collected samples ranged from basaltic andesite to rhyolitic pumice, with SiO2 = 52 – 73 %. Mafic rocks contain olivine (Fo70-75), clinopyroxene, Ca-rich plagioclase, and rare orthopyroxene. Some dredged samples contained xenoliths with less-differentiated olivine (~Fo85) and well-preserved basaltic volcanic glass. Olivine-melt composition suggests P-T conditions of ~1050 C and ~160 MPa. In contrast, pumice samples contain clinopyroxene, orthopyroxene, and Na-rich plagioclase, with the estimated conditions of ~900 C and 135-165 MPa. Thermodynamic modelling indicates the compositional trend cannot be explained by fractional crystallization. As such, the mixing of high-temperature mafic magma and low-temperature felsic magma within a pressure range of 134-165 MPa beneath Sofu Seamount triggered an explosive eruption in October 2023.

We compare and contrast physical properties (density, P-wave velocity) of volcaniclastic sediments with other uncemented marine sediments. We study cores collected by International Ocean Discovery Program (IODP) Expedition 398, which recovered more than 2200 m of volcaniclastic deposits from 12 sites and 28 holes from Santorini Caldera, Greece, and the surrounding rift basins in the South Aegean Volcanic Arc. The grain density (mass of solids divided by their volume, including any isolated vesicles) of volcaniclastic deposits is typically lower than that of volcanic glass and crystals and is sometimes less than 2 g/cm3, indicating the preservation of isolated gas-filled vesicles in erupted and then deposited materials. To complement bulk measurements, we also measured the total and isolated porosity in individual lapilli-sized volcanic clasts from four different volcanic deposits. We use xray computed tomography to image isolated pore space. Collectively, these measurements confirm that volcaniclastic sediments can preserve vesicle textures and isolated porosity for hundreds of thousands of years and at depths >500 m below sea-level and > 100 m below the seafloor Volcaniclastic deposits typically have higher P-wave velocities but lower bulk densities than oozes and other marine sediments. In volcaniclastic deposits, lapilli have higher P-wave velocities and lower bulk density than ash, the opposite trend of most sediment in which higher density is correlated with higher seismic velocity. We use granular physics models to show that the higher volcaniclastic P-wave velocity originates from two effects: 1) lower pore volume outside clasts that increases elastic moduli and P-wave velocity and 2) isolated gas vesicles in volcanic clasts that lower bulk density with proportionally less effect on elastic modulii. In volcaniclastic sediments there is relatively little change in physical properties to depths of several hundred meters below the seafloor, which we attribute to rough grain surfaces and lower intergranular (external) porosities that resist compaction and the decrease of intergranular pore space relative to background marine sediment. These trends lead to distinctive signatures of volcaniclastic sediments in reflection seismic images.

Data report: visible to shortwave infrared (VSWIR) spectroscopic scanning of selected core sections from Holes U1589A and U1591C, IODP Expedition 398, Hellenic Arc Volcanic Field

International Ocean Discovery Program Expedition 398 recovered more than 2,200 m of volcaniclastic deposits from 12 sites and 28 holes from Santorini Caldera, Greece, and the surrounding rift basins in the South Aegean Volcanic Arc. We compare and contrast discrete shipboard measurements of physical properties (density, P‐wave velocity) of these volcaniclastic sediments with other uncemented marine sediments in the cores. The grain density (mass of solids divided by their volume, including any isolated vesicles) of volcaniclastic deposits is typically lower than that of volcanic glass and crystals and is sometimes less than 2 g/, indicating the preservation of isolated gas‐filled vesicles in erupted materials. Volcaniclastic deposits typically have higher P‐wave velocities but lower bulk densities than oozes and other marine sediments. In volcaniclastic deposits, lapilli have higher P‐wave velocities and lower bulk density than ash, the opposite trend of most sediment in which higher density is correlated with higher seismic velocity. We use granular physics models to show that the higher volcaniclastic P‐wave velocity originates from two effects: (a) lower pore volume outside clasts that increases elastic moduli and (b) isolated gas vesicles in volcanic clasts that lower bulk density. In volcaniclastic sediments there is relatively little change in physical properties to depths of several hundred meters below the seafloor, which we attribute to rough grain surfaces and lower intergranular (external) porosities that hinder compaction and the decrease of intergranular pore space. These trends lead to distinctive signatures of volcaniclastic sediments in reflection seismic images.

Large explosive volcanic eruptions from island volcanoes deliver vast quantities of ash to the marine environment. While many of the transport pathways are understood, those from shoreline-crossing or submarine pyroclastic currents, and their transformation into water-supported gravity flows, remain poorly constrained. We report the discovery by International Ocean Discovery Program (IODP) deep drilling of a 200-meter-thick ash megabed buried in rift basins of the South Aegean Volcanic Arc. The >73–cubic kilometer ash deposit originates from the Kos Plateau Tuff caldera-forming eruption, which occurred 161 thousand years ago, >120 kilometers to the east. The ash forms a chemically uniform, graded megabed lacking bioturbation, interpreted as having been emplaced by a stream of eruption-fed turbidity currents. Bioclastic debris within the ash provides evidence of widespread destruction of marine ecosystems. Large volcanic eruptions can remodel the seafloor landscape, deposit thick ash turbidites, and destroy marine biota on island arc–wide scales in short-lived, catastrophic events.

Subseafloor in situ temperatures in a drilled hole in the Anhydros Basin, Aegean Sea, measured during International Ocean Discovery Program (IODP) Expedition 398, yielded a low heat flow (2) despite active magmatism and rifting in the region. The coldest and highest temperatures were 13.9°C at 52.5 m below seafloor (mbsf) and 15.5°C for the deepest measurement at 360.4 mbsf, respectively. Comparison of a heat transfer model with measurements suggests that sea bottom temperatures during the last glacial period were up to 10°C cooler than Holocene temperatures. The magnitude of Holocene warming co‐varies with the geothermal heat flow: if the former goes up the latter goes up. Low heat flow may arise from lateral removal of heat through deeper formations by gravity driven advection of fluids. Tectonic separation of the northwestern Anhydros Basin from the Christiana‐Santorini‐Kolumbo volcanic field may lead to minimal magmatic influences on heat flow.

Data report: core-seismic integration and time-depth relationships at IODP Expedition 398 Hellenic Arc Volcanic Field sites

Crystals and glass discharged as tephra during explosive volcanic eruptions provide a valuable window into magmatic processes. Kolumbo, a shallow submarine volcano on the Hellenic Arc of the southern Aegean Sea, recently erupted in 1650 CE and was both violently explosive and tsunamigenic, resulting in significant loss of life on neighboring Santorini. Developing a conceptual model of the magma storage and eruption processes at Kolumbo is crucial for improving preparedness in this volcanically active and densely populated region. Using in situ tephra deposits cored during the International Ocean Discovery Program Expedition 398, this study aims to reconstruct the temporal evolution of the magma plumbing system at Kolumbo over the two most recent eruptive units at Kolumbo (1650 CE and an earlier eruption). This research aims to (1) infer pre-eruption magmatic processes and eruption initiation mechanisms, (2) determine magma storage conditions, including pressure, temperature, and H₂O content, and (3) assess temporal variations in the magmatic system at Kolumbo. Methods include the traditional petrologic toolbox of major- and trace-element geochemistry, focusing on minerals and glass to delineate crystallization histories, identify zonation patterns, and infer pressures and temperatures of the crystallizing assemblages. Preliminary results suggest significant differences in eruption initiation mechanisms and magmatic histories between the two eruptive units. The 1650 CE tephra exhibits multiple populations of phenocrysts that preserve textures such as reverse zoning and sieving, consistent with mafic recharge into a silicic reservoir as suggested by previous research. Plagioclase-hosted melt inclusions reveal water content of 4.0–5.25 wt.% for 1650 CE at a storage temperature of 807–847ºC, determined using plagioclase-liquid hygrometry and thermometry (Waters and Lange, 2015; Putirka, 2008). Clinopyroxene-liquid barometry (Petrelli, 2020; Jorgenson, 2022) identifies two clinopyroxene populations, yielding pressures of 4 and 6 kbar, respectively (±0.3 kbar) and temperatures of 914–953ºC (±