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The North Atlantic and Arctic Oceans are unquestionably major players in the climatic evolution of the Northern Hemisphere and in the history of the Northern Hemisphere overturning circulation of the Atlantic Ocean. The establishment of the modern North Atlantic Water (NAW) transporting heat, salt, and moisture to the Northern Hemisphere has been indicated as one of the main forcing mechanisms for the onset of Northern Hemisphere glaciation. NAW controls the extent and dynamics of circum-Arctic and circum-North Atlantic ice sheets and sea ice in addition to deep water and brine production. How the ocean system and cryosphere worked during past warmer intervals of high insulation and/or high atmospheric CO2 content is still largely unknown and debated. The required information can only be attained by offshore scientific drilling in high-resolution continuous expanded sedimentary sequences identified on the western continental margin of Svalbard and eastern side of the Fram Strait, along the main pathway and northern penetration of the NAW flowing into the Arctic Ocean. The area around Svalbard is very sensitive to climatic variability and can be considered a sentinel of climate change. Furthermore, the reconstruction of the dynamic history of the marine-based paleo-Svalbard–Barents Sea Ice Sheet is important because it is considered the best available analog to the modern, marine-based West Antarctic Ice Sheet, for which the loss of stability is presently the major uncertainty in projecting future global sea level rise in response to the present global climate warming. 

The North Atlantic and Arctic Oceans are unquestionably major players in the climatic evolution of the Northern Hemisphere and in the history of the meridional overturning circulation of the Atlantic Ocean. The establishment of the modern North Atlantic Water (NAW) transporting heat, salt, and moisture to the Northern Hemisphere has been indicated as one of the main forcing mechanisms for the onset of Northern Hemisphere glaciation. NAW controls the extent and dynamics of circum-Arctic and circum-North Atlantic ice sheets and sea ice in addition to deep water and brine production. How the ocean system and cryosphere worked during past warmer intervals of high insulation and/or high atmospheric CO2 content is still largely unknown and debated. The required information can only be attained by offshore scientific drilling in high-resolution continuous expanded sedimentary sequences identified on the western continental margin of Svalbard (and eastern side of the Fram Strait) along the main pathway and northern penetration of the NAW flowing into the Arctic Ocean. The area around Svalbard is very sensitive to climatic variability and can be considered a sentinel of climate change. Furthermore, the reconstruction of the dynamic history of the marine-based paleo-Svalbard–Barents Sea Ice Sheet is important because it is considered the best available analog to the modern, marine-based West Antarctic Ice Sheet, for which the loss of stability is presently the major uncertainty in projecting future global sea level rise in response to the present global climate warming. 

The objectives of International Ocean Discovery Program 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. During the expedition, 10 primary and alternate sites that were originally proposed were drilled, 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 open the way to unraveling relationships between volcanic activity and crustal rift pulses. Two sites of condensed sequences 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, less than planned due to hole instability issues but deep enough to groundtruth the seismic stratigraphy and 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 scientific objectives once the laboratory work has been done. 

The North Atlantic and Arctic Oceans are unquestionably major players in the climatic evolution of the Northern Hemisphere and in the history of the Northern Hemisphere overturning circulation of the Atlantic Ocean. The establishment of the modern North Atlantic Water (NAW) transporting heat, salt, and moisture to the Northern Hemisphere has been indicated as one of the main forcing mechanisms for the onset of Northern Hemisphere glaciation. NAW controls the extent and dynamics of circum-Arctic and circum-North Atlantic ice sheets and sea ice in addition to deep water and brine production. How the ocean system and cryosphere worked during past warmer intervals of high insulation and/or high atmospheric CO2 content is still largely unknown and debated. The required information can only be attained by offshore scientific drilling in high-resolution continuous expanded sedimentary sequences identified on the western continental margin of Svalbard and eastern side of the Fram Strait, along the main pathway and northern penetration of the NAW flowing into the Arctic Ocean. The area around Svalbard is very sensitive to climatic variability and can be considered a sentinel of climate change. Furthermore, the reconstruction of the dynamic history of the marine-based paleo-Svalbard–Barents Sea Ice Sheet is important because it is considered the best available analog to the modern, marine-based West Antarctic Ice Sheet, for which the loss of stability is presently the major uncertainty in projecting future global sea level rise in response to the present global climate warming.