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1. Site U1598

   Kutterolf, S; Druitt, TH; Ronge, TA; Beethe, S; Bernard, A; Berthod, C; Chen, H; Chiyonobu, S; Clark, A; DeBari, S; et al (July 2024, Proceedings of the International Ocean Discovery Program Expedition reports)

   Site U1598 (proposed Site 14A) is located ~8 km northwest of Christiana Island and ~20 km southwest of Santorini (Figure F1) at 521.5 meters below sea level (mbsl). The drill site targeted the volcano-sedimentary fill of the Christiana Basin in two holes (U1598A and U1598B), with a maximum recovery depth of 95.7 meters below seafloor (mbsf) (all depths below seafloor are given using the core depth below seafloor, Method A [CSF-A], scale, except in Operations where the drilling depth below seafloor [DSF] scale is used) and core recoveries of 69% and 34%, respectively. The drill site targeted the volcano-sedimentary fill of the Christiana Basin. This basin was believed to have formed by subsidence along an ENE–WSW fault system before the changing tectonic regime activated the current northeast–southwest rift system in which the Christiana-Santorini-Kolumbo (CSK) volcanic field lies (Tsampouraki-Kraounaki and Sakellariou, 2018; Preine et al., 2022a, 2022b). The Christiana Basin is deeper than the Anhydros and Anafi Basins; its volcano-sedimentary fill potentially recorded the earlier volcanic history of the CSK volcanic field (including the products of Christiana and early Santorini), as well as the younger Santorini and possibly the Milos Volcano to the west along the Hellenic volcanic arc. The now-extinct Christiana Volcano produced lavas and tuffs of unknown ages (Aarburg and Frechen, 1999). An ignimbrite found on Christiani Island (one of the two small islands of the Christiana Volcano), Santorini, and the nonvolcanic island of Anaphi, called the Christiani Ignimbrite, was identified (Keller et al., 2010). Site U1598 was chosen to complement the previously drilled Site U1591. The aim was to retarget a thick pumice layer found at Site U1591 (uppermost ~65 mbsf; Figure F2), for which we had insufficient samples for our scientific aims, to increase our recovery of the interval. The site addresses scientific Objectives 1–4 and 6 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). 

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2. Site U1592

   Kutterolf, S; Druitt, TH; Ronge, TA; Beethe, S; Bernard, A; Berthod, C; Chen, H; Chiyonobu, S; Clark, A; DeBari, S; et al (July 2024, Proceedings of the International Ocean Discovery Program Expedition reports)

   Site U1592 (proposed Site CSK-09A) is located ~10 km southeast of Anhydros Island in the Anafi Basin at 693 meters below sea level (mbsl) (Figure F1). The aim at the site was to penetrate the entire volcano-sedimentary fill as far as the Alpine basement to reconstruct the evolution of the Anafi Basin: history of subsidence, presence of volcanic event layers in the basin sediments, and links between volcanism and crustal tectonics. We drilled to a maximum recovery depth of 519.8 meters below seafloor (mbsf) in two holes (U1592A and U1592B), terminating in limestone basement (all depths below seafloor [mbsf] are given using the core depth below seafloor, Method A [CSF-A], scale, except in Operations where the drilling depth below seafloor [DSF] scale is used). Average core recoveries were 71% (Hole U1592A) and 50% (Hole U1592B). The Anafi Basin potentially recorded the full volcanic history of Santorini (and any older centers) since rift inception, but it was envisaged to probably also contain few eruptive products from Kolumbo. Drilling enabled reconstruction of the volcanic, sedimentary, and tectonic histories of the Anafi Basin, allowing us to compare its evolution with that of the Anhydros Basin. The site was also chosen to develop a core-log-seismic integration stratigraphy and compare it with the recently published seismic stratigraphy for the basin (Preine et al., 2022a, 2022b) and the paleotectonic reconstruction of the region (Nomikou et al., 2016, 2018). The site transects six seismic packages of the Anafi rift basin, as well as the onlap surfaces between them (Nomikou et al., 2016, 2018; Preine et al., 2022a) (Figure F2). The Anafi Basin is crossed by many seismic profiles obtained in campaigns between 2006 and 2019, many of them multichannel (Hübscher et al., 2015; Nomikou et al., 2016, 2018). It is included within the area of the 2015 PROTEUS seismic tomography experiment, during which subbottom profiling, gravity, and magnetic data were also recorded (Hooft et al., 2017). The basin bathymetry had been studied in several marine campaigns, and fault distributions and throws had been mapped (Nomikou et al., 2016; Hooft et al., 2017). Previously published analyses of the seismic data suggested the following possible interpretations (from the bottom up; Preine et al., 2022a, 2022b): Units U1 and U2: sediment packages predating Santorini and Kolumbo volcanism; Unit U3: sediments and the products of the early Kolumbo volcanism and some of the Kolumbo cones; Unit U4: sediments associated with a major rift pulse; and Units U5 and U6: sediments and the products of Santorini activity, some of the Kolumbo cones, and the later eruptions of Kolumbo including the 1650 Common Era (CE) eruption. Units U3–U6 were believed to be of Pleistocene age, and Units U1 and U2 were believed to be possibly Pliocene. The site enabled us to test these interpretations by using the cores to reconstruct a near-complete volcanic stratigraphy consistent with both onshore and offshore constraints and pinned by chronological markers from biostratigraphy, magnetostratigraphy, and sapropel records. Benthic foraminifera from fine-grained sediments provided estimates of paleowater depths and, through integration with seismic profiles and chronologic data, of time-integrated basin subsidence rates. Coring at Site U1592 in the Anafi Basin addressed scientific Objectives 1–4 and 6 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). It was complemented by Site U1589 in the Anhydros Basin because each basin taps a different sediment distributary branch of the Christiana-Santorini-Kolumbo volcanic system. 

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3. Site U1589

   Druitt, TH; Kutterolf, S; Ronge, TA; Beethe, S; Bernard, A; Berthod, C; Chen, H; Chiyonobu, S; Clark, A; DeBari, S; et al (July 2024, Proceedings of the International Ocean Discovery Program Expedition reports)

   The principal aim at Site U1589 (proposed Site CSK-01A) was to reconstruct the evolution of the Anhydros Basin, including its history of subsidence, as well as to document the presence of volcanic event layers in the basin sediments and draw conclusions regarding the links between volcanism and crustal tectonics. The site is located about 10 km southwest of Amorgos Island at 484 meters below sea level (mbsl) (Figure F1). The drill site targeted the volcano-sedimentary fill of the Anhydros Basin. We received permission from the International Ocean Discovery Program (IODP) Environmental Protection and Safety Panel to touch the Alpine basement using an advanced piston corer/extended core barrel/rotary core barrel (APC/XCB/RCB) drilling strategy. The site involved three holes (U1589A–U1589C) and terminated in basement limestone at 612.4 meters below seafloor (mbsf) (all depths below seafloor are given using the core depth below seafloor, Method A [CSF-A] scale, except in Operations, where the drilling depth below seafloor [DSF] scale is used). Core recovery was good in Holes U1589A (78%) and U1589B (87%) and poor in Hole U1589C (24%). Site U1589 was chosen to sample the eruptive histories of both Santorini and the Kolumbo chain and was expected to yield volcaniclastics from many Kolumbo eruptions and the major Santorini eruptions. Many deposits from smaller Santorini eruptions were not expected at this distance from the volcano, in part due to flow blocking by the Kolumbo volcanic chain. Santorini has been active since 0.65 Ma, with many large explosive eruptions since about 0.36 Ma (Druitt et al., 2016). Kolumbo Volcano was known from seismic profiles to have had at least five eruptions (Hübscher et al., 2015), the last of which was in 1650 Common Era (CE) and killed 70 people on Santorini (Fuller et al., 2018). Seismic profiles provided constraints on the relative ages of the Kolumbo cones (Preine et al., 2022c) but not on the absolute ages. The site offered a near-continuous time series of volcanism in the area since rift inception. The site was also chosen to develop a core-log-seismic integration stratigraphy and compare it with the recently published seismic stratigraphy for the basin (Preine et al., 2022a) and the paleotectonic reconstruction of the region (Nomikou et al., 2016, 2018) to promote a holistic view of the Anhydros Basin evolution (Figure F2). The site transects all six seismic packages of the Anhydros rift basin, as well as the onlap surfaces between them (Nomikou et al., 2016, 2018; Preine et al., 2022a). The anticipated lithologies were undisturbed hemipelagic muds, volcaniclastics, turbidites, and finally continental basement rocks. A gravity core recovered 7 km to the east indicated that the uppermost sediments on site would consist of hemipelagic muds and volcaniclastic layers, as well as sapropels (Kutterolf et al., 2021). The Anhydros Basin is crossed by many seismic profiles obtained in campaigns between 2006 and 2019, many of them multichannel (Hübscher et al., 2015; Nomikou et al., 2016, 2018), and its southwestern part is included within the area of the 2015 PROTEUS seismic tomography experiment, during which subbottom profiling, gravity, and magnetic data were also recorded (Hooft et al., 2017). The basin bathymetry had been studied in several marine campaigns, and fault distributions and throws had been mapped (Nomikou et al., 2016; Hooft et al., 2017). Previously published analyses of the seismic data suggested the following possible interpretations (from the bottom up; Preine et al., 2022b, 2022c): Units U1 and U2: sediment packages predating Santorini and Kolumbo volcanism; Unit U3: sediments and the products of the early Kolumbo volcanism and some of the Kolumbo cones; Unit U4: sediments associated with a major rift pulse; and Units U5 and U6: sediments and the products of Santorini activity, some of the Kolumbo cones, and the later eruptions of Kolumbo including the 1650 CE eruption. Units U3–U6 were believed to be of Pleistocene age, and Units U1 and U2 were believed to be of possible Pliocene age. The site enabled us to test these interpretations by using the cores to reconstruct a near-complete volcanic stratigraphy consistent with both onshore and offshore constraints and pinned by chronological markers from biostratigraphy, magnetostratigraphy, and sapropel records. Benthic foraminifera from fine-grained sediments provided estimates of paleowater depths and, via integration with seismic profiles and chronologic data, of time-integrated basin subsidence rates. Coring at Site U1589 in the Anhydros Basin addressed scientific Objectives 1–4 and 6 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). It was complemented by Site U1592 in the Anafi Basin because each basin taps a different sediment distributary branch of the Christiana-Santorini-Kolumbo volcanic system. 

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4. Site U1599

   Druitt, TH; Kutterolf, S; Ronge, TA; Beethe, S; Bernard, A; Berthod, C; Chen, H; Chiyonobu, S; Clark, A; DeBari, S; et al (July 2024, Proceedings of the International Ocean Discovery Program Expedition reports)

   Site U1599 is located ~6 km north of Anafi Island within the upper reaches of the Anafi Basin at a water depth of 592 meters below sea level (mbsl) (Figure F1). Permission to drill in this location was granted by the International Ocean Discovery Program (IODP) Environmental Protection and Safety Panel during the expedition (proposed Site CSK-22A). Three holes (U1599A–U1599C) were drilled to a maximum recovery depth of 698.1 meters below seafloor (mbsf) (all depths below seafloor are given using the core depth below seafloor, Method A [CSF-A] scale, except in Operations, where the drilling depth below seafloor [DSF] scale is used), with average hole core recoveries ranging 51%–83% (Figure F2). Whereas Site U1592 is situated on the axis of the Anafi Basin and penetrated a thick basin fill including mass-transported material, Site U1599 is located on the southeast margin of the basin. It was chosen to offer a condensed sequence of tephra without quantities of mass-wasting debris. As such, it provided a complete stratigraphy of volcanic tephra from Santorini and Kolumbo. The same six seismic units (U1–U6, from the bottom up; Preine et al., 2022a, 2022b) present at Site U1592 are present at Site U1599. Site U1599 is located within the area of the 2015 PROTEUS seismic tomography experiment, during which subbottom profiling, gravity, and magnetic data were also recorded (Hooft et al., 2017). Drilling at Site U1599 enabled us to reconstruct a near-complete volcanic stratigraphy consistent with both onshore and offshore constraints and pinned by chronological markers from biostratigraphy, magnetostratigraphy, and sapropel records. Benthic foraminifera from fine-grained sediments provided estimates of paleowater depths and, via integration with seismic profiles and chronologic data, of time-integrated basin subsidence rates. Drilling in the Anafi Basin addressed scientific Objectives 1–4 and 6 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). It was complemented by Site U1589 in the Anhydros Basin because each basin tapped a different sediment distributary branch of the Christiana-Santorini-Kolumbo volcanic system. 

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5. Expedition 398 Scientific Prospectus: Hellenic Arc Volcanic Field

   https://doi.org/10.14379/iodp.sp.398.2022  

    Druitt, T.; Kutterolf, S.; Höfig, T.W. (March 2022, Scientific prospectus)

   The understanding of island arc volcanism and associated hazards requires study of the processes that drive such volcanism and how the volcanoes interact with their marine surroundings. What are the links and feedbacks between crustal tectonics, volcanic activity, and magma genesis? What are the dynamics and impacts of submarine explosive volcanism and caldera-forming eruptions? How do calderas collapse during explosive eruptions and then recover to enter new magmatic cycles? What are the reactions of marine ecosystems to volcanic eruptions? The Christiana-Santorini-Kolumbo (CSK) volcanic field on the Hellenic volcanic arc is a unique system for addressing these questions. It consists of three large volcanic centers (Christiana, Santorini, and Kolumbo), and a line of small submarine cones, founded on thinned continental crust in a 100 km long rift zone that cuts across the island arc. The marine rift basins around the CSK field, as well as the Santorini caldera, contain volcano-sedimentary fills up to several hundreds of meters thick, providing rich archives of CSK volcanic products, tectonic evolution, magma genesis and paleoenvironments accessible only by deep drilling backed up by seismic interpretations. We will drill four primary sites in the rift's basins and two additional primary sites inside the Santorini caldera. The expedition science has five main objectives, each with a leading testable hypothesis, and two secondary objectives. Deep ocean drilling will enable us to identify, characterize, and interpret depositional packages visible on seismic images, chemically correlate primary volcaniclastic layers in the rift fills with their source volcanoes, fill in the many gaps in the onshore volcanic records, provide a tight chronostratigraphic framework for rift tectonic and sedimentary histories, and sample deep subsurface microbial life. 

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