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1. Site U1590

   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 U1590 (proposed Site CSK-03A) is located 5 km northwest of the submarine Kolumbo crater on its flank in the Anhydros Basin at 397 meters below sea level (mbsl) (Figure F1). It lies on the intersection of Seismic Lines HH06-22 and HH06-34 (Figure F2). Drilling took place in two holes (U1590A and U1590B) to a maximum recovery depth of 627.8 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). Average core recovery in Hole U1590A was moderate (61%), but recovery in Hole U1590B was poor (14%). The seismic profiles across the Kolumbo edifice reveal five units interpreted as Kolumbo-derived volcaniclastics (K1–K5, from the base up; Figure F2) with Unit K5 representing the 1650 Common Era (CE) eruption (Hübscher et al., 2015; Preine et al., 2022). The submarine cones northeast of Kolumbo postdate Unit K2 on seismic profiles, but their products are not expected to be prominent in our drill cores. The aim of drilling on the flanks of Kolumbo was to penetrate the different seismically recognized volcanic eruption units from that volcano (K1, K2, K3, and K5 or their thin, lateral equivalents) as well as many eruption units from Santorini and traces from the submarine cones northeast of Kolumbo. This enabled characterization of the products of the Kolumbo eruptions and construction of a coherent stratigraphy for Santorini and the submarine Kolumbo volcano chain together. The anticipated lithologies were volcaniclastics, muds, and turbidites. Site U1590 lies at the foot of the Kolumbo edifice; it allowed us to drill Seismic Units K1, K2, K3, and K5 and therefore nearly the entire history of Kolumbo Volcano. Intercalated seismic units are believed to contain the products of Santorini eruptions, including potentially those of smaller magnitude than recorded at the more distal basin sites. 

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2. Site U1591

   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)

   Prior to arrival on site, a decision was made to replace the original primary site (proposed Site CSK-13A) by an alternate site (proposed Site CSK-20A); hence, the latter became Site U1591. This was done to pass through a slightly more complete suite of reflectors in the 800–900 ms two-way traveltime (TWT) interval. Site U1591 is located ~8 km northwest of Christiani Island and ~20 km southwest of Santorini (Figure F1) at 514 meters below sea level (mbsl). It was drilled in three holes (U1591A–U1591C) to a maximum recovery depth of 902.7 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). Average core recovery was similar in all three holes (U1591A= 66%; U1591B= 43%; U1591C= 58%). 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). 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 younger Santorini and possibly 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 Christiana Volcano), Santorini, and the nonvolcanic island of Anaphi, called the Christiani Ignimbrite, was identified (Keller et al., 2010). Six seismic units were previously recognized in the Christiana Basin (Preine et al., 2022a, 2022b; Figure F2). Site U1591 was chosen to pass through Seismic Units U1–U6, including volcaniclastics from Santorini and Christiana, and to target the top few meters of the prevolcanic basement below Unit U1. We received permission from the International Ocean Discovery Program (IODP) Environmental Protection and Safety Panel to drill to the Alpine basement at this site in an advanced piston corer/extended core barrel/rotary core barrel (APC/XCB/RCB) drilling strategy involving three holes. The aims of Site U1591 were (1) to better date the volcanic activity of Christiana using biostratigraphic and magnetostratigraphic means and determine whether the CSK volcanic field had Pliocene volcanism similar to the Milos Volcano farther west; (2) to relate the Christiana volcanism to subsidence along the ENE–WSW fault sets and to the activation of the northeast–southwest fault sets; and (3) to seek the submarine equivalent of the Christiani Ignimbrite. By using deeper coring (and seismic profiles) to reconstruct the volcanic, sedimentary, and tectonic histories of the Christiana Volcano, and possibly the Milos Volcano, we aimed to complement the Santorini and Kolumbo volcanic records of Sites U1589 and U1592 and therefore access a near-continuous time series of volcanism of the CSK volcanic field since rift inception. Site U1591 addressed scientific Objectives 1–4 and 6 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). 

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3. 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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4. 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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5. Site U1595

   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 U1595 (proposed Site CSK-08B) is located in the southern basin of the Santorini caldera at 291 meters below sea level (mbsl) (Figure F1). It was drilled in three holes (U1595A–U1595C) to a maximum recovery depth of 127.0 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). Average hole core recoveries range 46%–77%. Site U1594 addresses the same drilling objectives and lies northeast of Site U1595. Two additional sites (U1596 and U1597) lie in the northern caldera basin. Four seismic units (S1–S4) were previously recognized in the Santorini caldera (Johnston et al., 2015; Nomikou et al., 2016) (Figure F2). Seismic Units S1–S3 were thought to consist of muds and sands from cliff mass wasting (Unit S1); compacted (possibly lithified) sandy volcaniclastics from Kameni Volcano (Unit S2); and consolidated coarse blocky intracaldera tuffs, landslide debris, and/or flood gravels (Unit S3). Unit S4 was thought to be intracaldera tuff from the Late Bronze Age eruption. The four caldera sites were planned to sample Units S1–S3; test the published correlations between the two caldera basins; penetrate below Unit S3 into Unit S4; and address scientific Objectives 1, 4, 5, and 7 of the Expedition 398 Scientific Prospectus (Druitt et al., 2022). By drilling both caldera basins and exploiting our dense seismic reflection coverage, we gained access to the 3D architecture of the entire caldera fill. We also targeted the question of why the northern basin is 100 m deeper than the southern one. Finally, we tested whether Unit S3 consisted of flood debris from the caldera flooding event (Nomikou et al., 2016) or whether it was Late Bronze Age intracaldera tuff (Johnston et al., 2015). The intracaldera sites were used for microbiological work of Objective 7. 

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