Description: International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layers and enclosing sediments and to better constrain the thickness of the mafic ash layers. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory”; this was because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear part of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf. The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I–VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli tuff, and tuff breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100–260 m/My for the upper 1320 m (Units I–V). The proportion (40%) and grain size of tephra are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli tuff of fine grain size (clasts 〈 3 cm). These were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320–1806.5 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward. Alteration resulted in destruction of fresh glass from ~750 mbsf downward, but minerals are less altered. Because of the alteration, the deepest biostratigraphic datum was at ~850 mbsf and the deepest paleomagnetic datum was at ~1300 mbsf. Additional age control deeper than this depth is provided by an age range of 10.97–11.85 Ma inferred from a nannofossil assemblage at ~1403 mbsf and a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/–1.7 Ma, measured postcruise on a rhyolite peperite in Unit VI at ~1390 mbsf. Based on the seismic profiles, the Miocene–Oligocene hiatus (~17–23 Ma) was predicted to lie at ~1250 mbsf, but strata at that depth (Unit V, 1120–1312 mbsf) are much younger (~9 Ma), indicating that we recovered a thicker Neogene section of volcaniclastics and associated igneous rocks than anticipated. Our preliminary interpretation of shipboard geochemistry is that arc-front versus rear-arc sources can be distinguished in the upper, relatively distal 1320 m of section (Units I–V), whereas the lower, proximal 25% of the section (Units VI–VII) may be geochemically heterogeneous, suggesting that the rear-arc magmas only fully compositionally diverged after ~13 Ma.

Description: International Ocean Discovery Program (IODP) Expedition 385 drilled organic-rich sediments with sill intrusions on the flanking regions and in the northern axial graben in Guaymas Basin, a young marginal rift basin in the Gulf of California. Guaymas Basin is characterized by a widely distributed, intense heat flow and widespread off-axis magmatism expressed by a dense network of sill intrusions across the flanking regions, which is in contrast to classical mid-ocean ridge spreading centers. The numerous off-axis sills provide multiple transient heat sources that mobilize buried sedimentary carbon, in part as methane and other hydrocarbons, and drive hydrothermal circulation. The resulting thermal and geochemical gradients shape abundance, composition, and activity of the deep subsurface biosphere of the basin. Drill sites extend over the flanking regions of Guaymas Basin, covering a distance of ~81 km from the from the northwest to the southeast. Adjacent Sites U1545 and U1546 recovered the oldest and thickest sediment successions (to ~540 meters below seafloor [mbsf]; equivalent to the core depth below seafloor, Method A [CSF-A] scale), one with a thin sill (a few meters in thickness) near the drilled bottom (Site U1545), and one with a massive, deeply buried sill (~356–430 mbsf) that chemically and physically affects the surrounding sediments (Site U1546). Sites U1547 and U1548, located in the central part of the northern Guaymas Basin segment, were drilled to investigate a 600 m wide circular mound (bathymetric high) and its periphery. The dome-like structure is outlined by a ring of active vent sites called Ringvent. It is underlain by a remarkably thick sill at shallow depth (Site U1547). Hydrothermal gradients steepen at the Ringvent periphery (Holes U1548A–U1548C), which in turn shifts the zones of authigenic carbonate precipitation and of highest microbial cell abundance toward shallower depths. The Ringvent sill was drilled several times and yielded remarkably diverse igneous rock textures, sediment–sill interfaces, and hydrothermal alteration, reflected by various secondary minerals in veins and vesicles. Thus, the Ringvent sill became the target of an integrated sampling and interdisciplinary research effort that included geological, geochemical, and microbiological specialties. The thermal, lithologic, geochemical, and microbiological contrasts between the two deep northwestern sites (U1545 and U1546) and the Ringvent sites (U1547 and U1548) form the scientific centerpiece of the expedition. These observations are supplemented by results from sites that represent attenuated cold seepage conditions in the central basin (Site U1549), complex and disturbed sediments overlying sills in the northern axial trough (Site U1550), terrigenous sedimentation events on the southeastern flanking regions (Site U1551), and hydrate occurrence in shallow sediments proximal to the Sonora margin (Site U1552). The scientific outcomes of Expedition 385 will (1) revise long-held assumptions about the role of sill emplacement in subsurface carbon mobilization versus carbon retention, (2) comprehensively examine the subsurface biosphere of Guaymas Basin and its responses and adaptations to hydrothermal conditions, (3) redefine hydrothermal controls of authigenic mineral formation in sediments, and (4) yield new insights into many geochemical and geophysical aspects of both architecture and sill–sediment interaction in a nascent spreading center. The generally high quality and high degree of completeness of the shipboard datasets present opportunities for interdisciplinary and multidisciplinary collaborations during shore-based studies. In comparison to Deep Sea Drilling Project Leg 64 to Guaymas Basin in 1979, sophisticated drilling strategies (for example, the advanced piston corer [APC] and half-length APC systems) and numerous analytical innovations have greatly improved sample recovery and scientific yield, particularly in the areas of organic geochemistry and microbiology. For example, microbial genomics did not exist 40 y ago. However, these technical refinements do not change the fact that Expedition 385 will in many respects build on the foundations laid by Leg 64 for understanding Guaymas Basin, regardless of whether adjustments are required in the near future.

Description: Introduction This chapter of the International Ocean Discovery Program (IODP) Expedition 350 Proceedings volume documents the procedures and tools employed in the various shipboard laboratories of the R/V JOIDES Resolution during Expedition 350. This information applies only to shipboard work described in the Expedition Reports section of this volume. Methods for shore-based analyses of Expedition 350 samples and data will be described in the individual scientific contributions to be published in the open literature or in the Expedition Research Results section of this volume. This section describes procedures and equipment used for drilling, coring, and hole completion; core handling; computation of depth for samples and measurements; and sequence of shipboard analyses. Subsequent sections describe specific laboratory procedures and instruments in more details.

Description: International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, and 1.5 km west of Ocean Drilling Program (ODP) Site 792, at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including a distinctive black glassy mafic ash layer that may record a large-volume subaqueous eruption on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layer and its enclosing sediments and to better constrain its thickness. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory” because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc-front to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear-arc region of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf. The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I–VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli-tuff, and tuff-breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100–260 m/My for the upper 1320 m (Units I–V). The proportion (40%) and grain size of volcaniclastics are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli-tuff of fine grain size (clasts 〈3 cm). These volcaniclastics were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320–1806.5 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward. Alteration resulted in destruction of fresh glass from ~750 mbsf downward, but minerals are less altered. Because of the alteration, the deepest biostratigraphic datum was at ~850 mbsf and the deepest paleomagnetic datum was at ~1300 mbsf. Additional age control deeper than ~1300 mbsf is provided by an age range of 10.97–11.85 Ma inferred from a nannofossil assemblage at ~1403 mbsf and a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/−1.7 Ma, measured postcruise on a rhyolite peperite in Unit VI at ~1390 mbsf. Based on the seismic profiles, the Miocene–Oligocene hiatus (~17–23 Ma) was predicted to lie at ~1250 mbsf, but strata at that depth (Unit V, 1120–1312 mbsf) are much younger (~9 Ma), indicating that we recovered a thicker Neogene section of volcaniclastics and associated igneous rocks than anticipated. Our preliminary interpretation of shipboard geochemistry of solids is that arc-front versus rear-arc sources can be distinguished for individual intervals in the upper, relatively distal 1320 m of the section (Units I–V), whereas data for the lower, proximal 25% of the section (Units VI–VII) overlap and exceed the compositional fields for Neogene rear-arc seamounts and Quaternary arc-front volcanoes. This suggests that the compositional divergence between arc-front and rear-arc magmas only fully developed after ~13 Ma.