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1. Data report: marine tephra compositions in proximal and distal drill cores, IODP Expeditions 375, 372, and 329, ODP Leg 181, and DSDP Leg 90, offshore New Zealand, Southwest Pacific

   Pank, K. ; Kutterolf, S. ; Hopkins, J.L. ; Wang, K.L. ; Lee, H.Y. ( April 2023 , Proceedings of the International Ocean Discovery Program Expedition reports)

   We report on a total of 1005 samples analyzed for major and trace element compositions from marine sediments drilled along the Hikurangi subduction zone and within the incoming Pacific plate. The samples are from International Ocean Discovery Program Expeditions 375 and 372; Integrated Ocean Drilling Program Expedition 329; Ocean Drilling Program Leg 181; and Deep Sea Drilling Project Leg 90. All 1005 samples, resulting in a total number of ~20,200 individual measurements, were analyzed for major element compositions with the electron microprobe. A subset of 419 samples, resulting in a total number of ~1820 individual glass shard analyses, were analyzed for trace element compositions using the laser ablation-inductively coupled plasma-mass spectrometer. In total, ~640 samples were identified as primary ash layers based on their homogeneous geochemistry, visual appearance in the core pictures, and high amount of volcanic glass. Based on the biostratigraphy presented in the cruise reports and subsequent work, we can distinguish between Quaternary- and Neogene-derived tephras. The tephra layers of Quaternary age are mostly of rhyolitic composition with occasional andesitic, dacitic, and trachytic glass shards. The Neogene tephras are mostly of basaltic andesite, andesitic, and rhyolitic composition, with a few basaltic and trachytic tephras. Tephras of both age groups follow the calc-alkaline series trend with a tendency to shift into the high-K calc-alkaline series for tephras with >70 wt% SiO2. 

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2. Data report: marine tephra compositions in the deep drilling cores of the South China Sea, IODP Expeditions 349 and 367/368

    Schindlbeck-Belo, J.C. ; Dadd, K. ; Wang, K.L. ; Lee, H.Y. ( October 2022 , Proceedings of the International Ocean Discovery Program)

   We present geochemical major and trace element glass data for tephra samples from International Ocean Discovery Program (IODP) Expeditions 349 and 367/368 from four drilling sites in the South China Sea. Overall, we obtained data for 55 samples and identified 46 as tephra layers, with dominant volcanic glass shards in the component inventory (in the 63–125 µ fraction). In total, we performed 720 single glass shard analyses using an electron microprobe for major element compositions, as well as 130 single glass shard analyses using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) for trace element compositions. The compositions of the samples range from basaltic, (trachy-) andesitic to trachytic, and rhyolitic and fall mainly into the calc-alkaline and K-rich calc-alkaline magmatic series. One sample falls into the shoshonitic series. Tephras from Expedition 349 Site U1431 span the whole compositional range, whereas tephras from the other sites are limited to rhyolitic composition. Tephra ages, calculated applying sedimentation rates, range to ~2 Ma at Site U1431, ~0.8 Ma at Expedition 367 Site U1499, ~0.6 Ma at Expedition 368 Site U1501, and ~0.9 Ma at Expedition 368 Site U1505. 

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3. Rhyolite volcanism in the Marie Byrd Land volcanic province, Antarctica: New evidence for pyroclastic eruptions during latest Pliocene icesheet expansion

   https://doi.org/10.5194/egusphere-egu21-9003  

   Iverson, Nels ; Siddoway, Christine S. ; Zimmerer, Matt ; Smellie, John ; Dunbar, Nelia ; Gohl, Karsten ; Scientists, IODP Exp379 ( April 2021 , vEGU21, the 23rd EGU General Assembly)

   IODP Expedition 379 deep-sea drilling in 2019 (Gohl et al. 2021, doi:10.14379/iodp.proc.379.2021), offered an opportunity to obtain chronostratigraphic control for seismic reflection data for Amundsen Sea shelf and slope deposits that record Miocene to Present fluctuations in volume of the West Antarctic ice sheet. Here we report the age and interpret the provenance of a volcanic ash bed recovered at/near the Plio-Pleistocene boundary at 31.51 meters below sea level in Hole U1533B and 33.94 mbsf in Hole U1533D. With distinctive geochemistry and inferred wide regional distribution, the bed may serve as a reliable age marker. In Hole 1533B, the fresh tephra forms a discrete layer interstratified within uniform brown marine mud. The layer has a sharp base and upper boundary that is gradational over 5 cm into overlying mud. Color reflectance and density data aided identification of the tephra horizon (diffuse) in Hole 1533D, ~1000m away. A possible on-land source for ash is the Miocene to Pleistocene Marie Byrd Land volcanic province, comprising 18 large alkaline volcanoes dominated by effusive lavas. Products of pyroclastic eruptions are uncommon, mainly occurring as distal englacial, and probably marine, tephra. We undertook an offshore-onshore comparison by first characterizing samples of Site U1533 tephra from a petrographic and geochemical standpoint, using thin section observations, EMPA-WDS glass compositions, and 40Ar/39Ar dating. We then identified onshore exposures with similar characteristics. The offshore tephra are composed of coarse (50-300µm) cuspate glass shards with elongated vesicles. The glass composition is rhyolite, with 75-79wt.% SiO2, ~4wt.% FeO and 0.0wt.% MgO. Single-crystal feldspar 40Ar/39Ar dates are 2.55±0.12 and 2.92±0.02 Ma for U1533B and 2.87 ±0.45 Ma for U1533D. The geochemistry, shard morphology, discrete bed expression, and lateral continuity between Holes U1533B-U1533D indicate that the rhyolite tephra formed as airfall settled to the deep seabed. The ca. 2.55 Ma age based on youngest feldspar grains differs slightly from the 2.1 to 2.2 Ma result obtained from in-progress core bio-magnetostratigraphy. Rare exposures of rhyolite are found in the Chang Peak/Mt. Waesche centers, 1080 km from Site U1533. We obtained pumice sample MB.7.3 (prior-published age of 1.6±0.2 Ma), which displays elevated FeO and F content, and MB.8.1, a specimen of porphyritic cryptocrystalline lava. Single-crystal sanidine 40Ar/39Ar dates are 1.315±0.007 Ma (MB.7.3) and 1.385±0.003 Ma (MB.8.1). Site U1533 samples share a geochemical affinity with these on-land rhyolites, expressed as similar SiO2, CaO, TiO2, MgO and FeO content, suggesting an origin for Site U1533 tephra in the Chang-Waesche volcanoes. A possible explanation for the distinctly greater age, and observed contrasts in Al2O3, Na2O and F percentages, is that Site U1533 tephra are older and erupted from a source entirely concealed beneath subsequent eruptions and the ice sheet. Our results suggest that rhyolite volcanism initiated earlier, was of longer duration than previously known (2.92 to 1.315 Ma), and dispersed tephra far offshore. The finding is significant because ash and aerosols produced by large eruptions may influence regional climate. Antarctica cooled significantly and ice sheets expanded in latest Pliocene time (McKay et al. 2012, doi:10.1073/pnas.1112248109). 

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4. South China Sea Rifted Margin

   https://doi.org/10.14379/iodp.proc.367368.2018  

   Sun, Z. ; Jian, Z. ; Stock, J.M. ; Larsen, H.C. ; Klaus, A. ; Alvarez Zarikian, C.A. ( September 2018 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   The primary objectives of International Ocean Discovery Program (IODP) Expedition 367/368 to the northern South China Sea (SCS) margin were to (1) examine its history of continental breakup and (2) compare it with other nonvolcanic or magma-poor rifted margins with the broader goal of testing models for continental breakup. A secondary objective was to further our understanding of the paleoceanographic and environmental development of the SCS and southeast Asia during the Cenozoic. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C and are located in the continent–ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include the following: Determining the nature of the basement in crustal units across the COT of the SCS that are critical to constrain style of rifting, Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, Constraining vertical crustal movements during breakup, and Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Site U1499 on Ridge A and Site U1500 on Ridge B were drilled during Expedition 367. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively, but based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In addition, Expedition 368 added two more sites on the OMH (Sites U1504 and U1505). Expedition 367/368 completed operations at six of the seven sites (U1499–U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing without coring to 990 m because of mechanical problems with the drilling equipment that prevented the expedition, after 25 May 2017, from operating with a drill string longer than 3400 m. New alternate Site U1504, proposed during Expedition 367, met this condition. Original Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already-drilled Site U1501. At Site U1499, we cored to 1081.8 m in 22.1 days with 52% recovery and then logged downhole data from 655 to 1020 m. In 31 days at Site U1500, we penetrated to 1529 m, cored a total of 1012.8 m with 37% recovery, and collected log data from 842 to 1133 m. At Site U1501, we cored to 697.1 m in 9.4 days with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m in Holes U1502A (758 m) and U1502B (921 m), set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m and set 991.5 m of 10¾ inch casing, but no cores were taken because of a mechanical problem with the drawworks. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Sites U1503 and U1505, all sites were drilled to acoustic basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment, but drilling options were severely limited from 25 May to the end of the expedition by the defective drawworks limiting deployment of drill string longer than 3400 m. At Site U1499, coring ~200 m into the interpreted acoustic basement sampled sedimentary rocks, possibly including early Miocene chalks underlain by Oligocene polymict breccias and poorly cemented gravels of unknown age comprising sandstone pebbles and cobbles. Preliminary structural and lithologic analysis suggests that the gravels might be early to late synrift sediment. At Site U1500, the main seismic reflector corresponds to the top of a basalt sequence at ~1379.1 m. We cored 149.90 m into this volcanic package and recovered 114.92 m (77%) of sparsely to moderately plagioclase-phyric basalt comprising numerous lava flows, including pillow lavas with glass, chilled margins, altered veins, hyaloclastites, and minor sediment. Preliminary geochemical analyses indicate that the basalt is tholeiitic. Sampling of the Pleistocene to lower Miocene sedimentary section at Sites U1499 and U1500 was not continuous for two reasons. First, there was extremely poor recovery in substantial intervals interpreted to be poorly lithified sands, possibly turbidites. Second, we chose to drill down without coring in some sections at Site U1500 to ensure sufficient time to achieve this site’s high-priority deep drilling objectives. The upper Miocene basin sequence, which consists of interbedded claystone, siltstone, and sandstone can be correlated between the two sites by seismic stratigraphic mapping and biostratigraphy. At Site U1501 on the OMH, coring ~45 m into the acoustic basement sampled prerift(?) deposits comprising sandstone to conglomerate of presumed Mesozoic age. These deposits are overlain by siliciclastic synrift sediments of Eocene to Oligocene age followed by primarily carbonaceous postrift sediments of early Miocene to Pleistocene age. Site U1502 on Ridge A was cased to 723.7 m. No coring was attempted shallower than 380 m to save operational time and because of low expectations for core recovery in the upper Plio–Pleistocene sequence. At this site, we recovered 180 m of hydrothermally altered brecciated basalts comprising sheet and pillow lavas below deep-marine sediments of Oligocene to late Miocene age. At Site U1503 on Ridge C, 991.5 m of casing was installed in preparation for the planned deep drilling to ~1800 m. No coring was performed due to mechanical failures, and the site was abandoned without further activity except for installation of a reentry cone. Coring at Site U1504 on the OMH, located ~45 km east of Site U1501, recovered mostly foliated, greenschist facies metamorphic rocks below late Eocene(?) carbonate rocks (partly reef debris) and early Miocene to Pleistocene sediments. At Site U1505, we cored to 480.15 m through Pleistocene to late Oligocene mainly carbonaceous ooze followed at depth by early Oligocene siliciclastic sediments. Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted in the Eocene to Plio–Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results, although some nontrivial corrections can be expected from detailed postexpedition work on integrating seismic stratigraphic interpretations with detailed bio- and lithostratigraphy. As a result of the limited length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than originally planned, allowing Site U1505 (alternate to Site U1501) to be included. Despite this change in focus, Expedition 367/368 provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin or a margin possibly overprinted at a much later stage by plume-related magmatism. In this broader perspective, Expedition 367/368 accomplished a fundamental objective of the two-expedition science program. 

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5. Expedition 355 Preliminary Report: Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.pr.355.2015  

   Pandey, D.K. ; Clift, P.D. ; Kulhanek, D.K. ; Ando, S. ; Bendle, J.A.P. ; Bratenkov, S. ; Griffith, E.M. ; Gurumurthy, G.P. ; Hahn, A. ; Iwai, M. ; et al ( July 2015 , Preliminary report)

   null (Ed.)

   The Indian (southwest) summer monsoon is one of the most intense climatic phenomena on Earth. Its long-term development has been linked to the growth of high topography in South and Central Asia. The Indian continental margin, adjoining the Arabian Sea, offers a unique opportunity to investigate tectonic–climatic interactions and the net impact of these processes on weathering and erosion of the western Himalaya. During International Ocean Discovery Program Expedition 355, two sites (U1456 and U1457) were drilled in Laxmi Basin in the eastern Arabian Sea to document the coevolution of mountain building, weathering, erosion, and climate over a range of timescales. In addition, recovering basement from the eastern Arabian Sea provides constraints on the early rifting history of the western continental margin of India with special emphasis on continental breakup between India and the Seychelles and its relationship to the plume-related volcanism of the Deccan Plateau. Drilling and coring operations during Expedition 355 recovered sediment from Sites U1456 and U1457 in the Laxmi Basin, penetrating 1109.4 and 1108.6 m below seafloor (mbsf), respectively. Drilling reached sediment dated to 13.5–17.7 Ma (late early to early middle Miocene) at Site U1456, although with a large hiatus between the lowermost sediment and overlying deposits dated to <10.9 Ma. At Site U1457, a much longer hiatus occurs near the base of the cored section, spanning from 10.9 to ~62 Ma. At both sites, hiatuses span ~8.2–9.2 and ~3.6–5.6 Ma, with a possible condensed section spanning ~2.0–2.6 Ma, although the total duration for each hiatus is slightly different between the two sites. A major submarine fan draining the western Himalaya and Karakoram must have been supplying sediment to the eastern Arabian Sea since at least ~17 Ma. Sand mineral assemblages indicate that the Greater Himalayan Crystalline Sequence was fully exposed to the surface by this time. Most of the recovered sediment appears to be derived from the Indus River and includes minerals that are unique to the Indus Suture Zone, in particular glaucophane and hypersthene, most likely originating from the structural base of the Kohistan arc. Pliocene sandy intervals at Site U1456 were deposited in lower fan “sheet lobe” settings, with intervals of basin plain turbidites separated by hemipelagic muddy sections deposited during the Miocene. Site U1457 is more distal in facies, reflecting its more marginal setting. No major active lobe appears to have affected the Laxmi Basin since the Middle Pleistocene (~1.2 Ma). We succeeded in recovering sections spanning the 8 Ma climatic transition, when monsoon intensity is believed to have changed strongly, although the nature of this change awaits postcruise analysis. We also recovered sediment from a large mass transport deposit measuring ~330 and ~190 m thick at Sites U1456 and U1457, respectively. This section includes an upper sequence of slump-folded muddy and silty rocks, as well as underlying calcarenites and limestone breccias, together with smaller amounts of volcanic clasts, all of which are likely derived from the western Indian continental shelf. Identification of similar facies on the regional seismic lines in Laxmi Basin suggests that these deposits form parts of one of the world’s largest mass transport deposits. Coring of igneous basement was successful at Site U1457. Recovery of massive basalt and associated volcaniclastic sediment at this site should address the key questions related to rifting and volcanism associated with formation of Laxmi Basin. Geochemical analysis is required to understand the petrogenesis and thus the tectonic setting of volcanism that will reveal whether it is oceanic basalt or volcanic rock contaminated by underlying continental crust or continental flood basalt. However, the fact that the lavas are massive and have few vesicles implies water depths of eruption likely deeper than 2000 m. This precludes opening of the basin in the presence of a major mantle thermal anomaly, such as that associated with the Deccan Large Igneous Province. Other observations made at the two sites during Expedition 355 provide vital constraints on the rift history of this margin. Heat flow measurements at the two drill sites were calculated to be ~57 and ~60 mW/m2. Such heat flow values are compatible with those observed in average oceanic crust of 63–84 Ma age, as well as with the presence of highly extended continental crust. Postcruise analyses of the more than ~1722 m of core will provide further information about the nature of tectonic–climatic interactions in this global type area for such studies. 

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