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Drilling has been essential in illuminat-  previously identified Ice Ages and hinted at   have been debated ever since (Ryan, 2009).
          ing the origin and evolution of convergent   a cyclicity later tied to orbital forcing.   Additional drilling in the Mediterranean,
          margins. Not only are crust and sediment   Longer cores were needed to extend these   with better coring capabilities and refine-
          being subducted, but material from the   records. Today, compilations of oxygen   ment of the areas to be drilled, is needed.
          overlying plate is scraped off by the under-  (δ O) and carbon isotope values (δ C)   Among the new IODP proposals is “The
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          lying plate, termed subduction erosion.   from calcite secreted by benthic foramin-  Demise of a Salt Giant,” to examine the
          This process is ubiquitous at convergent   ifera are essential for past climate recon-  climate and environmental transitions dur-
          margins, recycling material deep into the   structions. Data compiled by Zachos et al.   ing the terminal Messinian salinity crisis.
          lower crust and mantle (e.g., Scholl and   (2001), Lisiecki and Raymo (2005), and
          von Huene, 2009). Trace elements and iso-  Cramer et al. (2009), include tens of thou-  Paleocene–Eocene Thermal Maximum
          topic tracers in volcanic rocks substantiate   sands of analyses from hundreds of cores   (PETM)
          the subduction and hence chemical recy-  collected by many scientists over decades.   At the start of the ODP, no one knew
          cling of ocean sediments (e.g., Plank and   These compilations and many new oceano-  about the PETM. This global warming
          Langmuir, 1993).                   graphic proxies document long- and short-  event challenges our understanding of the
            Crucial to the processes at subduction   term climate change and rates of change.   carbon cycle and is discussed in more than
          margins is illuminating the cause of large   But the changes go far beyond “just”    500 refereed papers. Its discovery led to
          earthquakes, identifying what happens at   climate. SOD has allowed us to identify   identification of multiple “hyperthermal
          the seismogenic zone when the release of   changes in ocean chemistry, better under-  events” of lesser magnitude in the
          pressure causes a major rupture. From   stand the effect of the asteroid impact at the   Paleocene–Eocene, thus a new view of
          cores and borehole measurements, varia-  K/Pg boundary, and, through continuous   climate instability in a greenhouse world.
          tions in lithologies, fluids, pore pressure,   ocean records, better understand evolution.  First identified in Site 690 (Leg 113,
          and heat flow have been documented.   All of these advances benefit from   Weddell Sea, Antarctica), it was character-
          These observations, in conjunction with dip   excellent resolution, provided by continu-  ized by extinction of benthic foraminifera
          angle, subducting seafloor topography, and   ing developments in paleomagnetics, bio-  (Thomas, 1989). Stable isotope data told an
          results from ocean bottom seismometers,   stratigraphy, X-ray core scanners, and   incredible story, a rapid deep-sea warming
          highlight this environment’s complexity.  orbitally tuned age models. Early in SOD,   combined with a massive negative carbon
            Multiple Legs (DSDP, ODP) and    holes were often spot cored. Then the com-  isotope excursion (Kennett and Stott, 1991)
          Expeditions (IODP) have focused on sub-  munity decided that holes generally should   and—as later recognized—widespread
          duction zones and backarc basins. The   be continuously cored. After the advent of   deep-sea carbonate dissolution due to
          Nankai Trough Seismogenic Zone     the HPC, higher resolution work became   ocean acidification (Zachos et al., 2005).
          Experiment (NanTroSEIZE) on board the   possible, but cores in a single hole do not   The details of this event are still debated,
          Chikyu is a multiyear project to core   provide a truly complete record. The top   but the deep ocean warmed ~5–8 °C in a
          through a plate boundary in an active fault   and bottom of each core is disturbed.   few tens of thousands of years, ca. 55.8 Ma
          area. In addition to cores, it is providing the   Today, multiple, overlapping holes at a    (Mudelsee et al., 2014). Even in the Arctic
          opportunity to monitor hydrologic and geo-  single site allow creation of a continuous   Ocean, already ~18 °C, temperatures rose
          dynamic properties of the subduction zone   composite core (Fig. 2). Here, highlights of   an additional 5–6 °C during the PETM
          through borehole observatories. All of this   two paleoceanographic achievements are   (Sluijs et al., 2006).
          information will help to develop models   briefly summarized.           Acidification and a δ C excursion were
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          that describe subduction zone process and                             linked to a massive discharge of carbon.
          should allow geoscientists to better explain   Messinian Salinity Crisis  Calculating how much carbon was released
          and predict earthquake behavior and    Leg 13 departed Lisbon, Portugal,    depends upon the source and its isotopic
          tsunami generation.                in 1973 to explore the origins of the   composition. This has been highly debated
                                             Mediterranean Sea and, in particular, to   and may be some combination of decom-
          Revolution #2: Climate and Ocean   identify reflectors on seismic profiles.   posing clathrates (Dickens et al., 1995),
          Change: Paleoceanography           Coring the pervasive and prominent   burning of organic matter (Kurtz et al.,
            The science of pre-Quaternary paleo-  M-Reflector proved problematic. The   2003), mantle-sourced volcanic CO 2, and
          ceanography arguably did not exist when   problem was caused by the unexpected   interaction of magma with organic matter
          the Glomar Challenger set sail in 1968.    presence of gravel, which jammed the drill   (Svensen et al., 2004; Gutjahr et al., 2017).
          The first International Congress of   pipe, and salt (dolomite, gypsum, anhy-
          Paleoceanography took place in Zurich,   drite, and halite), which was slow to   Revolution #3: Biosphere Frontiers:
          Switzerland, in 1983, and the American   impossible to penetrate. The M-reflector   Deep Marine Biosphere
          Geophysical Union began publishing   was caused by the top of a thick salt layer   From its initiation, there have been
          Paleoceanography (now Paleocean-   that accumulated in the late Miocene,    serendipitous discoveries by SOD, but
          ography and Paleoclimatology) in 1986.   3 km below the surface of the Atlantic   probably none was as unexpected as that of
          Early paleoceanographic research, based   Ocean. The initial interpretation was that   the deep biosphere. In the late 1980s, some
          on short cores, showed a marine expression   the Mediterranean dried up several times   researchers began to discover microbial
          of the Ice Ages recognized on land. Oxygen   during the Messinian (Hsü et al., 1973).   activity in deep marine sediments (e.g.,
          isotope records of foraminifera (Emiliani,   The details about the causes and extent of   Whelan and Tarafa, 1986). This discovery
          1957) documented more than the four   the desiccation and the role of tectonics   didn’t get celebrated the same way as the

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