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discovery of the hydrothermal vent com-  connections between Earth’s spheres in   to a changing environment. This informa-
          munities a decade earlier. Yet these com-  ways that will help to further revolutionize   tion will increase understanding of biodi-
          munities, invisible to the naked eye, con-  geoscience. Each theme is briefly described   versity and evolutionary paths on Earth
          tain abundant biomass. Just how much is a   in the following sections.  and aid expectations for the existence of
          matter of debate; Kallmeyer et al. (2012)                             life in other parts of the solar system.
          estimated there are roughly as many cells   Planetary Dynamics (Deep Processes
          in marine sediment as in the ocean or in   and Their Impact on the Earth’s   Earth in Motion (Geohazards)
          soil. Bar-On et al. (2018) refined the esti-  Surface Environment)      An increasing human population, espe-
          mate to 77 Gt of carbon (~14% of biomass   Plate tectonics is the driving force for   cially in coastal areas, puts more people in
          carbon) that reside in the deep subsurface,   Earth processes. Yet, despite decades of   harm’s way. This harm can come rapidly
          both land and marine.              research, some of the most basic mecha-  from earthquakes, tsunamis, volcanoes,
            Are they living cells? Metabolic rates of   nisms are not fully understood. What is   and landslides. This is an area where bore-
          these communities are orders of magnitude   clear is that the seemingly different com-  hole tools have a crucial role in furthering
          slower than those on the surface and hence   ponents of the Earth system are connected,   our understanding of the subsurface pro-
          barely detectable (e.g., D’Hondt et al., 2002;   and water and carbon are primary connec-  cesses that cause these changes. Long-
          Hoehler and Jørgensen, 2013). Unknown   tors. To understand these processes, geo-  term, in situ data collection can monitor
          and critical to this discussion is determin-  scientists need samples from the surface   the conditions of earthquake generation
          ing the connections between the geobio-  sediment, through the ocean crust to the   and biogeochemical cycling, allowing the
          sphere that humans inhabit and this barely   mantle. Although plate tectonics appears   rate of the different components of these
          explored marine world.             to be a fairly steady-state process today, it   changes to be better understood and pro-
            The accomplishments that have been   may not always have been so. For example,   viding the data for more accurate modeling
          made did not come easily. Even after    there is much to learn about the past   of these critical environments.
          samples are collected, there are challenges,   emplacement of large igneous provinces,
          such as characterizing bacteria that grow so   when massive amounts of mafic magma   FUTURE (POST-2023)
          slowly as to be virtually dormant. Despite   were discharged into the ocean and onto   Plans for the science program for the
          the challenges, there is now access to this   land, and abundant gases (CO 2, H 2S, and   next phase of SOD are already under way
          new world (Inagaki et al., 2015). Maybe   SO 2) were emitted into the atmosphere.  as many critical questions remain. For
          fifty years from now geoscientists will be                            example: How do subduction zones initi-
          shocked at our ignorance about such a    Climate and Ocean Change: Reading    ate and continents rift? How fast can sea
          critical component of Earth processes.  the Past, Informing the Future  level rise? How have marine microbial
            Gas hydrates are also part of deep bio-  The ocean ecosystem is changing in   communities responded to past changes
          sphere research. Several decades ago, they   response to elevated atmospheric CO 2:    in ocean chemistry and temperature? This
          were inferred from bottom simulating   it is becoming warmer, and the ocean is   is an opportunity for geoscientists from
          reflectors (BSRs), a puzzling reflector that   acidifying. The rate of modern changes   many disciplines and countries to become
          corresponded with the predicted base of   may be unprecedented (except for asteroid   involved in the continuation of an exciting
          the methane hydrate stability zone. First   impacts), but Earth has experienced simi-  international program that is critical to
          cored on Leg 67, sediment above the BSR   lar perturbations in the past. The carbon-  Earth science.
          indeed contained hydrates, and as the cores   ate compensation depth has risen and
          warmed and the gas expanded, the sedi-  fallen, and icebergs left rocky trails of   CONCLUSIONS
          ment left the core barrel and shot across   their passage. Through examination of   Although we have yet to drill through
          the deck. Special pressure core barrels   marine sediment cores, geoscientists can   the Mohorovicic Discontinuity, SOD has
          needed to be and were developed. Now   determine how the ocean (and ocean life)   radically changed and continues to change
          scientists on the JOIDES Resolution have   responded in the past. Collecting the   our view of Earth. Early SOD showed an
          cored gas hydrates in many environments,   detailed data necessary to better charac-  unexpectedly young ocean and confirmed
          especially on subduction margins. For   terize past changes will help climate mod-  plate tectonics as the underlying mecha-
          example, scientists on Expedition 311   elers better predict our future.  nism for Earth processes. Recent SOD
          (Cascadia Margin) determined that most of                             provides new information about elemental
          the marine hydrate is caused by the micro-  Biosphere Frontiers: Deep Life,   recycling and Earth history. Just as we
          bial reduction of CO 2 within the hydrate   Biodiversity, and Environmental   learn more about outer space and the ori-
          stability zone (Riedel et al., 2010). Looking   Forcing of Ecosystems  gin of the universe with better telescopes
          to the future, dissociation of hydrates   The unexpected finding of microbial   and planetary missions, we will learn more
          might produce slope instability and the   communities in the sea floor opens a wide   about our planet, the processes that drive
          release of methane.                variety of possibilities for exploration.   it, and the origin of life as drilling technol-
                                             Their very existence challenges our under-  ogy improves. The intrigue continues as
          PRESENT (2013–2023)                standing of the minimal conditions neces-  each seismic survey defines where to drill,
            Illuminating Earth’s Past, Present and   sary for life. Three major themes are being   each core sheds new light on Earth’s past
          Future (IODP, 2011) describes four themes   addressed: (1) sub-seafloor microbial com-  and helps predict its future, and borehole
          of focus for the current IODP. It highlights   munities; (2) the limits to life in the deep   instrumentation provides in situ monitor-
          opportunities to use SOD to explore   biosphere; and (3) community adaptation   ing of Earth’s processes.

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