VOL. 29, NO. 3–4 | MARCH-APRIL 2019 Holes in the Bottom of the Sea: History, Revolutions, and Future Opportunities Holes in the Bottom of the Sea: History, Revolutions, and Future Opportunities

Suzanne OConnell, Professor of & Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA, [email protected]

ABSTRACT about , ocean chemistry, (Scripps Institution of No other international scientific col- evolution, in harsh environments, [SIO]) and Harry Hess (Princeton laboration has contributed as much to our and climate change. University), both AMSOC members, knowledge of Earth processes as scientific Scientists from across the world have proposed to drill a deep hole to sample ocean drilling (SOD). These contributions benefited from and contributed to the pro- Earth’s mantle below a zone of seismic include geophysical surveys, core sam- gram. Geophysical site survey data, cores, velocity change, the Mohorovicic ples, borehole well logs, and sub-seafloor and associated information are available to Discontinuity (Moho): “.” observatories. After more than half a the global scientific community to study The National Science Foundation (NSF) century, involving thousands of scientists and sample. More than 1000 international may have been in favor of the project, from around the world, SOD has been scientists, ranging in age from early career because the 1957 International Union of instrumental in developing three geosci- to retired, are proponents on active propos- Geodesy and Resolution 11 ence revolutions: (1) plate tectonics, als for upcoming drilling. recommended that the Moho be drilled. (2) paleoceanography, and (3) the deep This article, by no means comprehen- The said they had the equip- marine . Without SOD, it is sive, highlights parts of the history and a ment and were looking for a place to drill unlikely that our current understanding few major discoveries of SOD. More (Bascom, 1961). They had just launched of Earth processes could have developed. complete histories are available in Ocean the Sputnik satellite. Their technological Building upon prior scientific results, Drilling: Accomplishments and Challenges advance alarmed many Americans the current science plan is guided by four (National Research Council, 2011), Earth and spurred investment in U.S. science interlinked themes: Planetary Dynamics, and Life Processes Discovered from and technology. Climate and Ocean Change, Biosphere Subseafloor Environments: A Decade of Despite the difficulties of SOD, Frontiers, and Earth in Motion. SOD has Science Achieved by the Integrated Ocean AMSOC, with NSF funding, took on the also been a leader in international collabo- Drilling Program (IODP) (Stein et al., challenge. Many technological improve- rations and the open sharing of samples, 2014), and Koppers et al. (2019). GSA Data ments were necessary for such a project: data, and information. Results from SOD Repository Table S1 (see footnote 1) a drilling platform that could hold station expeditions are open access and available provides URLs to detailed, preliminary in deep water and under different wave online. Almost 2.5 million samples have information for all SOD expeditions and and wind conditions (dynamic position- been taken from over 360 km of core legs, including co-chief scientists, sites ing), a way to retrieve cores through drill located in three repositories. Today about cored, and year. pipe so that the drill pipe could stay in half the members of scientific teams, place, and a sturdy drill bit that could including co-chief scientists, are women. HISTORY operate for days, even weeks. The first This program is needed in the future for drilling was accomplished in 1961 with the geoscientists to continue exploring our Pre-JOIDES (Joint Oceanographic barge, CUSS I (Continental, Union, Shell, planet to understand how it functions and Institutions for Deep Earth Sampling) and Superior). Unfortunately, after a prom- to create predictive models. SOD may be said to have originated ising start, organizational difficulties, and with the International Geophysical Year loss of political support, Project Mohole, INTRODUCTION (1957–1958), and an organization with the deemed too expensive, was abandoned in Scientific ocean drilling (SOD) cele- unlikely name American Miscellaneous 1966 (Hsü, 1992). brated its 50th birthday in 2018. As of Society (AMSOC). AMSOC included men December 2018, 283 expeditions (formerly at pivotal positions at oceanographic insti- The called legs) have been completed and tutions, oil companies, the Office of Naval Not everyone agreed that drilling a >1600 sites have been drilled (see Fig. S1 Research, and the United States Geological single deep hole in igneous rock was the in the GSA Data Repository1). These sites Survey. Bascom (1961) provides details best initial SOD research objective. In represent <0.0005% of the ocean floor, about AMSOC and the development of 1962, (University of yet have provided essential information their discussions about SOD. Miami) proposed that a drilling vessel be

GSA Today, v. 29, https://doi.org/10.1130/GSATG380A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

1 GSA Data Repository item 2019051, Table S1 and Figures S1–S7, is available online at www.geosociety.org/datarepository/2019. deployed to core continuously through deep-sea sediment (project LOCO, LOng COres). (Columbia University, also a member of AMSOC) supported obtaining multiple long cores, stating, “The entire record of terrestrial conditions from the beginning of the ocean is there in the most undisturbed form it is possible to find anywhere—and the dream of my life is to punch that hole 2000 feet deep and bring the contents to the lab to study them” (Gray, 1956). Thus, simultaneously with Project Mohole, a different kind of program was planned. Five U.S. oceanographic institu- tions established JOIDES (Joint Ocean- ographic Institutions for Deep Earth Figure 1. Multichannel Seismic Line 1027 in Baltimore Canyon, used to locate the best sites (902, Sampling). In 1965, using dynamic posi- 904, and 906) during Ocean Drilling Program Leg 150 (Mountain et al., 1994). Cored intervals are tioning, fourteen holes were cored in the shown in white. Also shown are the locations of Deep Sea Drilling Project Site 612 (Leg 95) and the Continental Offshore Stratigraphic Test (COST) B-3 well. The COST B3 well was used to establish eastern on board the first-order age and facies successions. These sediments were cored to establish a history of sea- Caldrill I. The results convinced the scien- level variations along the mid-Atlantic U.S. continental margin. Mission-specific drilling platform Expedition 313 continued the coring in modern water depths of 30–35 m and recovered nearshore tific community of the value of multiple and onshore sediments deposited during Neogene changes in global sea level. holes throughout the ocean. In 1967, Global Marine was contracted to design a ship expressly for the purpose of SOD. Seven days later, Leg 2 began coring The JOIDES Resolution With SIO as the prime contractor, the across the Atlantic. The scientific party of Deep Sea Drilling Project (DSDP), using eight included two female micropaleoen- Ocean Drilling Program (ODP, the Glomar Challenger, was launched tologists, Catherine Nigrini (SIO) and 1983–2003) (Bascom, 1961). Maria Bianca Cita (Instituto di Geologia, By the late 1970s, JOIDES began Universitá di Milano, Italy). This began a exploring alternative vessels. Major Deep Sea Drilling Project (DSDP, tradition of international participation improvements in heave compensation and 1968–1983) that was formalized in 1975: five coun- station-keeping ability offered better cores By 1968, several seminal papers on tries (Federal Republic of Germany, and an expansion of geographic areas that plate tectonics (e.g., Hess, 1962; Vine and France, Japan, the Soviet Union, and the could be explored. More research could be Matthews, 1963; Wilson, 1966) had been UK) became contributing international accomplished with expanded laboratories published. These papers were based on partners in the International Phase of and more scientists. The SEDCO/BP 471, sparse data, leaving many unconvinced. Ocean Drilling (IPOD). Today, 23 coun- an oil exploration vessel, was chosen, Interpretation of marine magnetic anoma- tries are part of the IODP consortium, highlighting the advances in technology lies (e.g., Pitman et al., 1968) and better and women comprise about half of the generated by the industry. earthquake data (e.g., Isacks et al., 1968) science team. Funding for the new program came from provided additional support for this the- Technological development continually the United States and member countries. ory. For many, final confirmation came improved drilling, coring, and logging. Texas A&M University was awarded the from SOD. In 1970, the first re-entry cone was contract. The SEDCO/BP 471 was con- As with Project Mohole, whose objec- deployed (Leg 15, Site 146, Venezuelan verted to a scientific drilling vessel, the tive was seismically defined, Leg 1 of the Basin), allowing the drill string to be JOIDES Resolution, and on 11 January Glomar Challenger and all subsequent pulled, a new bit attached, and coring in 1985 commenced operations. Unlike petro- expeditions have used seismic surveys to the same hole to continue. This enabled leum exploration, which relies heavily on determine the ideal place to drill for sci- the drilling of deep holes through sedi- well logs and cuttings to understand the ence (Fig. 1) and for safety reasons, such ment and into hard rock. In 1979 (Leg 64, of a location, SOD collects cores. as avoiding overpressured hydrocarbons. Guymas Basin), the hydraulic piston core For most JOIDES Resolution legs, a ship- Leg 1’s objectives in the central Gulf of (HPC) was deployed for the first time board scientific party would analyze cores were clear; identify the cause of (Fig. S2 [see footnote 1]). Today, the and document findings during two-month the reflectors on the seismic profiles, HPC and multiple offset holes at a single expeditions, working in two 12-hour shifts, especially the domes and knolls on the site are routine for paleoceanographic seven days a week. deep-sea floor, and recover the oldest expeditions. This allows construction of ocean sediment. Leg 1 of DSDP was a complete “composite” stratigraphic Integrated Ocean Drilling Program success. The domes were salt diapirs. records, essential for detailed paleoceano- (IODP, 2003–2013) The oldest sediment was Jurassic, much graphic research (Figs. 2 and S3–S5 Despite the capabilities of the JOIDES younger than expected. [see footnote 1]). Resolution, critical locations remained A Depth CSF (m) 01020304050

600 SI ) -6

0 1

400 2 3 5 6 Figure 2. The tops and bottoms of cores are usually disturbed and 4 there are often coring gaps between cores. To remedy this and cre- ate a complete stratigraphic sequence, a “composite” ideal core is created by splicing together data from different holes to so that 200 1 2 3 4 5 coring gaps in one hole are filled with core intervals from an adja- cent hole, trying not to use the tops and bottoms of cores. To gen-

Magnetic susceptibility (1 erate a composite core depth below seafloor (CCSF-A) at least two 0 1 2 3 4 5 or more holes are drilled at each site and each hole is slightly offset by depth from the other (Fig. S3 [see text footnote 1]). Hole core Depth CCSF-A (m) measurements, such as magnetic susceptibility and natural 01020304050 B gamma ray, and/or split core images and lithologic changes from the different holes are aligned using distinguishing features, such 600 as a sharp peak or a color change. (See Figs. S3–S5 for additional SI )

-6 examples.) (A) Example from Site U1333, central Pacific, showing 1 0 construction of a CCSF-A. In this example, magnetic susceptibility 400 2 3 5 6 4 measurements from different holes are aligned by depth; Hole U1333A (red), Hole U1333B (blue), Hole U1333C (green). Numbers indicate the order of cores taken in the hole. (B) Depth-shifted

2001 2 3 4 5 cores on composite depth scale (CCSF-A [m]) aligning distinguish- ing features. (C) The magnetic susceptibility records the different holes that are part of the splice are shown by the hole color along Magnetic susceptibility (1 the bottom. Along the top, core breaks (triangles) and hole desig- 0 1 2 3 4 5 nations are shown (from Pälike et al., 2010). C Depth CCSF-A (m) 01020304050

SI ) 600 -6 0 B A B C B C B C B C Revolution #1: Plate Tectonics 400 In the ocean, plate tectonic knowledge has come from geophysical studies, 200 submersible observations, and samples,

0 drill cores, and instrumented boreholes to Magnetic susceptibility (1 monitor in situ processes. As information leading to the wide acceptance of plate inaccessible. Without a riser system, with integrated approach is laid out in the pres- tectonics accumulated, one of the most its casing and the ability to circulate mud ent science plan: Illuminating Earth’s Past, convincing data sets came from DSDP and prevent blowouts, many thickly sedi- Present, and Future (IODP, 2011). Leg 3. “The most interesting finding from mented continental margins and subduc- the paleontological studies is the correla- tion zones were out of reach. Locations at SOME SOD HIGHLIGHTS tion of paleontologic ages of sediments some high latitudes with ice and in very SOD has been an engine for understand- immediately overlying the base- shallow water could not be explored. To ing Earth processes. Tens of thousands of ment with ages of basement predicted by meet these scientific needs, a new era of papers have been published, some among the sea-floor spreading hypothesis” SOD began in October 2003. The riser- the most highly cited in . (Maxwell et al., 1970, p. 445). equipped, Japanese-built Chikyu, capable Thousands of scientists from around the Below, briefly summarized, are some of drilling deep into heavily sedimented world, from undergraduates to emeritus, of the major results learned since Leg 3. margins, and the mission-specific drilling have been involved in the research, form- platforms (MSPs) operated though the ing international collaborations extending Rifted Margins (Also Called Passive European Consortium for Ocean Research beyond SOD. Margins) Drilling were added. Describing the results in a short paper is Continental rifting and ocean basin challenging and necessarily incomplete. formation are central processes of plate International Ocean Discovery Program Here, three areas where SOD contributed tectonics and continue to be an important (IODP, 2013–2023) to major revolutions in our understanding focus of SOD. This is an iterative process, Today, there is a much broader under- of Earth are described: plate tectonics, with geophysical surveys identifying standing of the interconnections among paleoceanography, and the deep marine prime drilling targets and SOD providing Earth’s spheres, and new fields of research biosphere. These topics are intertwined. the cores to determine the age and compo- have developed. Much has been learned Clearly, plate tectonics impacts ocean and sition of specific reflectors. Primary influ- about how Earth operates. But, the details climate history, which in turn affects the ences on rift development are related to necessary to reach the next level of under- deep biosphere; all are connected through mantle composition, thermal structure, and standing require interdisciplinary exper- carbon and water cycling. Future drilling tectonic stresses. This results in two end- tise, atmospheric scientists, computer will continue to enhance geoscientists’ member classifications, magma-rich and modelers, and biologists. Achieving deeper understanding of interconnected Earth magma-poor (non-volcanic) margins. knowledge has become more complex and processes from both a planetary and a Magma-rich margins were drilled dur- requires a new approach. This broadly human impact perspective. ing multiple expeditions (Legs 38, 81, 104, 1 km Tracer injection GeoB00-483 N20°E 3.4 Hole Hole U1362B 380 U1362A Hole Hole U1301A 1026B U1362A 400 Hole U1301B Seafloor Pillow basalt Hole U1362B Seafloor = 1026B 3.6 1027B 2,600 m U1301B 1027C 420 Massive basalt Depth (mbsl

440 3.8 ) Sediment 460 Basalt top

4.0 480 3.5 Depth (mbsf )

500 Tracer transport 3.7 Dominant fluid flow direction 4.2

520 Two-way travel time (s) Tracer 3.9

540 detection 4.4 4.1 Basaltic ocean Basalt top

560 Two-way travel time (s) Basalt flow 01 km 4.3 4.6

Figure 3. Schematic (not to scale) of borehole locations and experimental design for a tracer injection experiment conducted during and after Expedi- tion 327 to investigate transport rates in the upper ocean crust. Boreholes observatories were placed in ~3.5–3.6-m.y.-old crust on the eastern flank of the Juan de Fuca Ridge. Thicker sections of the drill pipe indicate open intervals where tracer was injected or sampled. The stratigraphy of the upper volcanic crust at Hole U1301B is shown on the left. Main diagram modified from Fisher et al. (2011), and stratigraphic column is from Becker et al. (2013). Experimental results indicate very rapid tracer transport (meters/day), with most of the flow occurring in a small fraction of the rock (Neira et al., 2016). See Figure S7 for additional information (see text footnote 1).

152, 163) with segments along the East submersible observations, and ophiolites spreading crust. These faults are also con- Greenland–Norwegian rifted margins was perceived as fairly simple: on duits for hydrothermal fluids that lead to (Fig. S6 [see footnote 1]), an area that con- the seafloor were fed by sheeted dike com- the formation of massive sulfide deposits stitutes part of the North Atlantic Igneous plexes that emanated from a magma cham- as observed in the Trans-Atlantic Geo- Province. An early surprise in drilling ber where gabbro crystalized and accumu- traverse hydrothermal field (Leg 158). these margins was that some of the well- lated over underlying mantle. There was, A critical component of modeling crustal defined, seaward-dipping reflectors west as there is today, discussion about the dif- accretion (and subduction) is understanding of Norway consisted of subaerial extrusive ferences between fast and slow spreading the role of hydrothermal circulation. To basalts. Later, 40Ar-39Ar dating of volcanic and the distinguishing features of backarc learn more about this plumbing system, rocks in southeast Greenland showed that basins. Now parts of the ocean crust have many holes have been instrumented with the breakup evolution spanned ~12 m.y. been sampled, boreholes instrumented, circulation obviation retrofit kits (CORKs). (Tegner and Duncan, 1999). and geophysical and submersible observa- CORKs seal off the borehole from the sea Magma-poor margin drilling has taken tions collected. The ocean floor is much floor, allowing in situ measurements of place on the once contiguous Newfound- more varied and complex than the early pressure and temperature, and show the land-Iberian margins (Legs 47, 149, and models suggested. extent of fluid circulation in young ocean 173, and Expedition 210), and in the South Multiple expeditions at Hole 504B (Alt et crust, which may be one of the most hydro- China Sea (Expeditions 349, 367, and 368). al., 1996) and Hole 1256D (Wilson et al., logically active formations on Earth These expeditions, with sites carefully 2006) in the eastern Pacific partially sup- (Becker and Fisher, 2000). Advanced located along seismic lines, have recovered port the model of layered ocean crust, with CORKs allow for additional measurements sediments, continental crust, a wide vari- basalts overlying sheeted dikes, which in (e.g., seismometers and fluid sampling), ety of igneous rocks, and even serpen- their deeper parts contain lenses of gabbro. ocean observatories, and experiments tinized mantle. Most recently, MSP The structure and composition of the (Fisher et al., 2011; Figs. 3 and S7 Expedition 381 cored the Corinth Rift in deeper crustal components was addressed [see footnote 1]). the Mediterranean. There, a syn-rift at the Hess Deep Rift on the East Pacific sequence is accessible, allowing the fault Rise (Leg 147 and Expedition 345). There, Convergent Boundaries and rift evolutionary history, including the the variety of textures and composition of Convergent boundaries (subduction deformation rates, to be determined. the plutonic rocks was larger than expected. zones) include island arcs, forearcs, backarc In the Atlantic, where spreading is basins, and accretionary prisms. Processes Ocean Crust and Lithosphere slower, investigators were initially sur- at these plate boundaries are more likely to Understanding the composition and prised by the presence of serpentine mixed impact humans than any other component structure of ocean crust, how it varies, and with basalts (Site 395, Leg 45). Many sub- of plate tectonics, generating large earth- how it is accreted is fundamental to under- sequent expeditions showed that mantle quakes, explosive volcanoes, and tsunamis. standing Earth’s basic evolution. In the late and lower crustal rocks, displaced upward The Chikyu has focused on the deeper por- 1970s, the structure of ocean crust gleaned along major normal faults, are integral tions of this plate boundary, especially in from geophysical surveys, dredge hauls, components of slow and ultra-slow the Nankai Trough and Japan Trench. 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- (δ18O) and carbon isotope values (δ13C) Among the new IODP proposals is “The 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 δ13C excursion were 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 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 CO2, 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 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 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 (CO2, H2S, and next phase of SOD are already under way new world (Inagaki et al., 2015). Maybe SO2) 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 CO2: 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 CO2 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. ACKNOWLEDGMENTS tracers during IODP Expedition 327, eastern Kurtz, A.C., Kump, L.R., Arthur, M.A., Zachos, This article was considerably improved thanks flank of Juan de Fuca Ridge, in Fisher, A.T, J.C., and Paytan, A., 2003, Early Cenozoic to discussions with and reviews from B. Clement, Tsuji, T., Petronotis, K., and the Expedition decoupling of the global carbon and sulfur S. D’Hondt, A. Fisher, J. Karson, J. McManus, 327 Scientists, Proceedings of the Integrated cycles: Paleoceanography, v. 18, p. 14-1–14-14, E. Thomas, B. Tucholke, D. Royer, GSA Today Ocean Drilling Program, v. 327: Tokyo, https://doi.org/10.1029/2003PA000908. editor G. Dickens, and two anonymous Integrated Ocean Drilling Program Manage- Lisiecki, L.E., and Raymo, M.E., 2005, A reviewers. 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2 Here, unless there is a specific figure or quote from a scientific ocean drilling volume, references to initial discoveries from Legs (DSDP and ODP) and Expeditions (IODP) are given only by their Leg/Expedition number. Table S1 (see footnote 1) provides links to a complete listing of Legs and Expeditions. Sluijs, A, Shouten, S., and Expedition 302 Tegner, C., and Duncan, R.A., 1999, 40Ar-39Ar ocean crust: Science, v. 312, p. 1016–1020, Scientists, 2006, Subtropical Arctic Ocean chronology for the volcanic history of the https://doi.org/10.1126/science.1126090. temperatures during the Palaeocene/Eocene southeast Greenland rifted margin, v. 163: Wilson, J.T., 1966, Did the Atlantic close and thermal maximum: Nature, v. 441, p. 610–613, Proceedings of the Ocean Drilling Project, then re-open?: Nature, v. 211, p. 676–681, https://doi.org/10.1038/nature04668. Scientific Results, p. 53–62, https://doi.org/ https://doi.org/10.1038/211676a0. Stein, R., Blackman, D.K., Inagaki, H., and 10.2973/odp.proc.sr.163.108.1999. Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Larsen, H.-C., editors, 2014, Earth and Life Thomas, E., 1989, Development of Cenozoic and Billups, K., 2001, Trends, rhythms, and Processes Discovered from Subseafloor deep-sea benthic foraminiferal faunas in aberrations in global climate 65 Ma to present: Environments: A Decade of Science Achieved Antarctic waters, in Crame, J.A., ed., Origins Science, v. 292, p. 686–693, https://doi.org/ by the Integrated Ocean Drilling Program and Evolution of the Antarctic Biota: An 10.1126/science.1059412. (IODP), v. 7, Developments in Marine Introduction: London, Geological Society, Zachos, J.C., Röhl, U., Schellenberg, S., Sluijs, Geology: Amsterdam, Elsevier, 829 p. p. 283–296. A., Hodell, D., Thomas, E., Nicolo, M., Raffi, Storey, M., Duncan, R.A., and Tegner, C., 2007, Vine, F.J., and Matthews, D.H., 1963, Magnetic I., Lourens, L.J., McCarren, H., and Kroon, Timing and duration of volcanism in the North anomalies over oceanic ridges: Nature, v. 199, D., 2005, Rapid acidification of the ocean Atlantic Igneous Province: Implications for p. 947–949, https://doi.org/10.1038/199947a0. during the Paleocene–Eocene Thermal geodynamics and links to the Iceland hotspot: Whelan, J., and Tarafa, M., 1986, Organic matter Maximum: Science, v. 308, p. 1611–1615, Chemical Geology, v. 241, p. 264–281, in Leg 96 sediments: Characterization by https://doi.org/10.1126/science.1109004. https://doi.org/10.1016/j.chemgeo.2007.01.016. pyrolsis, in Bouma, A.H., Coleman, J.M., Svensen, H., Planke, S., Malthe-Sorenssen, A., Meyer, A.W., et al., eds., Deep Sea Drilling Jamtveit, B., Myklebust, R., Eidem, T.R., and Project Initial Reports, v. 96: Washington, D.C., Rey, S.S., 2004, Release of methane from a U.S. Government Printing Office, p. 757–766, volcanic basin as a mechanism for initial https://doi.org/10.2973/dsdp.proc.96.146.1986. Manuscript received 12 July 2018 Eocene global warming: Nature, v. 429, Wilson, D.S., Teagle, D.A., Alt, J.C., and Shipboard Revised manuscript received 26 Nov. 2018 p. 542–545, https://doi.org/10.1038/nature02566. Scientists, 2006, Drilling to gabbro in intact Manuscript accepted 8 Dec. 2018