One of the fundamental premises of Illuminating Earth’s Past, Present, and Future: The International Ocean Discovery Program Science Plan for 2013-2023 (referred to hereafter as the “science plan”; IODP-MI, 2011) is that scientific ocean drilling has the potential to enable essential advances in multiple fields of scientific inquiry, as it has done so significantly in the past (see Chapters 2-4 of this report for discussion of accomplishments in previous scientific ocean drilling programs). In this chapter, the committee assesses the potential for the scientific challenges described in the science plan to lead to transformative scientific discovery. Because the committee’s intent was to provide guidance on how the plan could be most effectively implemented, recommendations are also offered on relevant areas of scientific ocean drilling inquiry that have the greatest potential for success.
The committee examined the potential for transformative discoveries that might result from research conducted within the framework of the science plan. The National Science Foundation (NSF) defines transformative research as research that “involves ideas, discoveries, or tools that radically change our understanding of an important existing scientific or engineering concept or educational practice or leads to the creation of a new paradigm or field of science, engineering, or education. Such research challenges current understanding or provides pathways to new frontiers.”1 Under this definition, the committee has considered those discoveries that might result from several different pathways, including: (1) new ways to interpret existing data, which lead to testable new theories or paradigms that can be assessed through the collection of new data (e.g., plate tectonics); (2) technological developments that enable previously impossible measurements (e.g., ability to date rocks with radiometric methods); (3) observations obtained in a frontier area (e.g., massive sulfide deposits beneath hydrothermal vents); and (4) novel integration of different data types or cross-disciplinary interchange (e.g., additional linkages between regional marine and on-land investigations of similar environments).
The first case of transformative discovery (new interpretations of existing data) is very difficult, if not impossible, to predict. Technological development will most directly lead to transformative discoveries through drilling to depths greater than have been reached previously or by coupling drilling with long-term time series measurements on scales not previously seen. Conventional drilling may lead to transformative science in regions that have not been previously drilled or sampled, while drilling transects of spatially related holes could lead to pathways for novel integration of data. Therefore, the committee focused on assessing the science plan through all of the pathways detailed above, except the first.
This chapter begins with a discussion of overarching comments regarding the science plan, including conclusions and recommendations. Following sections address each of the four research themes identified in the science plan. General comments on the theme are followed by a detailed analysis of each of the science plan challenges listed within the theme, assessment of other challenges or opportunities for transformative science beyond that in the science plan, synergies between the themes, and linkages between scientific ocean drilling, other NSF-supported science programs and facilities, and non-NSF programs. The wording of the themes and challenges after the overarching comments are taken directly from the science plan in order to easily map the science plan challenges to the committee’s assessment.
The science plan is divided into four research themes: climate and ocean change, biosphere frontiers, Earth connections (deep Earth processes), and Earth in motion (direct time series observations on human scales). Fourteen specific challenges, posed as questions, are included in these themes. In addition, chapters on education and outreach and on implementation are included. Although the document has no explicit vision statement, the science plan’s focus is clearly on new discoveries and better understanding of Earth science topics to meet emerging societal challenges and enhance decision making. This overarching focus is considerably different from the Integrated Ocean Drilling Program (IODP) Initial Science Plan (IODP, 2001), which was based on further exploration of the ocean and new scientific understanding of Earth systems, with the assumption that recognition of how to apply that knowledge to relevant societal issues would be automatic. The committee supports linking scientific ocean drilling to issues of societal relevance and commends the writers of the science plan for taking this approach. The climate and ocean change (e.g., climate change, sea level rise, ocean acidification) and Earth in motion (e.g., earthquakes and other geohazards) themes identify challenges with the most direct societal relevance, but relevant topics also exist in the biosphere and Earth connections themes.
Overall, the science plan presents a strong case for the continuation of scientific ocean drilling, with its possible benefits for science and society. The committee was particularly positive about the potential for transformative science resulting from studies of the subseafloor biosphere and about the importance of continuing paleoclimate studies that will provide analogs and likely constraints on global and regional changes predicted with future climate. It also agreed that the emphasis on sampling deeper into the crystalline basement will lead to better understanding of deep earth processes, especially if high percentages of intact core are recovered and if active tectonic processes are monitored in situ.
Each of the four themes within the science plan identifies compelling challenges with potential for transformative science that can only be addressed by scientific ocean drilling. Some challenges within these themes appear to have greater potential for transformative science than others.
The committee’s assessments of each theme and specific challenge identified in the science plan are discussed in greater detail in the following sections. The themes and challenges are pertinent and well-justified, although the committee was concerned that the science plan reads like a wish list with little guidance as to which of the 14 challenges were considered most important. The committee is aware that the writers of the science plan may not have been asked to prioritize, but given the financial constraints that the next phase of scientific ocean drilling is likely to face, it may now be appropriate for the scientific ocean drilling community to provide additional guidance on prioritization of potential drilling objectives. Such a prioritization could include guidance on which drilling objectives might be dependent on platform capabilities and availability, innovations in technology, challenges in obtaining supporting data (such as in high latitudes), or global political or safety concerns.
RECOMMENDATION: The scientific ocean drilling community should establish a mechanism to prioritize the challenges outlined in the science plan in a manner that complements the existing peer-review process.
The scientific ocean drilling programs have a history of making excellent use of legacy samples and data that have helped to quickly advance new areas of research (see in particular the discussion of accomplishments in the section on Abrupt Climate Change in Chapter 4). On the other hand, the science plan is justifiably focused on the importance of future drilling challenges and thus spends little time discussing the use of legacy information and samples.
Using legacy data and samples to their maximum capabilities will continue to increase the scientific value of the scientific ocean drilling programs. Expanded use of legacy materials could help, for example, with prioritization of drilling objectives in the next phase of scientific ocean drilling.
A more thorough future examination of the areas of natural integration among scientific ocean drilling objectives would also be valuable. Although several natural points of synergy between the challenges and themes are well described in the science plan, a more detailed examination of the areas where natural integration could occur between and among the science challenges would have further strengthened the science plan (see following sections for further discussion). An increase in efficiency and integration of multiple science objectives is one means by which resources can be most effectively used. For example, integrating multiple drilling objectives in the early planning stages of single expeditions would maximize scientific output in relation to costs. This approach is successfully used by the International Continental Scientific Drilling Program (ICDP)2 and polar expeditions. Integrating multiple objectives into a single expedition requires compromises, but certain practices, such as bringing expedition leaders together beforehand to ensure agreement on expedition goals and providing adequate planning to create a viable work plan, will make success more likely. Other potential approaches to increasing efficiency include evolving efforts to optimize ship tracks,
which also has possible implications for prioritization of science objectives.
RECOMMENDATION: From the earliest stages of proposal development and evaluation, possibilities for increasing program efficiency through integration of multiple objectives into single expeditions should be considered by proponents and panels.
Chapters 2-4 extensively discuss the vital role played by technology in achieving many scientific advances in previous scientific ocean drilling programs (Deep Sea Drilling Project [DSDP], Ocean Drilling Program [ODP], and IODP). Although the committee’s charge for this section was to assess the potential for transformative science in the science plan, it determined that transformative science is critically dependent on technological breakthroughs and could not be fully assessed without considering the need for continued technological development. Any future scientific ocean drilling program must continue to push the technological envelope. Previous scientific ocean drilling programs have shown great strength in this area, and the committee believes that the promise of continued innovation is high. Approaches such as dedicated engineering legs or establishment of sites with different lithologies to test and refine instruments could increase the potential for achievement of groundbreaking science.
RECOMMENDATION: Pathways for innovations in technology should be encouraged. In addition, setting aside some resources specifically to promote technological research and development could increase the potential for transformative science.
This theme identifies four key challenges related to Earth responses to changes in atmospheric carbon dioxide (CO2) concentration, sea level rise and diminishing ice sheets due to warming climate trends, regional precipitation pattern changes and potential impacts, and the ocean’s sensitivity to changes in chemistry. All of these challenges are relevant to future Intergovernmental Panel on Climate Change (IPCC) projections and can help provide constraints on the range of possible future impacts. They map well to the IPCC Fifth Assessment Report, which is currently in preparation, and also represent issues of significant uncertainty and high importance that will remain relevant for the follow-up Sixth Assessment Report.
The science described in these challenges has potential to be transformative and, in most cases, can only be achieved through a continuation of scientific ocean drilling. The climate theme has a strong focus on achievements, specifically those within the past 10 years, and suggests that more related work is needed, especially on certain targeted topics (e.g., polar regions, permanent El Niño states, monsoon patterns).
The science plan emphasizes the critical need to integrate proxy observations from paleoclimate data with numerical modeling of past climates to better understand climate system responses and feedbacks. In addition, there will be a need to develop new paleoclimate proxies and improve existing proxies. Both of these objectives will require strong linkages among the scientific ocean drilling, modeling, and climate proxy communities. The theme highlights the need for innovative thought and for collaboration with other drilling programs (e.g., Antarctic Geological Drilling program [ANDRILL], ICDP) to accomplish transects from the continental margin to the deep ocean, as well as the need for mission-specific platforms that have the flexibility to drill in difficult areas (e.g., shallow and ice covered regions) with increased core recovery. In many of these cases, the JOIDES Resolution may not be the optimum platform for future climate science needs. The next phase of scientific ocean drilling may need to consider new technology development or alignment with other programs that have common goals and appropriate technology.
The science plan also articulates well the case for the importance and societal relevance of these topics to decision makers. It acknowledges a number of international efforts (IPCC, International Geosphere-Biosphere Programme3 and the Past Global Changes project,4 European Project for Ice Coring in Antarctica, Scientific Committee on Antarctic Research5) that have developed strategies to address these challenges. One especially well-justified point is that of all paleoclimate drilling initiatives, only the next phase of scientific ocean drilling can consistently recover time-continuous, high-resolution records of warmer, high-CO2 climates in the deep past.
Challenge 1: How Does Earth’s Climate System Respond to Elevated Levels of Atmospheric CO2?
Sediment cores document the strong relationship between mean global atmospheric temperature and CO2 over millions to tens of millions of years, and demonstrate the potential for using warmer-than-present pre-Quaternary climates as a rich venue for exploring the behavior of Earth system models that are used for future projections (provided that relevant proxies can be convincingly shown to be reliable). Quantifying this relationship has been assisted by dramatic improvements in
the quality of proxy temperature and pCO2 reconstructions from scientific ocean drilling records. This challenge has great potential for societally relevant outcomes, especially in quantifying and understanding the processes involved in amplified temperature changes at the poles. Polar amplification is strongly linked with responses of ice sheets and sea level, which are addressed in the discussion of Challenge 2. The continued focus on this topic places scientific ocean drilling at the forefront of transformative science.
This challenge also identifies climate system sensitivity and the performance of IPCC climate models under different CO2 concentrations as areas where pre-Quaternary proxy temperature data are needed to increase understanding. Differences in climate sensitivity between data and models at low CO2 levels (<400 ppm) raises the question of other underappreciated but important feedbacks.
Challenge 2: How Do Ice Sheets and Sea Level Respond to a Warming Climate?
This challenge encompasses three areas of critical and transformative emphasis. The first area involves the need for more globally distributed atolls and coral reef records to constrain the rate, amplitude, and melt water source for sea level rise since the last glacial period, which is key to understanding the rate and pattern of future sea level rise. The second area pertains to spatially distributed sea level records for pre-Quaternary warm climate analogs such as the Pliocene (5-3 myr), which allow ice sheet-climate and proxy ice volume models to be verified. This issue is important, but difficulties exist with reconstructing sea level amplitudes by applying backstripping6 methods to continental margin drill core records. Getting these types of spatially distributed cores requires mission-specific expeditions and the ability to drill transects in difficult drilling environments, which can be costly and often need long lead times to develop successful research strategies and the necessary pre-cruise site survey information. Finally, the ability to locate drill cores proximally to ice sheets, which is the subject of the third area, is critical to the success of this challenge, because these sites provide direct physical evidence of variability in sea ice behavior and past ice sheet extent. This challenge is very important in terms of societal relevance, has potential for transformative discovery, and can only be achieved by continued scientific ocean drilling. Drilling near ice sheets and improved sediment recovery in high-latitude regions are important technical challenges that need to be addressed by the next phase of the program.
Challenge 3: What Controls Regional Patterns of Precipitation, Such as Those Associated with Monsoons or El Niño?
This challenge focuses on better understanding of regional climate variability, with specific focus on changes to the Pacific El Niño-Southern Oscillation (ENSO) due to warming and associated dramatic impacts on precipitation, drought, cyclonic activity, and the Asian monsoon. The aim of this topic is to quantify decadal and longer-term variability in regional climates and hydrological cycles, to support development of regional climate models on intermediate (10 yr-1 kyr) time scales, and to consider potential extreme events, such as floods and droughts. The challenge requires zonal and meridional transects of temperature gradients and their seasonal and longer-term variations, coupled with records of past intermediate and deepwater overturning. Sedimentary records from continental margins would need to be supported by synchronous continental records from lake sediments, ice cores, and speleothems.
Scientific ocean drilling in the equatorial Pacific identified weakening of the Pacific zonal temperature gradient and permanent El Niño-like conditions during the Pliocene. The reasons for these changes are unclear. The Pliocene is one of the best geological analogs for future warming, and, because El Niño has a major impact on global weather and moisture patterns, the study of this and similar past warmer climates has a high potential to improve understanding of the coupled ocean-atmosphere system. However, more data from scientific ocean drilling needs to be integrated with coupled ocean-atmospheric climate modeling. Although some progress can be made through the application of new temperature proxies to legacy cores, the large geographic scale of the problem requires that many new, strategically located drill cores be obtained to achieve the necessary high spatial and temporal resolutions. Because current numerical models cannot simulate permanent El Niño-like conditions without prescribing initial sea-surface temperatures, meeting this challenge requires close collaboration among groups working on model and proxy development. Developing hemispheric and regional temperature gradients for past climate states through targeted drilling is needed to achieve this and other theme challenges. The record of abrupt millennial-scale changes in ENSO above inter-annual variability, which has been identified in speleothems and corals, could also be explored through targeted high-resolution sediment cores. However, achieving the necessary resolution is generally difficult in oxygenated deep ocean regions, where bioturbation smoothes the records.
The prospect of achieving regional climate models on decadal time scales has lately been given considerable attention. Several European Union member states (e.g., France, the Netherlands, and Sweden) have invested in research funding and infrastructure and have launched a joint research program to look at modeling on this time scale. The U.S. sci-
6 “Backstripping” refers to a technique in which sediments are progressively removed from a basin in order to determine its subsidence and sedimentary history. In addition to sediment loading, corrections for changes in sea level, sediment compaction, and paleobathymetry are needed to separate out the sedimentary signal from thermal, crustal, and tectonic changes (Miall, 1997 and references therein).
entific community is aware of these climate modeling needs (e.g., Cane, 2010), and NSF supports the research through targeted calls within the context of climate, energy, and sustainability (e.g., decadal and regional climate prediction using Earth system models, water sustainability and climate change). A number of related programs exist within NSF’s Directorate for Geosciences; better integration between those programs and scientific ocean drilling could support advances in this area.
Finally, this challenge focuses on improved understanding of the role of tectonics and uplift on atmospheric circulation and precipitation patterns, as well as the influence of mountains and high terrain on erosion, precipitation, weathering, and CO2 drawdown through the silicate weathering cycle. Although having less immediate societal relevance, this much longer term process is important for understanding the Cenozoic evolution of atmospheric greenhouse gas concentrations, ocean nutrient production, and the carbon cycle.
Challenge 4: How Resilient Is the Ocean to Chemical Perturbation?
This challenge focuses on the carbon and nitrogen cycles, has the potential for transformative science, and is among the most societally relevant challenges in the climate theme. This challenge involves developing a better understanding of ocean resilience to large perturbations in atmospheric CO2 and ocean acidification, building on one of the major successes of IODP (see Box 4.2 on the Paleocene-Eocene Thermal Maximum [PETM]). The challenge identifies ocean eutrophication and oxygen depletion as key issues that can be addressed by scientific ocean drilling. In addition, the challenge poses a question about the length of time it takes for the ocean to neutralize carbonic acid following an abrupt increase in atmospheric CO2. Continued development and application of CO2 and carbonate system proxies (e.g., boron isotopes, boron/calcium ratios in carbonate fossils, carbon isotopic variations in alkenones, and other molecular organic compounds) will be required to address these issues. Although this challenge is of high importance, it is the most weakly justified within this theme. The science plan does not present a clear strategy for how future scientific ocean drilling will provide further insights on what is already known. Previous successful drilling of the PETM and the Eocene-Oligocene boundary (Coxall et al., 2005; DeConto et al., 2008), however, has demonstrated that the current scientific ocean drilling program has a strategy for and is capable of successfully addressing this challenge.
Synergies Between This and Other Science Plan Themes
There are linkages between the challenges in this theme and Challenge 7 in Biosphere Frontiers regarding the sensitivity of ecosystems to environmental change. Increased understanding of evolutionary events that are recorded in high-resolution ocean sediment cores and of the relationship to environmental change at a range of time scales has clear synergy with time series of climate proxies (e.g., temperature, pCO2) that document major environmental perturbations. Another area of connection is with Challenge 13 in Earth in Motion, which focuses on carbon flow and storage in the seafloor. Understanding carbon cycle perturbations in general and the Cenozoic evolution of atmospheric carbon dioxide in particular, especially their relationship to Earth’s surface temperature during past warmer times, clearly links the climate objectives discussed in this theme with borehole observatory science discussed in the Earth in Motion theme.
Linkages with NSF and Other Programs
The science plan attempts to acknowledge linkages with continental margin drilling programs such as ICDP, ANDRILL, and SHALDRIL (Shallow Drilling on the Antarctic Continental Margin). These programs are significant in the context of achieving transects from shallow marine to deep ocean settings, as well as in expeditions in seasonally ice-covered environments. However, the science plan provides little discussion of how these collaborative models could be advanced and implemented and how they could be balanced with mission-specific platforms. The science plan also identifies linkages that need to be developed between climate modeling programs and climate proxy development programs funded by NSF, including those at the National Center for Atmospheric Research, and other international partners and funding agencies. Finally, achievement of these challenges, especially Challenge 3, will require close integration with on-land climate and tectonics research communities that work on continental margins and mountain systems.
This theme outlines three main challenges: the origins and importance of subseafloor microbial communities; limits to life in the deep biosphere; and the relationship between marine ecosystems and changes in the environment. Of all of the challenges in this theme, some are quite novel, while others represent the continuation of present scientific ocean drilling work. This area of research is new, exciting, and cutting-edge, particularly with respect to microbiology within igneous rocks and linkages between marine organism evolution and environmental changes.
The possibility of an extensive population of microbes living beneath the seafloor raises many important and intriguing questions about the limits of microbial life, the role of marine microbes in essential biogeochemical cycles, and the origin and evolution of life on Earth and other planets. Scientists are presently very much in the exploratory phase
of some of these research areas, especially with regard to the application of next-generation “-omic” approaches (e.g., DNA and RNA metagenomic sequencing, proteomic analyses) that allow for detailed examination of the diversity, activity, and evolution of microbes in the subseafloor. However, exactly how to couple small-scale studies of subseafloor microbiology with large-scale biogeochemistry remains challenging. Because microbial biomass in marine rocks and sediments may have planetary-scale consequences, execution of these challenges is critical for success of the next phase of scientific ocean drilling.
Understanding the microbiology of the deep seafloor can only be accomplished through scientific ocean drilling. Given the large amount of data on sediment-hosted microbial communities that has already been collected by scientific ocean drilling, and the ability of microbiologists to now access “clean” crystalline rock, there is great potential for transformative science in this compelling area of research. This potential includes the interaction of microbial communities at the interface between sediments and crust, and the use of drill-hole observatories to advance understanding of subseafloor microbial communities.
Although not specifically identified in the science plan, identifying the synergies in understanding ecosystem dynamics in the deep sea—from microbes to viruses to eukaryotes, both living and fossil—is a fertile way to advance the science.
Challenge 5: What Are the Origin, Composition, and Global Significance of Subseafloor Communities?
The study of sediment microbiology has been an important aspect of both ODP and IODP, with scientists on numerous expeditions obtaining representative cores throughout the global ocean. Progress continues in the quantification and description of sediment-hosted microbial communities through a variety of microscopic and genetic tools, and the microbial community is poised to make significant strides in addressing some of the overarching questions related to the impact of microbial communities on geochemical transformations by moving beyond “who is there and how many” to “what is the activity, function, and contribution of sediment communities to carbon, sulfur, and iron cycling?” Specific missions and experiments to push our understanding of sediment communities to this next level have great potential for transformative science.
In contrast to sediment microbiology, the study of microbial life in igneous rocks is a relatively new facet of scientific ocean drilling, and there is yet to be a drilling expedition dedicated to igneous rock-hosted subseafloor microbiology. Although studies from the 1990s employed various DNA stains to drilling-recovered rocks and suggested the presence of microbes in basalt alteration zones, much less global sampling and detailed analyses of indigenous communities has been accomplished compared to the sedimentary microbial community. First-order questions concerning the origin and composition of igneous rock-hosted communities still need to be addressed, and the interactions between sediment and rock communities explored. As new observatory tools (CORKs; Circulation Obviation Retrofit Kits), instrumentation (in situ biosensors, tracer experiments), and hard-rock drilling techniques combine with microbiological and molecular techniques, exploration of the subseafloor habitat in oceanic rocks will advance considerably in the next phase of scientific ocean drilling.
Challenge 6: What Are the Limits of Life in the Subseafloor?
Marine microbes thrive in diverse and extreme environments that push the known limits of life. Challenge 6 would extend understanding of the limits of microbial life on earth in a transformative way, and its aspects (e.g., collection of extremely deep microbes) can only be accomplished through scientific ocean drilling. The committee believed that this challenge would integrate well with Challenge 5, given that the study of microbial life in rocks and sediments will necessarily include determining where in temperature-energy-pressure space these organisms are surviving. Although cataloging the diversity of life in these different extremes is important, in situ technologies such as biosensors or downhole activity enrichments, for example, could also be used to examine the limits of life. Laboratory experiments will also assist in defining the limits of life and are critical for strengthening field collections and experiments. Finally, although the connections between life in the subseafloor and the origins of life on Earth are intriguing, the committee believed that stronger experimental linkages could be made between currently measurable limits to life, laboratory simulations of abiotic carbon and energy generation in the subseafloor, and the origins of life.
Challenge 7: How Sensitive Are Ecosystems and Biodiversity to Environmental Change?
Understanding of abrupt environmental change and the role of CO2 on marine ecosystems has important implications for current anthropogenic change and great potential for transformative science with strong societal relevance. In particular, past environmental perturbations potentially driven by CO2 may have occurred at time scales similar to anthropogenically driven change, providing an opportunity to study biological consequences of rapid changes in atmospheric composition. Understanding the patterns and mechanisms of how marine ecosystems adapt to abrupt environmental change and longer-term orbitally forced environmental change also has great potential for transformative understanding of biological systems on Earth and is on the cutting edge of new science. This challenge provides justifi-
cation for the need to study marine ecosystem response and organismal evolution at large scales, which requires samples obtained through scientific ocean drilling. Such studies would complement studies of ecosystem and biodiversity sensitivity in non-marine samples (e.g., outcrops, etc), which are at smaller spatial scales than those possible in the oceans for Jurassic and younger age strata.
This challenge has strong links to the climate challenges in the previous theme. For example, Understanding Climate’s Influence on Human Evolution (NRC, 2010) highlights continent-ocean and climate-evolution linkages by proposing to drill not only lake strata spanning the time of human origin in the relevant geography, but also marine strata adjacent to Africa that received inputs from rivers with drainage areas covering the areas critical for human evolution. The goal is to link hominin evolutionary and ecological history with the high-resolution lacustrine record of environmental, particularly climatic, change within Africa and with the already developed global marine chronology.
Other Challenges and Opportunities
The science plan misses the potential opportunity to study (living) eukaryotes (such as fungi and protists) and viruses in the subseafloor biosphere, information on which is essential to truly understand subseafloor ecology and the ecosystem that may exist there. Furthermore, understanding living eukaryotes could perhaps lead to clearer linkages to the fossil eukaryotic community, with stronger overlap between Challenges 5 and 7. The subseafloor may host life forms that are completely unknown; new life-detection tools that do not depend on DNA, for example, could be employed for studying this potentially novel biosphere (e.g., NRC, 2007). The study of life in the subseafloor will also require changing the way that cores are stored after collection. Unlike most of the other disciplines involved in scientific ocean drilling, microbiology core samples need to be frozen and/or preserved with a fixative when collected. Most of the current core repository is not useful for microbiologists studying subseafloor life, which needs to be addressed in the next phase of the program. As suggested by the large number of proposals submitted to IODP that focus on subseafloor life and the funding of new programs focused on the deep biosphere (e.g., the Sloan Foundation’s Deep Carbon Observatory, the NSF-funded Center for Dark Energy Biosphere Investigations [C-DEBI]7), there is a large and growing group of scientists interested in studying microbial life in the subseafloor. Finally, justification for continued research in this area could be strengthened by consideration of two issues. The first is potential biotechnological applications of novel organisms and genes recovered from the subseafloor, especially under extremes of temperature and pressure. This is a strong point of interest for microbiology, with societal implications in the United States and beyond. The second is the unique technological challenge presented by microbiology during drilling operations in terms of sampling, processing, and instrument construction.
Synergies Between This and Other Science Plan Themes
There are natural synergies between Challenges 5 and 6 in this theme and Challenges 10 (Earth Connections; chemical exchanges between the ocean and the crust) and 14 (Earth in Motion; fluid flow in the crust), but they could have been better developed in the science plan. For example, the use of borehole CORK observatories to link hydrogeological, chemical, and microbiological observations is essential for integrating and understanding life within basaltic crust. Borehole observatories and the time series measurements they entail are discussed in great depth in Challenge 14, but they could have been more formally linked to the challenges in this theme. There are also obvious synergies between Challenge 7 and the climate theme regarding the sensitivity of ecosystems to environmental change, especially in understanding major climate perturbations that can impact the evolution of life.
Linkages with NSF and Other Programs
The challenges in this theme have many strong linkages to NSF programs and initiatives. Some important links include the National Deep Submergence Facility and the UNOLS (University-National Oceanographic Laboratory System) fleet. Many subseafloor microbial studies, including those with CORK operations, require deep submergence assets to sample and service the CORK instrumentation, adding cost and scheduling complexity. In addition, changes in marine environments can be expressed in continental environments and thus links exist with continental dynamics, sedimentary geology and paleobiology, and other areas within NSF (such as Systematic Biology and Biodiversity Inventories). Opportunities also exist with cross-cutting programs such as Dimensions of Biodiversity, C-DEBI, and ICDP. Programs of interest outside of NSF include the Sloan Foundation’s Deep Carbon Observatory and the Department of Energy’s methane hydrate and bioremediation programs.
This theme has four key challenges related to the composition and structure of the upper mantle, architecture of the ocean crust, chemical cycling between ocean crust and seawater, and relationships between subduction zones and continental crust. These challenges deal with the chemical and energy exchanges between the solid Earth, ocean, and
atmosphere, as well as their impact on the environment throughout geologic time.
This theme explores chemical and energy exchanges within Earth that lead to a distinctive layered internal structure, the tectonic activity that shapes the surface environment, and magmatic activity that builds the continents and ocean basins. The theme also deals with the importance of hydrothermal alteration in ocean crust, including how this process impacts seawater chemistry through time, as well as mantle rheology and magma generation in subduction zones. Finally, the theme also encompasses the importance of convection, both in the mantle where it may play a role in driving plate tectonics and in the outer core where it is responsible for generating Earth’s magnetic field.
One of this theme’s main messages is the great need to drill more deeply into and through intact ocean crust. Although the science plan emphasizes drilling to the Mohorovi?i? discontinuity (Moho), an ocean drilling objective with a long history (see Box 6.1), another aim could be to obtain good recovery of intact oceanic crust samples along the way. Some of the other objectives related to spatial variability in oceanic crustal structure and evolution could be better addressed with multiple, carefully chosen, shorter holes. Multiple, shorter holes could be very helpful in understanding ocean crust hydration (serpentinization), carbonation, and oxidation, where ultramafics that are out of equilibrium with their environment (usually through uplift processes) are altered through contact with water into serpentinite minerals.
Drilling intact ocean crust with high recovery rates would allow much to be learned, although a specific target of drilling to the Moho may not be technically viable and could be cost-prohibitive. Better understanding of oceanic mantle serpentinization is essential, and could be achieved through shorter holes drilled in disrupted ocean crust.
Challenge 8: What Are the Composition, Structure, and Dynamics of Earth’s Upper Mantle?
The single longest-running goal of scientific ocean drilling has been recovering samples from the Moho, prompting the very first ocean drilling project (Project Mohole; Box 1.3). Despite more than 50 years of scientific discoveries, there has been little progress toward that goal. Few drill cores penetrate deeply into crystalline ocean crust, and none has remotely approached the Moho. The science plan states that the scientific ocean drilling community is working concertedly toward this goal, and that doing so is scientifically significant. Several community workshop reports on this topic discuss the scientific merits in greater detail than that found in the science plan. Clearly, it seems that a substantial part of the scientific ocean drilling community is ready to embrace this project. The committee discussed this topic at length and recognized technological challenges that would have to be overcome before such a deep hole could be drilled (Box 6.1). Opinions vary about the utility of recovering samples of oceanic crust and mantle at just one location, even if directional drilling could be achieved. Mantle heterogeneity may preclude broad generalizations about processes, especially when based on samples recovered at a single location.
Nonetheless, first-order petrologic and geochemical questions could definitely be addressed by materials recovered from a hole to the Moho (often referred to as the “Mohole”). In addition to the significant technical challenges involved in drilling to the Moho, recovering Moho samples might not be the biggest scientific contribution from the drilling attempt. The presence of exposed, serpentinized peridotite at slow spreading ridges suggests that in some places the Moho is the boundary between mafic oceanic crust and ultramafic upper mantle, while in other locations it is a serpentinization front separating altered from unaltered peridotite. It would be valuable to have good estimates of the relative abundance of those two different kinds of Moho. The importance of recovering intact samples of oceanic crust and mantle may lie less in reaching the destination of the Moho, and more in the recovery of materials drilled along the way. Having the ability to determine the nature of petrologic and geochemical processes in the mantle that lead to the building of oceanic crust would be a significant development, possibly leading to transformative science.
Challenge 9: How Are Seafloor Spreading and Mantle Melting Linked to Ocean Crustal Architecture?
This challenge states that understanding linkages between seafloor spreading, mantle melting, and ocean crust architecture requires not only recovery of core material from intact crust from a single Moho location, but also shorter holes drilled through tectonically disrupted ocean crust. Fundamental differences between fast- and slow-spreading mid-ocean ridges and the processes that result in magmatic vs. amagmatic spreading are still poorly understood. These are first-order plate tectonic phenomena. The prevalence of detachment faulting at slow-spreading ridges is also a scientific topic of great significance. This challenge states that linking a series of shorter holes with geophysical investigations and seafloor mapping would add to the understanding of oceanic lithosphere structure that would be gained from drilling to the Moho. Because of the paucity of holes in crystalline basement, better understanding would almost certainly result. However, significant difficulties have been encountered when attempting to drill directly into crystalline ocean crust that lacks sediment cover, especially young volcanic crust, which could prevent progress on this front unless new technologies are developed.
The magnetic field is a shield that allows planets to maintain an atmosphere and is therefore a prerequisite for life on Earth. In addition, any major change in Earth’s magnetic field will impact important infrastructure such as telecommunications
Technological Challenges in Drilling to the Moho
The first scientific ocean drilling was undertaken as a challenge to drill to the Moho (NRC, 2000). The science plan for the next proposed phase of scientific ocean drilling has renewed this goal. At the beginning of Project Mohole, deepwater technology was in its infancy, with only a few ships capable of drilling in water depths greater than 1,000 m. Although Project Mohole engineers created new methods and technology to achieve scientific ocean drilling goals (see Box 1.3), they were unable to reach the depths needed to drill to the Moho. Since that time, the deepwater drilling and production industry has consistently drilled in water depths greater than 2,500 m, and the JOIDES Resolution reached greater water depths using a riserless drilling system. Advances in materials science, new control systems, and better automation have all contributed to greater abilities in floating production platforms and riser drillships, allowing them to move into very deep water (3,000-4,000 m). This leads to a question: using existing deepwater technology, how can the Moho be drilled with the highest probability of success?
Moho drilling involves two major risks: subsea risk arises from drilling very deep water, with issues regarding currents and open water surface conditions, and drilling risk arises from drilling in more than 6,000 m of rock with poorly known properties and composition (e.g., high temperatures). The risk of drilling rock at high temperatures and unknown conditions is a topic that would need to be addressed thoroughly before deciding on a target for Moho drilling.
A wide range of opinions exist regarding the viability of using existing scienti c ocean drilling platforms to drill the Moho. The committee believes that the technical capabilities of the riser drillship Chikyu are insufcient to ensure success in this endeavor.
Even though modern riser-equipped drill ships have achieved outstanding results, drilling on station for long periods of time is still challenging because of risks in handling the riser. Modern deepwater drill ships can drill to 6,000 m while maintaining station, even in severe weather conditions and strong currents. However, the long duration of a Moho project is significantly different than commercial deepwater drilling, and equipment failure over time could lead to operational delays and increased expenses associated with pulling the riser and the blowout preventers. By comparison, a fixed platform only needs to install the riser once, and blowout preventers are at the surface, mitigating subseafloor risk. Fixed platforms also have increased storage, better support facilities, and the ability to drill in extreme weather. Tethered deepwater platforms, for example spars or semi-submersibles, are types of fixed platforms that can be moored in water depths up 2.4501 and 3,000 m, respectively. A fixed or tethered platform may be superior to a mobile drilling platform for the purposes of Moho drilling.
If the goal of drilling to the Moho is to be achieved, additional resources for new technologies and perhaps alternate drilling platforms would need to be considered.
and power grids. Geomagnetism, another area of scientific study that will benefit from continued scientific ocean drilling, encompasses three aspects with the potential for transformative change. First, continuous sediment records collected from a select number of sites could yield time series of geomagnetic dipole declination, inclination, and relative paleointensity. These data, best retrieved through scientific ocean drilling, could provide benchmarks for evaluating numerical models that have been proposed to describe the fluid core motions thought to be responsible for the magnetic dipole and its reversals and excursions. Better models and more data would lead to increased understanding of the ongoing changes in intensity and direction of the present geomagnetic field. Second, validation of true dipole excursions from dating continental and marine sediments could provide a new high temporal resolution magnetic time scale, in conjunction with currently available geomagnetic polarity reversal stratigraphy, for dating climatic and environmental events. Third, the collection of continuous, intact cores of fast-spreading ocean crust, when supplemented by opportunistic sampling of peridotite and serpentinite from tectonically exposed upper mantle, could provide essential ground-truthing opportunities for proposed sources of marine magnetic anomalies. These cores could provide valuable information for modeling some anomaly features that are currently not well understood, including skewness, amplitude variations, and other parameters that allow for reconstruction of oceanic plate formation and aging.
Another opportunity for significant scientific breakthrough lies in improved documentation of the emplacement history of oceanic large igneous provinces (LIPs). Currently, some researchers theorize that LIP emplacement occurred over short time intervals of no more than 800 kyr, which could explain pulses of CO2 release into the atmosphere and possible relationships to mass extinction events. Very little scientific ocean drilling and core recovery has been done on submarine LIPs, and the paucity of materials has resulted in the inability to answer significant questions about their timing and origin. Increased activity on this topic, with significantly better core recovery, would lead to transformational understanding of oceanic LIPs and the implications of their emplacement.
Challenge 10: What Are the Mechanisms, Magnitude, and History of Chemical Exchanges Between the Oceanic Crust and Seawater?
A broad misconception in the geological research community is that the existence of many scientific ocean drilling sites throughout the ocean basins is equivalent to significant sampling of the crystalline crust and underlying upper mantle. This challenge underscores the fact that little drilling has occurred in crystalline basement and that core recovery has not been good. Designing drilling objectives to increase the volume and spatial distribution of hard rock cores will result in significant advances that are likely to transform understanding of the structure and composition of oceanic lithosphere.
As stated under Challenge 8, quantifying the volume of altered mafic crust and serpentinized ultramafic mantle that results from chemical exchanges between the solid Earth and seawater are significant pursuits likely to provide transformative understanding of the carbon cycle. For example, studying the distribution of intact vs. tectonically disrupted crust would lead to better understanding of the long-term carbon cycle. However, water-rock interaction has been a topic of concerted research for at least two decades. It is uncertain if truly transformative science can be accomplished by continued scientific ocean drilling investigations aimed at such targets. Broad estimates of geochemical and thermal exchanges between various reservoirs can be made with current data; refining those estimates could be a priority within the scientific ocean drilling community. However, the ability to place reasonable constraints on the volume of serpentinized mantle requires additional drilling and could dramatically transform understanding of carbon uptake in mantle rocks and the long-term carbon cycle. This would be one of the more significant ocean drilling contributions for better understanding of water-rock interaction.
The science plan states that the best strategy to address this challenge would be a series of drilling transects across ocean basins, from youngest to oldest oceanic crust and in a variety of different regions. The committee believes that much could be learned with such a strategy, but has concerns about such an ambitious plan under the current uncertain funding climate. If such a strategy is to be employed, it would be best to plan drill holes that could contribute to multiple drilling objectives in addition to studying chemical exchanges between seawater and crust.
Challenge 11: How Do Subduction Zones Initiate, Cycle Volatiles, and Generate Continental Crust?
Currently, most research focused on the area of subduction zone initiation, cycling, and generation of continental crust is based on observations and elemental analyses from ophiolites, which have mid-ocean ridge geochemical signatures overprinted by arc-like elemental distributions from younger magmatism. The proposed studies in the science plan invoke subduction initiation, at least in intra-oceanic settings, along fracture zones that are far from ridge systems and occur in significantly cooled lithosphere. Although this challenge’s concept is indeed important, the science plan does not provide a clear approach on how it could be tackled. Information supporting this challenge is vague, with no identification of potential drill sites and a lack of clarity about whether any exist, which makes it difficult to assess whether scientific ocean drilling could contribute to greater understanding in this field. Because of this lack of clarity, transformative science is not a likely outcome.
Cycling of volatiles in subduction zones has also been a topic of considerable research since cold seeps and mud volcanoes were discovered close to or within accretionary prisms. Observational data have been collected, leading to some quantification of fluxes. Certainly, additional work could result in new discoveries and further refine flux estimates. Scientific ocean drilling in this area would likely contribute to the very significant work that has already been done. Additionally, the NSF-funded GeoPRISMS (Geody-namic Processes at Rifting and Subducting Margins) and predecessor MARGINS programs have led to much work on understanding the “subduction factory.” As with subduction initiation, further work in the next phase of scientific ocean drilling would be useful but may not be transformative.
Synergies Between This and Other Science Plan Themes
The challenges in this theme have a natural fit with all of the challenges in the Earth in Motion theme (discussed in the next section). These challenges focus on processes controlling geohazard occurrence, the flow and storage of carbon in the ocean crust, and the role of fluids in seafloor processes. Challenge 11, which includes understanding how subduction zones initiate, has natural synergy with the study of mechanisms that cause earthquakes (Challenge 12) and implications for cycling of crustal fluids (Challenge 14). Further study of the architecture of Earth’s crust in Chal-
lenge 8 has links with Challenge 13, especially in the role of serpentinization.
Linkages with NSF and Other Programs
There is considerable room for cooperation between activities that would occur under this challenge and the NSF GeoPRISMS Program, which in its earlier existence as MARGINS actively supported research on the topics covered under Challenge 11. Together, GeoPRISMS and the next phase of proposed scientific ocean drilling could make more concerted progress on significant topics if their objectives are tightly integrated. In addition, natural linkages exist between this theme and those of ICDP, the InterRidge Program, and the Deep Carbon Observatory. For example, further investigation of submarine LIPs will benefit from continued integration with terrestrially based studies.
The fourth theme of the science plan concerns scientific objectives that require physical measurements or samples from boreholes as a function of time. Because physical and chemical properties of oceanic sediments and rocks change in response to fluid flow and subseafloor biological activity, the snapshots in time provided by conventional drilling and sampling are not adequate to answer many of the challenges presented in previous themes or explicitly discussed in the context of this theme. For example, temporal changes in fluid composition and flow rates must be measured to understand ecological changes beneath the seafloor, as in Challenge 7. Such measurements have been a part of past scientific ocean drilling programs since 1991, with the first CORK installation in Middle Valley on the Juan de Fuca Ridge (Becker and Davis, 2005). However, these measurements have been few and far between, mainly because of installation expenses and the need for extensive background studies to provide a geologic context for time series measurements. The relative importance of the information derived from the limited experiments to date suggests that demand will almost certainly grow in the next decade as new scientific questions develop from the information provided by DSDP, ODP, and IODP cores.
Recent developments in ocean observing systems that facilitate data communication to shore-based laboratories and in simpler, cheaper, specialized sensor packages are likely to enable further growth of borehole observatories.
The research objectives that require time series measurements are grouped into three closely linked challenges: understanding mechanisms related to major earthquakes, tsunami, and landslides; subseafloor carbon sequestration and cycling; and subseafloor fluid flow processes.
Challenge 12: What Mechanisms Control the Occurrence of Destructive Earthquakes, Landslides, and Tsunami?
The science plan notes that processes related to landslide, earthquake, and tsunami generation comprise “the only large-scale natural hazards for which no short-term prediction exists.” Any significant increase in the short-term predictive capability for great earthquakes and tsunamis would be truly transformative. Acquisition of samples from the depths at which slip originates and the ability to monitor physical and chemical changes in the fault zone would provide critical new information on how frictional properties of faults change with time as a result of diagenesis, changes in fluid pressure, or other factors. Many of the largest and most destructive faults are located offshore, so sampling and in situ measurements of these faults can only be achieved by scientific ocean drilling. Because of the great depth and potentially corrosive conditions in these locations, these are difficult and expensive objectives that can only be achieved at a small number of sites.
This challenge does a good job of enumerating both the difficulties and societal benefits of addressing this challenge. The likely heterogeneity of fault slip surfaces suggests the need to collect a global range of seismological, geodetic, and geologic measurements from active faults and incipient landslides. Although scientific ocean drilling will be required to collect some of these measurements, well-established links with other initiatives (e.g., continental drilling) will also be required to obtain the complementary information that is needed to fully address the challenge. For example, the NSF/ ICDP/EarthScope SAFOD (San Andreas Fault Observatory at Depth) effort to sample and instrument an active strand of the San Andreas Fault at seismogenic depth provides complementary information on the challenges of maintaining an observatory in a fault zone at several km depth as well as on alteration of fault zone rocks.
The past decade’s discovery of episodic tremor and slip and of very low frequency (slow) earthquakes in subduction zones also provides a potential specific new target for drilling. The occurrence of this broad spectrum of fault slip behaviors at the down-dip edge of subduction megathrusts is currently being studied primarily by land-based networks and a recently proposed ocean bottom seismic network in Cascadia.8 Similar processes may also occur at the up-dip edge of the megathrust, but remain poorly documented because of the lack of offshore seismic and geodetic stations everywhere except offshore Japan. The science plan does not clearly enunciate the contribution of scientific ocean drilling to understanding these phenomena. Another important aspect
of subduction zone geohazards that was not included in the science plan is the extension of paleoseismic histories further into the past. Although coring via less expensive platforms should remain the first approach to defining paleoseismic histories of large submarine faults, scientific ocean drilling will be needed to extend these records farther back in time. For example, short cores (less than 10 m long) have been successful at defining the earthquake history for the past ~12 kyr in Cascadia (Goldfinger et al., 2011). Longer time series are needed to develop robust models for earthquake recurrence patterns at major subduction megathrusts.
Scientific ocean drilling has the potential to improve forecasting and to provide early warnings of geohazards like earthquakes, tsunamis, and landslides. The science plan emphasizes installation of observatories at the base of landslides and at great depth in the seismogenic zone. However, conventional coring and logging that can be used to groundtruth seismic data, provide shallow holes for installation of arrays of buried geodetic and seismic instrumentation, and extend paleoseismic histories of major fault zones have equal, if not greater, potential for leading to transformative new insights. As the next phase of scientific ocean drilling moves forward, it would be helpful to consider a broader range of studies that could be designed in collaboration with other national and international geohazard programs (e.g., GeoPRISMS, U.S. Geological Survey).
Challenge 13: What Properties and Processes Govern the Flow and Storage of Carbon in the Seafloor?
This challenge concerns better understanding of the role of the subseafloor environment in the global carbon cycle. Three specific aspects of this problem are highlighted: distribution and dynamics of gas hydrates in marine sediments; the fate of carbon dioxide when it is injected into the seafloor (carbon sequestration); and the impact of hydration (serpentinization), carbonation, and oxidation of ultramafic rocks by seawater and dissolved CO2. All aspects of this challenge potentially have direct societal impacts. Gas hydrates have potential importance as an energy source, while their destabilization in response to environmental change may trigger underwater landslides with consequences for coastal communities. Ocean crust has been proposed as a possible repository for excess carbon dioxide, a growing concern due to global warming, and fluid-rock interactions may provide a way to trap excess carbon in deeply buried solids. Serpentinization, meanwhile, may drive or facilitate a number of tectonic processes because of the release of heat, increase in volume, and lowering of shear strength of serpentine minerals. Measuring rates at which fluids move through the subseafloor and at which rocks and sediments respond requires in situ time series observations, and can benefit from controlled perturbation experiments. This challenge emphasizes the flow and storage of CO2, but the implications for understanding the flux of carbon dioxide and other vola- tiles through the seafloor are actually much broader and are encompassed to some extent within the next challenge. By quantifying the role of water-rock interactions in the carbon cycle, this challenge would lead to transformative discovery. The volume of seawater flowing in the subseafloor aquifer is unusually large, but it is unknown how that impacts carbon exchange between water and rock. Similarly, the science plan discusses the role of serpentinization of exposed peridotite at slow-spreading ridges. Quantifying the elemental exchange associated with that process would lead to more accurate modeling and would add significantly to understanding of the carbon cycle.
Challenge 14: How Do Fluids Link Subseafloor Tectonic, Thermal, and Biogeochemical Processes?
This final challenge encompasses a wide range of topics that have already been partially discussed in the context of previous challenges. Recent studies have revealed that the amount of fluid being fluxed through the ocean crust is greater than previously thought and that changes in crustal fluid pressure and chemistry can change abruptly in response to distant earthquakes. The global implications of this apparently vigorous exchange between the crustal aquifer and the ocean remain to be explored, and there is little doubt that continued exploration will yield new surprises.
Calibration and verification of subsurface fluid flow requires direct measurement at more than one site so that more readily acquired proxies for fluid flow and chemical exchange in the subseafloor can be exploited, and results from local studies can be extended globally. Such calibration and verification requires direct sampling of the subseafloor fluid flow rates and compositional changes, information that cannot be obtained without scientific ocean drilling and the installation of long-term monitoring devices.
Significant potential exists for transformative scientific discovery related to studying subseafloor hydrology. Much will be gained from installation of monitoring networks related to seismic, geobiological, and other studies, but there is also a significant need for installations dedicated to long-term hydrological observations. Despite the availability of decades of data from terrestrial hydrology monitoring networks, significant aspects of the water cycle cannot be quantified. Developing such broadly based quantitative models of marine hydrogeology will take a significant investment of time, but the potential contribution to understanding a wide range of phenomena makes this long-term effort a high priority.
Other Challenges and Opportunities
Submarine geodesy is an aspect of this theme that is not discussed in the science plan, but one where long-term observatories in the subseafloor could potentially have a transformative impact. With the advent of widespread acqui-
sition of continuous GPS data on the continents, geodesy has transformed the understanding of the spectrum of fault slip behaviors, revealing a continuum of spatial and temporal scales. Development of marine geodetic networks on the sea-floor would require stable baselines for measurement, which could perhaps be associated with borehole observatories to measure position and strain as a function of time. A focus on this type of interdisciplinary instrumentation could facilitate transformative understanding of active tectonics in the ocean.
Synergies Between This and Other Science Plan Themes
As mentioned previously, Challenge 14 has natural synergy with topics discussed in all of the previous themes. This includes Challenges 1 and 4 (Climate and Ocean Change theme) on Earth’s response to higher CO2 concentrations and resilience to changes in ocean chemistry; Challenges 6 and 7 (Biosphere Frontiers theme) on the limits of life and ecosystem sensitivity in the subseafloor; and Challenges 10 and 11 (Earth Connection theme) on seawater-crust cycling and initiation of subduction zones. In addition, Challenge 13’s focus on carbon sequestration in the seafloor is related to the Climate and Ocean Change theme, especially with regard to a potential role for geoengineering in mitigating climate change.
Linkages with NSF and Other Programs
The scientific objectives of this theme dovetail with the objectives of many other national and international initiatives. There is a particularly strong symbiosis with the Ocean Observatories Initiative (OOI), which can enable high-resolution, real-time data flow from instruments that can be deployed in boreholes. It is essential that planning for these two programs be integrated so that boreholes and observing systems are collocated when appropriate. A comprehensive assessment of earthquake and tsunami hazards should take into account the full spectrum of observations from synergistic programs like EarthScope, GeoPRISMS, Ocean Bottom Seismography Instrument Pool Cascadia Initiative, NEPTUNE Canada, OOI, and the Dense Ocean Floor Network System for Earthquakes and Tsunamis.