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2 New Research Opportunities in the Earth Sciences T he vitality of the current Earth science research of biogeochemical and water cycles in terrestrial envi- community is manifestly evident in the numer- ronments. These address a range of grand challenge- ous strategic planning, Grand Challenges, and scale fundamental topics of both curiosity-driven and science vision documents that have been produced over strategic Earth Science. Key to many of these topics the past decade (a list of key documents is presented and to many other Earth science applications are geo- in Appendix A). Any attempt at comprehensive assess- chemical approaches to geochronology by exploiting ment of new research opportunities across the disci- the variety of stable and radiogenic isotopes that exist pline would quickly become unwieldy, and the finite in nature to provide relative and absolute dating of expertise of any committee would result in some over- geological materials and events. The expanding demand sights. The committee on New Research Opportunities for accurate sample dating for many of the research in the Earth Sciences (NROES), informed by personal opportunities motivates consideration of restructur- knowledge, myriad documents produced by workshops ing EAR-supported geochemical facilities that must and community organizations, and both solicited and simultaneously promote innovation of methodologies, contributed input from many researchers and program training of next-generation geochemists, and servicing managers (see Appendix B) has attempted to iden- the burgeoning demands for what are seldom routine tify specific areas in the basic Earth science research sample dating analyses. scope of the Division of Earth Sciences (EAR) of the National Science Foundation (NSF) that are particu- THE EARLY EARTH larly poised for rapid progress during the next decade. Seven primary topics involving complex dynamic Much of Earth’s present-day structure and signifi- geosystems that can only be fully quantified by inter- cant parts of its history can be traced back to events disciplinary approaches are highlighted in the fol - that occurred within the first few hundreds of million lowing sections organized by scale and disciplinary years after its formation. Understanding the processes participation related to the EAR Deep Earth Pro- involved in Earth accretion and early chemical differ- cesses and Surface Earth Processes sections: (1) the entiation is essential for establishing the initial thermal early Earth; (2) thermo-chemical internal dynamics conditions of the dynamical systems of the interior, the and volatile distribution; (3) faulting and deforma- volatile content of the planet, and the origins of the tion processes; (4) interactions among climate, Earth continents that have led to the current Earth system. surface processes, tectonics, and deep Earth processes; Recent progress on understanding the early Earth has (5) co-evolution of life, environment, and climate; been substantial, yet we have only begun the task of (6) coupled hydrogeomorphic-ecosystem response to resolving the timing, nature, and interrelationships natural and anthropogenic change; and (7) interactions of the most decisive events, including cataclysmic 13
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14 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES impacts; magma oceans; segregation of the core; early Geochemical and cosmochemical observations forms of continents, oceans, and the atmosphere; the provide important constraints on the timing and the onset of plate tectonics; and, of course, the origin of mechanisms of accretion and segregation of the core, life. Because Earth grew and differentiated rapidly, the a lthough several interpretations are possible. For energy available to the Earth system during its early example, in the Hafnium-Tungsten (Hf-W) system, history was far higher than today, permitting whole sets the excess radiogenic 180W in the silicate Earth rela- of physical and chemical processes without counter- tive to chondritic meteorites has been interpreted as parts in the modern Earth. The overarching challenge rapid accretion or alternatively as incomplete mixing here is to understand how Earth transitioned from its of the impactor with the growing Earth (Halliday, formative state into the hospitable planet of today (see 2008; Rudge et al., 2010). Similar interpretations have Box 2.1). Lessons learned from the early Earth will help come from other short-lived isotope systems, such us interpret the processes occurring in the hundreds of as 146Sm - 142Nd (O’Neil et al., 2008), which also extrasolar planetary systems now being discovered by have implications for the earliest crust. The fusion of astronomers. geochemistry and geophysics offers many promising avenues for better understanding formative processes that governed the early history of the Earth. Accretion of Earth Based on isotopic evidence from meteorites, what The birthplace of Earth was a protoplanetary accre- originated as occasional planetesimal collisions soon tion disk, a cloud of gas and dust surrounding the early began to run away, leaving a small number of rapidly Sun. Modern astronomy provides a glimpse of what growing planetary embryos. Improved chronologi- this environment may have been like, in the form of cal methods reveal that melting and differentiation debris disks that surround young stars, some of which occurred within a few million years of the formation have been imaged by the Hubble Space Telescope (see of the first solids, probably driven by collisions and Figure 2.1). Accretion disks are subject to instabilities assisted by now-extinct radioactive heat producers driven by powerful gravitational and electromagnetic such as 26Al and 60Fe. Accordingly, the assumption that forces that collect dust particles into planetesimals, Earth formed by a continuous influx of small particles typically 1-kilometer-sized objects that were the fun- made of pristine solar system condensates has given damental building blocks of Earth and the other terres- way to a much more dramatic model, in which Earth trial planets. Once a sufficient density of planetesimals was assembled by a relatively small number of trau- developed in the nebular cloud, increasingly violent matic collisions involving larger objects, some of these collisions began to dominate the accretion process, already having differentiated interiors and well as their forming an ever-smaller number of growing planetary own internal dynamics (Canup and Asphaug, 2001). embryos that swept up most of the remaining nebular Future progress on the processes and timing of Earth’s debris. growth in the coming decade will rely on a diversity of Although much effort has been directed toward approaches, including: understanding accretion from the perspective of solar system dynamics, many related processes that were • Application of new isotope techniques for dat- important for early Earth have not received the same ing methods attention. Accretion models, for example, often assume • Closer integration of isotope geochemistry with that colliding planetesimals simply adhere, ignoring astrophysical approaches to planetary formation effects like fragmentation, spin and precession, melt- • More comprehensive and more realistic dynami- ing, vaporization, condensation, and differentiation cal models of the accretion process (Chambers, 2004). There is mounting evidence for • Evolutionary studies of the chemistry and these processes, many of which bear directly on the physics of planetesimal-sized objects and plan- final chemistry and structure of the accreting body etary embryos (Halliday, 2004).
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15 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES BOX 2.1 Planetary Science Earth’s interior and surface environments are profoundly influenced by our position in the Solar System and interaction with the Moon and other planets. The Moon stabilizes the orientation of Earth’s spin axis and promotes climate stability, in stark contrast to, for example, Mars. Gravitational interactions between Earth and other planets, particularly Jupiter, cause small variations in the eccentricity, obliquity, and precession of Earth. While small, these variations are likely partly responsible for ice ages. The discovery of hundreds of planets orbiting other stars, so-called extrasolar planets, provides a new opportunity to understand whether the architecture of our solar system and presence of an Earth-like planet in the habitable zone are common. Planets around young stars may also offer a window into the earliest Earth. Limited information, however, is available about these planets, and in the best cases we know their mass, radius, eccentricity, and temperature and are able to detect some gases. Continued exploration of our own solar system has led to new, unexpected discoveries: an active dynamo on Mercury, eruptions on Enceladus (see Figure B2.1), and methane lakes on Titan. These discoveries provide new opportunities to test our understanding of the basic processes that govern planetary evolution and interactions between Earth systems, particularly the interior, the geodynamo, surface environments, and the atmosphere. Understanding these new discoveries and further exploration of our solar system are activities typically supported by the National Aeronautics and Space Administration (NASA). Nevertheless, there are opportunities to better understand Earth systems and the earliest Earth made possible by exploring other planetary objects. Collaboration between NASA and NSF in supporting such projects can only be positive. FIGURE B2.1 Ice geysers erupt on Enceladus, the bright and shiny inner moon of Saturn. This image presents a backlit view of the moon’s southern limb, where icy plumes were discovered by the NASA Cassini spacecraft mission in November 2005. Cryovolcanism is evidence that the 500-km- diameter Enceladus has active internal tectonics. SOURCE: NASA Jet Propulsion Laboratory/Space Science Institute.
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126 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES 16 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES FIGURE 2.1 Hubble Space Hubble Space images showing young stars surroundedyoung stars surrounded the birthplaces Figure 2.1 Telescope (HST) Telescope (HST) images showing by dust rings thought to be of planets like Earth. Top: The planet Fomalhaut b orbits the star Fomalhaut (25 light-years away in the southern constellation Piscis Australis) near a ring of dust similar to the Kuiper Belt that may contain bodies ranging from dust grains to objects the size of dwarf by dust rings thought to be the birthplaces of planets like Earth. Top: The planet planets. Bottom: Light reflected off a debris disk in cross section around the young star AU Microscopii, HD197481. SOURCES: Top: NASA, the European Space Agency (ESA), and Z. Levay. Bottom: NASA, ESA, and J. Graham. Fomalhaut b orbits the star Fomalhaut (25 light-years away in the southern constellation Piscis Australis) near a ring of dust similar to the Kuiper Belt that may Response to the Moon-Forming Impact dust grains to objectsThe size of dwarf planets. Bottom: the compositional similarity of Earth’s mantle contain bodies ranging from and the Moon and realization of the importance of AlthoughLconclusive evidence a still lacking that cross large impacts in the early Solar System, together with ight reflected off is debris disk in section around the young star AU ever-larger impacts dominated the later stages of Earth’s the large angular momentum present in the Earth- growth, the global dynamical and thermal implicationsTop: Moon system, have led Space Agencythat the Moon Microscopii, HD197481. SOURCES: NASA, the European to the theory of this process are not in doubt. Once Earth reached an formed as the result of a late cataclysmic impact of a appreciable mass, the and Z. Levay. Bottom:kinetic and (ESA), enormous amounts of NASA, ESA, and ars-sized object with the growing Earth (Wetherill, M J. Graham. gravitational potential energy released by large impacts 1990). Particle-based simulations of this giant collision dictate widespread melting, with regional and possibly (Canup, 2004; see Figure 2.2) predict that much of the global magma oceans extending to considerable depths preexisting layered structure of Earth was obliterated (Tonks and Melosh, 1993).
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controls on crystal fractionation, and the dynamic stability of the end product of magma ocean crystallization • The survivability of structures created by early differentiation 17 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES The Process of Planet Growth FIGURE 2.2 Two time slices in the animation of a glancing impact of a Mars-sized planetary embryo into the proto-Earth. The silicate mantles of both objects are shown in yellow, whereas their metallic cores are shown in red. The first image is slightly before Figure 2: Two time-slices in the animation of a glancing impact into the proto- the impact; the second is about two orbital rotations of the proto-Earth following impact. SOURCE: Reprinted from Canup (2004), Earth (from (Canup, 2004)). The silicate mantles of the planetesimals are shown with permission from Elsevier. in yellow while their cores are red. The first image is slightly before the impact whereas the second is taken after about two orbital rotations of the central object. and a substantial portion of the impacting material was early magma ocean on Mars is thought to be respon- thrown back into orbit, creating a post-impact accre- sible for the range in source compositions of Martian tion disk surrounding the proto-Earth, complete with meteorites (Borg and Draper, 2003). As is the case for a its own silicate vapor atmosphere. These simulations moon-forming impact, indisputable evidence for magma also predict that the Moon consists primarily of mate- oceans and their associated early atmosphere on Earth rial from the impacting object, and not material from remains elusive, although there is indirect evidence from proto-Earth. This computational model is challenged abundance patterns of the elements affected by core for- by remarkable similarity in oxygen isotopes found mation (Kleine et al., 2004), plus some isotopic evidence between lunar and Earth rocks, raising questions about for early mantle differentiation and atmosphere forma- the partitioning of material during impact. tion that are indicative of a magma ocean environment Despite its widespread acceptance, direct evidence (Moynier et al., 2010). What is more certain, however, is of a Moon-forming giant impact—the smoking gun that terrestrial magma oceans and the early atmosphere in Earth’s early history—remains elusive. Similarly, provided highly dynamical environments in which a our understanding of the events accompanying giant wide variety of chemical and physical processes were impacts and their consequences for the chemical and active, ranging from shock-wave heating to fracturing physical modification of the early Earth remain sketchy. and fragmentation, turbulent convection, percolation, Further delineation of the Moon-forming event and mixing, and a host of possible redox reactions. Under- its consequences for Earth are high priorities for the standing the evolution of a terrestrial magma ocean coming decade. requires answers to such basic questions as: • What is the relationship between impact and Terrestrial Magma Oceans magma ocean sizes? Magma oceans, an almost inevitable consequence of • What is the lifetime of a magma ocean, and how large planetary impacts given the energies involved, were is it coupled to the early atmosphere? first proposed to explain the plagioclase-dominated crust • Does a terrestrial magma ocean crystallize from of the Moon (Warren, 1985), and differentiation in an the bottom up or from the top down?
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18 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES • Was there a deep-mantle abyssal magma ocean? (Shearer, 2006). As is the case for many shallow layered • Do deep melts rise or sink in the early mantle? mafic intrusions on Earth, buoyancy-driven separation • What sequence of crystals form in a cooling of lower-density Ca- and Al-rich plagioclase from magma ocean? denser Mg- and Fe-rich silicates occurs on the Moon. • As a magma ocean crystallizes, is it stably strati- On Earth, however, the greater range of internal pres- fied, or will it overturn? sures introduces the likelihood of liquid-solid density • How did metals and silicates mix and then seg- crossovers (Mosenfelder et al., 2007; Stixrude et al., regate in magma oceans? 2009), so magma oceans may stabilize at both the top • What was the nature of mantle dynamics follow- and the base of the mantle (Labrosse et al., 2007), as ing magma ocean solidification? shown in Figure 2.3, significantly complicating their evolution. Providing answers to these questions will prob- ably require geodynamical modeling constrained by Core Formation improved understanding of the petrology of melts and element partitioning at high pressures and temperatures, In addition to the energy acquired from impacts, in parallel with interpretations of present-day seismic the segregation of the core released enormous amounts images of mantle heterogeneity in the deep mantle, of gravitational potential energy into the Earth system. where the chances are best of finding relics of this Isotope evidence generally points to early core forma- process still preserved. In addition, many of the issues tion (Yin et al., 2002), which is consistent with the raised by these questions are linked together, requiring magma ocean hypothesis, wherein growth of the core cross-disciplinary expertise. For example, separation of essentially kept pace with growth of the mantle. There immiscible liquids (in this case, iron from silicate melts) are several theories on how the core formed that are with greatly different densities happens rapidly in a compatible with large impacts and the existence of low-viscosity magma ocean, whereas buoyancy-driven magma oceans. One theory assumes that impacting segregation of silicates depends on the environmental cores fell through the magma ocean as large metal conditions. Because the moon’s interior spans a small masses, directly merging with Earth’s core (Halliday, range of pressures, the crystallization sequence of a sili- 2006). Another assumes that dispersed metal rained cate lunar magma ocean is reasonably well understood down through the magma ocean, collected at its base, FIGURE 2.3 Schematic evolution of progressive crystallization of surface and basal magma oceans (yellow) following Earth accre- tion and core formation, based on the assumed deep-mantle density crossover between melt and solid, leading to upward segrega- tion of melts in the upper mantle and downward migration of melts in the lower mantle. Core-forming metals are shown in orange; solid mantle is shown in gray, with circulation indicated by arrows. SOURCE: Labrosse et al. (2007). Reprinted by permission from Macmillan Publishers Ltd. ﬁg. 1
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19 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES then descended through the underlying crystalline deficient, mantle. The consensus view is that Earth’s mantle by several possible mechanisms, including frac- initial atmosphere, composed mostly of hydrogen, ture propagation, large metal diapirs, or metal-silicate was lost very early, perhaps during a T-Tauri phase of plumes (Ricard et al., 2009). The measured abundances solar activity or through hydrodynamic escape to space in the mantle of moderately siderophile elements such aided by the strongly ultraviolet-emitting young Sun as nickel (Ni) and cobalt (Co) indicate that some degree (Catling, 2006). As for the early composition of the sec- of chemical equilibration between core-forming met- ondary atmosphere, there is far too little in the way of als and mantle silicates took place, possibly at elevated direct evidence, although the decisive events in Earth’s pressure and temperature conditions (Chabot et al., early history point to some plausible scenarios. One 2005; Wood et al., 2006). Additional geochemical and possible consequence of the Moon-forming impact petrological constraints, better resolution of its tim- is rapid evolution from a hot silicate atmosphere to a ing and duration, and a fuller picture of the possible steam-dominated greenhouse atmosphere (Zahnle et dynamics are needed to constrain the core segregation al., 1988), and once the magma ocean solidified, liquid process. water could stabilize at the surface with carbon dioxide and methane dominating the climate (Kasting and Ono, 2006). A key unknown here is the capacity of the Early Earth’s Surface Environments mantle to sequester water, possibly in the presence of Evidence indicates that the accretion and major early whole-mantle convection. differentiation of Earth, including core formation, were largely complete within about the first 100 megayears Clues from the Early Crust (Myr). The ensuing 500-Myr time interval, the Hadean Eon, is often referred to as the geological dark age, Evidence for the earliest chapters in Earth’s his- because there is little preservation of this interval in tory comes from a variety of sources, including the the rock record. Yet it remains a crucial stage in Earth’s bulk composition of Earth and the Moon, the angular history because the transition to a habitable surface momentum of the Earth-Moon system, traces of short- environment occurred during this time. lived radioactive isotopes in meteorites and terrestrial There are few solid constraints on the Hadean rocks, terrestrial and lunar patterns of element abun- Earth and a host of first-order questions. Heat pro- dances, and perhaps most importantly, the oldest crustal duced during accretion and core formation, together rocks and minerals. The discovery of increasingly old with the higher concentrations of heat-producing crustal rocks (see Figure 2.4) provides a few tantalizing radioactive elements, point to a hot, possibly water- clues on the state of Earth’s surface in the late Hadean FIGURE 2.4 Images of Earth’s oldest crustal rocks. Left: The 4.28-Ga “faux-amphibolite” from the Nuvvuagittuq supracrustal belt in northern Quebec. Right: Zircons from Jack Hills Australia with ion-probe spots labeled by U-Pb ages. SOURCE: Left: O’Neil et al. (2008). Right: Wilde et al. (2001). Reprinted with permission from AAAS.
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20 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES and the earliest Archean. In terms of preservation, the ent rock, transported by fluvial systems, and deposited most diverse suite of ancient crustal rocks is found in sedimentary rocks of a younger age (see Box 2.2). in the Isua terrane in Greenland, with ages as great Advances in microanalytical techniques, especially ion as 3.8 Ga (Appel et al., 2001). These are moderately microprobes, have established ages around 4.3 Ga for metamorphosed but contain evidence to suggest that the oldest of these. The overarching inference from plate tectonic processes, liquid water oceans, and per- these oldest crustal materials is that by the late Hadean haps life forms were present. Still older are the Acasta and certainly by its end, Earth’s surface environment gneisses from north-central Canada, dated around 4 was rather equable, perhaps not dramatically different Ga (Bowring and Williams, 1999). The only known from the present (Mojzsis et al., 2001; Wilde et al., Earth materials that are unequivocally older are small 2001), so that some of the conditions for sustaining zircon grains that have been removed from their par- life were already in place. Other critical elements are BOX 2.2 Earth’s Oldest Solids: Hadean Zircons The oldest known terrestrial solids are zircon crystals. Zircons are extremely resistant to both chemical and physical destruction and hence have the potential to survive billions of years of reprocessing after their formation. Fortunately, they also carry a range of mineralogical, geochemical, and isotopic tracers that document their age and environment of formation. The oldest known zircons come from the Jack Hills region of Western Australia, where they are found in metamorphosed rocks originally deposited in a fan-delta setting (see Figure B2.2; Spaggiari et al., 2007). Although they span a range of ages, many of the Jack Hills zircons formed in the Hadean Eon (>3.8 Ga), and the oldest among them crystallized <250 million years after the birth of the solar system (e.g., Compston and Pidgeon, 1986). Because they provide a unique window into the early Earth, the Jack Hills crystals have been intensively studied in the past decade. A generally consistent story emerges from analyses of the trace element and isotopic composition of the zircons as well as the assemblage of mineral inclusions trapped within them (e.g., Wilde et al., 2001; Cavosie et al., 2005; Watson and Harrison, 2005; Trail et al., 2007; Hopkins et al., 2008; Harrison, 2009). The zircons appear to be igneous and formed at relatively low temperatures, suggesting crystallization from magma at or near water saturation. Inclusion mineralogy and oxygen isotope data indicate the magma may have formed from melting of a felsic protolith that had interacted extensively with an early hydrosphere, possibly an ocean. Geobarometry and thermometry of the inclusions and the zircons themselves suggest crystallization in an unusually cool geothermal gradient. These observations evoke an environment remarkably similar to the conditions under which modern granites form in subduction environments. Thus, it has been argued that within just a few hundred million years of the formation of the planet, a stable siliceous crust, an active hydrosphere, and a form of plate tectonics with marked similarities to the current regime had already been established. Further advances in this field may come from identification of new localities where extremely old rocks and detrital minerals occur. This task will require application of a variety of geochemical and petrological methods, especially in geochronology. The magnitude of the undertaking is illustrated by the work invested to identify the oldest zircons from the Jack Hills. Ion microprobe analyses of more than 100,000 individual zircons were required to identify the ~100 crystals with ages >4.2 Gyr (Holden et al., 2009). FIGURE B2.2 Jack Hills: a 4.06-billion-year-old Jack Hills zircon with mineral inclusions that characterize the parent magma’s protolith and melting/ crystallization conditions. SOURCE: Hopkins et al. (2008). Reprinted by permission from Macmillan Publishers Ltd.
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21 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES more problematic, however, particularly oxygen, which context can provide. An Early Earth initiative could does not appear to have been abundant then. This raises build on existing community organizations and fund- several fundamental questions, such as: ing programs, but distinct focus is required to catalyze coordinated momentum in this arena. • What is the critical oxygen concentration for early life forms? THERMO-CHEMICAL • What was the role of the late heavy bombard- INTERNAL DYNAMICS AND ment near 3.9 Ga on the terrestrial environment? VOLATILE DISTRIBUTION • At what time did the earliest continental crust stabilize? The elucidation of plate tectonics over the past • When did plate tectonics initiate, and what 50 years has provided a general framework for under- environmental effects did this transition have? standing shallow Earth structures, kinematics, and pro- cesses and for relating observations of the present Earth to those preserved in the geological record. The quests The Hadean Mantle and Core to fully quantify three-dimensional plate dynamics Most of the major questions posed for the early and to determine how distribution of materials at surface environment also involve the composition and Earth’s surface evolves with the internal dynamic sys- dynamics of the Hadean mantle, and some of these tem remain primary goals of the Earth sciences. The also involve the early state of the core. For example, dynamic configuration, thermal and chemical fluxes, the thermal and compositional stratification of the and driving forces within Earth’s interior are all of cen- mantle following the major phase of core segregation tral importance to understanding our planet’s evolution, (and magma ocean solidification) constitute the “initial but these must be deduced from observations made at conditions” for subsolidus mantle convection. In the the surface. An improved understanding of thermo- same way, conditions in the core inevitably changed chemical internal dynamics and volatile distribution once the major differentiation had occurred. Evidence within Earth also has important societal implications for these transitions can be found in the context of the for the mitigation of volcanic and earthquake hazards search for ancient rocks and minerals described previ- and for the discovery and development of mineral and ously. Geobarometry and geothermometry techniques geothermal resources. can infer mantle temperatures and pressures, and mag- Making progress has required parallel maturation netized samples provide information about the nature of a suite of disciplines that bring key information of the early geodynamo and also on the energetics of to light: seismology to image elastic and anelastic the Hadean deep Earth. p roperties and material heterogeneity throughout the interior, mineral physics to characterize thermo- elastic properties, phase equilibria, electronic transi- An Early Earth Initiative tions, and transport properties of Earth materials over This suite of topics involving the early Earth the full pressure-temperature range of the interior, emerges as a major research opportunity because there geodynamics to quantify dynamic behavior of deep have been significant advances in theory, observations, thermo-chemical systems and their surface manifes- and modeling capabilities across all of the related areas tations, geomagnetism to probe the flow field of the but little coordination of the research agenda. Develop- outer core material and to constrain temporal evolu- ing a community focus on these topics and coordination tion of the geodynamo, geochemistry to define internal of the interdisciplinary approaches is likely to accelerate chemical variability and timing of fractionation events, progress, much as has been the case for studies of the and geology to decipher the history of crustal forma- present-day deep Earth system. The complexity and tion and plate tectonics recorded by surface rocks. As energetics of the early Earth are distinct from today, observational, laboratory, and modeling capabilities and disciplinary approaches need to be informed by of these disciplines have expanded, the prospects for the geosystems perspective that an interdisciplinary major advances in our understanding of Earth’s internal
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22 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES dynamics have increased, and a concerted interdisci- h eterogeneities have remained sequestered in the plinary effort over the next decade holds the promise interior for billions of years, while others have rapidly of significant impact on fundamental questions such as: recycled to the surface. This multicomponent transport constitutes the primary interaction of the deep Earth • How long has plate tectonics been in operation, with the ocean, atmosphere, and crust over geological as we see it today? timescales. The internal convective engines provide • What is the style of mantle convection and strain energy for earthquakes, heat for volcanic activity, material flux between the upper and lower and power for the core geodynamo. Determining the mantles? magnitude, spatial distribution, and temporal variability • How is chemical heterogeneity distributed in of geochemical heterogeneities and pinpointing the the mantle, how and when was it created, and locations of internal reservoirs where they are seques- what is its role in the dynamic circulation? tered are key to understanding how the deep interior • What is the volatile budget of the deep Earth? contributes to Earth’s evolution (NRC, 2008). • How have the core and geodynamo evolved over A profound task is to fully understand the con- time? figuration of global circulation in the mantle and • What are the driving forces of plate tectonics its capacity to sequester chemical heterogeneities in and internal circulation? reservoirs. Evidence from mantle-derived isotopes has • When and how did the continents form? long been interpreted as favoring layering of the mantle, while most geodynamic interpretations and some seis- Specific topics for which there are clear opportu- mic interpretations favor mantle circulation that is at nities for making progress in the next decade include least partially continuous from top to bottom, with the (1) appraisal of geochemical heterogeneities in the deep transition zone providing some degree of resistance. mantle and their relationship to the dynamic system, Reconciling geochemical evidence favoring isolated (2) quantification of volatile fluxes and their distribu- mantle reservoirs, seismic evidence for down-welling tion in the mantle, and (3) determination of core evo- slab material in the lower mantle, and geodynamic lution. All three topics are central to determining the models that tend to favor extensive, although possibly thermo-chemical evolution of Earth. Progress is being intermittent, circulation remains at the heart of this made in these areas by concerted disciplinary and inter- long-standing controversy (Kellogg et al., 2004; Lay, disciplinary efforts. Breakthrough advances that resolve 2009; Olson, 2010). outstanding issues will require enhanced resolution of Quantifying the nature and dynamical influence fine-scale structures in the interior beyond what can of deep Earth chemical heterogeneities will require an now be achieved, and efforts to attain higher resolution interaction of multiple Earth science subdisciplines, from seismological, geodynamical, and mineral physics including geodynamics, petrology, mineral physics, geo- approaches will need to be undertaken. chemistry, and seismology. New opportunities naturally arise from these interactions. For example, improved resolution of mantle seismic heterogeneity provides Quantification of Geochemical Heterogeneities better constraints on candidate reservoirs and places and Their Role in Mantle Dynamics limits on their compositions and geodynamic behav- Earth’s mantle comprises an immense convec - ior. A dramatic example of a recent interdisciplinary tive system that circulate heat, volatiles such as water advancement on this topic is provided by the discovery and carbon dioxide, silicate melts, former lithospheric of two huge lower-mantle provinces with distinctive material, and a host of other chemical and isotopic spe- material properties (see Figure 2.5). These are the cies between the interior and the surface. Throughout Southern Pacific and African Large Low Shear wave Earth’s history chemical differentiation has produced Velocity Provinces (LLSVPs) with several thousand- continental and oceanic crust, much of which has been kilometer dimensions extending upward from the subducted or delaminated, generating compositional core-mantle boundary hundreds of kilometers (e.g., and isotopic mantle heterogeneity. Some chemical Ni et al., 2005; Wang and Wen, 2007). First detected
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23 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES FIGURE 2.5 Pattern of S-wave velocity anomalies (dVs) at the core-mantle boundary for model S20RTS (Ritsema and van Heijst, 2000). Red areas have lower than average S-wave velocity, and blue areas have higher than average S-wave velocity. The green curves outline the 20 percent of the core-mantle boundary area with the lowest S-wave velocities, and this corresponds to the two LLSVPs beneath southern Africa and the south-central Pacific that have been characterized by seismic tomography and waveform modeling studies over the past two decades. SOURCE: Reprinted from Thorne et al. (2004), with permission from Elsevier. by global seismic tomography, over the past decade motions suggest that the deep-mantle LLSVPs may these LLSVPs have been found to have abrupt lateral have persisted for at least 300 My, constituting a long- margins; stronger reductions of S-wave velocity than term connection between deep dynamics and surface P-wave velocity, indicating anomalously high incom- geology (Torsvik et al., 2006; Burke et al., 2008). Many pressibility; and anomalously high density—all sugges- questions about the composition and dynamics of these tive of hot, chemically distinct material (Garnero et al., huge chemical heterogeneities remain to be resolved, 2007; Garnero and McNamara, 2008; Trønnes, 2009). and petrological and geochemical investigations of Geodynamical modeling (see Figure 2.6) suggests surface materials are needed to evaluate possible deep that such massive hot dense piles of material can be compositions, but their discovery has driven models for localized by mantle circulation, with their margins pos- mantle evolution in totally new directions. sibly serving as loci for thermal boundary layer insta- Disciplinary advances underlying the progress bilities that rise through the mantle as well as accumu- in characterizing deep chemical reservoirs include lation zones for dense partially molten material right improved global seismic data sets accumulated from above the core-mantle boundary (e.g., Nakagawa and fixed and portable seismic networks; improved three- Tackley, 2004; McNamara and Zhong, 2005). Mineral dimensional (3D) waveform modeling and imaging physics experiments and theory now allow thermal and c apabilities for resolving complex, deep structures; chemical heterogeneity of these provinces to be esti- improved resolution of 3D thermo-chemical convec- mated based on predictions of elastic parameters (e.g., tion models enabled by faster computers and enhanced Murakami et al., 2004; Mao et al., 2006; Duffy, 2008; numerical codes; novel 3D petrographic analyses for Ohta et al., 2008; Shim, 2008). Next-generation experi- lower-mantle conditions enabled by 3D x-ray tomog- mental facilities will provide the ability to characterize raphy with nanoscale resolution; greatly expanded textures throughout the mantle pressure-temperature experimental determinations of deep-mantle properties (P-T) range, such as crystal-liquid wetting angles and enabled by synchrotron radiation facilities; and greatly shape-preferred orientations—features that provide improved molecular dynamics models implemented on direct constraints on mantle evolution. The locations of fast computer networks. The rapid accumulation of new large igneous provinces (LIPs) reconstructed for plate data, models, and properties positions the community
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60 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES were in their infancy a decade ago, such as Acoustic c arbon storage in the terrestrial environment, the Doppler Current Profilers (ADCPs), have matured water cycle, and a range of intertwined biogeochemical to the point where they are now available as off-the- cycles and atmospheric properties that feed back on shelf technology. Near-shore currents were previously climate and ecosystems (terrestrial and marine). This measured at discreet points that were interpolated and research opportunity differs from the earlier section on modeled to infer the flow field. The advent of ADCPs hydrogeomorphic-ecosystem response in that its roots allows true three-dimensional flow fields to be mea- are more in geochemistry than geomorphology. sured for the first time and is leading to significant EAR is poised to play a leadership role in compre- advances in coastal science. These technical advances hensive, uniquely integrated studies of the terrestrial and coupled ecogeomorphological-economic models environment in the face of human activity and climate represent first steps in what must be a transdisciplinary change. This work spans diverse programs within EAR effort among scientists studying coastal processes and and more broadly across diverse divisions and director- ecosystems, engineers, economists, and other social ates within NSF and other governmental agencies, such scientists to address the pressing problems facing as the USGS and DOE. An existing suite of obser- coastal environments. Advances in coastal sciences vatories provide insight into Earth’s ecosystems and would be accelerated by a dedicated NSF initiative that related dynamics. These natural laboratories include integrates physical, chemical, and biological processes the NSF Critical Zone Observatory (CZO) and Long with human activities and their interconnections across Term Ecological Research (LTER) programs and those coastal watersheds, into the coastal zone, and beyond to within the National Ecological Observatory Network the near-shore zone. This effort will necessarily involve (NEON) and the Free-Air Carbon Dioxide Enrich- several GEO divisions but is most naturally led by ment (FACE) program of the DOE. The EarthScope EAR because the majority of processes in question are facility also shows potential for providing data needed solid-earth processes. for ecosystem and water cycle studies through indirect measurements of soil moisture and snow cover from the EarthScope GPS network. These programs are BIOGEOCHEMICAL AND WATER CYCLES alike in their prioritization of integrated science, and IN TERRESTRIAL ENVIRONMENTS AND now, increasingly, these complementary programs are IMPACTS OF GLOBAL CHANGE philosophically and collaboratively bound together by Human land use, climate change, and energy common goals focused on common questions about demand are transforming geochemical and geobio - terrestrial ecosystems impacted under human influence logical systems and, in particular, the cycling of water, and climate change. carbon, and nitrogen in these settings. Humans are now managing and altering 50 percent of Earth’s land Integrated Soil, Water, and Biogeochemical surface—dubbed the “critical zone” in the Basic Research D ynamics in the Critical Zone Opportunities in Earth Science report (NRC, 2001)— and, in so doing, are transforming the physical, chemi- The dynamics of the critical zone—the dynamic cal, and biological states and feedbacks among essential interface between the solid Earth and its fluid envelopes components of the Earth surface system. Over the past (NRC, 2001)—are governed by the interplay between century, soil erosion rates have accelerated; metals and hydrological, geomorphic, biogeochemical, and biotic toxins have enriched and mobilized far beyond natural processes that transform and rearrange materials in the rates; agriculture has industrialized the nitrogen cycle; Earth surface environment. Plant growth, for example, freshwater usage has grown to exceed recharge in affects surficial weathering and hill slope form through major population centers; and natural ecosystems have bioturbation, fracture formation, alteration of hydro- been heavily overprinted by fragmentation, extinc- logical fluxes, soil carbon dioxide generation, and pro- tion, global-scale biogeographic shifts, and invasive fusion of organic weathering reagents. We are not yet species. At the same time, atmospheric temperature able to weave these and other individual processes into and carbon dioxide levels have increased, impacting a predictive conceptual model of critical zone evolution.
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61 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES This limitation is primarily due to incomplete knowl- ment of new technologies will be required to test these edge of couplings between the physical, chemical, and hypotheses. For example, geochemists now possess biological processes in the critical zone, including both powerful tools that permit the characterization of positive and negative feedbacks and their distribution fundamental processes and elemental, molecular, and in time and space. isotopic properties at scales from submicroscopic to An example of processes not adequately under- planetary, fueled in part by tremendous advances at stood at present literally lies beneath our feet. We lack the nanoscale and in computational and instrumental observation and theory of the weathering front (the toolkits. Among these advances are abilities to date pro- interface between regolith and bedrock) that strongly cesses in the critical zone at increasingly fine resolution influences processes in the critical zone. Coupled with using cosmogenic and uranium series isotope systems. the rapid development of soil ecology as a distinct dis- Two interdisciplinary techniques currently sup - cipline over the past several decades, this sets the stage ported by EAR also show significant promise. First, for significant advances in our understanding of how geodetic techniques are increasingly being used to life above ground and life below ground are adapted to measure changes in the components of the water cycle. each other and to spatially variable, hydrogeomorphic Long-term and seasonal subsidence can be observed via processes. The thin layer of weathered rock and soil GPS and InSAR, providing important constraints on that mantles Earth’s surface offers exceptional oppor- groundwater depletion due to withdrawals for irriga- tunities for research on both fundamental processes tion and municipal use. Gravity data measured using shaping landscapes and applied issues related to the satellites are being used to monitor changes in water geobiological basis for soil fertility. Chemical weather- storage at the basin scale that cannot be observed using ing and erosion of bedrock and soil influence climate, any other technique (Famiglietti et al., 2011). Second, river and groundwater chemistry, bedrock erodibility, GPS receivers in the EarthScope Plate Boundary and ecosystem properties. Despite the fundamental Observatory (PBO) are being used to measure critical importance of soil formation and fertility for life on environmental parameters such as soil moisture, snow Earth’s surface, soils and the breakdown of rock to form depth, biomass changes, and glacier retreat. These data soil remain among the least understood areas of the are valuable to both climate scientists and water man- Earth sciences. Quantifying the controls on rates of agers for drought and flood prediction. These PBO rock breakdown to form soils is needed to understand studies demonstrate how infrastructure developed for the processes of soil formation and how they vary in geophysical studies can simultaneously be used for different landscapes, climates, and tectonic regimes. water cycle studies funded through the hydrological Interdisciplinary studies of the critical zone are sciences within EAR, the Division of Atmospheric and yielding new ideas about the interactions of weather- Geospace Sciences (AGS), and non-GEO directorates ing, erosion, and biology in the critical zone. These such as the Directorate for Biological Sciences (BIO) include hypotheses concerning the evolution of the and the Office of Polar Programs (OPP). critical zone, such as that in relatively stable landscapes The payoffs of such investments in data acquisi- where biology drives weathering in the initial stages of tion are potentially enormous if the fluxes of energy, plant establishment while weathering drives biology water, and materials within and through the critical over the long term (Brantley et al., 2011). This work zone can be resolved and if fundamental insight can be also suggests that future land use change may impact provided into ecosystem and landscape evolution and critical zone processes more than climate change and resilience. The data sets and understanding developed that restoration efforts are likely to restore hydrological through such measurements will form the basis for functions on shorter timescales (decades or less) than coupled systems models that allow study of interac- biogeochemical functions and biodiversity (Brantley tions and feedbacks between biological and physical et al., 2011). processes in the critical zone through assimilation A substantial investment in in situ environmental of hydrological, meteorological, biogeochemical, and sensors, field instruments, geochemical tools, remote geomicrobiological measurements. sensing, surface and subsurface imaging, and develop- Q uantitative estimation of watershed carbon bal-
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62 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES ance provides a compelling example. Findings from BOX 2.9 the late 1980s to mid-1990s indicating that only ~30 Critical Zone Observatories percent of the carbon dioxide released by fossil fuel burning stayed in the atmosphere, with ocean uptake The Critical Zone concept, introduced in the 2001 NRC accounting for an additional ~30 percent, launched a report Basic Research Opportunities in Earth Science, provides a stampede of terrestrial ecosystem and surface Earth research framework for the portion of Earth most closely linked to scientists to every biome on Earth to look for the miss- society and terrestrial life. A network of Critical Zone Observatories ing sink for the remaining 40 percent. However, after 15 (CZOs) is being established to capitalize on this new research framework by providing locations and funding mechanisms for years of effort, a consensus has yet to emerge regarding integrated, multidisciplinary research. Five observatories are the spatial distribution of, or the processes responsible located in the continental United States and a sixth is in Puerto for, the 2 to 4 Pg C y–1 continental sink of the 1990s Rico, and each is in a different representative landscape. This (Solomon et al., 2007)—or the observation that con- CZO network is connected to an international network through tinents were likely a net carbon source in the 2000s. collaboration with a parallel effort in the European Union, and data One roadblock is that net ecosystem production (NEP) and infrastructure are open to all researchers. Past studies of the Critical Zone rarely were able to conduct long-term monitoring measured at local scales does not often extrapolate well efforts. Establishing semi-permanent observatories is allowing to larger scales (Ometto et al., 2005; Stephens et al., long-term studies to be conducted and has the potential to fill large 2007), very possibly due to lack of consideration of gaps in our knowledge of Critical Zone systems. Because human lateral export (Chapin et al., 2006; Lovett et al., 2006) agency plays such a large role in nearly every system of the Criti- and the details of spatial and temporal variability. The cal Zone, the traditional Earth science objective of constructing a importance of full watershed-scale carbon balances is universal model cannot be accomplished without including the influence of human activities. This is an evolution in thinking for illustrated by the one published study that accounted conventional Earth sciences, but it holds promise of transformative for both vertical carbon fluxes (via eddy covariance discoveries that will be both useful to society and add value to the tower) and lateral carbon exports via streams, demon- larger corpus of Earth science understanding. The CZO network strating that Net Ecosystem Exchange (NEE) went is designed to be the mechanism for making those discoveries. from a net sink of 0.278 Mg C ha–1 yr–1 to a net source of 0.083 Mg C ha–1 yr–1 when lateral stream fluxes were accounted for (Aufdenkampe et al., 2011). The integrated watershed studies needed to advance Responses and Feedbacks of Carbon, Nitrogen, and our understanding of the critical zone is a distinctive Water Cycles to Climate Change f eature of the CZO framework and their multi- disciplinary science teams. CZOs provide essential Each year about 120 Pg of carbon is exchanged data sets and a coordinated community of researchers between the atmosphere and terrestrial ecosystems w ho integrate hydrological, ecological, geochemi- t hrough photosynthesis and respiration. This is cal, and geomorphic processes from mineral grain to more than an order of magnitude larger than esti- watershed scales to illuminate the rich complexity of mates of exchange directly due to human activities interactions between the lithosphere, the pedosphere, ( 8.7 Pg C/year from fossil fuel combustion and the hydrosphere, the biosphere, and the atmosphere. 1.2 Pg C/year associated with land use change in 2008; CZO sites are establishing infrastructure for the inten- L e Quéré et al., 2009). As a result, global changes in sive data-gathering effort required to support their sources and sinks of carbon due to climate change science teams and the conceptual and mathematical could be at least as important to global carbon cycles models they develop (see Box 2.9). The development of as the total of all direct anthropogenic fluxes. Indirectly, more diverse observatory sites could facilitate compari- humans have and continue to be an important agent son and sensitivity studies that might then serve with of past and future climate change, primarily through reasonable confidence in a broader predictive mode fossil fuel burning. Identification of carbon sources and across non-observatory sites. sinks requires studies at landscape and regional scales, whereas most research to date on carbon cycling has
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63 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES been at global (e.g., Global Climate Models simula- The elemental stoichiometry wired into living tions) or local (e.g., flux tower) scales. organisms guarantees that the carbon cycle is coupled Environments in which climate change could with those of nitrogen and phosphorus, while processes trigger relatively rapid vegetation and landscape change, such as biological nitrogen fixation link nitrogen cycles such as permafrost areas and wetlands, are of particular to those of other elements, such as iron (e.g., Finzi et concern to regional and global carbon exchange. For al., 2011). Carbon, nitrogen, and other elemental cycles example, there are 23 × 10 6 k m2 of ice-rich per - respond variously to changes in temperature and pre- mafrost in the Northern Hemisphere, more than a cipitation, and their coupling creates a complex system third of which could be actively thawing by 2100, of interactions and feedbacks among elemental cycles, according to model projections (Grosse et al., 2011). ecosystems, and climate. The coupling between carbon An estimated 1,600 Pg C is stored in the top 3 m of and nitrogen cycles and climate change is one aspect ground in Northern Hemisphere permafrost regions of this system currently receiving considerable atten- (see Figure 2.23). Thawing of permafrost and associ- tion owing to uncertainties as to whether feedbacks ated microbial decomposition of organic carbon have between nitrogen and carbon cycles will act to buffer the potential to transfer large quantities of carbon or amplify the response of Earth’s climate to continued to the atmosphere, with estimates in the range of 50 to anthropogenic carbon dioxide flux to the atmosphere. 100 Pg by 2100 (Schuur et al., 2008). However, assess- The terrestrial nitrogen cycle has been dramatically ments of the vulnerability and resilience of perma- accelerated by industrial production of reactive nitrogen frost to warming and thawing, as well as potential for use as fertilizer, as well as by combustion of fossil carbon losses, are complicated by positive and negative fuels and cultivation of legumes. These three anthropo- feedbacks among snow cover, vegetation, soil, active genic sources of nitrogen are estimated to have added layer properties, and surface water and groundwater more nitrogen (187 Tg N/year in 2005; Galloway et (Grosse et al., 2011). Thawing of permafrost is also al., 2008) into the terrestrial environment during the likely to produce rapid landscape degradation, includ- past few decades than natural sources (110 Tg N/year; ing development of thermokarst, accelerated coastal see Figure 2.24; Gruber and Galloway, 2008). In addi- erosion, channel network expansion, and mass wasting tion, anthropogenic emissions of nitrous oxide (N2O, (Rowland et al., 2010). Improved understanding of the a greenhouse gas) directly contribute to stratospheric impacts of climate change on carbon, soil, ecosystem, ozone depletion and tropospheric N2O accumulation and landscape dynamics in permafrost regions will (Ravishankara et al., 2009), while emissions of nitro- require coordinated observation and modeling efforts gen oxides (NO x) indirectly contribute to tropospheric by multidisciplinary teams of scientists. ozone and aerosol formation (Arneth et al., 2010). FIGURE 2.23 Idealized cross section through northern permafrost regions indicating significant known and assumed carbon pools, including estimated carbon storage in petagrams (Pg C) for the terrestrial and marine portions of the permafrost system. SOURCE: Grosse et al. (2011).
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64 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES FIGURE 2.24 Natural (blue) and anthropogenic (orange) nitrogen fluxes for the terrestrial (left) and marine (right) nitrogen cycles. Illustrates major sources, sinks, and processes associated with production of reactive nitrogen and the coupling of the nitrogen cycles with those of carbon and phosphorus. Values are for the 1990s in Tg N/year. SOURCE: Gruber and Galloway (2008). Reprinted by permission from Macmillan Publishers Ltd. Understanding how all of this additional nitrogen cycles operating in soil-based, freshwater, and marine will affect climate, terrestrial ecosystems, and carbon systems. Quantifying changes in the water cycle associ- cycling is essential as we attempt to anticipate future ated with climate change is therefore a critical element environmental change and possible mitigation strate- of building an understanding of future changes in gies. For example, recent modeling studies indicate that biogeochemical cycles. nitrogen feedbacks represent an important control on Reconstruction of the monthly discharge of the changes in terrestrial carbon storage driven by increases largest rivers by Labat et al. (2004) indicates that global in atmospheric carbon dioxide, though the nature of continental runoff increased during the 20th century. this control varies between tropical, temperate, and Changes in runoff have been linked to changes in pre- high-latitude ecosystems and the magnitude of the cipitation, evapotranspiration, and land use. Modeling effect remains uncertain (e.g., Zaehle et al., 2010). of the relative contributions of precipitation, tempera- Nitrogen-related changes in carbon storage feed back ture, carbon dioxide concentration, land cover, and land into climate by regulating atmospheric carbon dioxide use to increases in river discharge in the 20th century levels. In addition, any changes in the C:N ratio of indicates that increases in precipitation are the domi- terrestrial plants and/or changes in rates or the geo- nant driver of global increases in discharge (Gerten et graphic distribution of biological nitrogen fixation and al., 2008). Precipitation is expected to increase with denitrification would alter regional and global carbon increasing temperature, though the rate of increase may cycles (Gruber and Galloway, 2008). Redistribution be moderated by the influence of tropospheric green- of nitrogen, carbon, and other elements in terrestrial house gas forcing and black carbon aerosols on pre- systems by runoff, land-atmosphere exchange, and cipitation (e.g., Frieler et al., 2011). Land use practices other surface processes connects the biogeochemical also contribute to increases in discharge, particularly
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65 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES in watersheds characterized by extensive agriculture or cycles caused by climate change as well as by land use deforestation. For example, there is a strong correlation change and other human impacts. between agricultural land cover in the Mississippi River basin and increased discharge under average precipita- Human Impacts on Water, Carbon, and tion conditions, with agricultural land use accounting Nitrogen Cycles for more of the increase in Mississippi River discharge in the past 50 years than do increases in precipitation Humans have altered the terrestrial water cycle (Raymond et al., 2008). This agriculturally enhanced through activities like reservoir construction, agricul- runoff can carry high concentrations of nitrogen, phos- ture, groundwater extraction, and urbanization. More phorus, and carbon (in the form of bicarbonate) that than half (52 percent) of the world’s largest rivers are impact the biogeochemistry of the receiving rivers and regulated by dams, including 85 percent of the most downstream marine systems. biogeographically diverse large river systems (systems The role of climate-related changes in evapotrans- that span five or more biomes; Nilsson et al., 2005). piration in the intensification of the water cycle is Regulation and fragmentation of rivers by dams also more challenging to sort out, in part because of feed- strongly impact sediment storage and the discharge of backs between evapotranspiration and soil moisture. terrestrial sediment to the coastal ocean. While surface Elevated atmospheric carbon dioxide has been tied to freshwater resources exceed global water demand at decreases in stomatal conductance (e.g., Leakey et al., present, variations in water availability and demand in 2009), which could lead to decreased evapotranspira- time and place result in regions of high water stress. tion and increased soil moisture (e.g., Gedney et al., In these water-stressed regions, groundwater with- 2006). However, several lines of hydrological evidence drawal often exceeds recharge, with recent estimates (water balance estimates, lysimeter and pan evapora- suggesting that groundwater depletion (withdrawal tion measurements, length of growing season) point in excess of recharge) has more than doubled since to an increase in evapotranspiration in temperate the 1960s (Wada et al., 2010). Virtual trade of water regions over the past 50 years (Huntington, 2008). used in the production of goods or services is likely to These results suggest that, at present, the effects of become increasingly important in supporting human higher temperatures are generally able to offset the populations in water-stressed regions, especially during effects of increased carbon dioxide on evapotranspi- drought, but may also facilitate unsupportable popula- ration, though their relative effects are likely to vary tion growth in regions of water scarcity (D’Odorico et geographically and may change with future changes al., 2010a). Accurate assessments of water availability, in climate and land cover. water demand, and sustainable water use require more While there are relatively long and spatially distrib- complete global hydrological data sets, compilations of uted records of runoff and precipitation, fundamental operational data regarding water use, and advances in hydrological parameters like soil moisture and evapo- modeling coupled with hydrological and socioeconomic transpiration are difficult to measure and, for the most systems. part, existing data are temporally and spatially sparse. Because of the centrality of the carbon cycle to To advance the science, measurements at points on climate, it is critical that the effects of human activities the landscape (e.g., from networks of flux towers) will on the carbon cycle be quantified, that the response of have to be integrated smoothly with areally distributed the carbon cycle to disturbance be determined, that estimates derived from remote sensing (e.g., satellite potential future impacts on carbon cycling and carbon measurements of soil moisture). All these measure- pools (e.g., ocean acidification and methane dynamics) ments will have to be coordinated through new data be evaluated, and that possible mitigation strategies be assimilation methods with new theory appropriate considered (Canadell et al., 2010). The potential for for landscape and regional scales. These and other rising atmospheric carbon dioxide levels to significantly new approaches to quantifying essential hydrological impact climate, ecosystems, and human populations parameters are necessary to resolve spatial and temporal has given rise to a variety of ideas for slowing rates of trends in the water cycle and related biogeochemical future increases in atmospheric carbon dioxide, rang-
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FEATURE 66 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES A safe operating space for humanity ing from energy-saving measures and use of renewable terrestrial processes. Impacts of reactive nitrogen on energy sources to schemes for increasing terrestrial and the environment are exacerbated by its cascading effect marine carbon storage (Gussow et al., 2010). Proposed as it moves through the environment, such that each Identifying and quantifying planetary boundaries that must not be transgressed could help preven engineered approaches to reducing atmospheric carbon molecule of nitrogen can contribute to multiple envi- activities from causing unacceptable environmental change, argue Johan increases, andnd colleagu ronmental problems. Future population RockstrÖm a dioxide include ocean iron fertilization, large-scale for- estation using nonnative species, and injection of car- improvements in standards of living, will likely add to A bon dioxide in deep-sea sediments andhas undergone many this anthropogenic nitrogen load through growing use lthough Earth aquifers. Geo- SUMMARY engineering proposals for periods of significant environmen- of energy, additional demand for food production, and carbon storage can involve ● New approach proposed for defining preconditions for tal change, the planet’s environment substantial risks, possible unintended consequences, has been unusually stable for the past 10,000 improvements in diet. Policies and practices for nitro- development and potentially limited 1–3. This period of stability — known to gen use must balance the certain biophysical thresholds could have dis years benefit (Bala, 2009; Finzi, excesses and inefficiencies ● Crossing geologists as the Holocene — has seen human associated with nitrogen use in much of the developed 2011). Both the American Meteorological Society consequences for humanity civilizations arise, develop and thrive. Such (AMS) and the American Geophysical Union (AGU) the world with the need hreefood in interlinked planetary boundaries have al ● T for of nine other parts of the world stability may now be under threat. Since overstepped have adopted position statements on geoengineering Industrial Revolution, a new era has arisen, (Galloway et al., 2008). the Anthropocene4,the intended and in which human actions that recommend further research on Understanding these and other anthropogenic have become the main driver of global envi- industrialized forms of agriculture, human boundaries define the safe op unintended Earth system response to This could see human impacts on the environment could dam- for humanity with respect to the ronmental change5. geoengineering activities have reached a level that requires integrated, inter- proposals and coordinated, interdisciplinary study of the disciplinary studies of climate,desirable and are cycles, age the systems that keep Earth in the biogeochemical associated with the activities push the Earth system outside stable environmental state of theissues Holocene, water, ecosystems, and humans. In particular, it is Holocene state. The result could be irrevers- physical subsystems or proces the relevant scientific, social, legal, and ethical with consequences that are detrimental or ible and, in some cases, abrupt environmental Earth’s complex systems somet (AGU, 2009; AMS, 2009). even catastrophic for large parts of the world. important that to a state less conducive to processes and change, leading Earth scientists identify smoothly to changing pressures Humans have also significantly impacted other During the Holocene, environmental thresholds that, when crossed, would lead to irreversible be the exceptio human development6. Without pressure from this will prove to change occurred naturally and Earth’s regu- ahumans, the Holocene is expected to continue Rockström etsubsystems of E nd unacceptable environmental change. the rule. Many biogeochemical cycles. As noted above, industrial latory capacity maintained the conditions for at least several thousands of years7. a nonlinear, often abrupt, way production of fertilizer,enabled human development.and that fossil fuel combustion, Regular al. (2009) suggest that this threshold hasticularly sensitive around thres already been cultivation of legumes are currently adding availability and cPlanetary boundaries atmospheric carbon dioxide, variables. If these t temperatures, freshwater more new rossed with respect to certain key the reactive nitrogen to iogeochemical flows all than within a rela- nTo meet the challenge of maintaining the Figure 2.25). b the environment stayed natural itrogen cycle, and biodiversity loss (see crossed, then important subsyst tively narrow range. Now, largely because of Holocene state, we propose a framework monsoon system, could shift in a rapidly growing reliance on fossil fuels and based on ‘planetary boundaries’. These often with deleterious or pot disastrous consequences for hu Most of these thresholds can Climate change a critical value for one or more n tio llu ed) Oc ables, such as carbon dioxide c ean po tiﬁ al uan ac Not all processes or subsystems ic q id iﬁ t ot m well-defined thresholds, alth ye e Ch ca t actions that undermine the resi ion (n processes or subsystems — for and water degradation — can in ) iﬁed ozo (not yet quant g Stra epletion aerosol load ic in that thresholds will also be cro r Atmosphe ne d tospheric processes, such as the climate sy We have tried to identify the processes and associated thresh crossed, could generate unacc ronmental change. We have fou processes for which we believ loss ﬂow eoch (bio N it r cycl en sary to define planetary bound ity g bo e og e ers change; rate of biodiversity lo un mic le u div da a l and marine); interference with ry o Bi ) Ph and phosphorus cycles; stratos cy pho os cr depletion; ocean acidification; e us fre s shw nd water use; change in land use; n la at i Glo er use Change lution; and atmospheric aeroso bal Fig. 1 and Table). In general, planetary bounda for control variables that are ei Figure 1 | Beyond the boundary. The inner green shading represents the proposed safe operating FIGURE 2.25 Earth-system processes and their proximity to crossing threshold conditions that lead to unacceptable environmentalthresholds — distance from space for nine planetary systems. The red wedges represent an estimate of the current position for change. Processes are indicated outside boundaries in three systems (rate of biodiversity loss, climate change and human of with evidence of threshold be each variable. The each sector. Green colors denote safe operating conditions. The heights the red-colored wedges represent the status of each with the nitrogen cycle), have already been exceeded. In this figure, climate change, the nitrogen cycle, process with respect to safe operating conditions. at dangerous levels — for proc interference and biodiversity loss have crossed the threshold of unacceptable environmental change. SOURCE: Rockström et al. (2009). Reprinted 472 by permission from Macmillan Publishers Ltd. © 2009 Macmillan Publishers Limited. All rights reserved 472-475 Opinion Planetary Boundaries MH AU.indd 472
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67 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES RECENT ADVANCES IN Profound new insights into the rates of geo- GEOCHRONOLOGY chemical and biological processes are possible with ages precise to a small fraction of a percent. For example, A common theme running through previous sec- Maloof et al. (2010) recently investigated a portion of tions of this chapter is the growing reliance on geo- the early Cambrian period associated with the appear- chronology to provide quantitative estimates of the ance of the first calcite biomineralizing organisms and age, duration, and rate of events and processes over an associated dramatic change in global carbon cycling, many different timescales. As a result of improve- as indicated by a large d13C shift of marine carbonate ments in analytical methods and in the theoretical (see Figure 2.26). Dates of multiple ash fall zircons underpinnings and calibrations of a variety of dating show that the event occurred at 525.34 ± 0.09 Ma, and methods, the past few years have seen transformative the adjustment in global carbon cycling occurred in 506 advances in many approaches to geochronology. Areas ± 126 kyr. The rate of this event suggests that these of notable growth include surface exposure dating using changes arose from biological diversification occurring rare isotopes produced by cosmic rays, determination at that time. of cooling histories of rocks (thermochronometry), The ability to obtain extremely accurate and reli- extremely high precision dating of volcanic ashes, and ably inter-calibrated ages allows previously impossible high-throughput dating of detrital minerals. These geo- high-precision cross-correlation of events recorded in chemical techniques provide quantitative estimates of different localities. For example, Schoene et al. (2010) time that are an essential complement to dates and rates dated the end-Triassic mass extinction to 201.32 Ma established using magnetostratigraphy and increas - in sedimentary sections in both Peru and Nevada ingly reliable methods of cyclostratigraphy (counting and determined that the extinction was complete in of orbitally paced oscillations recorded in sedimentary <300 kyr. Additional dates from the Central Atlantic rocks). Magmatic Province yielded precisely the same age, Recent work greatly improving the ability to extract providing compelling evidence of a linkage between the extremely precise and accurate ages from both the U/Pb extinction and massive volcanic eruptions. and 40Ar/39Ar methods underscore recent advances Similar advances have occurred in 40Ar/39Ar dating, and illustrate likely future directions both in terms of which is important because not all samples of inter- method development and application. est contain datable zircons. Furthermore, the ability to date coexisting minerals by two different high- High Precision–High Accuracy precision methods allows the detection of possible age Radiometric Dating biases arising from such factors as daughter product loss, inheritance, and magma residence time. Much of Given the wide applicability of the U/Pb and the improvement in 40Ar/39Ar dating has arisen from 40Ar/39Ar methods, especially to dating ashfalls in refinements to the 40K decay constant (Renne et al., sedimentary sequences, recent improvements have had 2010) and to the ages of the standards that are essential and will continue to have a major impact on the Earth to the method. As an important example of standard sciences. In the case of U/Pb dating, a remarkable series calibration, Kuiper et al. (2008) assigned extremely pre- of discoveries culminating in the work of Mattinson cise and accurate ages from the astronomical timescale (2005) has revealed an analytical approach by which (counting of Milankovitch cycles) to ashfall sanidines the consequences of Pb loss on zircon U/Pb dates can in Miocene sediments. These sanidines were analyzed be almost entirely removed. This new approach permits for 40Ar/39Ar ratio and then used to back-calculate the routine determination of U/Pb dates with a precision true age of the widely used Fish Canyon sanidine stan- of better than 0.1 percent. Geochronologists are also dard (the new age of this standard of 28.201 ± 0.046 Ma continuing to reduce other sources of error, includ- is remarkably more precise than the previously adopted ing spike calibration, instrumental mass fractionation, value of 28.02 ± 0.56 Ma). decay constants, and the magma chamber residence Productive interplay between astronomical dating time of zircon crystals prior to eruption and deposition. and the improved accuracy of the 40Ar/39Ar and U/Pb
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68 NEW RESEARCH OPPORTUNITIES IN THE EARTH SCIENCES FIGURE 2.26 An example of the new insights possible with ages precise to a small fraction of a percent. Chart shows carbon isotope variability in marine carbonate in the early Cambrian period. High-precision U/Pb zircon ages of intercalated tuffs shown in boxed numbers (in Ma). SOURCE: Maloof et al. (2010). Reproduced with permission of Geological Society of America. chronometers is likely to continue in the coming years. high-accuracy-age tie point at the K-T boundary. This S uch interplay is nicely illustrated by work on the new tie point provides a new and more robust (but not Cretaceous-Tertiary (K-T) boundary (Kuiper et al., yet definitive) age anchor on which to pin the astro- 2008). While excellent cyclostratigraphy is apparent in nomical timescale. some K-T boundary sections (see Figure 2.27), there Geochronology has its roots in analytical geo- is ambiguity in precisely how to map the sedimentary chemistry and has greatly benefited from improvements signals to the independently computed astronomical in instrumentation and in a refined understanding of forcings. Improved dating accuracy has provided a new, the underlying geochemical principles. Geochronology
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confirm the thermally undisturbed nature of the samples (14). We calculate an astronomically calibrated one of the most important biotic crises in Earth FCs age for each experiment propagating only analytical uncertainties. The weighted mean FCs age and history. The K-T boundary section at Zumaia, standard analytical error for BGC and VU data are displayed separately and as a combined-age probability Spain, which magnetostratigraphically covers the FCs NEW RESEARCH The 28.201 ± 0.012 IN THE for FCs isSCIENCES an intercalibration factor of R astro of diagram. OPPORTUNITIES Ma age EARTH converted to 69 4.3644 ± 0.0018 for a Tastro at 6.500 Ma. This translates to 28.201 ± 0.046 Ma, including decay-constant interval from the younger part of polarity interval uncertainties and the uncertainty in the astronomical ages of ±10 ky. C29r well into C26r, has been astronomically tuned and the boundary has been assigned an age of 65.777 Ma (33). The astronomical age of (33) is Fig. 3. Photo of the uncertain for two reasons: (i) the use of the poten- upper part of the Zumaia tially unstable very-long-period 2.4-My eccentricity section below the San cycle as the starting point for the tuning; and (ii) Telmo chapel. Both the the matching of basic marl/limestone cycle pack- 100-ky limestone beds ages [the E-cycles of (33)] to successive 100-ky 405-max 405-max 29 to 42 of (33) and the eccentricity minima in the target curve, which is 405-max large-scale clusters of less certain (and stable) than the 405-ky eccentricity 405-max precession-related basic minima (fig. S2). cycles that mark succes- According to (33), the 405-ky cycle is not ex- 31 30 29 sive 405-ky eccentricity 35 34 33 32 pressed, or only very weakly present at Zumaia. maxima are indicated Nevertheless, this cycle can be identified on 42 41 39 38 37 36 (see also figs. S3, a to c). photographs, in the field, and in the lithologic log 40 The phase relation with of Zumaia of (33) through differences in the thick- eccentricity is unambig- ness and expression of marls intercalated between uous: The marly intervals 100-ky limestone beds (Fig. 3 and fig. S3). Details in between the 405- of the cycle pattern confirm the phase relations and 100-ky limestone between the sedimentary cycles and eccentricity as beds often reveal dis- inferred by (33). Small-scale precession-related cy- tinct precession-related cles are less well developed in the limestone beds of cycles, which is consistent with eccentricity maxima because eccentricity determines precessional amplitude. Eccentricity minima are eccentricity-related cycles, indicating that these FIGURE marked by ilankaovitch cycles at the Zumaya K-T boundary section, Spain. beds. 2.27 M weakly developed precession-related cycles and are dominated by limestone High-precision radiometric dates permit improved beds indeed correspond to eccentricity minima be- assignment of the absolute ages of these cycles and hence a more accurate geological timescale. SOURCE: Kuiper et al. (2008). Reprinted with permission from AAAS. 502 25 APRIL 2008 VOL 320 SCIENCE www.sciencemag.org is a vibrant research subdiscipline, and the next decade many cases—for example, in surface exposure dating will likely see continued advances in this area. How- and thermochronometry—sophisticated models are ever, as the fidelity, availability, and diversity of dat- essential to extract the full meaning from the data. ing methods expand, the need for close collaboration Thus, continued and robust advances in geochronology among those who develop techniques and make the will involve a broad cross section of the Earth science measurements with those who select key samples and community. interpret results is becoming increasingly apparent. In
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