1. Overview
The Deep Carbon Observatory (DCO) is a ten-year research program to investigate the quantities, movements, forms, and origins of carbon in Earth. More than 90% of Earth’s carbon may reside in the planet’s deep interior, and DCO’s overarching mission is to understand Earth’s
entire carbon cycle—beyond the atmosphere, oceans, and shallow crustal environments, which have drawn most previous research attention—to include the deep carbon cycle [
1,
2]. A decade of focused research has led to major discoveries by DCO scientists on the physical, chemical, and biological roles of carbon in Earth.
To pursue its multidisciplinary mission, DCO connects more than 1200 scientists worldwide, broadly grouping them into four Science Communities: Extreme Physics and Chemistry, Reservoirs and Fluxes, Deep Energy, and Deep Life (Table 1). The program also emphasizes four crosscutting activities that bridge community boundaries—namely, data science, instrumentation, field studies, and modeling and visualization—and several cross-community groups that provide essential services to the entire DCO community (Fig. 1).
In addition to its scientific advances, DCO has created an enduring legacy of interdisciplinary and international community building, successfully establishing a diverse, dynamic, and collaborative community of geologists, physicists, chemists, and biologists in more than 50 countries. In particular, DCO has focused on cultivating the next generation of deep carbon researchers by supporting early career scientists who will carry on the tradition of exploration and discovery for decades to come.
The vision, guiding questions, and scientific goals of DCO were initially framed at an international Deep Carbon Cycle Symposium at the Carnegie Institution for Science in 2008. This symposium led to a successful proposal to the Alfred P. Sloan Foundation to establish a decadal research program on deep carbon science from 2009 to 2019. The Alfred P. Sloan Foundation pledged seed funding of 50 million USD over ten years to foster DCO. DCO has leveraged the support from the Alfred P. Sloan Foundation with more than 500 million USD in support from other sources, including international organizations, national science agencies, foundations, and the private sector.
2. The Extreme Physics and Chemistry Community
DCO’s Extreme Physics and Chemistry Community is dedicated to improving our understanding of the physical and chemical behavior of carbon at extreme conditions, as found in the deep interiors of Earth and other planets. This Community is making transformational advances in our understanding of carbon in minerals, magmas and melts, and aqueous fluids at extreme conditions. DCO scientists have published new results on the properties of carbon-bearing minerals at high pressures and temperatures, including the structure, compressibility, and elasticity of carbonate minerals, carbides, carbon dioxide (CO
2) ices, and clathrates. One discovery is the formation of phases with sp
3 hybridization of carbon to yield tetrahedral coordination of carbon by oxygen at elevated pressure and temperature. Under ambient conditions, CO
2 is a linear molecule; however, at sufficiently high pressure, CO
2 transforms into a polymerized framework structure in which carbon is tetrahedrally coordinated with four oxygen atoms
[3]. Dense forms of polymeric CO
2 are potential reservoirs of carbon in planetary interiors. The stabilization of tetrahedrally coordinated carbon has profound implications if this carbon substitutes for tetrahedrally coordinated silicon in silicate minerals. This substitution mechanism has been verified experimentally
[4], and indicates a potential reservoir for carbon in Earth and planetary interiors. DCO scientists have also conducted extensive research on carbonate minerals under extreme conditions. Under ambient conditions, carbon forms trigonal planar structural units in carbonate minerals. At high pressure, carbonate minerals transform into denser structures in which carbon is tetrahedrally coordinated with four oxygen atoms [
5–
11]—a discovery that has important implications for the stability and properties of carbonates in the deep carbon cycle [
12,
13]. Complementary experimental studies of carbon in iron at extreme conditions provide conflicting evidence about the elusive role of carbon in Earth’s core [
14–
16].
Magma ocean processes set the initial distribution of carbon and conditions for further development of Earth’s deep carbon cycle
[17]. Magmas are also the main agent for transporting carbon from Earth’s interior to its surface
[18]. Conversely, carbon influences deep Earth dynamics by inducing melting and mobilization of structurally bound mineral water
[19]. Melting may also have a major impact on the recycling of subducted carbon into the deep mantle. The melting-phase relations of recycled oceanic crust suggest that slabs undergo melting and loss of carbonate components in the transition zone, creating a barrier to deep carbon subduction
[20]. Recent experimental studies indicate that carbonatitic liquids provide a potentially significant pathway for carbon recycling at shallow depths beneath arcs
[21].
DCO is making rapid advances in our understanding of the role of aqueous fluids in the deep carbon cycle. Until recently, thermodynamic models developed to understand water-rock interactions in Earth’s upper and middle crust could not be extended to deeper environments, largely because the dielectric constant of water at extreme conditions was unknown. DCO scientists removed this barrier by conducting first-principles calculations of the dielectric constant of water under extreme conditions and the transport of carbonates in the deep Earth
[22]. These calculations were used in combination with experimental results
[23] to develop the Deep Earth Water (DEW) model
[24]. Initial results from the DEW model demonstrate that organic carbon plays an important role in subduction zone fluids
[25] and that diamonds can form due to pH shifts in deep fluids
[26]. The role of carbon in setting the pH of subduction zone fluids can now be assessed, with implications for volatile and metal cycles
[27]. This approach is transforming our view of global geochemical transport
[28].
3. The Reservoirs and Fluxes Community
DCO’s Reservoirs and Fluxes Community is dedicated to identifying the principal deep carbon reservoirs, determining the mechanisms and rates by which carbon moves among these reservoirs, and assessing the total carbon budget of Earth. Its Diamonds and Mantle Geodynamics of Carbon (DMGC) group has created an international infrastructure to study Earth’s deep interior through the unique record preserved in diamonds. DMGC aims to advance deep Earth exploration by studying natural diamonds and diamond-forming fluids and melts in order to elucidate carbon mobility in Earth through geologic time
[29]. Because this effort hinges, in part, on sample availability and a coordinated effort toward sample research, DMGC is developing registered sample collections and a geochemical database of diamonds and diamond inclusions.
The discovery of native metals, metal carbides, and reduced volatiles in large gem-quality diamonds indicates that these diamonds formed from metallic liquid in Earth’s deep mantle, and provides broad implications for Earth’s evolution
[30]. In contrast, highly saline fluids from a subducting slab are the source for certain fluid-rich fibrous diamonds; the data imply a strong association between subduction, mantle metasomatism, and fluid-rich diamond formation, as well as pointing to the important effect of subduction-derived fluids on the composition of the deep lithospheric mantle
[31]. The discovery of hydrous ringwoodite included within an ultradeep diamond provides direct evidence of a hydrous mantle transition zone, which may have a key role in terrestrial magmatism and plate tectonics
[32]. The discovery of calcium silicate perovskite in a diamond that formed 780 km below Earth’s surface provides the first direct observation of Earth’s fourth most abundant mineral; the isotopic composition of carbon in the surrounding diamond, together with the pristine high-pressure CaSiO
3 structure, indicate the recycling of oceanic crust into the lower mantle
[33]. Blue boron-bearing diamonds contain mineral inclusions indicating that these diamonds formed in oceanic lithosphere that was subducted into Earth’s lower mantle. Blue diamonds indicate a pathway for the ultradeep recycling of carbon, water, and other materials from the crust to the lower mantle and back to the surface
[34].
The Reservoirs and Fluxes Community’s Deep Earth Carbon Degassing (DECADE) initiative aims to determine accurate global fluxes of volcanic CO2 to the atmosphere. To achieve this goal, DECADE has launched an intensive field-based effort to install CO2-monitoring networks on 20 of the world’s 150 most actively degassing volcanoes. DECADE also conducts laboratory-based studies, which focus on using gas samples and melt inclusions to provide empirical constraints on carbon degassing to the atmosphere.
High-frequency gas monitoring at Turrialba Volcano in Costa Rica revealed CO
2 precursors to eruptions, which lays the foundation for improved volcanic eruption forecasts
[35]. An international team led by DECADE members demonstrated that the Ambrym basaltic volcano in the Vanuatu Arc in the southwest Pacific Ocean ranks among the top-three known persistent emitters of volcanic gas at the global scale
[36]. There is growing evidence that continental rifts represent a major source of deep carbon released to Earth’s surface from both volcanoes and fault zones [
37–
40]. New measurements of olivine-hosted melt inclusions from the Mid-Atlantic Ridge indicate that upper mantle carbon content varies by almost two orders of magnitude globally, which can affect the dynamics of melting, the style of volcanism, and the evolution of Earth’s atmosphere via planetary outgassing
[41]. For the first time in a global study, DCO scientists found evidence for higher carbon output (
m(CO
2)/
m(S
total), where
m(CO
2) and
m(S
total) are the amounts of CO
2 and total sulfur, respectively) in volcanic arcs where carbonate sediment subducts on the seafloor
[42]. A synthesis of global carbon and helium isotopic data from arc volcanoes concludes that the carbon isotope composition of mean global volcanic gas is considerably heavier than the canonical mid-ocean ridge basalt value; this result indicates that reworking of crustal limestone is an important source of volcanic carbon, which has implications regarding the return of carbon to the deep mantle and for Earth’s past climate
[43].
Modeling and visualization of Earth’s deep carbon cycle through deep time contributes significantly to DCO’s overall goals. A reevaluation of carbon fluxes in subduction zones indicates that almost all of the carbon in subducting sediments and oceanic plates may be extracted in fluids and melts, with relatively little carbon being returned to the convecting mantle
[44]. Plate tectonic reconstructions establish connections between the deep carbon cycle and the concentration of CO
2 in the atmosphere over geologic time scales [
45–
47]. Numerical models of the role of volatiles in reactive melt transport in the asthenosphere indicate that CO
2 and water, despite their low concentration, have an important control on the extent and style of magma genesis, as well as on the dynamics of melt transport and the stranding of carbon at the lithosphere–asthenosphere boundary; these findings have significant implications for deep Earth degassing [
48,
49].
4. The Deep Energy Community
DCO’s Deep Energy Community is dedicated to developing a fundamental understanding of environments and processes that regulate the volume and rates of the production of abiogenic hydrocarbons and other organic species in the crust and mantle through geological time. This research is transforming our understanding of methane (CH4) and includes novel information about its origin, provenance, and formation temperature. Revolutionary advances in instrumentation make it possible to discriminate between methane produced by abiotic synthesis and that produced by biological processes.
The Panorama mass spectrometer is the first instrument capable of resolving the two doubly substituted mass-18 isotopologues of methane (“clumped isotopes”)—
13CH
3D and
12CD
2H
2—at natural abundances
[50]. A paper by Edward Young and 23 co-authors from 14 institutions in eight countries
[51] reports the first resolved measurements of two doubly substituted isotopologues of methane in gases collected from diverse geologic settings around the globe, including major natural gas fields, ultramafic complexes, and ancient waters from deep underground mines in Precambrian Cratons. If thermodynamic equilibrium is achieved, then Δ
13CH
3D and Δ
12CD
2H
2 serve as two independent, intramolecular thermometers. If thermodynamic equilibrium is not achieved, then temperature cannot be determined by this method; however, the data may provide the means for distinguishing between abiotic and biotic origins of methane. Disequilibrium isotopologue ratios can provide information about methane formation mechanisms and serve as tracers that provide insights into mixing, diffusion, kinetics, and other processes
[51].
Ubiquitous dissolved methane in hot-spring fluids emanating from submarine hydrothermal vents is a potential carbon source for microbial communities living at and below the seafloor and in the water column. Methane clumped isotope analyses indicate a hot (270–360 °C), deep, and abiotic origin for methane at seafloor hot springs in unsedimented oceanic crust
[52]. This important finding was made possible by a novel tunable infrared laser direct absorption spectroscopy instrument
[53], which was developed with DCO support. Using the same instrument, microbial methane from a broad range of natural and cultured samples produced non-equilibrium clumped isotope signals
[54]. The clumped isotope anomalies place new constraints on biogeochemical sources, sinks, and budgets of methane.
Computational studies have identified a novel mechanism for abiotic methane formation: The effects of confinement on carbon dioxide methanation in nanopore-controlled chemical reactions can shift thermodynamic equilibrium toward methane production
[55]. This mechanism for producing abiotic methane may be applicable to interactions between seawater and oceanic crust, and could explain the origin of methane observed in some submarine hydrothermal vent systems.
Experimental studies have been conducted on abiotic methane formation under a wide range of conditions, including reducing environments that develop during serpentinization of ultramafic rocks. Using isotopically labeled CO
2, some experiments have shown that earlier claims of CH
4 production by serpentinization at low temperatures were incorrect
[56]. However, other experiments have shown that CO
2 reduction in the presence of extant hydrogen (H
2) vapor leads to significant methane production at low temperatures
[56]. Similarly, the production of methane in serpentinized ultramafic rocks can be catalyzed at low temperatures by ruthenium (Ru)-bearing chromite minerals [
51,
57].
DCO researchers have studied the production of H
2 and CH
4 during serpentinization at field sites around the world. For example, International Ocean Discovery Program (IODP) Expedition 357: Atlantis Massif Serpentinization and Life successfully cored a transect across the Atlantis Massif on the flank of the Mid-Atlantic Ridge. This expedition examined the role of serpentinization in driving seafloor hydrothermal systems, sustaining microbial communities, and sequestering carbon
[58].
In a remarkable paper, DCO scientists documented abiotic synthesis of amino acids in the oceanic lithosphere
[59]. Amino acids formed abiotically during a late alteration stage of massif serpentinites beneath the Atlantis Massif. This discovery has significant implications for the origins of life, ancient metabolisms, and the functioning of the present-day deep biosphere.
Ancient groundwaters in Precambrian Cratons are now recognized as important sources of H
2, which is available to support the subsurface biosphere to depths of several kilometers
[60]. The contribution of Precambrian continental crust to global H
2 production had been greatly underestimated. If H
2 production via both radiolysis and hydration reactions is taken into account, H
2 production rates from the Precambrian continental lithosphere are comparable to estimates from marine systems
[61]. Incorporating H
2 production from the Precambrian continental lithosphere could double existing estimates of global H
2, increasing the habitable volume of Earth’s crust.
Research on molecular hydrogen is not limited to Earth. DCO scientists are members of the teams that discovered molecular hydrogen and higher hydrocarbons escaping from Saturn’s moon Enceladus, which has a layer of ice covering a subsurface ocean [
62,
63]. The discovery of molecular hydrogen indicates that water is reacting with rocks on the floor of the alien ocean
[62]. The process responsible for producing molecular hydrogen on Enceladus might resemble hydrothermal vents on Earth’s seafloor. The discovery of complex carbon compounds emanating from Enceladus
[63] suggests that the moon’s ocean may contain the raw ingredients necessary for life.
5. The Deep Life Community
DCO’s Deep Life Community is dedicated to assessing the nature and extent of the deep microbial and viral biosphere. This Community has expanded the known boundaries of Earth’s microbial and viral biosphere, and investigated the interactions and processes that govern how ecosystems survive and evolve. DCO has united researchers approaching overarching deep life questions from varying perspectives by: ① studying the subsurface biosphere in the sediments and rocks of both the continents and the seafloor; ② exploring what genomes can reveal about the limits and possible origins of life; and ③ investigating the response of deep life to a range of physical and chemical extremes.
The lower boundary of the deep sedimentary biosphere was explored by DCO researchers participating in IODP Expedition 337: Deep Coalbed Biosphere off Shimokita
. They discovered a microbial ecosystem producing methane from Miocene coalbeds nearly 2.5 km below the seafloor
[64]. The assemblages appear to be the descendants of microbes buried in terrigenous sediments up to 20 million years ago. Despite low cell numbers, these microbes are actively growing at rates that range from months to over 100 years—some of the slowest microbial biosynthesis rates ever directly measured by incubation
[65]. Other DCO researchers found microbial communities exhibiting aerobic respiration in sediments 75 m below the seafloor in the South Pacific Gyre, a finding that suggests that oxygen and aerobic communities may occur in sediments over 15%–44% of the Pacific and perhaps 9%–37% of the global seafloor
[66].
The select survival of taxa capable of coping with the severe energy stresses characteristic of subsurface environments may cause these taxa to become the founders of more common communities such as Bathyarchaeota, which may play an important role in carbon cycling
[67]. Therefore, microbial communities existing at extremes, such as the elevated temperatures in hydrogen-rich hydrothermal vent systems on the Mid-Cayman Rise, are critical to understanding the limits of deep life
[68]. For example, members of the Bathyarchaeota are among the most abundant, diverse, and widely distributed Archaea in marine subsurface habitats globally. A metagenomic study has shown that Bathyarchaeotal subgroups employ versatile metabolisms, which in turn supply substrates for heterotrophic and methanogenic community members
[69]. Another metagenomic study indicates that distinct evolutionary pressures correlate with genes related to nutrient uptake, biofilm formation, or viral invasion, a finding that is consistent with distinct evolutionary histories between geochemically different hydrothermal vent fields
[70].
Based on observations of Archaea and bacteria at 77 worldwide locations with different marine ecosystems, DCO researchers determined that methane seep communities exhibit lower diversity than communities in other ecosystems. The surviving assemblages reflect the most favorable microbial metabolisms at methane seeps and distinguish the seep microbiome from other seafloor microbiomes
[71]. Although only a few species of methanotrophs occur at all seeps worldwide, these microorganisms seem to greatly influence the methane budget of the ocean.
DCO also investigates the deep terrestrial continental biosphere. Based on a compilation of cell concentration and microbial diversity data from continental subsurface localities around the globe, DCO researchers estimated that the continental subsurface hosts (2–6) × 10
29 cells, and found that bacteria are more abundant than Archaea and that their community composition is correlated to sample lithology
[72].
Researchers investigating a subsurface lithoautotrophic microbial ecosystem (SLiME) in the ancient Witwatersrand Basin found, to their surprise, that sulfur-driven autotrophic denitrifiers were the dominant microbial group. Further analysis revealed that metabolic community cooperation enabled less typical metabolic reactions to prevail and stabilize the ecosystem
[73]. Modeling also suggests that food—not dissolved oxygen—limits eukaryotic population growth at 1.4 km depths in 12 300-year-old palaeometeoric fissure water in South African mines [
74,
75]. Microbial communities occurring in connection with hydraulic fracturing during hydrocarbon resource development in the deep subsurface also provide clues to the deep terrestrial biosphere
[76]. Microbes exist as deep as 2500 m below land surface and exhibit salt tolerance, metabolic capacity without electron acceptors, and evidence of active viral infection.
Through the DCO Census of Deep Life (CoDL), DNA sequencing has provided new and value-added information about carbon cycling, deep biosphere evolution, and the connection between ecosystems and environments both continental and marine. For example, CoDL researchers used 16S DNA sequencing to identify bacterial and archaeal taxa associated with eight minerals from a sub-seafloor microbial observatory on the Juan de Fuca Ridge, 280 m below the seafloor. They confirmed that distinct communities colonize different minerals, and that these communities group by mineral chemistry
[77].
DCO’s Extreme Biophysics group has approached the challenges of life existing in these extreme environments from a completely different angle. By focusing on molecular-level adaptation to life under extreme conditions, these researchers are advancing our knowledge of the basic chemistry and physics of the component biological structures and systems that define the limits of life [
78,
79].
6. Synthesis and future opportunities
DCO is synthesizing and integrating research across its four Science Communities to realize a new understanding of deep carbon science and fully capture DCO’s achievements. This synthesis process aims to elevate the collaborative efforts of the global research initiative. Synthesis products and activities include cross-community research projects, such as Biology Meets Subduction and Carbon Mineral Evolution, as well as workshops, meetings, visualizations, special issues of journals, and books. The culmination of these activities will occur at an international conference, Deep Carbon 2019: Launching the Next Decade of Deep Carbon Science.
To help launch the next decade of deep carbon science, DCO scientists are developing a broad portfolio of activities that will extend beyond the culmination of the initial decadal program in 2019. A biennial Gordon Research Conference on Deep Carbon Science is planned as a sustainable successor to DCO international science meetings. A biennial Gordon Research Seminar on Deep Carbon Science for early career scientists could become a successor to DCO Summer Schools and DCO Early Career Scientist Workshops. Carbonates at High Pressures and Temperatures (CarboPaT), a research consortium supported by the German Research Foundation to study carbonates at extreme conditions, will continue to provide a platform for deep carbon science in Germany. In the United Kingdom, the Natural Environment Research Council has established a research program on Volatiles, Geodynamics, and Solid Earth Controls on the Habitable Planet. Science for Clean Energy is a European consortium led by DCO researchers with support from the European Union’s Horizon 2020 program. The next decade of deep life research will be facilitated by a new International Center for Deep Life Investigation at Shanghai Jiao Tong University in China as well as a cluster of excellence titled The Ocean Floor—Earth’s Uncharted Interface in Germany. These and other initiatives will help propel the next decade of deep carbon science.
7. Conclusion
In 2009, DCO was an ambitious experiment in scientific programs with no guarantee of success. Since then, DCO has evolved into a network of more than 1200 scientists that spans the globe and transcends traditional scientific disciplines. DCO is a science incubator that has launched new research groups, science communities, international scientific collaboration and partnerships, major research projects, field expeditions, scientific instruments, and companies. Perhaps most importantly, DCO has built an enduring legacy in its diverse, dynamic, and collaborative community of interdisciplinary scientists. DCO’s management and community-building innovations are keys to the program’s scientific success. Based on its success in achieving fundamental advances in deep carbon science, DCO may serve as an effective model for tackling large-scale, interdisciplinary, and international science questions.
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