Deep Carbon

Professor Marie Edmonds is a volcanologist at the University of Cambridge. She is interested in the role of magmatic volatiles in magma genesis, volcanic eruptions, and volatile geochemical cycling. Dr. Robert Hazen is a geologist at Carnegie Science and executive director of the Deep Carbon Observatory. His latest research has focused on the co-evolution of the geospheres and biospheres, and mineral diversity and distribution. Marie and Robert apply their research to help understand the chemical and biological roles of carbon in Earth.

The processes that control the movement of carbon from microfossils on the seafloor to erupting volcanoes and deep diamonds, in a cycle driven by plate tectonics, are reviewed.

Subseafloor microbial activities are central to global biogeochemical cycles, affecting Earth’s surface oxidation, ocean chemistry, and climate. Here the authors review present understanding of subseafloor microbes and their activities, identify research gaps, and recommend approaches to fill those gaps.

The seabed is a hostile environment for most microorganisms, and unique microbial communities are found in deeply buried marine sediments. In this Review, Orsi highlights which and how microorganisms survive and the differences between oxic and anoxic subseafloor sediments.

Methane metabolism has a central role in the global carbon cycle. In the Review, Tyson and colleagues discuss the enzymatic pathways responsible for archaeal methane metabolism and highlight the evolutionary relationships of key enzymes with recently discovered alkane-oxidizing archaea.

Continental rifts are stores and sources of abundant carbon, according to calculations of carbon storage, enrichments and mobilization in rift systems. Continental rift systems are likely to play an important role in Earth’s deep carbon cycle.

The abundance of microorganisms in the continental subsurface may have been overestimated, according to a review compilation of data from subsurface localities around the globe.

Hydrothermal vents are unique habitats for chemosynthetic bacteria and archaea and the animals that live in symbiosis with them. In this Review, Dick explores the challenges and opportunities that vent ecosystems provide for microbial life and their relationship to biogeography.

In the forearc regions of Costa Rica, helium and carbon isotope data reveal that about 20 per cent less carbon is being transported into the deep mantle than previously thought.

Experiments show that carbonated oceanic crust subducting into the mantle will intersect the melting curve at depths of about 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle.

Microorganisms metabolise methanol using either a methanol methyltransferase or a methanol dehydrogenase. Here, the authors use proteomics and stable isotope fractionation to show that a thermophilic sulfate-reducing bacterium, isolated from the deep subsurface, uses both pathways.

The warm subseafloor at deep-sea hydrothermal vents hosts diverse microbial communities. Here, Anderson et al. reconstruct 73 metagenome-assembled genomes from two geochemically distinct hydrothermal vent fields, showing different patterns of genomic variation among diverse microbial taxa.

Anaerobic archaea enriched in thermophilic microbial consortia completely degrade butane by modifying mechanisms which were hitherto thought to be specific to methane metabolism.

Using transcriptome data from marine subsurface sediments, expressed microbial enzymes are shown to be potential targets for secretion by Bacteria, Archaea and Fungi, providing insights into nutrient cycling in the subsurface environment.

High-resolution imaging techniques show that aromatic amino acids such as tryptophan formed abiotically and were subsequently preserved at depth beneath the Atlantis Massif of the Mid-Atlantic Ridge, supporting the hydrothermal theory for the origin of life.

Little is known about the microbial ecology of the deep seabed. Here, Dong et al. predict metabolic capabilities and microbial interactions in deep seabed petroleum seeps using shotgun metagenomics, sediment geochemistry, metabolomics, and thermodynamic modelling.

Hydrogen build-up in geological nuclear waste repositories poses risks, but it may be alleviated by H₂ consumption by deep subsurface microbial communities. Here, the authors inject H₂ in a borehole and use metagenomics and metaproteomics to identify a carbon cycle driven by autotrophic H₂oxidizers.

The contribution of marine archaea to the ocean's carbon cycle is unclear. Here, Li et al. analyse the genomes and transcriptomes from five deep-sea archaeal groups to reveal their metabolic characteristics, suggesting a crucial role in modulating the carbon cycle in deep oceans.

Research on microbes that inhabit the Earth's subsurface is mostly based on metagenomic information only. Here, Probst et al. combine metagenomics with ultrastructural and functional analyses to study the biology of a group of uncultivated subsurface archaea, the SM1 Euryarchaeon lineage.

The amount of carbon stored in closed hidden reservoirs is unknown. Here the authors use a computational approach to study the evolution of carbon species and observe polymerization of carbon atoms at high pressures, illustrating the potential for a significant carbon reservoir in the Earth’s deep interior.

Carbon migration in the deep Earth is still not fully understood. Here, the authors show that immiscible isobutane formsin situfrom transformation of aqueous sodium acetate at 300 °C and 2.4–3.5 GPa, indicating that hydrocarbon fluids may play a major role in carbon transfer in the deep carbon cycle.

The composition of natural calcium silicate perovskite, the fourth most abundant mineral in the Earth, found within a diamond indicates an origin from oceanic crust subducted deeper than 700 kilometres into the Earth’s mantle.

The cause of diamond precipitation has previously been attributed to poorly understood redox changes at depth. Here, the authors propose that a drop in pH during water–rock interactions leads to diamond formation as a consequence of the migration of reactive fluids at elevated temperatures and pressures.

The lowermost mantle and transition zone are increasingly oxidized at depth, according to analyses of the oxidation state of iron in majoritic garnet inclusions from deep diamonds.

The nature and stability of carbon dioxide under extreme conditions relevant to the Earth’s mantle is still under debate, in view of its possible role within the deep carbon cycle. Here, the authors perform high-pressure experiments providing evidence that polymeric crystalline CO₂ is stable under megabaric conditions.

The carbon abundance in the Earth’s mantle is enhanced relative to sulfur. Experiments suggest that the accretion of a differentiated planetary body to the growing Earth could explain the silicate Earth’s carbon and sulfur budgets.

The composition of natural calcium silicate perovskite, the fourth most abundant mineral in the Earth, found within a diamond indicates an origin from oceanic crust subducted deeper than 700 kilometres into the Earth’s mantle.

Alteration of ultramafic rocks plays a role in hydrocarbon production, but little is known about this process at depth. Here, the authors provide evidence that alteration of carbonated ultramafic rocks at high-pressures are an important source of abiotic methanogenesis with implications for deep C mobility.

Degassing of large amounts of CO₂ from continental rifts may have contributed to greenhouse climate episodes over the past 200 million years, according to numerical models.

Earth degassing of CO₂-rich fluids contributes significantly to the global carbon budget but its link to tectonic regimes remains unclear. Here, the authors use global geological datasets to show that there is a positive spatial correlation between CO₂ discharges and extensional tectonic regimes.

Experiments show that carbonated oceanic crust subducting into the mantle will intersect the melting curve at depths of about 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle.

Mineral inclusions in blue boron-bearing diamonds reveal that such diamonds are among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements.

Current estimates of dissolved CO₂ in subduction-zone fluids based on thermodynamic models rely on a very sparse experimental data base. Here, the authors show that experimental graphite-saturated COH fluids interacting with silicates at 1–3 GPa and 800 °C display unpredictably high CO₂ contents.

The origin of carbon-rich magmas is unclear. Boron isotopic analysis of carbonatite magmas that formed over the past 2.6 billion years reveals a link to carbon recycled during tectonic plate subduction.

Thermodynamic calculations suggest that condensed carbonaceous matter should be the dominant product of abiotic organic synthesis during serpentinization of the oceanic crust at Mid-Ocean Ridges. Here the authors report natural occurrences of such carbonaceous matter formed during low temperature alteration.

Subduction of oceanic crust introduces huge amounts of carbonates into Earth’s mantle, contributing to the global carbon cycle. Here, based on high-pressure-temperature experiments, the authors present a reversible temperature-induced transition from aragonite to amorphous CaCO₃.

X-ray diffraction, Raman and infrared spectroscopic evidence for the inclusion of water-rich ringwoodite in diamond from Juína, Brazil, indicates that, at least locally, the Earth’s transition zone is hydrous to about 1 weight per cent.

Carbonated silicate melts are expected to exist in the mantle, but have been elusive in nature. Geochemical analyses of rocks from the South China Sea identify such melts formed in the mantle and erupted at the surface through thin lithosphere.

Geochemical data from inclusions within diamonds from the Northwest Territories, Canada, indicate that saline fluids are parental to silicic and carbonatitic deep mantle melts, via fluid–rock interaction; a subducting plate under western North America is suggested to be the source of the fluids.

The formation of Bermuda sampled a previously unknown mantle reservoir that is characterized by silica-undersaturated melts enriched in volatiles and by a unique lead isotopic signature, which suggests that the source is young.

Seismic images of Earth’s crust and uppermost mantle around the Mariana trench show widespread serpentinization, suggesting that much more water is subducted than previously thought.

A thermodynamic model of fluid pH and its variability in Earth’s mantle and subducting crust highlights chemical feedbacks that connect deep Earth to surface processes.

Estimates of the carbon content of Earth’s mantle and magmas vary. Analysis and modelling of gas emissions at Hawai‘i indicate that the amount of carbon in the Hawaiian mantle plume and CO₂ in Hawaiian lavas is 40% greater than previously thought.

Transfer of CO₂ from Earth’s interior to the atmosphere happens largely by volcanic degassing. Measurements of CO₂ emissions from faults in the East African Rift system imply that tectonic degassing is also important for deep carbon release.

Little is known about the deep carbon cycle during the Archaean. High- pressure and -temperature experiments indicate that the subduction of organic carbon on a hotter, younger Earth was efficient, helping to sequester carbon in Earth’s interior.