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Deep Carbon

Deep carbon refers to the carbon found beneath the subsurface of the Earth, where ninety percent of the Earth’s carbon resides. This vital part of the carbon cycle impacts the oceans, atmosphere and ultimately life on Earth. Despite this, there is still much unknown about the behaviour of carbon in the Earth’s interior. Advances include the quantification of carbon fluxes at subduction zones, the role of microorganisms in subsurface carbon cycling and the carbon budget of Earth’s deep interior. This Collection contains Reviews and Research from Nature, Nature Communications, Nature Reviews Microbiology, Nature Geoscience and Nature Microbiology that cover the latest advances in deep carbon science.

This Collection is editorially independent, produced with financial support from a third party. About this content.

Core collection

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.

Q&A | Open Access | | Nature Communications

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.

Review Article | | Nature

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.

Review Article | Open Access | | Nature Communications

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.

Review Article | | Nature Reviews Microbiology

Andrew Thomson et al. present experiments showing that carbonated oceanic crust subducting into the mantle will intersect the melting curve at depths of about 300 to 700 km, creating a barrier to direct carbonate recycling into the deep mantle. The low-degree partial melts produced would be highly reactive with reduced ambient mantle, producing diamond. The authors conclude that this deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

Letter | | Nature

Deep life

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

Research on the anaerobic oxidation of natural gas has largely been focused on methane as the most abundant constituent. It is less clear how short-chain alkanes—including ethane, propane, n-butane and iso-butane, which together make up about 20% of natural gas—are anaerobically metabolized. Sulfate-reducing bacteria are the only organisms known to date to anaerobically oxidize short-chain hydrocarbons. Gunter Wegener and colleagues identify an anaerobic thermophilic enrichment culture composed of dense consortia of archaea and bacteria that uses a pathway similar to anaerobic methane oxidation, which was previously thought to be specific for C1-compounds, to oxidize butane. Archaea activate butane, and reducing equivalents are channelled to sulfate-reducing partner bacteria. Similar consortia are detected in marine subsurface sediments, suggesting that this pathway may be widespread in nature.

Article | | Nature

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.

Article | | Nature

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.

Article | Open Access | | Nature Communications

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.

Article | | Nature Communications

Deep Earth

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

Within the Earth's transition zone and lower mantle, the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3) is thought to be the main host of calcium, as well as the heat-producing elements potassium, uranium and thorium. Despite being considered as the fourth most abundant mineral in the Earth, it has never been found in nature. Fabrizio Nestola and co-authors document the perovskite-structured polymorph of CaSiO3 included within a diamond from Cullinan kimberlite mined in South Africa. The authors conclude that the bulk composition of material within the diamond is consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle, providing additional evidence for the recycling of oceanic crust and carbon from the surface to lower-mantle depths.

Letter | | Nature

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

Reservoirs and fluxes

Within the Earth's transition zone and lower mantle, the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3) is thought to be the main host of calcium, as well as the heat-producing elements potassium, uranium and thorium. Despite being considered as the fourth most abundant mineral in the Earth, it has never been found in nature. Fabrizio Nestola and co-authors document the perovskite-structured polymorph of CaSiO3 included within a diamond from Cullinan kimberlite mined in South Africa. The authors conclude that the bulk composition of material within the diamond is consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle, providing additional evidence for the recycling of oceanic crust and carbon from the surface to lower-mantle depths.

Letter | | Nature

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

Andrew Thomson et al. present experiments showing that carbonated oceanic crust subducting into the mantle will intersect the melting curve at depths of about 300 to 700 km, creating a barrier to direct carbonate recycling into the deep mantle. The low-degree partial melts produced would be highly reactive with reduced ambient mantle, producing diamond. The authors conclude that this deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

Letter | | Nature

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.

Letter | | Nature

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

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.

Article | Open Access | | Nature Communications

It is not clear just how much water resides within the solid Earth, and where it is to be found, with many indirect measurements yielding conflicting results. Here Graham Pearson and co-authors present evidence from a diamond inclusion from Juína, Brazil, for the first known terrestrial occurrence of ringwoodite — a high-pressure polymorph of olivine first identified in meteorites and thought to be a major constituent of the Earth's mantle transition zone. The water-rich nature of this inclusion provides direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent.

Letter | | Nature

Most of the diamonds found near the Earth's surface were formed at depths of more than 150 km in the roots of old continents. Chemical impurities bottled up in 'dirty' diamonds — clear monocrystalline diamonds overgrown by a microinclusion-bearing fibrous coat — therefore hold valuable information about these deep, inaccessible regions of the Earth. Yaakov Weiss and co-authors present geochemical data from inclusions within a suite of eleven diamonds from the Ekati mine from the Northwest Territories, Canada. The data contain a clear chemical evolutionary trend that indicates the involvement of highly saline solutions in the formation of silicic and carbonatitic deep mantle melts. The chemistry of the saline fluids and the timing of host diamond formation suggest a subducting plate under western North America as the source of the fluids, implying a strong association between subduction, mantle metasomatism and fluid-rich diamond formation. This new model provides a context for resolving the effects of the compositional spectrum of mantle fluids, which alter the deep lithosphere globally and play key roles in diamond formation.

Letter | | Nature

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.

Letter | | Nature

Matthieu Galvez and co-authors present thermodynamic predictions of fluid–rock equilibria that tie together models of subduction-zone thermal structure, mineralogy and fluid speciation. They find that the pH of fluids in subducted crustal lithologies is uniform and confined to a mildly alkaline range, controlled by rock volatile and chlorine contents, but that the pH of mantle wedge fluids exhibits marked sensitivity to minor variations in rock chemistry. They conclude that this sensitivity of fluid chemistry to carbon, alkali metals and halogens illustrates a feedback between Earth's atmosphere–ocean chemistry and the speciation of subduction-zone fluids via the hydrothermally altered oceanic lithosphere.

Letter | | Nature

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.

Article | | Nature Geoscience