Ocean-Floor Resources

3.3.1 Mineral carbonation and CO

sequestration in the oceanic lithosphere

The ocean crust is a natural sink for CO₂ (Alt and Teagle, 1999) and during hydrothermal alteration and weathering it releases Ca and Si, which increase the production of carbonate and biomass in the oceans and hence play a role in the carbon cycle. A new fundamental question is: How does natural mineral carbonation in previously little-studied lithologies such as mantle peridotite affect the global carbon cycle? Natural carbonation of peridotite could be an important sink for carbon from the ocean into subduction zones. Ocean drilling programs have played no role as yet in the study of natural mineral carbonation processes and conditions in the ocean crust. Yet, much of what we wish to know about natural mineral carbonation systems can be achieved via ocean drilling. Learning about mineral carbonation from natural analogues can take place within IODP expeditions even if their primary objectives lie elsewhere.

The study of the deeper parts of active hydrothermal systems, particularly those where rapid, ongoing peridotite carbonation has been demonstrated, should be given a high priority. These include mid-ocean ridge systems (e.g., Lost City, Rainbow), carbonated peridotite basement at rifted continental margins, oceanic lithosphere altered during the aging process, and carbonated peridotite in serpentine seamounts in western Pacific fore-arc settings. Of exceptional interest is sampling of actively forming, large (>1 m) bodies of fully carbonated peridotite (listwanites) in near-seafloor peridotites, because full carbonation is the goal of enhanced, in situ mineral carbonation processes and it is vital to understand the combination of pressure, temperature, mechanical properties, and fluid composition that lead to this outcome in natural systems.

In studying natural mineral carbonation in these environments, we need to characterize relevant mineral carbonation reactions (reactants, products, temperature, pressure, fluid composition). Theoretical and experimental work along these lines has just started (Andreani et al., 2009; Kelemen and Matter, 2008), but we also need to

determine mineral carbonation rates in ocean lithosphere by relating carbonate abundance to age in drill cores (e.g., Alt and Teagle, 1999). Additionally, we need to understand the controls of oxygen fugacity buffers, pH, and the presence of chemical or biological catalysts on those rates. Finally, we need to determine fluid-flow rates and multi-scale porosity and permeability characteristics in the ocean crust.

There are many advantages to subseafloor geological capture and storage of CO₂, including the safe release of saline pore fluids displaced by CO₂ injection. Mineral carbonation is important for long-term storage of CO₂ during injection of the gas into pore space in sediments, but is the least well understood component of the overall process. We have much to learn from natural systems about favored geochemical and geomechanical pathways for mineral carbonation. Identifying and studying natural environments where rates of carbonation are increased will be critical in the evaluation of promising methods of in situ mineral carbonation. Ocean drilling can provide ‗proof of concept‘ for these options, which may be complementary to, or even preferable to, injection of supercritical CO₂ into pore space in sedimentary rocks. Questions to be addressed include: What are the fundamental mechanisms of mineral carbonation in natural systems and how can they be utilized to design systems for enhanced, in situ CO₂ sequestration? What are the conditions of pressure, temperature, fluid composition, rock composition, and physical properties that maximize the rate of mineral carbonation processes in different environments proposed for geological CO₂ sequestration? How will changes to the natural system (increasing pCO₂, increasing fluid-flow rates) affect the natural system?

3.3.2 Subseafloor resources

CO₂ sequestration within pore spaces and reactive lithologies represents only one subseafloor resource exploitable for human society. Apart from hydrocarbons (including gas hydrates, which are more extensively covered in section 5.1.3), polymetallic sulfide mineralizations constitute a potentially important type of subseafloor resource. Others include geothermal energy and genetic resources (e.g., enzymes from thermophilic microorganisms).

Gas hydrates

The largest fraction of hydrocarbons on Earth is stored in gas hydrates, but many details of the global distribution and abundance of gas hydrates have yet to be elucidated. What are the controls on gas hydrate localization and concentration?

Progress on this front depends on our ability to determine the nature of bottom-simulating seismic reflectors (BSRs) as indicators of gas hydrate occurrences.

Furthermore, the source of the carbon in gas hydrates and the rate at which these deposits form is relatively poorly understood. In terms of assessing the exploitability of gas-hydrate deposits, it is crucial to study the effect of mining of gas hydrates on continental slope stability.

Conventional oil and gas

Large hydrocarbon deposits form in rifted continental margin settings. It is uncertain to what extent igneous activity affects the maturation of conventional oil and gas deposits. Non-volcanic rifted margins constitute very favorable conditions for

hydrocarbon formation, both in terms of their structure and thermal states. The basement underlying hydrocarbon-bearing sedimentary sequences is often inaccessible, but slow- and ultraslow-spreading ridges may be a good analogue that can be utilized by developing synergies between the hydrocarbon and lithosphere research communities.

Deep-seafloor volcanogenic-hosted massive sulfide deposits

From a research perspective, submarine massive sulfide accumulations have long been of interest because it is believed they represent modern analogues for volcanogenic-hosted massive sulfide (VMS) deposits on land, including many world-class copper and gold deposits. From studies of active seafloor hydrothermal systems, much can be learned about the formation of VMS deposits. Drilling is critical to examine the structure, thermal profiles, source rocks, fluid pathways, reaction/precipitation processes, and the controls on sulfide composition and abundance. Questions revolving around subseafloor sulfide mineralizations and their potential use as a metal resource include the following: What are the size and magnitude of massive sulfide accumulations and how can we detect them remotely and ground-truth them directly? Can massive sulfide accumulations form without creating a seafloor expression of high-temperature fluid upflow? What is the influence of the underlying volcanic crust/sediment composition and magma degassing processes on sulfide composition (base metal content)? What controls the sub-surface pathways of fluids and their precipitation/reaction products?

What is the source of the reduced sulfur and how does it vary along the hydrothermal upflow path? What are the rates of formation of seafloor sulfide accumulations? What are the mechanisms for focusing, precipitating, and preserving seafloor and subseafloor sulfides? What is the role of biomediation in the formation and enrichment of metals in seafloor and subseafloor sulfide?

Further questions relate to specific settings. For instance, what is the heat source for ultramafic-hosted sulfide mineralizations and hydrothermal systems? Is it mafic intrusions or serpentinization? Are volcanic rifted margin sequences, including their intrusive bodies and their mineralization, analogues for other LIPs?

Sustainability issues need to be considered when entertaining thoughts about seabed mining of metal sulfides. How fragile are the seabed and sub-seabed ecosystems? What is the comparative environmental footprint of seabed mining compared with land-based mining?

Bio-prospecting

A key question related to bio-prospecting of the subseafloor biosphere is whether it is feasible and desirable. In any case, researching the biological involvement in massive sulfide formation and base metal concentration both at the seafloor and in the subseafloor should be considered a high priority. Of special interest in this regard are microbes, which can catalyze the dissolution or precipitation/immobilization of various elements and thereby mediate the extraction or recovery of desired metals and metaloids.

Sub-seafloor geothermal energy

While the thermal power output from ridges and arcs is immense (several Terawatts), a critical question is whether subseafloor geothermal energy resources are a

viable future energy resource. A related question is over what time frames focused and potentially usable geothermal resources on ridges are sustainable.

Drilling is critical for ground-truthing geophysical methods of detecting subseafloor resources. Drilling is also required to access deeper portions of the seafloor beneath zones of mineralization. Moreover, drilling is the only means of assessing the thermal and permeability structure of the seafloor into and beneath zones of hydrocarbon or metal accumulations.