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Likewise, the deep interior plays a critical role in the global carbon cycle and
carbon can also alter physical properties of the mantle yielding feedbacks between
carbon cycling and mantle dynamics (see Figure 2.7). As is true for H
2
O there are
great uncertainties in the distribution and flux of carbon (e.g., Dasgupta and
Hirschmann, 2010). Most of Earth's carbon is stored in rocks, with much of that
carbon in the mantle. Most mantle carbon is stored in high-pressure minerals, and
volcanic processes provide the mechanisms for transferring some of this carbon to the
atmosphere, while subduction provides the main mechanism for its return to the
interior. Because the mantle carbon reservoir is thought to be large, resolving the
internal component of the global carbon cycle is vital to interpreting the record of
long-term climate changes. The broad scope of understanding Earth's carbon cycle
from crust to core will require the expertise of geologists, physicists, chemists, and
microbiologists. For example, discoveries of microbial life deep in the crust beneath
both the oceans and continents indicate a rich subsurface biota that by some estimates
may rival all surface life in total biomass. The subduction of tectonic plates and
volcanic outgassing are primary vehicles for carbon fluxes to and from deep within
Earth, but the processes and rates of these fluxes—as well as their variation
throughout Earth's history—remain poorly understood. For example:
•
Is biologically processed carbon represented in deep Earth reservoirs?
•
What are the physical and chemical processes that govern carbon's distribution in
Earth?
•
How do carbon's elemental character and behaviors impact its various roles in the
Earth system?
The current opportunity to improve our understanding of volatile fluxes in the
interior also derives from improvements in high-resolution imaging of internal
structures and material properties with seismology and magnetotellurics, especially in
regions of both active and ancient subduction, in new petrological and volcanic
constraints on subduction zone volatile fluxes, in high-resolution 3D geodynamical
modeling capabilities for subduction zones with volatile transport and mineralogical
reactions, and in mineral physics characterization of the myriad hydrous phases,
dehydration processes, and influence of volatiles on rheology and the elastic
properties imaged by seismology. Concerted community efforts to study subduction
zones such as GeoPRISMs bring together diverse research communities that can
address the volatile budget and flux problem, and large-scale studies of upper-mantle
structure such as those conducted under the Continental Dynamics and EarthScope
programs now regularly cast interpretations of seismic models in terms of coupled
thermal, volatile, and chemical heterogeneities rather than solely thermal models (see
Figure 2.8). With great expansions of seismological databases that can be anticipated
over the next decade, in parallel with improved characterization of rheological and
elastic attributes that reflect volatile presence and abundance, significant progress on
mapping volatile distributions and resolving volatile fluxes can be anticipated with
sustained research investment.
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