Deep Carbon Observatory Thematic Institute (TI) “Carbon from the Mantle to the Surface” was held at the Sutardja Dai Hall of University of California, Berkeley, CA on 1 to 3 July 2015. The goal of this Thematic Institute was to bring an interdisciplinary community of geoscientists to discuss the forms and fluxes of carbon in Earth and its long-term fate on the planet. The DCO TI took place in partnership with the National Science Foundation-sponsored Cooperative Institute for Dynamic Earth Research (CIDER) Summer Program scheduled from 6 to 31 July. The aim of the DCO TI was a state of the art understanding of fluxes and uncertainties that addresses one of DCO’s decadal goals: is the net flux of carbon into or out of Earth? If carbon is disappearing into Earth via plate subduction, does this affect the surface abundances over geological time? Or is carbon returned efficiently to surface pools via metamorphism, migration, magmatism, and eruption? A further goal was to assess not only the fluxes and reservoirs and their uncertainties, but also the processes that affect these estimates and how they may have varied through geologic time. The complete agenda is listed here.
The meeting was attended by about 85 participants, including senior and early career faculty members and scientists, graduate students, and postdoctoral researchers from the US and from academic institutions overseas, with 20 to 30 DCO TI participants also attending the CIDER Summer Program. The three-day meeting was divided into three broad themes: Day 1 on Carbon In the Mantle, i.e., how much carbon enters the mantle from surficial reservoirs via subduction; Day 2 on Carbon Rising, i.e., processes that lead to extraction of carbon from the mantle; and Day 3 on Carbon Output, i.e., how much carbon is getting liberated from the solid Earth. The meeting included a series of 10- to 25-minute research presentations with ample time for discussion, questions, and answers. The meeting also included poster presentations and 5-minute rapid-fire talks by students and post-docs.
Additionally, the meeting featured a session on the final day in which four breakout groups were tasked with highlighting the priority research questions of the three themes of the meeting—Carbon in the Mantle, Carbon Rising, and Carbon Outputs, as well as reflecting on how these themes could be integrated.
Some of the key challenges identified in the breakout groups include:
1) How significant is the flux of carbon coming from non-ridge sources, such as plume, continental intraplate, and diffuse fluxes?
2) How do we constrain how much carbon coming from the present-day mantle is primordial or recycled?
3) What are the consequences of a net outflux of carbon from the mantle at the present day? Similarly, what are the consequences of a net influx of carbon into the mantle at the present day? How will we test these scenarios?
4) How does the rise of carbon and carbon-bearing fluid/melt in rift systems relate to continental break-up?
5) What was the original budget of carbon in the mantle and how heterogeneous was the distribution?
A more detailed summary the collected thoughts from these groups are as follows:
Carbon in the Mantle and Carbon Rising
One of the most frequently raised points in the meeting was the key role of oxidation state of the mantle, as reflected in the breakout feedback. Key questions included how oxidation states vary with pressure, how heterogeneous it may be in different mantle environments, and what the carbon fertility of different mantle source regions is.
Other questions touched on uncertainties in the amounts of carbon residing in the continental lithosphere, how heterogeneous carbon distributions were during Earth’s formation, and whether the core plays any role now or in the past to the distribution of carbon in the mantle. Whether the abundance of carbonatitic melt in the mantle is sufficiently voluminous to be detected with geophysical methods was also addressed.
Additionally questions focused on how much carbon is entering the mantle, to what depths and with what oxidation state, and what the destiny is of subducted organic carbon. The group also considered volatile and redox heterogeneity in the oceanic crust entering a subduction zone. With regard to serpentinization of oceanic crust by bending fault at outer rise of subducting plates – is carbonation involved, and if so, what happens to carbon released by deserpentinization during subduction? Does hydrocarbon generation occur during serpentinization? Key to this discussion is knowledge of the age of subducting crust over Earth’s surface today and in the past, as this controls the carbon flux entering the mantle. Looking at timescales, key questions include what is the residence time of carbon in the mantle, and whether there is an unambiguous geochemical signature for primordial carbon. How does redox evolve through deep time?
The main questions to address in terms of carbon output include the sources and processes of carbon flux in places like East African rift system, and how it relates to continental rifting and breakup. Closely related to this is the question of the relative flux from MORB and ridge flank degassing vs. rift eruption. Diffuse degassing is clearly a major component of the carbon output story, with main questions including how much is released from the lithosphere by diffuse output, igneous intrusion and rifting.
Crowdsourcing measurements of widespread diffuse sources of CO2 on flanks of volcanoes and tectonic areas/faults could be a solution to the practical issues of measuring large areas. There are almost certainly unaccounted fluxes e.g., springs in Italy and Yellowstone. We need to extrapolate widely but accurately for global flux models, using smart statistical methods. We need to know starting CO2 concentrations better in many magma types, in particular in arcs and OIB. We need much better knowledge of magma fluxes, particularly the intrusive to extrusive ratio. In exposed arc crustal sections, we need to know magnitude of losses by delamination.
One feature of the three-day meeting was how frequently different talks used the schematic diagram from Peter Kelemen depicting carbon budgets entering and exiting the mantle. An ongoing international group effort to provide estimates for budgets, fluxes, and reservoirs in the global C cycle model seems an optimal integration strategy. Knowledge of carbon isotope fractionation could be extended to other systems such as melting models to improve our understanding and interpretation of carbon isotope signatures.
A key strategy in integrating the meeting themes is numerical modelling, through combination of complimentary phase equilibrium codes and mass transport models that can be used together with geodynamic simulations, allowing investigation of multiple scales. This would also produce the raw materials useful for 3D visualization for education, outreach, and scientific discovery.
The group considered integrating approaches in focus areas, as in the Margins program, e.g., integrated mantle to surface studies at volcanoes including deep, long period earthquakes, volcanic gas data, other geophysical data, better location of earthquakes, and locations where CO2/SO2 last equilibrate with magma. Finally, linking biological processes with silicate weathering could yield deep multidisciplinary insights. Highlights of presentations and posters are summarized here.
These questions and synergies developed during the Thematic Institute will guide Deep Carbon Observatory efforts over the next five years.