ABSTRACT

Subduction zones are the interface between Earth?s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively moved between terrestrial reservoirs by plate tectonics. The efficiency of volatile transfer controls the chemical state of Earth?s interior and exterior, including the atmospheric composition. In turn, the distribution of carbon and other volatiles in Earth?s surface reservoirs has enabled conditions favorable for life on Earth. Despite the importance of carbon, its fluxes, sources, and sinks remain under-constrained in subduction-related fluids, with an almost complete absence of studies linking it to underground biological processes. The deep carbon mass balance has previously been estimated by comparing subducting slab inputs to arc volcano degassing outputs, with the difference representing the amount of carbon that is transported into Earth?s deep mantle. However, slab and mantle-derived carbon can also be sequestered in the form of hydrothermal minerals (e.g., calcite, aragonite) and by microbial uptake in the crust of the overriding plate, effectively masking an unknown portion of the carbon output from the subduction zone. The lack of constraints on these key processes, however, limits the understanding of the overall efficiency of the deep carbon cycle. This project involves substantive collaboration with colleagues in Chile, including an international workshop, and the work of several U.S. students will be supported.

This project will characterize the extent of mineralogical and biological carbon sequestration along the geologically well-studied Andean Convergent Margin (ACM). The PIs hypothesize that calcite precipitation sequesters significant amounts of carbon in the ACM, particularly where the crust is thickest in the Central Volcanic Zone (CVZ). Furthermore, this project will test if subsurface microbial communities sequester carbon into biomass through chemosynthesis, forming an additional sink for carbon. Finally, the PIs will systematically assess how geochemical transformations and microbial communities vary as a function of subduction parameters (i.e., carbon input from the slab, upper plate thickness and lithology, as well as slab dip angle), which can then be used to compare results to a range of global convergent margins. To accomplish these goals, the PIs will conduct a field expedition to the CVZ in 2022 to collect fluid and gas samples from ~15 natural springs, seeps and fumaroles in the forearc and arc. They will measure helium and carbon isotopes, microbial chemosynthesis rates, and contributions of chemosynthesis to total microbial biomass in all samples. CVZ results will be interpreted alongside published and unpublished datasets from the ACM and other convergent margins globally (i.e., Central America). If subsurface geochemical and biological carbon sinks are found to be substantial in the ACM, it will add to growing evidence that carbon sequestration is widespread in the overlying crust of convergent margins. Such a finding could fundamentally alter the canonical understanding of deep carbon cycling between Earth?s surface and mantle. This project involves substantive collaboration with colleagues in Chile, including an international workshop, and the work of several U.S. students will be supported.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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