Helium, inorganic and organic carbon isotopes of fluids and gases across the Costa Rica convergent margin

Carbon and other volatiles are transported from Earth’s surface into the mantle at subduction margins. The efficiency of this transfer has important implications for the nature and scale of geochemical heterogeneities in Earth’s deep (mantle) and shallow (crustal) reservoirs. However, the proportion of volatiles released outside of the arc (i.e., in the forearc and backarc) are not well-constrained compared to fluxes from the volcanic-front. In a recent study1, we used helium and carbon isotope data from deeply-sourced springs along two cross-arc transects to show that carbon is likely sequestered within the crust by calcite deposition and incorporated into biomass through microbial chemolithoautotrophy. As carbon moves between Earth’s surface (atmosphere and ocean), crust and mantle, it is involved in a number of geological, geochemical, and biological cycles, each of which operates on dramatically different temporal and spatial scales. Helium, on the other hand, acts as a tracer of deep subsurface inputs, since it is not subject to biological or chemical transformations. One of the most important physical processes linking the deep and shallow carbon cycles is subduction, which transports surface-derived carbon into the mantle. During subduction, volatile-rich fluids are released from the downgoing slab (oceanic crust and upper mantle). These fluids are thought to migrate through the overlying mantle-wedge and crust, and are ultimately released across the forearc, volcanic arc-front and backarc. The main goal of the “Biology Meets Subduction” project1 was to understand how biogeochemical processes shape the isotope signatures observed in surface manifestations in a subduction setting – namely Costa Rica. Until now, few studies have combined geological and biological observations to determine the dominant processes across a regional scale, despite the fact that much of the forearc subsurface is at low temperatures (<100 °C) that are conducive to microbial life.