Biogeodynamics: bridging the gap between surface and deep Earth processes.

Life is sustained by a critical and not insubstantial set of elements, nearly all of which are contained within large rock reservoirs and cycled between Earth's surface and the mantle via subduction zone plate tectonics. Over geologic time scales, plate tectonics plays a critical role in recycling subducted bioactive elements lost to the mantle back to the ocean-biosphere system, via outgassing and volcanism. Biology additionally relies on tectonic processes to supply rock-bound 'nutrients' to marine and terrestrial ecosystems via uplift and erosion. Thus, the development of modern-style plate tectonics and the generation of stable continents were key events in the evolution of the biosphere on Earth, and similar tectonic processes could be crucial for the development of habitability on exoplanets. Despite this vital 'biogeodynamic' connection, directly testing hypotheses about feedbacks between the deep Earth and the biosphere remains challenging. Here, I discuss potential avenues to bridge the biosphere-geosphere gap, focusing specifically on the global cycling and bioavailability of major nutrients (nitrogen and phosphorus) over geologic time scales.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.