Connecting the deep Earth, lithosphere and the atmosphere in space and time

It has long been evident that surface tectonics, deep Earth dynamics and long-term climate and sea level change are inextricably linked, but the fundamental framework of forces and the complexity of feedbacks are still poorly known. That is largely due to our inability to build quantitative, integrative Earth models far back in time. Conceptually, the link between plate tectonics and the deep Earth’s mantle can be viewed as a simple mass-balance: subducted lithosphere slabs restore mass to the mantle and trigger the return flow toward the surface ─ including mantle plumes ─ rising from the margins of two large low shear-wave velocity regions in the deep mantle, dubbed TUZO and JASON. These thermochemical provinces have likely been quasi-stable for hundreds of millions, perhaps billions of years, and plume heads rise through the mantle in about 30 Myrs or less. LIPs provide a direct link between the deep Earth and the atmosphere but environmental consequences depend on both their volumes and the composition of the crustal rocks they are emplaced through. LIP activity can also alter the plate tectonic setting by creating and modifying plate boundaries and hence changing the palaeogeography and its long-term forcing on climate.

The long-term climate is largely controlled by plate tectonic forcing, both via sources and sinks ─ e.g. silicate weathering ─ which can be enhanced by extensive blankets of LIP-lava on short (<1 Myr) and potentially much longer time-scales. Subduction fluxes derived from full-plate models provide a powerful means of estimating plate tectonic CO2 degassing (sourcing) through time. These correlate well with zircon age frequency distributions and zircon age peaks clearly correspond to intervals of high subduction flux associated with greenhouse conditions. Lows in zircon age frequency, however, are more variable with links to both icehouse and greenhouse conditions.

Degassing estimates based on subduction flux rates are apparently too low to match proxy CO2 levels for parts of the Mesozoic and notably the Early Cenozoic. Implementing intra-oceanic subduction histories, adding fluxes from continental rifts and/or explicitly considering the intersection of continental magmatic arcs with carbonate-filled basins through time would almost certainly increase plate tectonic degassing and further minimize existing model-proxy CO2 mismatches. But there are many other parameters to consider ─ including the silicate weathering parametrization ─ which can be improved by better global paleogeographies to estimate theoretical weatherability, and confirmed by biome maps.