MODELING METAMORPHISM AND GLOBAL CLIMATE CHANGE

Modeling global paleotemperatures through the greenhouse effect requires quantifying present and past global CO₂ degassing. Arc volcanism provides a significant CO₂ source. CO₂ released from volcanic arcs largely originates from metamorphic decarbonation of subducted slabs. Most models of the global carbon cycle have assumed that the global flux of CO₂ from arc volcanoes is linearly related to subduction rate. In addition to other oversights, this assumption ignores the fact that the extent and depth range of subduction zone metamorphic decarbonation depends upon bulk-rock compositions and P-T paths (geotherms) along the tops of subducted slabs. Fast subduction yields cold slab surface geotherms whereas warm slab surfaces accompany slow subduction. Coupling this with our modeling of subduction zone metamorphic decarbonation (Kerrick and Connolly; Ernst Symposium, this conference), the flux of CO₂ to arc magmas in mantle wedges is inversely proportional to subduction rate! Thus, alternative proxies for paleo-CO₂ degassing are mandatory. The CO₂ flux from volcanoes can be linked to the volumetric rate of magma extrusion. Accordingly, the volumetric rate of volcanism through time provides a potential alternative proxy for paleo-CO₂ degassing.

Quantification of contemporary global CO₂ fluxes suggests that there is a marked deficiency (several terramoles/yr) in the rate of CO₂ emission from volcanoes compared to the rate of atmospheric CO₂ drawdown by silicate weathering. To balance the global carbon cycle, this implies that there are globally significant fluxes of “non-volcanic” CO₂ (i.e., not from the craters or flanks of volcanoes). Enhanced CO₂ emission occurs in high heat flow regions undergoing extensional tectonism. In some of these regimes (e.g., the Salton Trough in southern California, and the northern California Coast Ranges), geochemical signatures suggest that the CO₂ originated by metamorphic decarbonation. Faulting accompanying extensional tectonism provides efficient pathways for the focused expulsion of CO₂ to the Earth’s surface. Analogous geotectonic settings are implied for the discharge of large amounts of non-volcanic CO₂ in the past. Accordingly, expulsion of large quantities of metamorphic CO₂ from the British Columbia Cordilleran belt may have contributed to global warming during the Paleocene.