IMPLICATIONS OF EARTH'S SECULAR COOLING FOR CARBON CYCLING AND CLIMATE

Observations across a range of scales, sub-outcrop to regional and late Cenozoic to full earth history, suggest that seafloor uplift-exhumation events (SUE) transform subduction CO₂ sources into silicate weathering-organic C burial sinks. The “right rocks,” mantle and mantle-derived melts containing plagioclase, pyroxene, and especially olivine, when delivered to the surface subside upon cooling and escape significant exhumation until horizontally translated through erosion fronts above converging thick, buoyant lithosphere. Ferruginous and/or dolomitic carbonates, silica, and black shales in distal foreland basin deposits, in particular, may directly record the activity of CO₂ sinks in the upper plate. Passive margin end dates and marine ⁸⁷Sr/⁸⁶Sr, reflecting both antecedent non-radiogenic (upper plate) and subsequent radiogenic (lower plate) sources, may be supportive of this mechanism. Is SUE the link between Earth’s secular cooling, manifested by lithospheric thickening, oceanic crustal thinning, and seafloor compositional evolution (komatiites rare post-Archean), and Earth’s climate history? Archean climate, despite a dim sun and abundant komatiite, was warm, implying an ineffective carbon sink. Hypothetically, seafloor largely escaped erosion, sinking into weak granitic diapirs, until lower-plate lithosphere became sufficiently rigid (supported by change in structural style) in the late Archean. The organic carbon reservoir, limited in size by phosphorus, then grew as nutrients were released; the oxygen produced functioned as a built-in glacial “off switch” by iron oxide armoring of the mafic minerals. Following the carbon cycle instability and ice ages of the Archean-Proterozoic transition, the “boring billion” (1.8-0.8 Ga) was warm and coincident with anorthosite magmatism (cause of high pCO₂?). During the Neoproterozoic, thinning of oceanic crust led to peridotite-floored thrust sheets. Again, mantle fertilization enabled the biosphere to grow, and iron oxide armoring both turned off glaciations and enabled pO₂ to build further. While the SUE hypothesis has yet to be rigorously tested by quantifying C fluxes, the circumstantial evidence remains tantalizing.