Evidence from the joint interpretation of proxy data as well as geodynamical and biogeochemical modeling results point to complex interactions between sea level drawdown, volcanic degassing, and atmospheric CO2 that hampered the climate system’s decent into the last ice age. Ice core data shows that atmospheric CO2 dropped abruptly into glacial Marine Isotope Stage (MIS) 4 at ~71 ka, while Antarctic temperatures display a more gradual decline between ~85 ka to ~71 ka across the MIS 5/4 transition. Based on 2D and 3D geodynamical simulations, we show that a ~60-100 m sea level drop associated with the MIS 5/4 transition led to a significant increase in magma and possibly CO2 flux at mid-ocean ridges (MOR) and oceanic hotspot volcanoes. The MOR signal is assessed with 2D thermomechanical models that account for mantle melting and resolve the flux of incompatible carbon dioxide. These models have been run at different spreading rates and integrated with the global distribution of opening rates to compute global variations in magma and CO2 flux across the MIS 5/4 transition. 3D plume models have been used to quantify the impact of a dropping sea level on oceanic hotspot melting and CO2 release. Here a wide range of simulations with differing plume fluxes, lithospheric thicknesses as well as speeds, and plume excess temperatures have been integrated with data from ~40 hotspots in order to compute a global signal. Biogeochemical carbon cycle modeling shows that the predicted increase in volcanic emissions is likely to have raised atmospheric CO2 by up to 15 ppmv, sufficient to explain the bulk of the decoupling between temperature and atmospheric CO2 during the global change to pronounced glacial conditions across the MIS 5/4 transition.
EPIC Alfred Wegener Institut