CO2 emissions due to melt-carbonate interactions triggered the Miocene Climatic Optimum

The Miocene Climatic Optimum (ca. 17−14 Ma) is an enigmatic climatic reversal that interrupted the long-term cooling trending of the Cenozoic. The forcing for this warming event remains unclear. Here we present direct evidence of melt−carbonate interactions in the Himalaya that produced globally significant CO2 emissions, and quantify resulting contributions to the atmosphere. The interactions occurred at high temperatures (685−828°C) in the lower crust (1.2−1.4 GPa) during the middle Miocene. Decoupled Sr−O isotope ratios suggest that calc-silicate zones were produced by infiltration of eclogite-derived granitic melts into carbonates during exhumation. Carbon budget estimates indicate that ~1270 kg of CO2 was liberated during the formation of each cubic meter of calc-silicates, and <4% of the carbon remained at the reaction site. Area extents of granite plutons vary from <10−1705 km2 in the Himalaya and each pluton contributed to carbon outfluxes of up to 1361× 103 kg/yr. As such, deep Earth degassing due to melt−carbonate interactions in the Himalayan orogen released 0.14−0.18 Pg/yr CO2 during the middle Miocene. With such degassing rates sustained over 0.1 Myr, corresponding atmospheric CO2 levels reached 400–600 ppmv within 0.1–0.3 Myr, ultimately enough to trigger the Miocene climate warming.