Enhanced magmatism played a dominant role in triggering the Miocene Climatic Optimum

The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum.