Did high Neo-Tethys subduction rates contribute to early Cenozoic warming?

Abstract

The 58–51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9–50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric <i>p</i>CO₂, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO₂ at arc volcanoes. To analyze the impact of Neo-Tethys closure on early Cenozoic warming, we have modeled the volume of subducted sediments and the amount of CO₂ emitted along the northern Tethys margin. The impact of calculated CO₂ fluxes on global temperature during the early Cenozoic have then been tested using a climate carbon cycle model (GEOCLIM). We show that CO₂ production may have reached up to 1.55 × 10¹⁸ mol Ma⁻¹ specifically during the EECO, ~ 4 to 37 % higher that the modern global volcanic CO₂ output, owing to a dramatic India-Asia plate convergence increase. The subduction of thick Greater Indian continental margin carbonate sediments at ~ 55–50 Ma may also have led to additional CO₂ production of 3.35 × 10¹⁸ mol Ma⁻¹ during the EECO, making a total of 85 % of the global volcanic CO₂ outgassed. However, climate modeling demonstrates that timing of maximum CO₂ release only partially fits with the EECO, and that corresponding maximum <i>p</i>CO₂ values (750 ppm) and surface warming (+2 °C) do not reach values inferred from geochemical proxies, a result consistent with conclusions arising from modeling based on other published CO₂ fluxes. These results demonstrate that CO₂ derived from decarbonation of Neo-Tethyan lithosphere may have possibly contributed to, but certainly cannot account alone for early Cenozoic warming. Other commonly cited sources of excess CO₂ such as enhanced igneous province volcanism also appear to be up to 1 order of magnitude below fluxes required by the model to fit with proxy data of <i>p</i>CO₂ and temperature at that time. An alternate explanation may be that CO₂ consumption, a key parameter of the long-term atmospheric <i>p</i>CO₂ balance, may have been lower than suggested by modeling. These results call for a better calibration of early Cenozoic weathering rates.