Abstract
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 pCO2, 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 pCO2 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 pCO2 and temperature at that time. An alternate explanation may be that CO2 consumption, a key parameter of the long-term atmospheric pCO2 balance, may have been lower than suggested by modeling. These results call for a better calibration of early Cenozoic weathering rates.
Highlights
Based on paleotemperature proxies, a trend of decreasing global temperatures throughout the Late Mesozoic and Cenozoic has long been identified (e.g, Shackelton and Kennett, 1975; Zachos et al, 2001, 2008; Cramer et al, 2009; Friedrich et al, 2012)
We aim to test whether Neo-Tethyan closure, which was obviously associated with widespread arc volcanism, may or may not have had an impact on global warming during the Late Paleocene-Early Eocene (LPEE) and the EECO, keeping in mind that this hypothesis hardly conforms to available carbon isotope records during the LPEE
Excess CO2 fluxes calculated at minimum (15 %) and maximum (60 %) efficiencies correspond to extreme scenarios that very likely encompass true excess CO2 fluxes related to Neo-Tethys closure
Summary
A trend of decreasing global temperatures throughout the Late Mesozoic and Cenozoic has long been identified (e.g, Shackelton and Kennett, 1975; Zachos et al, 2001, 2008; Cramer et al, 2009; Friedrich et al, 2012). Carbonates indicate that from ∼ 58.0 to 52.5 Ma this warming was characterized by a 2 per mil negative shift in marine and terrestrial δ13C, referred to as the Late Paleocene-Early Eocene (LPEE) by Komar et al (2013) This drop in δ13C suggests an additional source of depleted CO2 (i.e enriched in 12C) or/and decreased net organic carbon burial (Hilting et al, 2008; Komar et al 2013). Among several other hypotheses, it was suggested that Neo-Tethys closure may have strongly controlled Cretaceous and early Cenozoic climates, up to the EECO, through the subduction of tropical pelagic carbonates (δ13C ∼ 0 ‰) under the Asian plate and their recycling as CO2 at arc volcanoes (Edmond and Huh, 2003; Kent and Muttoni, 2008; Johnston et al, 2011) These authors argued that the tropical latitudes of the northern Neo-Tethys could have favored deposition of carbonaterich pelagic sediments on the Tethyan seafloor. In detail, Kent and Muttoni (2008) suggested that the Indian plate dominated this “carbonate subduction factory”, with a major decrease in CO2 production as India and Asia collided some
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