Volcanic CO2 emissions from subduction of the tropical Paleo-Tethyan Ocean contributed to the early Permian deglacial warming

Abstract

Subduction of oceanic crust is a critical tectonic process that controls the recycling of carbon from the lithosphere to the atmosphere through volcanism and thus influences the evolution of Earth's climate. However, the potential contribution of volcanic CO₂ from arc systems to the long-term warming in the Paleozoic has not been quantified. The early Permian subduction zones of the eastern Paleo-Tethyan orogens formed in tropical latitudes. To evaluate the causal link between arc volcanic CO₂ emissions and warming in the early Permian, we study the early Permian volcano-sedimentary successions in the Ailaoshan Belt of southwest China, which constitutes part of the eastern Paleo-Tethyan orogens. Volcanic rocks separating the lower and upper sedimentary units yield a zircon U-Pb age of 284 ± 2 Ma, and along with the youngest detrital zircon age group, determine an early Permian age (ca. 290−275 Ma) for the sampled successions. For the sedimentary rocks, abundant volcanic lithic fragments are observed and a predominant detrital zircon age group of 300−270 Ma is defined. Zircons of this age group can be classified into two geochemical types in terms of variations in trace element ratios and Hf isotopes. Combining with sandstone modal compositions, two types of volcanic source rocks that represent arc and back-arc volcanism are distinguished. These provenance data support a westward subduction model for the early Permian Paleo-Tethyan Ocean along the Ailaoshan Belt where subduction may have initiated in the late Carboniferous (ca. 320 Ma) and strengthened in the early Permian (ca. 295 Ma). Based on sedimentological, magmatic and structural correlations, the early Permian subduction zone in the Ailaoshan Belt can be extended to the north along the Jinshajiang Belt and to the south along the Song Chay and Song May belts, forming a ∼1700 km continental arc system, allowing the entire length of the tropical Paleo-Tethyan Ocean continental arc system to reach ∼6350 km. Peak volcanism from this arc system is concomitant with the early warming event at 290−280 Ma. Considering the prevailing carbonate deposition in the adjacent blocks, it is plausible to infer carbonate-rich sediments in these early Permian subduction zones because modern observations reveal high carbonate contents in the subducted sediments, which then generate high volcanic CO₂ flux. Assuming the length-averaged volcanic CO₂ emission rate (0.0018 – 0.0059 Mt yr⁻¹ km⁻¹) based on modern tropical continental arc systems with carbonate-rich sediments on the down going plate, we find that the early Permian low latitude arc systems (∼6350 km) related to the subduction of eastern Paleo-Tethyan Ocean would have released ∼31,000 to 102,000 Gt C during the 10 Myr peak in volcanic activity. This volcanic CO₂ degassing estimate is much higher than that (∼5300−17,800 Gt C) from the three Circum-Tethyan large igneous provinces, which were also active from ca. 290 to 280 Ma and may have enhanced the Early Permian volcanic warming effect. This result is integrated with a long-term carbon cycle model (LOSCAR) simulation and implies a potential CO₂ contribution from the tropical Paleo-Tethyan continental arc system to the early Permian atmospheric pCO₂ increase and warming.