New perspectives on deep carbon cycling

The proto-atmosphere serves as a crucial starting point for the carbon cycle. Estimations based on atmospheric data from Mars and Venus suggest that Earth’s proto-atmosphere contained >110 bar of CO₂ and >2.6 bar of nitrogen. The proto-atmosphere had over 1000 bar of water vapor during the magma ocean stage, assuming the proto-ocean had a volume of two oceans of water. During this stage both water and carbon dioxide were in a supercritical state at the magma-atmosphere interface. Intense serpentinization reactions occurred due to rock-water interaction, producing abundant hydrogen. Consequently, nitrogen is reduced to ammonia, and carbon dioxide to methane, forming carbonate simultaneously. The proto-atmosphere dominated by methane, ammonia, and hydrogen formed a significant amount of amino acids through lightning. This process is essential not only to the origin of life, but also to the early carbon-nitrogen cycle on Earth. By the Hadean eon, a large amount of CO₂ was sequestered as carbonate and organic material. Subsequently, it mainly entered the deep mantle through mantle overturn or subduction. In the mantle transition zone, carbonate undergoes “Redox freezing”, where carbonate is reduced to diamond through oxidation of ferrous iron in the melt. In the lower mantle, Fe²⁺ undergoes disproportionation reactions, forming Fe³⁺ and metallic iron. Among these, Fe³⁺ mainly resides in bridgmanite, thereby increasing the oxygen fugacity of the lower mantle, while metallic iron falls to the Earth’s core. The distribution of carbon in the mantle is crucial for deep carbon cycling. The density curves of diamond and mantle peridotite melt intersect at the bottom of the mantle transition zone (about 660 km). This density crossover leads to the accumulation of diamond during the magma ocean stage. When materials such as subducting slabs enter the lower mantle, compensatory upwelling of lower mantle material occurs. Bridgmanite enters the upper mantle, decomposes, releasing Fe³⁺ ions and oxidizes diamond to carbonate, which under thermal disturbance from kimberlite and igneous carbonatites, moves upward. This carbonate layer may have caused significant topographic fluctuations at the 660 km boundary. Currently, diamond in this layer may still not have been completely oxidized to carbonate or carbon dioxide, serving as a redox buffering layer. This is a key factor in constraining deep carbon cycling. Subduction zones are important pathways for facilitating the cycling. Processes in the Earth’s deep carbon cycle significantly influence the carbon content of surface reservoirs. The fluctuations in atmospheric CO₂ content since the Neogene are closely linked to the uplift of the Tibetan Plateau and the subduction of the western Pacific Plate. Around 60 million years ago, the closure of the Neo-Tethys Ocean led to subduction of the Indian passive margin. The massive sediments on the Indian margin carried down large amounts of carbonate and organic material into the mantle, and the resulting volcanism released large amounts of greenhouse gases such as CO₂ and methane into the atmosphere. The subduction of the Neo-Tethys Ocean passive margin weakened at about 51 Ma, and subduction of the western Pacific began. The depth of the western Pacific Ocean generally exceeds the carbonate compensation depth, and the amount of carbonate carried by subducting oceanic crust is minimal. Consequently, the input of subducted carbonate decreased significantly, leading to a substantial reduction in CO₂ emissions from volcanoes. Based on volcanic data from the past 12,000 years, the average rate of volcanic eruptions in subduction zones is estimated to be about 3 cubic kilometers per year. The weathering rate of volcanic ash is much higher than that of continental crust materials such as granite. The calcium, magnesium, and other ions provided by weathering of global volcanic ash are equivalent to the flux of global rivers into the oceans. The increase in volcanic ash and the decrease in CO₂ emissions from subduction zones have led to a decrease in atmospheric CO₂ levels, which is a key factor in the sustained global cooling since 51 million years ago.