The cooling of Earth's climate during the Cenozoic is linked to decreasing atmospheric CO₂ concentrations. Two main hypotheses attempt to explain this decrease: increased CO₂ consumption by silicate weathering due to Tibetan Plateau uplift, and decreased CO₂ release from Earth's interior due to the India-Asia collision's shutdown of the Neo-Tethyan decarbonation subduction factory. Both hypotheses posit the India-Asia collision as the ultimate cause of Cenozoic atmospheric CO₂ variations. Understanding the India-Asia collision processes is crucial for constraining the controls on Cenozoic atmospheric CO₂ changes. Cenozoic magmatism and metamorphism in the Tibetan Plateau are reliable recorders of the India-Asia collision evolution. Magmatic and metamorphic degassing, linked to plate tectonics, are significant components of the Earth's deep carbon cycle. A comprehensive understanding of these processes in Tibet can provide critical constraints on explaining atmospheric CO₂ variations. Recent studies suggest that the Indian-Asian convergence rate and the subducted Neo-Tethyan lithosphere's magnitude and recycling efficiency influence magmatic CO₂ emissions and global climate. Enhanced magmatic CO₂ degassing in the early Cenozoic, due to carbonate-rich subduction, is proposed as a cause of the Paleocene-Eocene Thermal Maximum. Studies have also modeled the climatic effects of Neo-Tethyan arc extinction, suggesting waning volcanic degassing led to long-term cooling. The importance of metamorphic CO₂ emissions in controlling atmospheric CO₂ levels and Cenozoic climate evolution, however, remains poorly constrained. This study aims to quantify the relationship between Tibet's tectonic evolution and global atmospheric CO₂ variations by developing a model to calculate Tibetan magmatic and metamorphic CO₂ outgassing fluxes over the last 65 million years. This model is constrained by new and existing geochemical and geochronological data for magmatic rocks in the Plateau. The study hypothesizes that magmatic-metamorphic CO₂ degassing from the Neo-Tethyan Ocean closure and the India-Asia collision drove global Cenozoic CO₂ variations and climate changes.

Previous research has explored the link between Cenozoic magmatism and atmospheric CO2 variations. Studies indicate that Cenozoic magmatism and metamorphism in the Tibetan Plateau record the India-Asia collision's evolution. Magmatic and metamorphic degassing are vital parts of Earth's deep carbon cycle. Convergence rates and the efficiency of recycled Neo-Tethyan lithosphere are proposed as controls on magmatic CO2 emissions and climate change. Early Cenozoic enhanced degassing due to carbonate-rich subduction might explain the Paleocene-Eocene Thermal Maximum. Models suggest Neo-Tethyan arc extinction led to long-term cooling by reducing volcanic degassing. However, the role of metamorphic CO2 emissions remains unclear, with a lack of continuous quantitative calculations of magmatic and metamorphic outgassing fluxes across the Cenozoic.

To address the knowledge gap, the authors developed a Continental Collision-Derived CO₂ Flux Model (CCFM). This model estimates the total flux of CO₂ emissions from magmatism and metamorphism during the Neo-Tethyan Ocean's closure and the India-Asia collision throughout the Cenozoic. The CCFM uses a time series based on mean eruptive ages from volcanic fields in the Tibetan Plateau, derived from a compilation of published geochronological data. The model accounts for potential errors and inconsistencies in the data. The CCFM also incorporates geochemical data (major elements, trace elements, Sr-Nd-Pb isotopes) for magmatic rocks to determine the compositions of mantle source regions and estimate the proportions of end-members (depleted MORB mantle (DMM), global subducting sediments (GLOSS), Indian and Asian-derived silicate and carbonate components). Three-component mixing calculations of Sr-Nd-Pb isotope compositions and trace element modeling were used to estimate the proportions of these end-members and degrees of partial melting. The model calculates CO2 emission fluxes based on these geochemical parameters, along with estimates of slab thickness, width, convergence rates, and upwelling rates. The model considers both magmatic CO2 degassing from volcanic activity and passive degassing from hydrothermal systems, with distinct calculation methods for each. The model also considers metamorphic CO2 production through a carbonate dissolution reaction, particularly significant in the early stage of the collision. The model outputs a time series of collision-derived CO2 degassing rates and calculates uncertainties through standard deviation reflecting the range of values for different volcanic fields. The results were compared to the global atmospheric CO2 curve to assess the correlation.

The study divides Cenozoic magmatism in Tibet into three stages: Stage 1 (65-55 Ma) Andean-type igneous rocks from Neo-Tethyan subduction; Stage 2 (55-25 Ma) large-scale lava flows and pyroclastic deposits from mantle transition zone (MTZ)-derived carbonated asthenospheric mantle plume (CMP) upwelling induced by Indian slab subduction; and Stage 3 (25 Ma-present) small-volume K-rich lava flows from dual polarity subduction. These stages correlate with three stages of atmospheric CO2 variations: Stage 1 (intermediate CO2); Stage 2 (elevated CO2); and Stage 3 (low CO2). The CCFM results show a strong correlation between the modeled CO2 emission rates and the global atmospheric CO2 curve. The model reveals a three-stage evolution of mantle source region composition (increasing carbonate content in Stage 2, decreasing in Stage 3) and CO2 emission fluxes. Stage 2 exhibits much higher fluxes (up to ~7 Pg/year) due to subduction of a carbonate-rich component, consistent with the high atmospheric CO2 levels. The model suggests that metamorphic CO2 outgassing (from a carbonate dissolution reaction) significantly contributed to the atmospheric CO2 increase in the early part of Stage 2, and magmatic outgassing dominated later. Stage 3 shows intermediate CO2 emissions, reflecting decreased carbonate input and lower melting degrees. A peak CO2 degassing rate of ~7 Pg/year occurred around 52 Ma. The current CO2 flux from dormant volcanic fields in Tibet (~15 Mt/year) agrees with the model's estimates. The model suggests that the India-Asia collision substantially contributed to Cenozoic CO2 outgassing, exceeding contributions from ocean lithosphere subduction. This study shows that the India-Asia collision is a critical factor driving atmospheric CO2 variations and climate change.

The strong correlation between magmatic activity, source region composition, modeled CO2 outgassing, and atmospheric CO2 concentrations supports a causal link between the India-Asia collision and global CO2 levels, driving long-term paleoclimate changes. The three stages of tectonic evolution, CO2 variations, and paleoclimate changes are interconnected. Stage 1's Neo-Tethyan subduction yielded low CO2 flux and intermediate atmospheric CO2 levels. Stage 2's extensive magmatism, with high carbonate input and high recycling efficiency, resulted in high CO2 outgassing, causing elevated atmospheric CO2. Stage 2 is further subdivided: Stage 2(a) (55-50 Ma) with a plume-wedge interaction significantly increasing CO2; Stage 2(b) (50-25 Ma) with decreasing CO2 due to waning magmatic activity. Stage 3's dual continental subduction led to intermediate CO2 flux and low atmospheric CO2 levels. The study differentiates between India-Asia collision-derived CO2 degassing (Stages 2-3) and Neo-Tethyan subduction-related CO2 emissions (Stage 1), based on rock types, geochemical compositions, and CO2 degassing mechanisms. While previous studies pointed to continental arc volcanism's role, this study emphasizes the India-Asia collision's dominant contribution to Cenozoic CO2 outgassing. The differences in subducted material composition (higher carbonate in Stage 2 from Indian subduction) and resulting CO2 fluxes support the collision's impact. The model's consistency with current CO2 flux from dormant volcanic fields strengthens its validity.

This study demonstrates that deep Earth degassing, primarily magmatic and metamorphic CO2 emissions from the India-Asia collision, significantly controlled Cenozoic atmospheric CO2 variations and paleoclimate changes. The India-Asia collision's CO2 flux far exceeded that from ocean lithosphere subduction. Future research should investigate the interplay between CO2 production and consumption rates across all three stages to fully reconcile fluctuations in atmospheric CO2 levels. Further refinement of paleoaltimetry data and its correlation with silicate weathering rates will improve our understanding of the Tibetan Plateau's role in the carbon cycle.

The model relies on existing geological data, and uncertainties associated with age dating and geochemical analyses propagate through the model calculations. The model does not explicitly account for all potential factors affecting atmospheric CO2 levels (e.g., biological processes, oceanic processes, other volcanic sources). The interpretation of the three stages of magmatism and their correlation with atmospheric CO2 changes are based on the current understanding of the geological record, which might be further refined by future research.