Carbon is an important element in the Earth’s interior. It is present in minerals, melts, and fluids under extreme conditions. Carbon trapped in diamonds and carbonate-bearing rocks in subduction zones are examples of the continuing exchange of substantial carbon between Earth’s surface and its interior.

We through  first-principles calculations simulation found that CaC₂O₅ and its high-pressure variants may  implications for Earth’s structure, carbon cycle and geodynamics. Here we share with you some of the discovery process related to this study.

The first thing that caught our attention was the CaC₂O₅-I-42d synthesized by König et al. in 2022 through high-pressure diamond pressure chamber (DAC) synthesis. We found that this is a new type of carbonate mineral that can exist in high-temperature and high-pressure environments. So we conducted more literature searches on this mineral and discovered the six CaC₂O₅-minerals structures required for the calculation in this article.

Based on the summary of previous research, we found that they did not specifically analyze the impact of CaC₂O₅-minerals on the mantle.  Analyzing the impact of minerals on the mantle through their elastic properties has always been a widely recognized method in the scientific community. Therefore, we calculated the lattice parameters, electronic properties, and elastic properties of these six minerals, and summarized the main points of this article based on these properties.

The most exciting thing for us is the significant density and wave velocity changes that occur when CaC₂O₅-Cc is converted to CaC₂O₅-I-42d near 660 km. There are still many different opinions on the origin of wave velocity at 660 km in the mantle transition zone, but our discovery undoubtedly provides new evidence for the origin of wave velocity anomalies in this region.

Moreover, the composition and genesis of large shear wave low-speed zones (LLSVPs) have always been a focus of analysis in the field of high-temperature and high-pressure research. We are also honored to contribute new knowledge and ideas to the research in this region.

Based on previous research and our calculations, we propose a deep carbon cycle model of the Earth based on CaC₂O₅-minerals. This model provides a new understanding of the ways in which carbon cycles occur within the Earth and the formation of internal materials. Without the research results of Yao et al., König et al., and Sagatova et al., we would not be able to establish a model. We express our deep gratitude to the aforementioned workers and our highest respect to the pioneers in the study of the Earth's interior.

Looking back at this research, I recall the importance of innovation and collaboration in scientific research. I am grateful for the support and guidance of my supervisors, colleagues and collaborators, as well as the editors and anonymous reviewers. 

In conclusion, this study, the structural, electronic, and elastic properties of CaC₂O₅s, including those of CaC₂O₅-Cc, CaC₂O₅-Fdd2, CaC₂O₅-C2-l, CaC₂O₅-C2, CaC₂O₅-I2d, and CaC₂O₅-Pc, were studied under high pressure by first-principles simulation.  Scientists from China University of Geoscience (Beijing)and  Institute of Earthquake Forecasting (China Earthquake Administration) provides different insights for understanding the origin of the wave velocity discontinuity at the depth of 660 km in the mantle transition zone and at the LLSVPs. It also sheds light on the deep carbon cycle model of the Earth.