显生宙长时间尺度碳循环演变的模拟：现状与展望 (Simulating the long-term carbon cycle in the Phanerozoic Current status and future developments)

长时间尺度碳循环演变控制了大气CO2的含量。显生宙以来大气CO2含量的变化及其对地表气温的控制，是古气候地球化学研究的前沿领域。地球系统箱式模型被广泛用于揭示长时间尺度碳循环和古气候变化的过程与机制。以COPSE和GEOCARB模型为代表的早期长时间尺度碳循环模型，在应用于显生宙大气CO2含量变化研究上成效显著，但因无法表达地球三维地表的影响，制约了其进一步发展。新发展的SCION模型基于COPSE模型，结合了GEOCLIM模型中运用的FOAM气候模型数据集，实现了大陆风化的动态表达，进而更准确地表征了长时间尺度的碳循环演变。然而，最新版SCION模型模拟的大气CO2含量变化，仍与大气CO2的地质指标记录存在不一致之处。采用多箱式海洋替代单一箱式海洋，区分硅酸盐岩性对风化的影响，完善营养物质循环，优化构造古地理和陆地植物演化的表达等，有望提高对长时间尺度碳循环源汇体系的限定和显生宙大气CO2模拟的准确性。

Over geological timescales, Earth’s atmospheric CO2 concentration is determined by the long-term carbon cycle. Here the principal CO2 sources are tectonic degassing and the weathering of carbonate or organic-rich rocks, and the sinks are the deposition of carbonate minerals and organic carbon in sediments. The global carbon cycle has a self-regulation mechanism because the weathering of silicate rocks, which provides key elements for marine carbonate formation, is temperature dependent and can remove more carbon when Earth’s CO2 levels and temperature rise. Nevertheless, changes in carbon inputs and outputs can drive substantial variation in CO2 levels over geological time, and this has been the most important factor controlling the long-term variations in global average surface temperature over the Phanerozoic. Therefore, reconstructing the long-term carbon cycle and Phanerozoic changes in atmospheric CO2 levels is an important area of geochemical research.

Earth system box models have traditionally been used to study the long-term carbon cycle and to understand what controls changes in CO2 concentration. Early box models, such as the GEOCARB and COPSE (Carbon-Oxygen-Phosphorus-Sulphur-Evolution) models, mathematically expressed the physical and chemical properties of the Earth’s surface in a dimensionless way, using a single box for the ocean or land surface and a single value for global temperature or rate of precipitation. These models succeeded in reconstructing some aspects of atmospheric CO2 variations during the Phanerozoic but were limited because they cannot represent Earth’s surface in 3D, so could not properly represent continental weathering processes. The GEOCLIM model improved on this method by using a large dataset of physical climate model simulations to approximate a 3D climate and land surface for a set of specific times in Earth’s history, which allowed it to evaluate weathering processes and simulate uplift-driven glaciation in the late Palaeozoic.

A further step was made in the SCION (Spatial Continuous Integration) model to approximate between different continental maps and therefore run the 3D weathering functions continuously over geological time by a method of interpolation. The variations of Phanerozoic atmospheric CO2 levels predicted by the SCION model show the following major features: (1) High atmospheric CO2 content (>1000 ppm) from the Cambrian to Silurian period; (2) a significant decrease of atmospheric CO2 concentration in the carboniferous and a following recovery in the Permian; (3) low atmospheric CO2 content in Jurassic; (4) a peak in CO2 concentration during the Cretaceous and a decline through the Cenozoic. However, the modelling results still show some inconsistencies with the proxy records of atmospheric CO2, such as a higher atmospheric CO2 prediction during the Palaeozoic and a lower atmospheric CO2 prediction during the Jurassic. Future work is needed to more accurately simulate the Earth’s carbon cycle in these models. This includes revising the plate tectonic and paleogeographic boundary conditions to modern standards, replacing the one-box ocean with a multi-box or 3D ocean component, distinguishing the more reactive silicate lithologies on the continental maps to better represent their influences on silicate weathering, and improving the expression of land plant evolution and biogeography.