风化限制理论的铀同位素验证

The theory of weathering limitation suggests that weathering flux is limited by the supply of fresh rock and thus is correlated to erosion rate under condition of low erosion rate due to the long regolith residence time and thus a congruent weathering depletion. Instead, weathering is limited by kinetic factors such as temperature and precipitation amount due to sufficient exposure of fresh rock and thus a low weathering intensity under condition of high erosion rate. Supply limitation is the foundation for the uplift driven changes of silicate weathering drawdown of atmospheric CO2 and thus climate. The kinetic limitation generates a negative feedback between atmospheric CO2 concentration and weathering flux, which acts as a key mechanism that maintains the Earth’s habitability and balance of carbon cycle on tectonic time scale. However, proof for the theory of weathering limitation is illusive. Literature investigations show that the relationship between weathering intensity and weathering time is very different between supply limited and kinetic limited weathering regimes. The uranium isotope of dissolved river water is a good proxy for the weathering intensity that can avoid the complicate influencing factors such as lithology. Meanwhile, the uranium comminution age of riverine sediment can reflect the catchment scale regolith age. This work, based on Chinese catchments with known erosion rate in Taiwan Island and eastern margin of Tibetan Plateau, tests the theory of weathering limitation regimes using the 234U/238U ratios of river water and sediment as proxies for weathering intensity and weathering age, respectively.

风化限制理论认为：当剥蚀速度较低时，岩石在风化带的存留时间长，表现为一致性完全风化，化学风化通量受新鲜岩石的供应限制，与剥蚀速度成正比；当剥蚀速度较高时，岩石充分暴露，风化强度低，化学风化通量受温度、降水量等动力学因子限制。供应限制是构造抬升影响硅酸盐风化吸收大气CO2进而改变气候的关键。动力学限制形成了化学风化通量与大气CO2含量之间的负反馈，是维持构造时间尺度地球宜居性和碳循环平衡的关键。但是，风化限制理论模型并未得到充分实证。调研发现，动力学限制和供应限制所对应的风化强度与风化年龄的关系不同。河水的铀同位素组成可以排除岩性等复杂因素的影响，是风化强度的良好指标；同时河流沉积物的铀同位素破碎年代可反映流域尺度风化带的年龄。本研究选取我国台湾和青藏高原东缘具有大量物理剥蚀数据的小流域，利用河水和沉积物的234U/238U比值分别反映风化强度和风化年龄，验证风化限制理论。

风化限制理论认为，化学风化在低剥蚀地区受剥蚀速率控制，为供应限制；在高剥蚀速率地区受温度、降水量等因素控制，为动力学限制。供应限制是构造抬升影响硅酸盐风化吸收大气CO2进而改变气候的关键。动力学限制形成了化学风化通量与大气CO2含量之间的负反馈，是维持构造时间尺度地球宜居性和碳循环平衡的关键。但是，风化限制理论模型并未得到充分实证。本项目选取我国台湾和青藏高原东缘具有大量物理剥蚀数据的小流域，通过分析河水的铀同位素值和总结全球小流域河水的铀同位素的数据，探索剥蚀速率对铀同位素和化学风化的控制，发现铀同位素与剥蚀速率的U形关系，同时结合铀同位素计算模型，证实了经典的风化限制理论。在本项目的资助下，研究团队进一步探索了在新生代全球变冷的背景下，高原隆升持续驱动大陆风化增强的原因，揭示了滑坡过程对大陆风化和全球气候的显著影响。项目团队成员还开展了铀同位素机理和大陆风化过程中Li同位素分馏机理的研究。在本项目的支持下，项目组成员以第一/通讯作者发表9篇论文，其中SCI论文8篇，包括Geology、EPSL、GCA、GRL等NI论文6篇。本项目的研究结果将有利于风化限制理论的广泛运用，并为经典的风化限制理论与构造抬升驱动气候变化理论之间的矛盾提供可能的解释。