河流铀同位素的控制机理及其对风化限制理论的启示意义

Controling Mechanism of Riverine Uranium Isotope and Its Implication for Weathering Limitation Theory

经典风化限制理论认为,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,即为"供应限制";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,即为"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CO_2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CO_2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。本文将介绍利用流域溶解态铀同位素证明风化限制理论的原理与研究进展。全球数据总结发现,流域铀同位素与物理剥蚀速率之间在整体上呈现U形函数关系,可用经典风化限制理论解释。但河流的铀同位素还受岩性、气候、地貌等其他因素的影响。利用已知风化年代的单一岩性流域是河流铀同位素验证风化限制理论的重要途径。

The classic theory of weathering limitation suggests that the chemical weathering flux is limited by the supply of fresh rocks and thus is proportionally correlated to the erosion rate as rocks were completely weathered due to the long regolith residence time at relatively low erosion rate.This is called the supply limitation.However,at relatively high erosion rate,chemical weathering flux is limited by kinetic factors such as temperature and amount of rainfall due to sufficient exposure of fresh rocks but low weathering intensity.This is called the kinetic limitation.The supply limitation is the key factor for the climate change resulted from the atmospheric CO2absorption in the process of silicate weathering which was influenced by the tectonic uplift.The kinetic limitation generates a negative feedback between the atmospheric CO2concentration and chemical weathering flux.It is the key to maintain the earth’s habitability and balance of carbon cycle on tectonic time scale.However,the theoretical model of weathering limitation has not been sufficiently substantiated.This paper will introduce the principle and research progress for testing the weathering limitation theory using the riverine dissolved uranium isotope,(234U/238U)Riv.A review of global data indicates that there is a generally U-shaped functional relationship between the(234U/238U)Rivand catchment scale denudation rate(D).This can be explained by the classic weathering limitation theory.However,the(234U/238U)Rivis also complicatedly controlled by other factors including lithology,climate,landscape and so on.It would be an important way to verify the weathering limitation theory by using the riverine uranium isotope of a single lithological drainage basin with known weathering age.