The reactive transport of Li as a monitor of weathering processes in kinetically limited weathering regimes
The reactive transport of Li as a monitor of weathering processes in kinetically limited weathering regimes
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DOI：
10.1016/j.epsl.2019.01.034
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发表时间：
2019-04-01
影响因子：
5.3
通讯作者：
Bickle, Mike J.
中科院分区：
文献类型：
--
作者：
Bohlin, Madeleine S.;Bickle, Mike J.
描述Li浓度和同位素比演变的反应输运方程的解析解，在一维流动路径中，反应化学计量沿着流动路径是恒定的。这些解决方案被认为适用于快速侵蚀的集水区的化学风化。解可以用二维数来描述；1)描述反应速率与流体停留时间乘积的Damkohler数；2)描述二次矿物中再沉淀Li的比例为流体二次矿物分配系数与二次矿物沉淀质量分数乘积的净分配系数。在进入流道的水是稀释的情况下，Li浓度将沿着流道增加，直到它们达到由净分配系数决定的极限值。同时，锂-7/锂-6同位素比值将增大至烃源岩比值减去次生矿物流体锂同位素分馏因子的极限值。锂同位素比超过这个极限值的水一定是随着反应化学计量学和/或分配系数的变化而演变的，这样在某一点上，锂对次生矿物的净去除超过了原生矿物溶解提供的去除。模拟结果表明，化学风化速率的多重控制因素（温度、降雨、侵蚀速率、水文）不能单独从Li浓度和同位素比值数据中推断出来，它们只提供了两个独立的约束条件。在根据潜在的气候变量解释海洋Li记录时应谨慎。该模型通过对恒河源头阿拉克南达河流域河水和河床砂的一组锂浓度和同位素比值测量来说明。这说明了如何用Damkohler数和净分配系数的值来追踪风化过程。来自具有相似岩性和气候的集水区的水样沿着近似恒定的净分配系数的等高线分散，反映了相似的反应化学计量，但具有更多可变的Damkohler数，反映了流动路径长度、流体通量和/或反应速率的变化。来自较低、较温暖、侵蚀速度较慢的流域的样品具有较高的Li-7/Li-6同位素比率和较低的Li浓度，必须至少反映两个阶段的风化过程，在这个过程中，反应化学计量和/或Li流体-矿物分配系数沿着流动路径变化，因此净Li在后期被去除。(C) 2019 Elsevier B.V.版权所有
Analytical solutions to reactive-transport equations describing the evolution of Li concentrations and isotopic ratios are presented for one-dimensional flow paths where reaction stoichiometry is constant along the flow path. These solutions are considered appropriate for chemical weathering in rapidly eroding catchments. The solutions may be described by two dimensionless numbers; 1) a Damkohler number describing the product of reaction rate and fluid residence time, and 2) a net partition coefficient which describes the fraction of Li re-precipitated in secondary minerals as the product of a fluid secondary mineral partition coefficient and the mass fraction of secondary mineral precipitates. In settings where water entering flow paths is dilute, Li concentrations will increase along the flow path until they reach a limiting value determined by the net partition coefficient. Simultaneously, Li-7/Li-6 isotopic ratios will increase to a limiting value of the source rock ratio minus the secondary mineral fluid Li-isotopic fractionation factor. Waters with Li-isotopic ratios in excess of this limiting value must have evolved with a change of reaction stoichiometry and/or partition coefficient along the flow path such that at some point net removal of Li to secondary minerals exceeds that supplied by dissolution of primary minerals. The modelling shows that the multiple controls on chemical weathering rates (temperature, rainfall, erosion rate, hydrology) cannot be inferred from Li concentration and isotopic ratio data alone, which only provide two independent constraints. Caution should be exercised in interpretation of oceanic Li records in terms of potential climatic variables. The model is illustrated by a set of Li concentration and isotopic ratio measurements on river waters and bed sands in the Alaknanda river basin which forms the headwaters of the Ganges. This illustrates how values of the Damkohler number and net partition coefficient can be used to trace weathering processes. Water samples from catchments with similar lithologies and climates scatter along contours of approximately constant net partition coefficient, reflecting similar reaction stoichiometries, but with more variable Damkohler numbers reflecting variations in flow path length, fluid flux and/or reaction rate. Samples from the lower, warmer and less rapidly eroding catchments have high Li-7/Li-6 isotopic ratios with lower Li concentrations and must reflect at least a two-stage weathering process where reaction stoichiometry and/or Li fluid-mineral partition coefficients change along the flow path so that net Li is removed in the later stages. (C) 2019 Elsevier B.V. All rights reserved.