Contribution of Deep Groundwater to Weathering Budget in a Rapidly Eroding Mountain Belt, Taiwan

Résumé : Quantitative understanding of the controls on weathering rates is critical to deciphering the importance of weathering-driven climate feedbacks, but such understanding has remained elusive because of the tight coupling between major processes at the Earth’s surface. Two decades of research have identified a series of key parameters, and a major next step is to be able to model weathering rates on the basis of predictive principles. A significant hurdle to developing such models is the limited knowledge about where weathering reactions take place. Models are built on basic assumptions that weathering is dominated by processes in the relatively shallow zone of soil and saprolite formation, but catchment systems are hydrologically complex and often have significant flowpaths well below the saprolitic zone. Understanding the relative importance of these flowpaths for weathering is thus crucial as it suggests that a portion of the weathering reactions may be controlled by geothermal conditions rather than surface conditions. We analyse a 6-yr time series of water chemistry of the Liwu River, Taiwan, in order to infer the relative importance of deep and shallow weathering in a steep, well-drained and rapidly eroding basin system. Temporal variation in river chemistry is used to develop a 3-components mixing model describing hydrologic and geochemical contributions to the river flow, such that the data can be used to quantify weathering processes along water flowpaths as well as groundwater input and its associated deep chemical weathering flux to the river.