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.