TYPHOON-DRIVEN WEATHERING AND EROSION FLUXES IN TAIWAN
N. HOVIUS1*, A. GALY1, D. CALMELS1, P. MEUNIER1, R.H. HILTON2, H. CHEN3, M.-C. CHEN4,
S.J. DADSON5, AND M.-J. HORNG6
1
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
(*correspondence: nhovius@esc.cam.ac.uk)
2
Laboratoire de Géochimie et Cosmochimie, IPGP, 4 Place Jusieu, 75252 Paris, France.
3
Department of Geosciences, National Taiwan University, Taipei 106, Taiwan ROC.
4
Taroko National Park, Fu-Su Village, Hualien 972, Taiwan ROC.
5
Centre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, OX10 8BB,
UK.
6
Water Resources Agency, Ministry of Economic Affairs, Taipei, Taiwan ROC.
Taiwan is a typical example of an active mountain belt in which rapid crustal shortening and rock
uplift are countered by fluvial incision and landsliding. Such mountain belts are
disproportionately important in the erosion of sediment from the continents. Erosional mass
fluxes have a long established geophysical and stratigraphic significance, and increasingly their
role in Earth’s carbon cycle is recognized. The erosion of Taiwan is driven by earthquakes and
typhoons. The biggest trigger events cause instantaneous erosion and seed a weakness in the
landscape that is removed over time in predictable fashion. This gives rise to patterns of erosion
that can not be understood in terms of bulk characteristics of climate, such as average annual
precipitation. We illustrate this with an example from the epicentral area of the Chi-Chi
earthquake, where rates of hillslope mass wasting and fluvial sediment transport have increased
sharply due to the earthquake, and then decreased to reach background values after about six
years. In this time, the distribution of landslides in the ridge-and-valley topography has changed
systematically, reflecting the topographic site effects on incoming seismic waves and the
subsequent typhoon-driven cascading of sediment down hillslopes, which are superposed on the
long-lived topographic and geological controls on slope failure.
Rapid erosion of the mountain catchments of Taiwan does not only maintain a steep landscape
and fast advection of rock mass through the orogen, but it also encourages the draw down of CO2
by refreshing of the weathering zone, and the harvesting of particulate organic carbon (POC)
from soil and standing biomass. Combined, these processes are responsible for the stability of
Earth’s atmospheric composition on geological time scales. In Taiwan as in other active mountain
belts, the POC route appears to dominate, and typhoon precipitation plays a special role in this
mechanism. It combines the triggering of landslides with entrainment of organic litter in overland
flow. This gives rise to very high suspended sediment concentrations in rivers at a time when
modern POC is flushed from hillslopes. Hyperpycnal discharge of this material promotes rapid
and long-term burial of POC in turbidites. Moreover, the removal of organic matter effectively
limits the availability of nutrients, and thus the accumulation of biomass in mountain forest, in an
intricate pattern of feedbacks that connect tropical cyclone weather with landsliding and natural
vegetation.