Tropical-cyclone-driven erosion of the terrestrial biosphere from mountains

The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle. More than one third of this organic carbon flux comes from sediment-laden rivers that drain the mountains in the western Pacific region. This region is prone to tropical cyclones, but their role in sourcing and transferring vegetation and soil is not well constrained. Here we measure particulate organic carbon load and composition in the LiWu River, Taiwan, during cyclone-triggered floods. We correct for fossil particulate organic carbon using radiocarbon, and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge. Floods have been shown to carry large amounts of clastic sediment. Non-fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation. We estimate that on decadal timescales, 77–92% of non-fossil particulate organic carbon eroded from the LiWu catchment is transported during large, cyclone-induced floods. We suggest that tropical cyclones, which affect many forested mountains within the Intertropical Convergence Zone, may provide optimum conditions for the delivery and burial of non-fossil particulate organic carbon in the ocean. This carbon transfer is moderated by the frequency, intensity and duration of tropical cyclones

Effects of earthquake and cyclone sequencing on landsliding and fluvial sediment transfer in a mountain catchment

Patterns and rates of landsliding and fluvial sediment transfer in mountain catchments are determined by the strength and location of rain storms and earthquakes, and by the sequence in which they occur. To explore this notion, landslides caused by three tropical cyclones and a very large earthquake have been mapped in the Chenyoulan catchment in the Taiwan Central Range, where water and sediment discharges and rock strengths are well known. Prior to the MW 7·6 Chi-Chi earthquake in 1999, storm-driven landslide rates were modest. Landslides occurred primarily low within the landscape in shallow slopes, reworking older colluvial material. The Chi-Chi earthquake caused wide-spread landsliding in the steepest bedrock slopes high within the catchment due to topographic focusing of incoming seismic waves. After the earthquake landslide rates remained elevated, landslide patterns closely tracking the distribution of coseismic landslides. These patterns have not been strongly affected by rock strength. Sediment loads of the Chenyoulan River have been limited by supply from hillslopes. Prior to the Chi-Chi earthquake, the erosion budget was dominated by one exceptionally large flood, with anomalously high sediment concentrations, caused by typhoon Herb in 1996. Sediment concentrations were much higher than normal in intermediate size floods during the first 5 years after the earthquake, giving high sediment yields. In 2005, sediment concentrations had decreased to values prevalent before 1999. The hillslope response to the Chi-Chi earthquake has been much stronger than the five-fold increase of fluvial sediment loads and concentrations, but since the earthquake, hillslope sediment sources have become increasingly disconnected from the channel system, with 90 per cent of landslides not reaching into channels. Downslope advection of landslide debris associated with the Chi-Chi earthquake is driven by the impact of tropical cyclones, but occurs on a time-scale longer than this study. Copyright © 2008 John Wiley & Sons, Ltd

Prolonged seismically induced erosion and the mass balance of a large earthquake

Large earthquakes deform the Earth's surface and drive topographic growth in the frontal zones of mountain belts. They also induce widespread mass wasting, reducing relief. The sum of these two opposing effects is unknown. Using a time series of landslide maps and suspended sediment transport data, we show that the MW7.6 Chi-Chi earthquake in Taiwan was followed by a period of enhanced mass wasting and fluvial sediment evacuation, peaking at more than five times the background rate and returning progressively to pre-earthquake levels in about six years. Therefore it is now possible to calculate the mass balance and topographic effect of the earthquake. The Choshui River has removed sediment representing more than 30% of the added rock mass from the epicentral area. This has resulted in a reduction of surface uplift by up to 0.25 m, or 35% of local elevation change, and a reduction of the area where the Chi-Chi earthquake has built topography. For other large earthquakes, erosion may evolve in similar, predictable ways, reducing the efficiency of mountain building in fold-and-thrust belts and the topographic expression of seismogenic faults, prolonging the risk of triggered processes, and impeding economic regeneration of epicentral areas

The sources and fluxes of dissolved chemistry in a semi-confined, sandy coastal aquifer: The Pingtung Plain, Taiwan

International audienc

Climatic and geomorphic controls on the erosion of terrestrial biomass from subtropical mountain forest

[1] Erosion of particulate organic carbon (POC) occurs at very high rates in mountain river catchments, yet the proportion derived recently from atmospheric CO2 in the terrestrial biosphere (POCnon-fossil) remains poorly constrained. Here we examine the transport of POCnon-fossil in mountain rivers of Taiwan and its climatic and geomorphic controls. In 11 catchments we have combined previous geochemical quantification of POC source (accounting for fossil POC from bedrock), with measurements of water discharge (Qw) and suspended sediment concentration over 2 years. In these catchments, POCnon-fossil concentration (mg L−1) was positively correlated with Qw, with enhanced loads at high flow attributed to rainfall driven supply of POCnon-fossil from forested hillslopes. This climatic control on POCnon-fossil transport was moderated by catchment geomorphology: the gradient of a linear relation of POCnon-fossil concentration and Qw increased as the proportion of steep hillslopes (>35°) in the catchment increased. The data suggest enhanced supply of POCnon-fossil by erosion processes which act most efficiently on the steepest sections of forest. Across Taiwan, POCnon-fossil yield was correlated with suspended sediment yield, with a mean of 21 ± 10 tC km−2 yr−1. At this rate, export of POCnon-fossil imparts an upper bound on the time available for biospheric growth, of ∼800 yr. Over longer time periods, POCnon-fossil transferred with large amounts of clastic sediment can contribute to sequestration of atmospheric CO2 if buried in marine sediments. Our results show that this carbon transfer should be enhanced in a wetter and stormier climate, and the rates moderated on geological timescales by the regional tectonic setting

Earthquake-triggered increase in sediment delivery from an active mountain belt

In tectonically active mountain belts, earthquake-triggered landslides deliver large amounts of sediment to rivers. We quantify the geomorphic impact of the 1999 Mw 7.6 Chi-Chi earthquake in Taiwan, which triggered >20,000 landslides. Coseismic weakening of substrate material caused increased landsliding during subsequent typhoons. Most coseismic landslides remained confined to hillslopes. Downslope transport of sediment into the channel network occurred during later storms. The sequential processes have led to a factor-of-four increase in unit sediment concentration in rivers draining the epicentral area and increased the magnitude and frequency of hyperpycnal sediment delivery to the ocean. Four years after the earthquake, rates of hillslope mass wasting remain elevated in the epicentral area

Links between erosion, runoff variability and seismicity in the Taiwan orogen

The erosion of mountain belts controls their topographic and structural evolution1, 2, 3 and is the main source of sediment delivered to the oceans4. Mountain erosion rates have been estimated from current relief and precipitation, but a more complete evaluation of the controls on erosion rates requires detailed measurements across a range of timescales. Here we report erosion rates in the Taiwan mountains estimated from modern river sediment loads, Holocene river incision and thermochronometry on a million-year scale. Estimated erosion rates within the actively deforming mountains are high (3–6 mm yr-1) on all timescales, but the pattern of erosion has changed over time in response to the migration of localized tectonic deformation. Modern, decadal-scale erosion rates correlate with historical seismicity and storm-driven runoff variability. The highest erosion rates are found where rapid deformation, high storm frequency and weak substrates coincide, despite low topographic relief