The impact of glacial/interglacial climate changes on fluvial and mass-wasting processes in the Taiwan's mountains

Abstract:

The Taiwan orogenic belt, located in Southeastern Asia, is under monsoon

climate, frequently attacked by tropical typhoons, and characterized by rapid

tectonic uplift with high seismicity. Researchers have been linking the Taiwan’s

landscapes to active tectonic uplift. In this study, we show the significance of

glacial/interglacial climate changes in shaping the landscapes. We focus on the

mountain areas that have never been glaciated. Based on >400 radiocarbon

dates (70 of which >12 ka), we find that both the slope and fluvial activities

were generally low during the glacial time. Still, extensive alluviation had

occurred at certain time periods, forming large debris slopes or alluvial fans

(typically along mountain fronts), and causing significant aggradation along

some major rivers. In contrast, with numerous landslides and debris flows, river

incision has dominated during the postglacial time. Episodic river aggradation

with alluvial-terrace development (typically at tributary mouths) also occurred

during this time period, but was less extensive than previously. Some huge

postglacial alluvial terraces have been proved sourced from the colluviums

deposited in the glacial time. We attribute the low landscape activities of the

glacial period to the dryness during the period. However, even in this time

rare but severe rainfall events must have occurred to trigger some extensive

alluviation. In contrast, the increase in both rainfall and typhoon frequency

during the postglacial time drastically increased the slope instability and

sediment yield. The great stream power, along with the sufficient coarse debris

acting as erosion tools, ensured the rapid river incision during this time.