Weathering Processes Across Extreme Erosional Gradients: Do Landslides Matter?

Abstract:

A process-based understanding of weathering in actively eroding mountain belts is vital to understand how linkages between erosion and weathering affect global biogeochemical cycles on a range of timescales. Here we present surface water chemistry data from Southern Taiwan that demonstrates the impact of variable erosive processes on weathering budgets on a large range of scales, from tens of metres to large catchments (>50km²). Southern Taiwan is an excellent example of a number of gradients in erosive processes, with relief and median slope increasing from the southernmost small hills to mountainous threshold-hillslopes with up to 2.5km of relief approximately 100km to the north. Furthermore, Typhoon Morakot (2009) triggered extremely extensive landsliding in some catchments within this zone, allowing distinctions to be drawn between average topographic characteristics of catchments and the erosive processes (i.e. mass wasting) at work therein. Landslides play an important role in localising weathering in deposits with high internal surface area and slow throughflow of fluids, creating sites of rapid weathering which can be a first order control on catchment solute budgets in watersheds where landslides deposits and scars exceed 2% of drained area. Variation in the detailed chemistry of landslide seepages – particularly the carbonate/silicate weathering balance – indicates that this process has a different impact on inorganic weathering-driven carbon cycling than slower erosive processes; a strong positive correlation between landslide-affected area and Ca²⁺:Si ratios on catchment scale suggests rapid erosion is not strongly coupled to CO₂ drawdown. Rapid oxidation of sulphides – ubiquitous in many rapidly eroding mountain belts – within highly fragmented landslide deposits, and associated sulphuric-acid driven weathering, further complicates the effect landsliding has on the carbon cycle.