Oxidation of sulfides and rapid weathering in recent landslides

Abstract

Abstract. Linking together the processes of rapid physical erosion and the resultant chemical dissolution of rock is a crucial step in building an overall deterministic understanding of weathering in mountain belts. Landslides, which are the most volumetrically important geomorphic process at these high rates of erosion, can generate extremely high rates of very localised weathering. To elucidate how this process works we have taken advantage of uniquely intense landsliding, resulting from Typhoon Morakot, in the T'aimali River and surrounds in southern Taiwan. Combining detailed analysis of landslide seepage chemistry with estimates of catchment-bycatchment landslide volumes, we demonstrate that in this setting the primary role of landslides is to introduce fresh, highly labile mineral phases into the surface weathering environment. There, rapid weathering is driven by the oxidation of pyrite and the resultant sulfuric-acid-driven dissolution of primarily carbonate rock. The total dissolved load correlates well with dissolved sulfate -the chief product of this style of weathering -in both landslides and streams draining the area (R 2 = 0.841 and 0.929 respectively; p < 0.001 in both cases), with solute chemistry in seepage from landslides and catchments affected by significant landsliding governed by the same weathering reactions. The predominance of coupled carbonate-sulfuric-acid-driven weathering is the key difference between these sites and previously studied landslides in New Zealand (Emberson et al., 2016), but in both settings increasing volumes of landslides drive greater overall solute concentrations in streams.Bedrock landslides, by excavating deep below saprolite-rock interfaces, create conditions for weathering in which all mineral phases in a lithology are initially unweathered within landslide deposits. As a result, the most labile phases dominate the weathering immediately after mobilisation and during a transient period of depletion. This mode of dissolution can strongly alter the overall output of solutes from catchments and their contribution to global chemical cycles if landslide-derived material is retained in catchments for extended periods after mass wasting.

Understanding terrestrial organic carbon export : a time-series approach

“…These measurements of OC petro loss from soils suggest a median atmospheric CO 2 flux of 5.6 t C km −2 yr −1 to 17.1 t C km −2 yr −1 (Figure 6.S2A), consistent with previous estimates for Taiwan: (i) ≤12 t C km −2 yr −1 calculated by comparing riverine OC petro yield and TSS yield (Figure 6.S2B; , and (ii) 7 t C km −2 yr −1 to 13 t C km −2 yr −1 calculated using dissolved rhenium yield as a proxy for OC petro oxidation (Figure 6.S2B; . While similar within uncertainty, our estimates are slightly lower than catchment-specific rhenium-based values, and may suggest additional OC petro oxidation in locations not captured by our soil samples such as landslide colluvium (Emberson et al, 2016) and/or during fluvial transit (Galy et al, 2008a). This flux is on the same order of magnitude as CO 2 drawdown due to OC bio export from rivers combined with subsequent burial in marine sediments [Taiwan average: (21 ± 10) t C km −2 yr −1 ; Figure 6.S2C; Hilton et al, 2012] as well as that due to weathering of silicate minerals (LiWu River: 7.04 t C km −2 yr −1 ; Figure 6.S2D;Calmels et al, 2011).…”

“…High rates of soil erosion and landscape turnover by bedrock landslides (Hovius et al, 2000;Lin et al, 2008;Hilton et al, 2012) results in a continuous exposure of bedrock material to chemical weathering (Hilton et al, 2012;Emberson et al, 2016). The dominant lithologies are metasedimentary, decreasing in metamorphic grade from the Tananao schist on the east (peak metamorphic temperature ≈500 • C) to the Lushan sedimentary formation on the west (≤150 • C; Figure 6.S1; .…”

Weathering of Reactive Mineral Phases in Landslides Acts as a Source of Carbon Dioxide in Mountain Belts

“…Other studies have demonstrated how weathering of detrital calcite in the WSA prevents fast overall weathering there from serving as an effective drawdown for atmospheric CO 2 (Chamberlain et al, ; Jacobson et al, ). In a previous study we have shown that this is driven by weathering in landslide deposits (Emberson et al, ). In the two Taiwanese catchments we can take this further and suggest that the presence of sulfides in the bedrock combined with carbonate‐dominated weathering acts as a source for carbon dioxide rather than a sink.…”

Storage and weathering of landslide debris in the eastern San Gabriel Mountains, California, USA: Implications for mountain solute flux

“…For example, Jin et al () observed elevated river solute fluxes following widespread landsliding during the Wenchuan earthquake of 2008. Elevated solute fluxes were linked to recent landsliding in both the Southern Alps (Emberson, Hovius, Galy, & Marc, ) and southern Taiwan (Emberson, Hovius, Galy, & Marc, ); both studies found the effect of landslides on solute fluxes dampened on decadal timescales.…”