Chemical weathering in active mountain belts controlled by stochastic bedrock landsliding

Abstract

Alps of New Zealand that weathering in bedrock landslides controls the variability in solute load of these mountain rivers. We find that systematic patterns in surface water chemistry are strongly associated with landslide occurrence at scales from a single hillslope to an entire mountain belt, and that landslides boost weathering rates and river solute loads over decades. We conclude that landslides couple 25 erosion and weathering in fast-eroding uplands and, thus, mountain weathering is a stochastic process that is sensitive to climatic and tectonic controls on mass wasting processes. . Lack of rain prior to and during sampling meant that landslide scarps were dry, but persistent seepage from landslide debris suggested a low hydraulic conductivity of these deposits. Seepage from two larger landslides of 1. (Tables S1, S3), but the elevated TDS in landslide seepage is systematic across all sites.Measured TDS values of WSA rivers plot on a mixing line between dilute runoff from soil covered slopes and concentrated landslide seepage (Fig. S2) This has also allowed us to assess how landslide age can affect the river solute load. (Table S7), despite important temporal variations in landslide rates (Fig. S6). The strongest correlation with river TDSmodern is obtained when all landslides since 1980 are combined. Thus, the very high landslide rates during 1980-1985 still disproportionately affect the river chemistry, and the effect of the degradation of these older slides on weathering is less than that of the order-of-magnitude drop in landslide rate since then. 150Nevertheless, the weaker correlation between river TDSmodern and landslide rates prior to 1980 indicates that the landslide weathering boost dissipates on decadal time scales. Both lines of evidence suggest that, in the WSA, the timescale over which landslides affect the weathering budget is about 30-60 years. by the process at a given time.Our data show that landslides, with associated expansive mineral surface area in debris and extended, slow hydrological pathways, provide an optimal weathering environment, the volume of which is a first-order control on the dissolved load of rivers draining the 165 WSA. Where bedrock-involved landsliding dominates, it provides an effective link between physical erosion and chemical weathering. The temporal and spatial stochasticity of landslide-driven erosion will be reflected in the weathering budget, and can explain the major part of spatial and temporal variability of solute transport in mountain rivers.

Oxidation of sulfides and rapid weathering in recent landslides

“…The typhoon generated more than 22 000 landslides West et al, 2011), with an average density of 35 000 m 2 km −2 , orders of magnitude more than under less exceptional meteorological conditions. This presents a rare opportunity to study an extensive population of recent, synchronous landslides, which is important because the rapid weathering in landslides might decay over a decadal timescale to levels indistinguishable from that on surrounding hillslopes (Emberson et al, 2016). Having a constant, known date of initiation for sampled landslides reduces any systematic changes to weathering that would be introduced by variable timing of the mass wasting.…”

“…It also suppresses the possibility of amalgamation of multiple landslides into single mapped polygons, which blights automatic mapping . In our analysis we draw a distinction between older landslides and those generated by Typhoon Morakot, prompted by our previous work (Emberson et al, 2016) that has shown that the chemical output from individual slides is likely to significantly decay over a decadal timescale. It should be noted that the proportion of landslides predating Typhoon Morakot is dwarfed by the mass wasting caused by this exceptional typhoon in much of the T'aimali catchment.…”

“…Errors in the mapped landslide area are assumed to be < 20 %, which are propagated to the total volume estimated, as well as the catchment-wide volume estimates. This follows the approach described in Emberson et al (2016), which allows direct comparison of the estimated landslide volumes in Taiwan and the WSA (Fig. 4).…”

“…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.…”

“…It has been shown in the western Southern Alps (WSA) of New Zealand (Emberson et al, 2016) that this can give rise to strong gradients in dissolved solid concentrations in surface waters on hillslopes, with highest concentrations in seepage from landslides, and that the rate of landsliding is an important control on the chemistry of the rivers draining the mountain belt. Localised landslide weathering, prone to the spatial and temporal variability of mass wasting, contrasts with common models in which weathering is controlled by steady and distributed erosion of a laterally continuous soil and regolith mantle (Dixon and von Blanckenburg, 2012;Hilley et al, 2010;Ferrier and Kirchner, 2008;West, 2012;Lebedeva et al, 2010;D.…”