Denudation Rate Changes Along a Fast‐Eroding Mountainous River With Slate Headwaters in Taiwan From 10Be (Meteoric)/9Be Ratios

Friedhelm Von Blanckenburg,Shouye Yang,Hella Wittmann,Kai Deng

doi:10.1029/2019jf005251

Abstract

AbstractThe novel 10Be (meteoric)/9Be system, where 10Be is delivered by precipitation and stable 9Be is released by weathering, provides denudation rates over weathering‐erosion timescales. The new tool is applicable to quartz‐poor lithologies, for example, mafic rock and claystone, which are not readily accessible by the commonly used in situ‐produced 10Be in quartz. We provide a first application of this proxy to a tectonically active mountainous river, the Zhuoshui River in Taiwan. Taiwan Rivers supply a disproportionately high suspended and dissolved flux to the oceans and are often underlain by fine‐grained shale/slate. 10Be (meteoric)/9Be‐derived denudation rates (Dmet) from the Zhuoshui Catchment are highest in the slate‐dominated headwaters (4–8 mm/year), and much lower (1–2 mm/year) along the midlower reaches with mixed lithologies. At the basin‐wide scale, we find a poor correlation between Dmet and basin‐averaged channel steepness despite a small climatic gradient. Because large lithological heterogeneities exist in this basin, we invoke a lithological effect to explain this poor correlation. Relying on a revised stream power incision model that incorporates rock erodibility, the resulting lithology‐ and runoff‐adjusted ksn (kLrsn) can be reconciled with denudation rates with the highest erodibility predicted to prevail in the Miocene slate of low metamorphic grade and high fracture density. This model suggests that the lithological heterogeneity can alter the coupling between surface denudation and channel morphology. On a broader perspective, the successful application of the 10Be (meteoric)/9Be proxy shows its applicability as a tracer for erosion and sediment transport processes in fast‐eroding mountain belts underlain by slate lithologies.

Highlights

Small mountainous rivers discharging to the oceans have been in the focus of sediment flux studies for many years because of their disproportionately high sediment discharge (Hovius et al, 1997; Milliman & Syvitski, 1992)
Taiwan Rivers supply a disproportionately high suspended and dissolved flux to the oceans and are often underlain by fine‐grained shale/slate. 10Be/9Be‐derived denudation rates (Dmet) from the Zhuoshui Catchment are highest in the slate‐dominated headwaters (4–8 mm/year), and much lower (1–2 mm/year) along the midlower reaches with mixed lithologies
Given that the modeled results meet the three criteria mentioned above, we propose that the lithological heterogeneity and corresponding variability in erodibility can alter the coupling between denudation rates and fluvial channel morphology

Summary

Introduction

Small mountainous rivers (area 1 km) discharging to the oceans have been in the focus of sediment flux studies for many years because of their disproportionately high sediment discharge (Hovius et al, 1997; Milliman & Syvitski, 1992). Considering that quartz‐poor slate dominates the alpine region of Taiwan (Garzanti & Resentini, 2016), in situ 10Be‐derived denudation rates may not fully represent basin‐wide erosion processes In this case, a recently developed denudation proxy, the 10Be (meteoric)/9Be ratio, provides an alternative choice, as it does not rely on quartz lithologies. The values of these metrics are relatively stable and high in the central part (e.g., 125–275 km from the southern tip of this island), indicating the mature stage of arc‐continent collision and plausible topographic steady state, that is, the balance between long‐term uplift and denudation In comparison, these metrics gradually decrease southward and northward of the mature collision zone, corresponding to initial and postcollision stages (Chen & Willett, 2016; Stolar et al, 2007). Longer‐term methods include reservoir sedimentary records (Montgomery et al, 2014) over centennial scales, in situ‐produced cosmogenic 10Be measured in quartz of detrital sediments over the timescale of several hundreds to thousands of years (Derrieux et al, 2014; Fellin et al, 2017; Siame et al, 2011), and low‐

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