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

Abstract

The 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.

Comparison of basin-scale in situ and meteoric <sup>10</sup>Be erosion and denudation rates across a rainfall, slope, and elevation gradient at George River, northeast Tasmania, Australia

“…Moreover, the similarity of Dm and ε-based sediment fluxes for central and eastern tributaries provides support our hypothesis that 10 Bem/ 9 Bereac based denudation rates should more-closely replicate 10 Bei-based erosion rates in small river basins where geological heterogeneity is minimized. Our findings also, generally, support the continued exploration and application of 10 Bem/ 9 Bereac denudation rates in geomorphological studies (Dannhaus et al, 2017;Deng et al, 2020;Portenga et al, 2019;Rahaman et al, 2017;Wittmann et al, 2012Wittmann et al, , 2015. Interestingly, measured values of E are systematically ~4-5x greater than ε, and thus while E may be influenced by the same geomorphological processes in George River as ε and Dm, it does not appear to reflect accurate rates of landscape change.…”

“…Conceptually, and with regards to 10 Be, Portenga et al (2019) suggested that if soil thickness approximates the zone of 10 Bei production (<2 m) and if pH values in the environment are high (>3.9, Graly et al, 2010) such that no 10 Bem desorbs from sediment grain coatings, erosion and denudation rates derived from measurements of 10 Bei and 10 Bem/ 9 Bereac should be comparable and should measure landscape dynamics similarly. Although replication between 10 Bei and 10 Bem erosion and denudation rates at individual sites is poor (Portenga et al, 2019;Rahaman et al, 2017;Wittmann et al, 2015), average rates from 10 Bei and 10 Bem/ 9 Bereac erosion and denudation datasets tend to be similar in magnitude, and similar spatial patterns of landscape change emerge from both 10 Bei and 10 Bem datasets across large regions (Deng et al, 2020;Portenga et al, 2019;Wittmann et al, 2015). Further use of the 10 Bem/ 9 Bereac denudation method in landscapes where 10 Bei erosion can be measured and compared is important for evaluation the veracity of 10 Bem erosion and 10 Bem/ 9 Bereac denudation calculations.…”

“…Recently derived equations allow erosion rates and denudation rates to be calculated from measurements of 10 Bem and the chemically-similar, non-cosmogenic 9 Be, which is weathered out of mineral grains ( 9 Bereac; Willenbring and von Blanckenburg, 2010;von Blanckenburg et al, 2012). 10 Bem erosion and 10 Bem/ 9 Bereac denudation rates have been used to quantify landscape evolution over a variety of spatial scales for long-established river basins (Dannhaus et al, 2018;Deng et al, 2020;Harrison et al, 2021;Portenga et al, 2019;Rahaman et al, 2017;Wittmann et al, 2012Wittmann et al, , 2015 and has shown particular promise in quantifying landscape dynamics in quartz-poor landscapes (Deng et al, 2020;Rahaman et al, 2017).…”

“…Thus, a secondary goal of this study is to compare 10 Bem erosion and denudation rates to 10 Bei erosion rates as a means of assessing the efficacy of the 10 Bem erosion and 10 Bem/ 9 Bereac denudation methods in a landscape that minimizes geological heterogeneity, which otherwise may introduce scatter to larger datasets covering larger, more geologically-diverse landscapes (i.e. Deng et al, 2020;Portenga et al, 2019;Rahaman et al, 2017). Although George River has a simple bedrock geology, it also has a long history of intensive lode and placer tin mining that has, in the past, disturbed its fluvial systems (Knighton, 1991;Preston, 2012).…”

Coseismic Uplift of the 1999 Mw7.6 Chi‐Chi Earthquake and Implication to Topographic Change in Frontal Mountain Belts

“…Erosion rates along Choushui River are ~6–12 mm/a (Dadson et al, 2003; Kao & Milliman, 2008), which is comparable to the uplift rate of ~8.5 mm/a over an earthquake cycle (Figure 4). On the contrary, denudation rates derived from 10 Be (Deng et al, 2020; Derrieux et al, 2014) are as low as 1–2 mm/a suggesting that landslide debris may require a longer time than an earthquake cycle to be carried out of catchments by being preserved as river terraces or buried in the valley (Hsieh et al, 2014; Milliman et al, 2017). Because the denudation rates represent catchment‐wide erosion, which links to erosion upstream in deeper mountains rather than landslide erosion in frontal mountains, the low denudation rates may not reflect erosion due to coseismic landslides.…”