ALBERT

Description: The tectonically active Taiwan orogen features numerous rivers that yield a high amount of sediment with fluxes exceeding 104 t/km2/yr. Amongst these, the landslide-dominated Liwu River is well studied regarding its dynamic surface processes. However, the quantification of denudation in the Liwu Basin is still an ongoing task as rates obtained to date are subject to substantial differences depending on methods that differ in their spatio-temporal scales. We constrain an upper limit of global denudation using the cosmogenic nuclide 10Be(meteoric) and its ratio to stable 9Be. Meteoric cosmogenic 10Be is delivered to Earth’s surface by precipitation, whereas stable 9Be is released from rock weathering. In contrast to in situ cosmogenic 10Be measured in quartz, the 10Be(meteoric)/9Be ratio can be analyzed in quartz-poor settings. 10Be(meteoric)/9Be-derived denudation rates (Dmet) vary from 8.1 to 〉30 mm/yr in the Liwu mainstem, and from 3.4 to 21.5 mm/yr in the tributaries. These new Dmet are among the highest cosmogenic nuclide-derived rates ever measured. Most of these rates agree with rates from sediment gauging or channel incision. We propose that stochastic landsliding plays a major role in denudation processes here. Using a soil-bedrock mixing model and published riverine organic 14C data as a soil tracer, we estimate the fractional contribution of bedrock landslide material to mainstem sediments to be 55-97%, which explains the magnitude and large variability (4-fold) in Dmet. We demonstrate the complexity associated with denudation rates determination in landslide-dominated routing systems; but also the potential of 10Be(meteoric)/9Be for tracing stochastic landsliding processes.

Description: Meteoric cosmogenic 10Be is a powerful tracer to quantify dates and rates of Earth surface processes over timescales of 103-105 yrs. A prerequisite for its applications is knowledge of the flux at which 10Be, produced in the atmosphere, is delivered to the Earth surface. Four entirely independent approaches are available to quantify this flux: 1) General Circulation Models (GCM) combined with 10Be production functions and aerosol dynamics; 2) 10Be in precipitation collections; 3) 10Be inventories in dated soil profiles; 4) riverine 10Be exported in solid and dissolved forms. We compiled and reprocessed published globally distributed 10Be flux data from each of these methods and compared them with each other after normalization to a common atmospheric production rate. Based on precipitation records, we propose a simple framework to discriminate between two delivery effects on 10Be fluxes. In the additive effect water vapor and 10Be are continuously accumulating during long-distance transport, leading to an increase in 10Be flux with precipitation rate. In the dilution effect, the 10Be flux is delivered from proximal vapor sources, limited by the rate of 10Be introduction from the stratosphere and independent of precipitation rate. Both effects are mostly present in combination, and the relative weight of either effect depends on vapor condensation rate and on the ratio of vapor condensation area to precipitation area. A comparison between precipitation-derived fluxes and GCM-derived fluxes shows that half of the precipitation estimates are 〉2 times greater than GCM-derived fluxes. By comparison, soil- and GCM-derived fluxes agree within a factor of 2 for more than half (∼57%) of the dataset, and the remaining soil estimates (∼43%) are much lower than GCM-derived fluxes. 71% of 10Be flux estimates from riverine export using 10Be (meteoric)/9Be ratios also agree with GCM-derived fluxes within a factor of 2. We explain the precipitation-derived fluxes that commonly exceed all other estimates by short-term stochasticity in precipitation events that might introduce a measurement time-interval bias towards higher fluxes. This bias is not present over longer-term (103-105 yrs) flux estimates like those from soil profiles. Soil-derived fluxes might still present an underestimation when retention of 10Be in soil is incomplete. We recommend producing more 10Be depositional flux data from soil inventories with full Be retention, as these generate in our view the most relevant estimates for applications of meteoric 10Be on millennial-scale Earth surface processes.

Description: 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.