Debris‐flow and anthropogenic disturbance on 10Be concentration in mountain streams with contrasting structural geomorphic connectivity

<p>This study examines the sensitivity of 10Be concentrations to debris‐flow and anthropogenic perturbations in steep mountain streams of the Eastern Alps, characterized by contrasting structural geomorphic connectivity. Using cosmogenic 10Be as a tracer for functional geomorphic connectivity, we conduct sampling replications across four seasons in Gadria, Strimm and Allitz Creek. Sampling sites encompass a range of structural connectivity configurations, including the conditioning of a sackung, all assessed through a geomorphometric index (IC). By combining information on contemporary depth of erosion and sediment yield, disturbance history and post‐LGM (Last Glacial Maximum) sedimentation rates at the Gadria fan, we constrain the effects of debris‐flow disturbance on 10Be concentrations along Gadria Creek. Here, we argue that bedrock weakening imparted by the sackung promotes high depth of erosion. Consequently, we show that debris flows recruit sediment beyond the critical depth of spallogenic production (e.g., >3m), which in turn, episodically, due to predominantly muogenic production pathways, lowers 10Be concentration by a factor of 4, for over 2 years. In contrast, steady erosion in Strimm Creek yields very stable 10Be concentrations through time. In Allitz Creek, we observe two‐ to fourfold seasonal fluctuations in 10Be concentrations, which we explain as the combined effects of water diversion and hydraulic structures on sediment mixing. We further show that 10Be concentration correlates inversely with the IC index, where sub‐basins characterized by high concentrations (long residence times) exhibit low IC values (structurally disconnected) and vice versa, implying that, over millennial time scales a direct relation exists between functional and structural connectivity, and that the IC index performed as a suitable metric. In terms of landscape evolution, we argue that the sackung, by favouring intense debris‐flow activity across the Holocene, as recorded by the paraglacial sedimentary wave associated with the formation of the Gadria fan, has aided rapid postglacial reshaping of the Gadria source basin, which currently exhibits a topographic signature characteristic of unglaciated debris flow-dominated systems.</p>

Short Communication: Increasing vertical attenuation length of cosmogenic nuclide production on steep slopes negates topographic shielding corrections for catchment erosion rates

Interpreting catchment-mean erosion rate from in situ produced cosmogenic 10Be concentration in stream sands requires calculating the catchment-mean 10Be surface production rate and effective mass attenuation length, both of which can vary locally due to topographic shielding and slope effects. The most common method for calculating topographic shielding accounts only for the effect of shielding at the surface, leading to catchment-mean corrections of up to 20 % in steep landscapes, and makes the simplifying assumption that the effective mass attenuation length for a given nuclide production mechanism is spatially uniform. Here I evaluate the validity of this assumption using a simplified catchment geometry to calculate the spatial variation in surface skyline shielding, effective mass attenuation length, and the total effective shielding factor for catchments with mean slopes ranging from 0° to 80°. For flat catchments (i.e., uniform elevation of bounding ridgelines), the increase in effective attenuation length as a function of hillslope angle and skyline shielding leads to a catchment-mean total effective shielding factor of one, implying that no topographic shielding factor is needed when calculating catchment-mean vertical erosion rates. For dipping catchments (as characterized by a plane fit to the bounding ridgelines), the catchment-mean total effective shielding factor is also one, except for cases of extremely steep range-front catchments, where the shielding correction is counterintuitively greater than one. These results indicate that in most cases, topographic shielding corrections are inappropriate for calculating catchment-mean erosion rates, and only needed for steep catchments with non-uniform distribution of quartz and/or erosion rate. By accounting only for shielding of surface production, existing shielding approaches introduce a slope-dependent systematic error that could lead to spurious interpretations of relationships between topography and erosion rate.

Bayesian inversion of a CRN depth profile to infer Quaternary erosion of the northwestern Campine Plateau (NE Belgium)

The rate at which low-lying sandy areas in temperate regions, such as the Campine Plateau (NE Belgium), have been eroding during the Quaternary is a matter of debate. Current knowledge on the average pace of landscape evolution in the Campine area is largely based on geological inferences and modern analogies. We performed a Bayesian inversion of an in situ-produced 10Be concentration depth profile to infer the average long-term erosion rate together with two other parameters: the surface exposure age and the inherited 10Be concentration. Compared to the latest advances in probabilistic inversion of cosmogenic radionuclide (CRN) data, our approach has the following two innovative components: it (1) uses Markov chain Monte Carlo (MCMC) sampling and (2) accounts (under certain assumptions) for the contribution of model errors to posterior uncertainty. To investigate to what extent our approach differs from the state of the art in practice, a comparison against the Bayesian inversion method implemented in the CRONUScalc program is made. Both approaches identify similar maximum a posteriori (MAP) parameter values, but posterior parameter and predictive uncertainty derived using the method taken in CRONUScalc is moderately underestimated. A simple way for producing more consistent uncertainty estimates with the CRONUScalc-like method in the presence of model errors is therefore suggested. Our inferred erosion rate of 39 ± 8. 9 mm kyr−1 (1σ) is relatively large in comparison with landforms that erode under comparable (paleo-)climates elsewhere in the world. We evaluate this value in the light of the erodibility of the substrate and sudden base level lowering during the Middle Pleistocene. A denser sampling scheme of a two-nuclide concentration depth profile would allow for better inferred erosion rate resolution, and including more uncertain parameters in the MCMC inversion.

Constraining Quaternary erosion of the Campine Plateau (NE Belgium) using Bayesian inversion of an in-situ produced ¹⁰Be concentration depth profile

The rate at which low-lying sandy areas in temperate regions, such as the Campine area (NE Belgium), have been eroding during the Quaternary is a matter of debate. Current knowledge on the average pace of landscape evolution in the Campine area is largely based on geological inferences and modern analogies. We applied Bayesian inversion to an in-situ produced 10Be concentration depth profile in fluvial sand, sampled on top of the Campine Plateau, and inferred the average long-term erosion rate together with three other parameters, i.e., the surface exposure age, inherited 10Be concentration and sediment bulk density. The inferred erosion rate of 44 ± 9 mm/kyr (1σ) is relatively large in comparison with landforms that erode under comparable (palaeo-)climates elsewhere in the world. We evaluate this value in the light of the erodibility of the substrate and sudden base level lowering during the Middle Pleistocene. A denser sampling scheme of a two-nuclide concentration depth profile would allow to include more parameters in the model inversion and further reduce their uncertainty.

Simulating ice core ¹⁰Be on the glacial–interglacial timescale

10Be ice core measurements are an important tool for paleoclimate research, e.g. allowing for the reconstruction of past solar activity or variation in the natural 14C production rate. However, especially on multi-millennial timescales, the share of production and climate induced variations of respective 10Be ice core records is still up to debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn-transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow pit measurements are used for model calibration and validation. Being specifically configured for polar 10Be, this tool thus allows for a straight-forward investigation of production and non-production related modulation of this nuclide. We find that the polar 10Be ice concentration does not record a globally mixed cosmogenic production signal. In fact, the geomagnetic modulation of Greenland 10Be is up to 50% lower than in case of the global atmospheric 10Be inventory. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable to reproduce the largest portion of the observed 10Be changes. However, model-measurements differences exhibit multi-millennial oscillations with amplitudes up to 87% of the mean observed Holocene 10Be concentration. Focusing on the (12–37) kyr b2k (before the year 2000 AD) period, mean model-measurements differences of 30% cannot be imputed to production changes. However, unconsidered climate-induced changes could likely explain the model shortcomings. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) kyr b2k measurements-model differences.