Shaping the Huara Intrusive Complex in the Hyperarid Atacama Desert—Erosional Near‐Stasis Contrasting High Topographic Gradients

The Atacama Desert is one of the driest and oldest deserts on Earth, with extremely low precipitation rates (<2 mm/yr). Mostly abiotic hyperarid environmental conditions prevail, and surface processes act at extremely low rates over the long‐term. To gain knowledge about the rate of surface processes and age of landscapes in desert environments, terrestrial cosmogenic nuclide derived erosion rate estimates can be used. Within the Huara Intrusive Complex, situated in the hyperarid core of the Atacama Desert, basin‐averaged bedrock erosion rates from channel sediments are extremely low, that is, less than 1 m/Myr. Such low rates indicate that fluvial processes operate very slowly or are almost absent. Bedrock erosion rates of channel knickpoints, however, reveal one to two orders of magnitude higher erosion rates (2–12 m/Myr). Erosion rates are remarkably low when compared to the steep surrounding topography. Tectonic uplift creates higher gravitational potentials for surface processes, controlling the overall erosion rate capacity. However, erosion itself is taking place by local precipitation capable of exceeding thresholds for surface activity. In the Atacama Desert, this happens only due to rare severe precipitation events, explaining the extremely low erosion rates. The efficiency of these events is modulated by local intrinsic processes and conditions, such as high infiltration capacities of Atacama soils and/or large channel boulder accumulations. Due to the virtual absence of these precipitation events capable of erosion, the landscape appears to be in hibernation.Plain Language Summary: The Atacama Desert is one of the driest and oldest deserts on Earth, with extremely low precipitation rates (<2 mm/yr). Surface processes operate at very low rates and on small spatial scales. To understand the rates of surface activity, cosmogenic nuclides are a widely used tool to constrain the exposure duration of sediments on the Earth's surface. Within the Huara Intrusive Complex, situated in the hyperarid core of the Atacama, basin‐averaged erosion rates from channel sediments are extremely low. Such low rates indicate that the transport of sediment is very slow or almost absent, revealing a landscape in hibernation. Bedrock erosion rates in channels, however, are one to two orders higher. The studied catchments have been subject to Quaternary tectonic activity, which can explain higher bedrock erosion rates. Rare precipitation events, typical for desert environments, have to be strong enough to provoke erosion. Processes associated with extreme long‐term aridity modulate the erosive impact of precipitation events; for example, CaSO4‐rich soils soak up water preventing surface runoff or channel boulder accumulations buffer surface flow reducing the capacity to erode.Key Points: Extremely low basin erosion rates (<1 m/Myr) in the hyperarid Atacama prevailed since the Pliocene, contrasting high relief topography. Higher bedrock erosion rates indicate that the tectonic activity is the active landscape forming mechanism. The capacity to erode is reduced by the effects of atmospheric deposition, soil inflation, and channel boulder accumulations.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://www.crc1211db.uni-koeln.de/search/view.php?doiID=6

A photogrammetry-based approach for soil bulk density measurements with an emphasis on applications to cosmogenic nuclide analysis

The quantification of soil bulk density (rho(B)) is a cumbersome and time-consuming task when traditional soil density sampling techniques are applied. However, it can be important for terrestrial cosmogenic nuclide (TCN) production rate scaling when deriving ages or surface process rates from buried samples, in particular when short-lived TCNs such as in situ C-14 are applied. Here, we show that soil density determinations can be made using structure-from-motion multi-view stereo (SfM-MVS) photogrammetry-based volume reconstructions of sampling pits. Accuracy and precision tests as found in the literature and as conducted in this study clearly indicate that photographs taken from both a consumer-grade digital single-lens mirrorless (DSLM) and a smartphone camera are of sufficient quality to produce accurate and precise modelling results, i.e. to regularly reproduce the true volume and/or density by > 95 %. This finding holds also if a freeware-based computing workflow is applied. The technique has been used to measure rho(B) along three small-scale (< 1 km) N-S transects located in the semi-arid to arid Altos de Talinay, northern central Chile (similar to 30.5 degrees S, similar to 71.7 degrees W), during a TCN sampling campaign. Here, long-term differences in microclimatic conditions between south-facing and north-facing slopes (SFSs and NFSs, respectively) explain a sharp contrast in vegetation cover, slope gradient and general soil condition patterns. These contrasts are also reflected by the soil density data, generally coinciding with lower densities on SFSs. The largest differences between NFSs and SFSs are evident in the lower portion of the respective slopes, close to the thalwegs. In general, field-state soil bulk densities were found to vary by about 0.6 g cm(-3) over a few tens of metres along the same slope. As such, the dataset that was mainly generated to derive more accurate TCN-based process rates and ages can be used to characterise the present-day condition of soils in the study area, which in turn can give insight into the long-term soil formation and prevailing environmental conditions. This implies that the method tested in this study may also being applied in other fields of research and work, such as soil science, agriculture or the construction sector

Development of a steep erosional gradient over a short distance in the hyperarid core of the Atacama Desert, northern Chile

Although generally considered to exist under hyperarid conditions over the long term, landscapes in many parts of the Coastal Cordillera of northern Chile have undergone fluvial erosion. Small-scale drainage systems in this mountain range are mostly isolated from river networks and associated processes originating in the Precordillera or the High Andes to the east, thus providing natural laboratories to investigate the interplay between erosion, atmospheric deposition, tectonics and local (micro-) climatic conditions. In this study, we present a set of cosmogenic Be-10 and Al-26 derived, catchment-wide erosion rates along a short (2.5 km) E-W transect on the northern rim of the Rio Loa Canyon in the Coastal Cordillera of northern Chile (latitude 21.4 degrees S). Here, a flat sedimentary gravel surface, which was deposited before the Middle Miocene, becomes increasingly dissected and changes into a badland-like topography to the west. The Be-10 erosion rates increase by approximately an order of magnitude from east to west, reflecting (1) localised tectonic movements, (2) geologically recent base level lowering, (3) time-integrated (micro-) climatic gradients and (4) the presence/absence of gyperete. These findings are corroborated by analysis of geomorphologic parameters, which point towards the presence of two fundamentally different erosional regimes in this small study area. These regimes are sharply delineated along a topographically modest tectonic ridge. To the west, a detachment-limited erosion regime prevails, while in the east a transport-limited regime is dominant. The presence or absence of gyperete, whose prevalence is governed by (micro-) climatic conditions, generally appears to reflect the respective erosional regimes. The erosion rates we infer point to a long-term process of differential drainage evolution in the study area, likely on timescales of millions of years