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Hillslopes represent areas of predominant denudation and constitute the transition and trajectory to floodplains; they play a crucial role in understanding the longterm landscape evolution of desert environments. However, although hillslope processes are known to be very slow or even stagnant in (hyper-) arid environments, process mechanisms under the virtual absence of water are poorly understood, and process rates are essentially unknown. Based on irrigation experiments, different monitoring techniques including drone-based high-resolution digital elevation models, geomorphological, stratigraphical, geochronological (OSL), sedimentological and geochemical investigations, as well as mu CT scans of sediment cores, this contribution presents detailed insights into the chronostratigraphy of tongue-shaped, 50 m-long and 30 m-wide fine sediment lobes located along a 10-30 degrees steep thrust-related slope east of the Salar Grande (Atacama, Chile). Irrigation experiments were performed to gain insights into precipitation thresholds for surface runoff and hillslope dynamics. Although artificial rainfall intensities were similar to 46 mm/h, infiltration was 100%, and the experiments did not initiate surface runoff or (detectable) slope material displacements. In addition, a distinct stratigraphic pattern with buried paleo-surfaces and paleo-biological surface crusts suggests increased hillslope activity during the late Pleistocene, potentially driven by changes of (fog-induced) humidity, salt-related shrink and swell processes, or paleo-seismic activity. While a variety of geo-bio-archives document periods of increased precipitation in the Andean parts of the Atacama Desert throughout the Quaternary, evidence for contemporaneous paleoclimatic changes from areas disconnected to Andean precipitation fluctuations (i.e., the Coastal Cordillera) is scarce. In this regard, the investigated landforms potentially represent one of only few sediment records recording paleoclimatic

Colloids and their subset nanoparticles are key soil constituents for nutrient and Organic Carbon (OC) storage and transport, yet little is known about their specific role in overall transfer of elements under hyper-arid conditions. We analyzed the Water Dispersible Colloids (WDCs) of two adjacent soil profiles, located either on the active (named: Fan) or passive (named: Crust) sections of an alluvial fan. Colloidal particles ( < 500 nm) were fractionated using Asymmetric Field-Flow-Field Fractionation (AF(4)), which was coupled online to an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) and an Organic Carbon Detector (OCD) to detect the composition of size-fractionated colloids. Three size categories of particles were identified: nanoparticles (0.6-24 nm), fine colloids (24-210 nm), and medium colloids (210-500 nm). The two profiles differed distinctively in vertical WDC distribution and associated phosphorus (P) content. Fractograms of the Crust profile predominantly showed fine colloids, whereas the medium-sized colloids dominated those of the Fan. Furthermore, the highest colloid content in the Crust profile was found at the surface, while in the Fan, colloids accumulated at 10-20 cm depth, thus overall reflecting the different genesis and infiltration capacities of the soils. Despite very low concentration of colloidal P in these hyper-arid soils, a strong correlation between colloidal P and calcium (Ca), Silica (Si), aluminum (Al), iron (Fe), and OC content were found. This also revealed Ca-phosphates as the primary P retention from, with the association of P to phyllosilicates and Fe/AI (hydr-) oxides as the main soil colloidal fractions. Overall, our results did highlight that small local scale differences in topographic-derived distribution of water flow pathways, defined the formation of the crust-like surfaces, and ultimately the overall movement and distribution of nanoparticles and colloids in soil profiles under hyper-arid conditions.

Geological records indicate that the hyper-aridity in the Atacama Desert has prevailed since at least the mid-Miocene, with shorter periods of increased humidity punctuating long-term aridity. The 7-m-high accumulation of colluvial sediments at the Salar Grande (21 degrees S/70 degrees W) studied here provides a key palaeoclimate record to understand hillslope dynamics and its relation to humid periods. While 1 degrees Be surface exposure ages point to long-term surface stability of the flat upslope surface, a combination of humidity-driven soil creep, overland flow and soil creep related to seismic shaking, caused denudation of the hillslope and accumulation of several metres of colluvium over much shorter timescales during the last 130 ka. A robust chronology for the hillslope sediments has been established by using Infrared Stimulated luminescence (pIR-IR225 protocol) on K-feldspar extracts from nine samples collected within the accumulation. A series of tests has been carried out to confirm the suitability of the method. The estimated ages indicate accelerated sedimentation at 35-80 ka, and 100-130 ka, which are interpreted as periods with more humid climate conditions than present. These findings agree with climate variations in the hyper-arid Atacama indicated by the activity of coastal alluvial fans and river catchments.

Although a number of studies have pointed out the remarkable slowness of Earth surface processes in the Atacama Desert, process mechanisms under such extremely limited water availability are poorly understood, and process rates remain unknown. This paper revisits the discussion on the formation of the prominent Atacama-specific hillslope zebra (stone) stripes, previously interpreted to result from palaeo-overland flow (Owen et al., 2013). Compared to this study, our data document different stripe characteristics with regard to stripe form and orientation as well as sorting- and bedding-patterns of stripe-confining surface gravel units. We found a remarkable form-concordance between zebra stripes and deposits from experiments on segregation-induced granular fingering. Hence, we propose a combination of seismic shaking and instantaneous dry granular free surface flows as the key mechanism for zebra stripe formation. Our findings underline the potential significance of seismicity in shaping Atacama landscapes, which bear important analogies to extraterrestrial surfaces. (C) 2019 Elsevier B.V. All rights reserved.

Marine conglomerates at high elevation on the flanks of ocean islands are usually interpreted as evidence of mega-tsunamis generated by volcano flank collapses, although their origin is sometimes debated (elevated littorals vs. tsunami). In this review, we introduce case studies of well-documented examples of tsunami conglomerates in Hawaii (Pacific Ocean), the Canary and Cape Verde Islands (Atlantic Ocean), and Mauritius Island (Indian Ocean). Other less-documented marine conglomerates are also presented as tsunami candidates. Then, we build a comprehensive picture of the general characteristics of these conglomerates and the different methods that can be applied to date them. Different perspectives of research are proposed, especially on the use of tsunami conglomerates as proxies for better constraining numerical models of ocean island flank collapses and associated tsunamis. We also discuss the possible links between volcano growth, flank instability, and climate.

Although extreme-wave events are frequent along the northwestern coast of Western Australia and tsunamis in 1994 and 2006 induced considerable coastal flooding locally, robust stratigraphical evidence of prehistoric tropical cyclones and tsunamis from this area is lacking. Based on the analyses of X-ray computed microtomography (mu CT) of oriented sediment cores, multi-proxy sediment and microfaunal analyses, optically stimulated luminescence (OSL) and C-14-AMS dating, this study presents detailed investigations on an allochthonous sand layer of marine origin found in a back-barrier depression on the NW Cape Range peninsula. The event layer consists of material from the adjacent beach and dune, fines and thins inland, and was traced up to similar to 400 m onshore. Although a cyclone-induced origin cannot entirely be ruled out, the particular architecture and fabric of the sediment, rip-up clasts and three subunits point to deposition by a tsunami. As such, it represents the first stratigraphical evidence of a prehistoric, mid-Holocene tsunami in NW Western Australia. It was OSL-dated to 5400-4300 years ago, thus postdating the regional mid-Holocene sea-level highstand. (C) 2015 Elsevier B.V. All rights reserved.