Hyperarid coasts develop under relatively high air humidity and abundant sea salt aerosols, resulting in physical and chemical weathering processes that distinctly differ from those present in inland deserts. However, neither the geomorphic effects of the weathering processes nor the timescales on which they alter depositional surfaces are sufficiently understood. In this study we therefore integrate spectral, textural, and gravelometric analyses based on high to very high resolution satellite and aerial remote sensing to assess the degree of post-depositional alteration of a multi-stage alluvial fan at the coast of the Atacama Desert. To further deduce the present-day as well as past intensities of responsible weathering processes we establish a morphochronology of the fan's evolution using cosmogenic 10Be exposure dating. Spectral, textural, and gravelometric parameters relatively date a succession of fan generations, assuming the weathering of fan surface sediments prior to deposition can be largely excluded. Under coastal hyperaridity, effects on the spectral surface signatures are primarily explained by the development of weathering rinds and related formation of secondary iron oxides. Spectral iron oxide ratios show little to no change during the Holocene but strongly increase during the Late Glacial, while over longer timescales an asymptotic increase is suggested. Clast breakdown can largely be attributed to salt weathering causing sublinear to roughly linear decreases in mean size, coarseness, and size variation of coarse clasts. Governed by clast breakdown as well as lateral remobilization of clasts, nano-scale surface roughness exhibits a non-monotonic, peak-shaped relationship with age, whereas the micro-relief experiences continuous smoothing due to denudation. Findings are summarized in a conceptual model showing that in hyperarid coastal environments fan surface weathering results over timescales of 104 to 105 a in a fully developed desert pavement. Two hypothetical scenarios are introduced which differ based on whether desert pavements remain stable over timescales exceeding the Late Pleistocene or not. Late Quaternary weathering intensity along the coast of the Atacama Desert is found to be related to sea surface temperature variability and sea level jointly driving the oceanic moisture supply.
Hyperaridity is the major limiting factor of Earth-surface processes and biological activity in the Atacama Desert of northern Chile, one of the oldest and driest deserts on Earth. On geological timescales, however, the general aridity, which is thought to have onset during the Oligocene to Miocene, has been punctuated by distinct pluvial periods. Also nowadays sporadic but severe rainfall events, like during the flood in 2015, occur within the hyperarid core of the Atacama Desert. During the Miocene and Pliocene, such wetter conditions caused lake formation in parts of the Central Depression and Coastal Cordillera, but also amplified surface processes as well as changes in vegetation dynamics. Unfortunately, due to the limited number and heterogeneity of suitable paleoclimate archives, the long-term precipitation history of the central Atacama Desert and its drivers are still a matter of controversy. Here we present a first quasi-continuous (on time periods of 10-100 kyr) record of the Mid-Miocene to present paleoclimatic and environmental history of the central Atacama Desert obtained from an endorheic clay pan. Due to its location in the Coastal Cordillera, the investigated clay pan is assumed to have been decoupled from Andean ground and surface-water inflow, and thus to have recorded only local and regional precipitation variations on different timescales. The investigated 52 m-long sediment sequence exhibits significant changes in the sedimentological, geochemical paleontological, and mineralogical properties. Preliminary data from the recovered Mid-Miocene sediments imply permanent lacustrine conditions with alternating evaporation cycles that point to significantly different hydrological and/or climatic conditions in Coastal Cordillera than today. A major lithological shift, accompanied by a well-preserved paleosol, documents a significant local groundwater lowering, probably due to fault activation during the mid-Miocene. Subsequent high sedimentation of coarse-grained alluvial deposits indicates alternating periods of wetter than present precipitation, though still arid, conditions with episodic shallow lake phases during the Miocene. A distinct drop in the sedimentation rate and a synchronous transition to fine-grained distal alluvial deposits highlights a significant change in the moisture availability in the Coastal Cordillera during the Pliocene and Pleistocene, probably marking the final onset of hyperarid conditions in the central Atacama Desert.
The impact of Quaternary climate change on landscape evolution, and more specifically the timing of incision of the overdeepened Alpine valleys, remains difficult to quantify with existing thermochronometric methods. Thermochronometers are used to determine rates of rock cooling, however most techniques are insensitive to temperature changes <60 °C that occur within the last kms of Earth’s crust. Recording cooling rates within this temperature range is essential if the impact of glacial-interglacial cycles on rock exhumation is to be resolved.Electron spin resonance (ESR) thermochronometry applied to quartz minerals has the potential to span this thermal (and temporal) gap. We are developing this method by building upon previous studies (e.g. Scherrer, 1993) with the ultimate aim of constraining the timing of incision of the Rhône valley. Preliminary data from the Japanese Alps (King et al., 2020) indicate that ESR thermochronometry could resolve rates of <1 mm/yr over Quaternary timescales.To determine a rock cooling history using ESR thermochronometry, signal accumulation and signal thermal loss must be robustly determined within the laboratory. We have collected a series of geological samples including rocks from boreholes that have known isothermal histories to investigate the potential of this technique. Our objective is to use the latter rocks to confirm the validity of our laboratory measurements and data-fitting/numerical models. Specifically, we have investigated known-thermal history samples from the MIZ1 borehole (Japan) and the KTB borehole (Germany) as well as samples from Sion in the Western European Alps.Preliminary data reveal that the ESR dose response and thermal decay of different quartz samples is highly variable. Whereas the Al-centre of some samples exhibits linear dose response to laboratory irradiation up to 15 kGy, the Al-centre of other samples exhibits exponential, or double-exponential growth and saturates at doses of 3-4 kGy. The Ti-centre of most samples is well described by a single saturating exponential function, however samples from the MIZ1 borehole exhibit pronounced sub-linearity in the low-dose response region. Furthermore, whereas for some samples the Al-centre is less thermally stable than the Ti-centre, for other samples the inverse is observed. These observations suggest that a uniform measurement protocol and data-fitting approach may not be appropriate for quartz ESR data.Inversion of two KTB samples yielded temperatures within uncertainty of borehole temperature, however results for the MIZ1 borehole are more variable and can only recover temperature at best within ~10%. Investigations into the cause of the poor results for the MIZ1 borehole are ongoing (i.e. measurement protocol, data-fitting/numerical model) and will be discussed. Preliminary data from Sion are promising and reveal consistent cooling rates. Scherer, T., Agel, A., and Hafner S. S.: Determination of uplift rates using ESR investigations of quartz, KTB Rep. 93-2. Kontinentales Tiefbohrprogram der Bundesrepublic Deutschland Niedersächs. Landesamt Bodenforsch., Hannover, 121–124, 1993.King, G.E., Tsukamoto, S., Herman, F., Biswas, R.H., Sueoka, S., Tagami, T. Electron spin resonance (ESR) thermochronometry of the Hida range of the Japanese Alps: validation and future potential. Geochronology 2, no. 1 (2020): 1-15.   
More than 30 years after the first Electron Spin Resonance (ESR) dating application to optically bleached quartz grains by Yokoyama et al. (1985), the absence of standardization for reporting methodology and age results remains an obstacle for the development and recognition of the ESR dating method within the Quaternary scientific community. To overcome this issue, the present work proposes some basic guidelines which should hopefully be useful not only for the ESR dating community, but also for any potential reviewers who may not be familiar with the specificities of this field.
<p>The Japanese Alps uplifted throughout the Quaternary and reach elevations of up to 3,000 m today. However, understanding the interaction between rates of Earth surface processes, tectonics and climate is challenging, partly due to the difficulties of measuring changes in the rates of Earth surface processes at the timescale of glacial-interglacial cycles. In particular, the youth of the Japanese Alps has made measurement of their exhumation histories complicated. To help resolve this issue, we investigate the potential of ultra-low temperature thermochronometers based on the luminescence and electron spin resonance (ESR) of feldspar and quartz minerals, respectively. We focus on the Tateyama region in the Hida range, which was glaciated during the late Quaternary period. In total, eight samples were analyzed by luminescence and ESR thermochronometry. While most luminescence signals have already reached their upper dating limit, ESR signals do constrain exhumation rates.</p><p>We measured the ESR dose response and thermal decay properties of all samples, specifically targeting the Al and Ti centres. In general, thermal stability is higher for the Ti centre than the Al centre, resulting in ESR ages of between 0.5-0.9 Ma, although the natural intensity of the Ti centre is close to or above the upper dating limit. In contrast, the Al signal still grows with time and is suitable for determining finite exhumation rates. Initial inversions reveal rock cooling rates on the order of 80 &#176;C/Ma, which can be inverted to preliminarily rates of rock exhumation of <3 mm/a within the past 1 Ma. In the next step, we will relate these rates to the climatic (glacial) and tectonic history of the Tateyama region.</p>
