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  • English  (4)
  • 1
    Publication Date: 2020-10-12
    Description: In the last two decades, the use ofin situcosmogenic nuclides for the quantification of exogenic processes and thedetermination of exposure ages of landforms has seen a fast and broad expansion. Among the group of terrestrialcosmogenic nuclides that can be used to study geomorphic processes (e.g.10Be,26Al,36Cl,3He,21Ne and22Ne),in situ-produced10Be is the most widely used, especially for the quantification of denudation rates. However,there are a number of problematic issues related to the use of cosmogenic nuclide techniques in rapidly evolvinglandscapes because of the typically low10Be abundancies. The difficulties encountered in these settings are mainlyrelated to (1) the mass of clean quartz that can be obtained and thus the total amount of10Be available, and (2) thebackgrounds of the sample preparation and measurement processes. In order to improve measurements in thesecircumstances, a series of steps can be taken into consideration during field work and sample preparation to helpimprove the final results. We discuss the quality of the blanks, blank corrections, and the limits of detection of thetechnique in the specific case of low concentration samples. Based on a number of different synthetic scenarios,we demonstrate the importance of blank corrections and utility of determination limits, and we highlight how theseparameters may affect the reliability and meaningfulness of the results. This information in turn helps to illustratehow low-concentration data should be interpreted and reported.
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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  • 2
    Publication Date: 2020-12-14
    Description: Preserved remnants of fluvial activity in deserts constitute evidence for changing boundary conditions. The Atacama Desert of northern Chile is the global end-member for aridity, so the history of relict stream networks in this region is a record of how landscapes develop under extreme conditions. On Pampa de Tana in northern Chile (19.4°S), a series of channel forms that are presently inactive but in the past flowed westward are incised into the surface of a fault bounded, topographically elevated portion of the El Diablo Formation, a regionally extensive, relict pediment. We measure cosmic-ray produced 10Be, 26Al and 21Ne in fluvial deposits to date the timing of abandonment of three channels and couple this with topographic profile information from a SPOT-6 derived, 2 m resolution digital elevation model. We find two of the channels were abandoned approximately 〉5.6 Myr and 2.0 Myr ago. One channel is still capable of flow and has ages suggesting it was fluvially active within the last few hundred thousand years. Using the paleochannel ages measured here and published ages for the end of aggradation of the El Diablo Formation we estimate the rates of fluvial channel incision before channel abandonment, and uplift rates on the faults after channel abandonment. Maximum uplift rates of ~12 m/Myr over the last 2 Myr are found. In general, while rates of uplift are relatively low they are several-fold more rapid than the rates of fluvial incision prior to channel abandonment. This implies that westward channel flow was interrupted by uplift of topography above a blind NW-SE striking reverse fault that affects the Central Depression, an alluvial forearc basin. We consider also that shrinkage of the upstream catchment area by stream capture, promoted via headward erosion and lateral expansion of adjacent canyons (quebradas) could be a factor in the abandonment of the channels on Pampa de Tana. Our results highlight the polygenetic nature of this landscape and show that relatively minor amounts of fault displacement in hyperarid regions can have implications for stream network evolution. Even subtle topographic uplift upstream should be taken into account when fluvial deposits are used as proxies for long-term environmental conditions.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 3
    Publication Date: 2020-11-30
    Language: English
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  • 4
    Publication Date: 2022-08-26
    Description: The formally named SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt flow that is preserved in the desert climate of northern Arizona, USA. The flow is independently dated with an 40Ar/39Ar age of 72 ± 4 ka (2σ) and has undergone negligible erosion and/or burial, making its surface an ideal site for direct calibration of cosmogenic nuclide production rates. Production rates for cosmogenic 26Al have been determined from SP flow quartz in this study and are combined with production rates for 10Be, 14C, and 21Ne (Fenton et al., 2019) to yield a suite of production rate ratios. The error-weighted mean, sea-level, high latitude (SLHL) total reference production rate of 26Al is 25.8 ± 2.5 at/g/yr (2σx; standard error) using time-independent Lal (1991)/Stone (2000)(St) scaling factors. The St scaled spallogenic 26Al rate is 25.0 ± 2.4 at/g/yr integrated over the past 72 ka. This rate overlaps within 2σ uncertainty with other St-scaled production rates in the literature. SLHL spallogenic 26Al production rates in SPICE quartz (SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations) are nominally lower if time-dependent Sf, Sa, and Lm scaling factors are used, yielding values of 22.9 ± 2.2 at/g/yr, 22.6 ± 2.2 at/g/yr, and 24.1 ± 2.2 at/g/yr (2σx), respectively. All 26Al production rates in SP flow quartz overlap within 2σ uncertainty, regardless of time independent or time dependent scaling. Production rate ratios for cosmogenic 26Al/10Be, 26Al/14C, and 26Al/21Ne are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2σx; standard error) of 6.7 ± 0.6, 2.23 ± 0.20, and 1.51 ± 0.13, respectively. This study suggests that, similar to cosmogenic 21Ne and 10Be production rates in SP flow quartz, production rates of cosmogenic 26Al in quartz do not significantly increase when integrated over 72 ka, a time span which includes the period of decreased magnetic strength from 20 to 50 ka.
    Language: English
    Type: info:eu-repo/semantics/article
    Format: application/pdf
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