Browsing by Author "Bourlès, D"

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    Carbonate and silicate intercomparison materials for cosmogenic 36Cl measurements
    (Elsevier, 2019-09-15) Mechernich, S; Dunai, TJ; Binnie, SA; Goral, T; Heinze, S; Dewald, A; Schimmelpfennig, I; Keddadouche, K; Aumaître, G; Bourlès, D; Marrero, S; Wilcken, KM; Simon, KJ; Fink, D; Phillips, FM; Caffee, M; Gregory, LC; Phillips, R; Freeman, SPHT; Shanks, R; Sarikaya, MA; Pavetich, S; Rugel, G; Merchel, S; Akçar, N; Yesilyurt, S; Ivy-Ochs, S; Vockenhuber, C
    Two natural mineral separates, labeled CoCal-N and CoFsp-N, have been prepared to serve as intercomparison material (ICM) for in situ-produced cosmogenic 36Cl and natural chlorine (Clnat) analysis. The sample CoCal-N is derived from calcite crystals in a Namibian lag deposit, while the sample CoFsp-N is derived from a single crystal of alkali-feldspar from a Namibian pegmatite. The sample preparation took place at the University of Cologne and a rotating splitter was used to obtain homogeneous splits of both ICMs. Forty-five measurements of CoCal-N (between 1 and 16 per facility) and forty-four measurements of CoFsp-N (between 2 and 20 per facility) have been undertaken by ten target preparation laboratories measured by seven different AMS facilities. The internal laboratory scatter of the 36Cl concentrations indicates no overdispersion for half of the laboratories and 3.9 to 7.3% (1σ) overdispersion for the others. We show that the CoCal-N and CoFsp-N splits are homogeneous regarding their 36Cl and Clnat concentrations. The grand average (average calculated from the average of each laboratory) yields initial consensus 36Cl concentrations of (3.74 ± 0.10) × 106 at 36Cl/g (CoCal-N) and (2.93 ± 0.07) × 106 at 36Cl/g (CoFsp-N) at 95% confidence intervals. The coefficient of variation is 5.1% and 4.2% for CoCal-N and CoFsp-N, respectively. The Clnat concentration corresponds to the lower and intermediate range of typical rock samples with (0.73 ± 0.18) µg/g in CoCal-N and (73.9 ± 6.8) µg/g in CoFsp-N. We discuss the most relevant points of the sample preparation and measurement and the chlorine concentration calculation to further approach inter-laboratory comparability. We propose to use continuous measurements of the ICMs to provide a valuable quality control for future determination of 36Cl and Clnat concentrations. © 2019 Elsevier B.V.
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    New chronology for the southern Kalahari Group sediments with implications for sediment-cycle dynamics and early hominin occupation
    (Cambridge University Press, 2017-01-20) Matmon, A; Hidy, AJ; Vainer, S; Crouvi, O; Fink, D; Erel, Y; ASTER Team; Arnold, M; Aumaître, G; Bourlès, D; Keddadouche, K; Horwitz, LK; Chazan, M
    Kalahari Group sediments accumulated in the Kalahari basin, which started forming during the breakup of Gondwana in the early Cretaceous. These sediments cover an extensive part of southern Africa and form a low-relief landscape. Current models assume that the Kalahari Group accumulated throughout the entire Cenozoic. However, chronology has been restricted to early–middle Cenozoic biostratigraphic correlations and to OSL dating of only the past ~ 300 ka. We present a new chronological framework that reveals a dynamic nature of sedimentation in the southern Kalahari. Cosmogenic burial ages obtained from a 55 m section of Kalahari Group sediments from the Mamatwan Mine, southern Kalahari, indicate that the majority of deposition at this location occurred rapidly at 1–1.2 Ma. This Pleistocene sequence overlies the Archaean basement, forming a significant hiatus that permits the possibility of many Phanerozoic cycles of deposition and erosion no longer preserved in the sedimentary record. Our data also establish the existence of a shallow early–middle Pleistocene water body that persisted for > 450 ka prior to this rapid period of deposition. Evidence from neighboring archeological excavations in southern Africa suggests an association of high-density hominin occupation with this water body. © University of Washington
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    New chronology for the southern Kalahari Group sediments: implications for sediment cycle dynamics and early hominin occupation
    (The Geological Society of America, 2014-10-19) Matmon, A; Hiny, AJ; Crouvi, O; Fink, D; Erel, Y; Arnold, M; Aumaître, G; Bourlès, D; Keddadouche, K; Horwitz, LK; Chazan, M
    The Kalahari Group covers an extensive part of the southern African continent and forms a low-relief landscape dominated by extensive unconsolidated sand. Current depositional models assume that the Kalahari Group sediments accumulated gradually throughout the Cenozoic, but an absence of absolute chronology beyond ~60 ka has left this premise untested. Here, we challenge this age model with new cosmogenic burial ages obtained from a 55 m section of Kalahari Group sediments at Mamatwan Mine near the southern edge of the Kalahari basin. Our results indicate that the majority of the existing section was emplaced rapidly at ~1 Ma. At this time the basin filled to its present level and established the Kalahari sand belts, which fostered the modern savannah. Our data suggest a dynamic landscape, with punctuated cycles of erosion and deposition, in contrast to the accepted concept of a stable basin filling slowly throughout the Cenozoic. The sedimentology and cosmogenic nuclide measurements from the lower Mamatwan Mine section reveal the existence of an extensive Early to Middle Pleistocene water body, persisting at least 420 ka prior to the rapid filling event at ~1 Ma. This water body is contemporaneous with a significant hominin occupation as evidenced by neighboring archaeological excavations. We thus provide the first evidence of association of the high-density hominin occupation in southern Africa with an extensive water body. © 2014 The Geological Society of America (GSA)
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    Seismic origin of the Atacama Desert boulder fields
    (Elsevier BV, 2015-02-15) Matmon, A; Quade, J; Placzek, CJ; Fink, D; Arnold, M; Aumaître, G; Bourlès, D; Keddadouche, K; Copeland, A; Neilson, JW
    Extensive fields of large boulders are common around the base of hills in the Atacama Desert. How these boulders are transported from nearby hillslopes is unclear given the lack of rainfall of the region. Here we document the central role of seismicity, not runoff, in transporting and smoothing >1 ton boulders all across the hyperarid core of the Atacama Desert. The generally granitoid boulders emerge as corestones on hillslopes at an erosion rate of 0.1-1 m Ma(-1). Thereafter, physical and cosmogenic isotopic evidence suggests that boulders slide and bounce rather than roll down hills and onto adjacent flats. In the transport process, the largest boulders are split and the smaller ones are weathered to grus, narrowing average boulder mass to similar to 2 tons (<1 m(3)). At the base of hills, the boulders bunch together and rub during the frequent earthquakes in the region, producing distinctive smoothing around boulder mid-sections, and silt moats around the boulder bases. Our measurements show a strong correlation between boulder field density and rubbing, and only when the density exceeds 60-70% does rubbing become common. Except for slow removal by rubbing, the boulders seem to undergo no further erosion while in the flats. Exposure times for some boulders are >12 Ma, making them among the oldest continuously exposed features on the Earth. Boulder rubbing is just one geologic feature among many in the Atacama that underscore the role that seismicity probably plays in shaping landscapes of the waterless worlds of the solar system. © 2015, Elsevier B.V.