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首页> 外文期刊>Quaternary geochronology >Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment
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Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment

机译：干旱地区流域混合砂中的宇宙成因核素浓度与单个河流卵石之间的不一致

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摘要

Based on cosmogenic ~(10)Be and ~(26)Al analyses in 15 individual detrital quartz pebbles (16e21 mm) and cosmogenic ~(10)Be in amalgamated medium sand (0.25e0.50 mm), all collected from the outlet of the upper Gaub River catchment in Namibia, quartz pebbles yield a substantially lower average denudation rate than those yielded by the amalgamated sand sample. ~(10)Be and ~(26)Al concentrations in the 15 individual pebbles span nearly two orders of magnitude (0.22 ± 0.01 to 20.74 ± 0.52×10~6 ~(10)Be atoms g~(-1) and 1.35 ± 0.09 to 72.76 ± 2.04×10~6 ~(26)Al atoms g~(-1), respectively) and yield average denudation rates of w0.7 m Myr~(-1) (~(10)Be) and w0.9 m Myr~(-1) (26Al). In contrast, the amalgamated sand yields an average ~(10)Be concentration of 0.77 ± 0.03×10~6 atoms g~(-1), and an associated mean denudation rate of 9.6 ± 1.1 m Myr~(-1), an order of magnitude greater than the rates obtained for the amalgamated pebbles. The inconsistency between the ~(10)Be and ~(26)Al in the pebbles and the ~(10)Be in the amalgamated sand is likely due to the combined effect of differential sediment sourcing and longer sediment transport times for the pebbles compared to the sand-sized grains. The amalgamated sands leaving the catchment are an aggregate of grains originating from all quartz-bearing rocks in all parts of the catchment. Thus, the cosmogenic nuclide inventories of these sands record the overall average lowering rate of the landscape. The pebbles originate from quartz vein outcrops throughout the catchment, and the episodic erosion of the latter means that the pebbles will have higher nuclide inventories than the surrounding bedrock and soil, and therefore also higher than the amalgamated sand grains. The order-of-magnitude grain size bias observed in the Gaub has important implications for using cosmogenic nuclide abundances in depositional surfaces because in arid environments, akin to our study catchment, pebble-sized clasts yield substantially underestimated palaeo-denudation rates. Our results highlight the importance of carefully considering geomorphology and grain size when interpreting cosmogenic nuclide data in depositional surfaces.
机译：根据15个碎屑石英卵石（16e21 mm）和成因〜（10）Be在混合介质砂（0.25e0.50 mm）中的宇宙成因〜（10）Be和〜（26）Al分析，全部从在纳米比亚的高布河上游集水区，石英卵石的平均剥蚀率大大低于混合砂样的平均剥蚀率。 15个卵石中的〜（10）Be和〜（26）Al浓度跨近两个数量级（0.22±0.01至20.74±0.52×10〜6〜（10）Be原子g〜（-1）和1.35± 0.09至72.76±2.04×10〜6〜（26）Al原子g〜（-1））的平均剥蚀率分别为w0.7 m Myr〜（-1）（〜（10）Be）和w0。 9 m Myr〜（-1）（26Al）。相反，混合砂产生的平均〜（10）Be浓度为0.77±0.03×10〜6个原子g〜（-1），相关的平均剥蚀率为9.6±1.1 m Myr〜（-1），数量级大于合并的鹅卵石获得的速率。卵石中〜（10）Be和〜（26）Al与混砂中〜（10）Be之间的不一致可能是由于卵石的差异性沉积物沉积和更长的沉积物传输时间的综合作用所致。沙粒。离开集水区的混合砂是来自集水区所有部分所有含石英岩石的谷物的聚集体。因此，这些沙子的宇宙成因核素清单记录了景观的总体平均下降速度。卵石起源于整个集水区的石英脉露头，而后者的偶发侵蚀意味着卵石的核素存量要高于周围的基岩和土壤，因此也要高于汞齐沙粒。在Gaub中观察到的数量级晶粒尺寸偏差对于在沉积表面中使用宇宙成因的核素丰度具有重要意义，因为在干旱环境中，类似于我们的研究流域，卵石大小的碎屑会大大低估古生物剥蚀率。我们的结果强调了在解释沉积表面的宇宙成因核素数据时，仔细考虑地貌和晶粒尺寸的重要性。

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