Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
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DOI：
10.1016/j.epsl.2020.116166
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发表时间：
2020-04
影响因子：
5.3
通讯作者：
H. Goring-Harford;J. Klar;Hannah K. Donald;C. Pearce;D. Connelly;R. James
中科院分区：
文献类型：
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作者：
H. Goring-Harford;J. Klar;Hannah K. Donald;C. Pearce;D. Connelly;R. James
河流是海水中铬（Cr）的主要来源，古代海洋沉积物的Cr同位素特征被广泛认为可以提供陆地上是否存在氧化风化过程的记录。然而，这是假设河水的δ 53 Cr值忠实地转移到海洋中，而在河口混合带没有改变。为了验证这一假设，我们确定了无机Cr (Cr （III)+ Cr (VI)）的浓度和δ 53 Cr值，以及在英国Beaulieu河河口混合带收集的水样中的Cr形态。溶解无机Cr的δ 53 Cr值为- 0.59 ~ 1.68‰，Cr （VI）浓度为0.39 ~ 1.83 nmol kg - 1, Cr （III）浓度为0.11 ~ 3.21 nmol kg - 1。Cr （VI）浓度和δ 53 Cr值随盐度线性增加，而Cr （III）浓度随盐度线性降低。因此，δ 53 Cr、Cr （III）和Cr （VI）在河口混合带均表现为保守行为，波留河水体δ 53 Cr特征仅在河与海水端元混合时发生改变。河流水端δ 53 Cr平均值（- 0.39±0.08‰）低于其他河流的平均值，这可能与缺氧风化过程释放的有机结合Cr （III）的输入有关。紫外辐射回收的Cr具有较低的δ 53 Cr值（- 0.11 ~ - 0.75‰）。虽然缺氧风化过程的Cr输入不太可能是今天海洋中Cr的重要来源，但这表明在局部尺度上，氧化风化以外的其他过程可能对河口和沿海水域的δ 53 Cr值产生影响。沿海海水端部δ 53 Cr值（1.6±0.4‰）也高于深海开阔海域，这是由于Cr的原位生物地球化学循环，这些因素在海相沉积δ 53 Cr记录的解释中需要考虑。
Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ 53 Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ 53 Cr values of inorganic Cr (Cr (III)+ Cr (VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ 53 Cr values of dissolved inorganic Cr ranged from− 0.59 to 1.68‰, Cr (VI) concentrations from 0.39 to 1.83 nmol kg− 1 and Cr (III) concentrations from 0.11 to 3.21 nmol kg− 1. Both Cr (VI) concentrations and δ 53 Cr values increased linearly as a function of salinity, while Cr (III) concentrations decreased linearly with salinity. Thus δ 53 Cr, Cr (III) and Cr (VI) all showed conservative behaviour in the estuarine mixing zone, and the δ 53 Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average δ 53 Cr value of the river water endmember (− 0.39±0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr (III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ 53 Cr values (− 0.11 to− 0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ 53 Cr values of estuarine and coastal waters on the local scale. The δ 53 Cr value of the coastal seawater endmember (1.6±0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ 53 Cr records.