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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Radiogenic Hf isotopic compositions of continental eolian dust from Asia, its variability and its implications for seawater Hf
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Radiogenic Hf isotopic compositions of continental eolian dust from Asia, its variability and its implications for seawater Hf

机译：亚洲大陆风积尘的放射性Hf同位素组成，变异性及其对海水Hf的影响

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摘要

The inorganic silicate fraction extracted from bulk pelagic sediments from the North Pacific Ocean is eolian dust. It monitors the composition of continental crust exposed to erosion in Asia. ~(176)Lu/~(177)Hf ratios of modern dust are subchondritic between 0.011 and 0.016 but slightly elevated with respect to immature sediments. Modern dust samples display a large range in Hf isotopic composition (IC), -4.70 < ε_(Hf) < 16.45, which encompasses that observed for the time series of DSDP cores 885/886 and piston core LL44-GPC3 extending back to the late Cretaceous. Hafnium and neodymium isotopic results are consistent with a dominantly binary mixture of dust contributed from island arc volcanic material and dust from central Asia. The Hf-Nd isotopic correlation for all modern dust samples, ε_(Hf) = 0.78ε_(Nd) + 5.66 (n = 22, R~2 = 0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in ε_(Hf) of Asian dust exceeds that predicted on the basis of corresponding ε_(Nd) values (-4.7 < ε_(Hf) < +2.5; -10.9 < ε_(Nd) < -10.1). This is attributed to: (1) the fixing of an important unradiogenic fraction of Hf in zircons, balanced by radiogenic Hf that is mobile in the erosional cycle, (2) the elevated Lu/Hf ratio in chemical sediments which, given time, results in a Hf signature that is radiogenic compared with Hf expected from its corresponding Nd isotopic components, and (3) the possibility that diagenetic resetting of marine sediments may incorporate a significant radiogenic Hf component into diagenetically grown minerals such as illite. Together, these processes may explain the variability and more radiogenic character of Hf isotopes when compared to the Nd isotopic signatures of Asian dust. The Hf-Nd isotope time series of eolian dust are consistent with the results of modern dust except two samples that have extremely radiogenic Hf for their Nd (ε_(Hf) = +8.6 and +10.3, ε_(Nd) = -9.5 and -9.8). These data may point to a source contribution of dust unresolved by Nd and Pb isotopes. The Hf IC of eolian dust input to the oceans may be more variable and more radiogenic than previously anticipated. The Hf signature of Pacific seawater, however, has varied little over the past 20 Myr, especially across the drastic increase of eolian dust flux from Asia around 3.5 Ma. Therefore, continental contributions to seawater Hf appear to be riverine rather than eolian. Current predictions regarding the relative proportions of source components to seawater Hf must account for the presence of a variable and radiogenic continental component. Data on the IC and flux of river-dissolved Hf to the oceans are urgently required to better estimate contributions to seawater Hf. This then would permit the use of Hf isotopes as a monitor of past changes in erosion.
机译：从北太平洋上层浮游沉积物中提取的无机硅酸盐成分是风尘。它监测亚洲遭受侵蚀的大陆壳的成分。现代粉尘的〜（176）Lu /〜（177）Hf比在约0.011至0.016之间呈亚软骨形，但相对于未成熟的沉积物略有升高。现代粉尘样品的Hf同位素组成（IC）范围很广，-4.70 <ε_（Hf）<16.45，其中包括在DSDP岩心885/886和活塞核LL44-GPC3的时间序列中观察到的延伸到后期白垩纪。 island和钕同位素的结果与岛弧火山材料贡献的粉尘和中亚地区的粉尘占优势的二元混合物相吻合。所有现代尘埃样品的Hf-Nd同位素相关系数ε_（Hf）=0.78ε_（Nd）+ 5.66（n = 22，R〜2 = 0.79），比迄今报道的地面储层要平坦。此外，亚洲尘埃的ε_（Hf）的变异性超过了根据相应的ε_（Nd）值预测的变异性（-4.7 <ε_（Hf）<+2.5; -10.9 <ε_（Nd）<-10.1）。这归因于：（1）固定了锆石中重要的非放射性Hf，并通过在腐蚀循环中移动的放射性Hf进行了平衡;（2）化学沉积物中Lu / Hf比升高，这在一定的时间下会导致与从其相应的Nd同位素组分中获得的Hf相比，Hf具有放射源性;（3）海洋沉积物的成岩作用重置可能会将大量的放射源性Hf成分掺入成岩作用的矿物中，例如伊利石。与亚洲尘埃的Nd同位素特征相比，这些过程在一起可以解释Hf同位素的变异性和更多的放射源特征。风尘的Hf-Nd同位素时间序列与现代尘埃的结果一致，除了两个样品的Nd极具放射源性Hf（ε_（Hf）= +8.6和+ 10.3，ε_（Nd）= -9.5和- 9.8）。这些数据可能表明Nd和Pb同位素未解析的粉尘源贡献。输入到海洋的风尘的Hf IC可能比以前预期的更易变，更具有放射成因。但是，在过去的20 Myr中，太平洋海水的Hf签名变化不大，尤其是在亚洲大约3.5 Ma的风尘通量急剧增加的情况下。因此，大陆对海水Hf的贡献似乎是河流而不是风积。当前关于源成分与海水Hf相对比例的预测必须考虑存在可变的和放射源的大陆成分。迫切需要有关河流溶解的Hf的IC和通量的数据，以更好地估计对海水Hf的贡献。然后，这将允许使用of同位素作为过去侵蚀变化的监测器。

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