Radiogenic Hf isotopic composition of continental eolian dust in Pacific Ocean sediments

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. 176Lu/177Hf 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 < epsilon-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, epsilon-Hf= =0.78 epsilon-Nd = +5.66 (n =22, R**2 =0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in epsilon-Hf of Asian dust exceeds that predicted on the basis of corresponding epsilon-Nd values (34.76 epsilon-Hf < +2.5; -10.96< epsilon-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 (epsilon-Hf =+8.6 and +10.3, epsilon-Nd =39.5 and 39.8). 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.

从北太平洋块状远洋沉积物（bulk pelagic sediments）中提取的无机硅酸盐组分即为风尘（eolian dust），其可用于示踪受侵蚀作用暴露的亚洲大陆地壳组成。现代风尘的¹⁷⁶Lu/¹⁷⁷Hf比值低于球粒陨石值（subchondritic），介于0.011至0.016之间，但相较于未成熟沉积物略高。现代风尘样品的Hf同位素组成（Hf isotopic composition, IC）跨度极大，εHf值介于-4.70至+16.45之间，这一范围覆盖了深海钻探计划（DSDP, Deep Sea Drilling Project）885/886岩芯及活塞岩芯LL44-GPC3的时间序列数据，其时间跨度可追溯至晚白垩世。铪与钕同位素结果表明，风尘主要由岛弧火山物质贡献的组分与中亚来源的风尘构成二元混合体系。所有现代风尘样品的Hf-Nd同位素相关关系为εHf = 0.78εNd + 5.66（样本量n=22，决定系数R²=0.79），其相关趋势相较于目前已报道的陆地储层相关曲线更为平缓。此外，亚洲风尘的εHf值变化范围超出了基于对应εNd值预测的结果（εHf波动范围为-34.76至+2.5；εNd波动范围为-10.96至-10.1）。该现象可归因于以下三点：(1) 锆石（zircon）中固定了大量非放射成因Hf组分，而侵蚀循环中可移动的放射成因Hf则与之保持平衡；(2) 化学沉积物中Lu/Hf比值升高，随时间推移，其Hf同位素特征相较于由对应钕同位素组分推导的预期值更具放射成因；(3) 海洋沉积物（marine sediments）的成岩重置（diagenetic resetting）作用可能将显著的放射成因Hf组分引入伊利石（illite）等成岩成因矿物中。综上，相较于亚洲风尘的钕同位素特征，上述过程可解释其Hf同位素的变化性与更高的放射成因特征。风尘的Hf-Nd同位素时间序列与现代风尘的结果基本一致，仅两个样品例外：其相较于对应Nd同位素组成具有极强的放射成因Hf（εHf=+8.6与+10.3，εNd=-39.5与-39.8）。这些数据可能指示了一种未被钕与铅同位素识别的风尘源区贡献。输入海洋的风尘Hf同位素组成可能比此前预期的具有更强的变化性与更高的放射成因特征。然而，在过去2000万年中，太平洋海水的Hf同位素组成几乎未发生显著变化，尤其是在约3.5 Ma前后亚洲风尘通量急剧增加的时期。因此，海洋水体Hf的大陆来源似乎以河流输入为主，而非风尘输入。当前关于海洋水体Hf源组分相对占比的预测，必须考虑可变且具放射成因特征的大陆组分的影响。目前亟需开展针对河流溶解态Hf的同位素组成与通量的相关研究，以更精准地估算其对海洋水体Hf的贡献，进而实现利用Hf同位素示踪过去侵蚀作用的变化。