Dust production and deposition in Asia and the north Pacific Ocean over the past 12 Myr
Dust production and deposition in Asia and the north Pacific Ocean over the past 12 Myr
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DOI：
10.1016/s0012-821x(00)00083-2
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发表时间：
2000-05
影响因子：
5.3
通讯作者：
T. Pettke;A. Halliday;C. Hall;D. Rea
中科院分区：
文献类型：
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作者：
T. Pettke;A. Halliday;C. Hall;D. Rea
北太平洋中部现代远洋沉积物的硅酸盐组分主要是来自亚洲中部的风成尘。ODP 885/886站点（44.7°N，168.3°W）的1100万年沉积记录可以评估这些灰尘及其来源如何响应晚新生代气候和构造而发生变化。提取的风成部分含有不同量（>70%）的粘土矿物，其次是石英和斜长石。在1100万年的记录中，大部分记录的Nd同位素组成（Nd=-8.6到-10.5）和Sm/Nd比值（0.170到0.192）都很一致，这表明青藏高原北部盆地和戈壁沙漠的混合物源很好，早在最古老的亚洲黄土形成之前，这就是尘埃的来源。<2.9 Ma的样品的Nd值高达−6.5，表明年轻的堪察加火山弧成分的混合物≤35 wt%。Pb和Nd在侵蚀旋回中的相关性使我们将没有人为污染的亚洲黄土的Pb同位素组成限制为206 Pb/204 Pb =18.97± 0.06，207 Pb/204 Pb =15.67±0.02，208Pb/204Pb=39.19±0.11.87Sr/86Sr（0.711-0.721）和Rb/Sr比值（0.39-1.1）随尘埃矿物学的不同而变化，提供的年龄指示值为250 Ma左右。6个尘埃样品的40 Ar/39 Ar年龄一致，均在200 Ma左右，与K-Ar年龄一致。沉积在夏威夷现代尘埃的Ar年龄。这些数据反映了伊利石形成的加权平均年龄。自上新世晚期以来，从伊利石≥蒙皂石（含大量高岭石）到富含伊利石和绿泥石、不含高岭石的组合的变化记录了源区化学风化强度的变化。这种风化作用显然没有干扰K-Ar系统，而只是引起Rb-Sr数据的分散。我们建议，当蒙皂石的形式在伊利石的费用，K和Ar是定量损失从什么变成蒙皂石，但定量保留在相邻的伊利石层。40 Ar/39 Ar年龄数据，因此，是不敏感的蒙皂石形成过程中的化学风化，但日期的伊利石，主要含钾相在灰尘的成岩生长。在过去的1200万年里，北太平洋的尘埃通量增加了一个数量级以上，记录了中亚的严重干旱。然而，这种气候变化并没有改变尘埃的最终来源，化学风化的新生产物始终从属于东亚和太平洋沉积尘埃中机械侵蚀改造的组合。
The silicate fractions of recent pelagic sediments in the central north Pacific Ocean are dominated by eolian dust derived from central Asia. An 11 Myr sedimentary record at ODP Sites 885/886 at 44.7°N, 168.3°W allows the evaluation of how such dust and its sources have changed in response to late Cenozoic climate and tectonics. The extracted eolian fraction contains variable amounts (>70%) of clay minerals with subordinate quartz and plagioclase. Uniform Nd isotopic compositions (ϵNd=−8.6 to −10.5) and Sm/Nd ratios (0.170–0.192) for most of the 11 Myr record demonstrate a well-mixed provenance in the basins north of the Tibetan Plateau and the Gobi Desert that was a source of dust long before the oldest preserved Asian loess formed. ϵNdvalues of up to −6.5 for samples <2.9 Ma indicate ≤35 wt% admixture of a young, Kamchatka-like volcanic arc component. The coherence of Pb and Nd in the erosional cycle allows us to constrain the Pb isotopic composition of Asian loess devoid of anthropogenic contamination to206Pb/204Pb=18.97±0.06,207Pb/204Pb=15.67±0.02,208Pb/204Pb=39.19±0.11.87Sr/86Sr (0.711–0.721) and Rb/Sr ratios (0.39–1.1) vary with dust mineralogy and provide an age indication of ∼250 Ma.40Ar/39Ar ages of six dust samples are uniform around 200 Ma and match the K–Ar ages of modern dust deposited on Hawaii. These data reflect the weighted age average of illite formation. Changes from illite≥smectite with significant kaolinite to illite- and chlorite-rich, kaolinite-free assemblages since the late Pliocene document changes in the intensity of chemical weathering in the source region. Such weathering evidently did not disturb the K–Ar systematics, and only induced scatter in the Rb–Sr data. We propose that when smectite forms at the expense of illite, K and Ar are quantitatively lost from what becomes smectite, but are quantitatively retained in adjacent illite layers.40Ar/39Ar age data, therefore, are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite, the major K-bearing phase in the dust. Over the past 12 Myr, the dust flux to the north Pacific increased by more than an order of magnitude, documenting a substantial drying of central Asia. This climatic change, however, did not alter the ultimate source of the dust, and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern Asia and the Pacific Ocean.