The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition
The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition
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DOI：
10.2475/ajs.302.9.774
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发表时间：
2002-11
影响因子：
2.9
通讯作者：
S. Poulton;R. Raiswell
中科院分区：
文献类型：
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作者：
S. Poulton;R. Raiswell
对来自 34 条主要河流（地理分布广泛）和 36 条冰川融水流的悬浮沉积物进行了检查，了解其在不同操作定义的铁组分中的变化； FeHR（可溶于连二亚硫酸盐的氧化铁）、FePR（可溶于沸腾的 HCl，但不溶于连二亚硫酸盐的铁）和 FeU（少于可溶于沸腾的 HCl 的总铁）。河流颗粒物显示 FeHR 和总铁 (FeT) 之间存在密切关联，反映了化学风化的影响，化学风化作用从总铁中衍生出氧化铁，并使其与总铁保持密切关联。与此相一致的是，大陆尺度平均FeHR/FeT比值随着径流比（单位面积平均河流径流量/单位面积平均降水量）的变化而变化。相比之下，化学风化作用的减弱不会产生冰川颗粒中 FeHR 与 FeT 的可识别关联，相反，FePR 和 FeU 都与 FeT 密切相关，基本上反映了原始矿物学。对河流颗粒物和海洋沉积物的全球平均成分进行比较表明，后者的 FeHR、FePR 和 FeT 含量较低，但 FeU 含量较高。河流和冰川颗粒数据与文献中对自生、热液、大气和沿海侵蚀铁通量的估计相结合，生成 FeHR、F e PR、F e U 和 FeT 的全球预算。该预算表明，河流颗粒物和海洋沉积物之间的差异可以通过以下方式解释： (i) 通过沉积到河漫滩、盐沼和河口等内岸水库中，优先去除河流颗粒通量中的 FeHR； (ii) 将所得河流颗粒与 FeHR 耗尽的冰川颗粒混合。内岸沉积物的初步测量结果与上述（i）一致。显生宙和现代正常海洋沉积物具有相似的铁形态特征，这意味着铁循环存在长期稳态。这种稳定状态可以通过冰川平衡反馈来维持，其中，当海平面高时，富含 FeHR 的河流颗粒物被更有效地捕获（小冰块，冰川侵蚀减少），或者当海平面低时，与大量 FeHR 耗尽的冰川颗粒物混合（大冰块，冰川侵蚀增强）。对 FeHR 稳态的进一步重要控制是通过微生沉积物和铁矿石的形成进行的，这也为富含 FeHR 的沉积物提供了依赖于海平面的汇。
Suspended sediments from 34 major rivers (geographically wide- spread) and 36 glacial meltwater streams have been examined for their variations in different operationally-defined iron fractions; FeHR (iron oxides soluble in dithionite), FePR (iron soluble in boiling HCl but not in dithionite) and FeU (total iron less that soluble in boiling HCl). River particulates show a close association between FeHR and total iron (FeT), reflecting the effects of chemical weathering which derive oxide iron from, and retain it in close association with, total iron. Consistent with this, continental- scale average FeHR/FeT ratios vary with runoff ratios (average river runoff per unit area/average precipitation per unit area). By contrast, the diminished effects of chemical weathering produce no recognizable association of FeHR with FeT in glacial particulates, and instead both FePR and FeU are closely correlated with FeT, reflecting essentially pristine mineralogy. A comparison of the globally-averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in FeHR, FePR and FeT but enriched in FeU. The river and glacial particulate data are combined with estimates of authigenic, hydrothermal, atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for FeHR ,F e PR ,F e U and FeT. This budget suggests that the differences between riverine particulates and marine sediments can be explained by; (i) preferentially removing FeHR from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains, salt marshes and estuaries; and (ii) mixing the resulting riverine particulates with FeHR- depleted glacial particulates. Preliminary measurements of inner shore sediments are consistent with (i) above. Phanerozoic and modern normal marine sediments have similar iron speciation characteristics, which implies the existence of a long-term steady state for the iron cycle. This steady state could be maintained by a glacioeustatic feedback, where FeHR-enriched riverine particulates are either more effectively trapped when sealevel is high (small ice masses, diminished glacial erosion), or are mixed with greater masses of FeHR-depleted glacial particulates when sealevel is low (large ice masses, enhanced glacial erosion). Further important controls on the steady state for FeHR operate through the formation of euxinic sediments and ironstones, which also provide sealevel-dependent sinks for FeHR-enriched sediment.