Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere
Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere
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DOI：
10.1016/j.epsl.2019.115851
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发表时间：
2019-12
影响因子：
5.3
通讯作者：
I. Bindeman;G. Bayon;J. Palandri
中科院分区：
文献类型：
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作者：
I. Bindeman;G. Bayon;J. Palandri
大陆风化作用伴随着粘土和其他次生矿物的形成，因此它们的δ 18 O和Δ 17 O值在一定程度上反映了大气降水（δ 18 O MW）和年平均温度（MAT）的特征。我们从地质历史上的风化产物中提取气候信息的能力依赖于针对现代气候条件进行测试和校准的分析方法。我们在这里介绍了来自全球45条河流的粘土大小的沉积物的三重氧同位素分析，以及相应的富含粉砂和砂的碎屑组分的δ 18 O分析，这些碎屑组分总共覆盖了从热带到极地地区约25%的排入海洋的大陆面积。大多数研究的粘土非常接近风化产物，无论基岩类型如何，始终具有高δ 18 O特征，并与当地大气沃茨平衡。由于碎屑稀释作用较大，淤泥平均仅轻1.9‰。总的来说，不同气候区的散装粘土的同位素变化不大;我们将这一观察结果归因于温度对粘土-水分馏的相反影响以及温度与δ 18 O MW之间的水文关系。将测得的粘土δ 18 O和Δ 17 O值（对碎屑贡献进行了校正）数学反演为MAT和δ 18 O MW，为每个研究的流域编制，返回令人满意的估计值。在全球范围内，粘土中的三重O同位素似乎以水为主，在相应的风化温度下几乎完全受δ 18 O MW的控制，与蒸发有关的影响较小。利用河流砂、δ 18 O粉砂与碎屑比例的相关性以及不同岩石类型地表露头的估计值，我们还得到了风化作用下暴露硅酸盐结壳的+11.5 ‰估计值。全球平均，沉积物通量加权粘土δ 18 O和Δ 17 O值分别为+14.80 ‰和-0.164 ‰。这些值是显着偏向O同位素签名的东南亚和西太平洋地区，其特点是非常高的沉积物通量的海洋。使用粘土和粉砂两种粒度组分，输出到世界海洋的硅酸盐风化δ 18 O总加权签名为− 2.59‰，比之前的估计高出近50%，得出无冰世界水圈估计值为− 0.78‰。总体而言，现代河流粘土代表了大陆现代风化条件的快照，以及与水圈MAT和δ 18 O相关的一级气候特征。这意味着在地质记录捕获的页岩中测量到的δ 18 O的增加和Δ 17 O的逐步减少：在大陆出现时演变的全球水文循环，全球MAT或成岩条件的减少，以及通过水化低δ 18 O、高Δ 17 O板的俯冲而使地幔再水化而减少海洋质量。
Continental weathering is accompanied by formation of clays and other secondary minerals and their δ 18 O and Δ 17 O values should hence reflect to some extent signatures of meteoric water (δ 18 O MW) and mean annual temperatures (MAT). Our ability to extract climate information from weathered products across the geologic history relies on analytical methods tested and calibrated against modern climate conditions. We here present triple-oxygen isotope analyses of clay-size sediments from 45 rivers worldwide, as well as δ 18 O analyses of corresponding silt-and sand-rich detrital fractions, which altogether cover about 25% of the continental area that drained into the oceans, extending from the tropics to polar regions. The majority of studied clays closely approximate weathering products, always having high-δ 18 O signatures regardless of the bedrock type, and in equilibrium with local meteoric waters. Silts are only∼ 1.9‰ lighter on average due to greater detrital dilution. Overall, bulk clays from across different climatic regions do not vary much isotopically; an observation which we attribute to opposing effects of temperature on clay-water fractionation and hydrologic relationship between temperature and δ 18 O MW. Mathematical inversion of measured clay δ 18 O and Δ 17 O values (corrected for detrital contribution) into MAT and δ 18 O MW, compiled for each studied watershed, returns satisfactory estimates. Globally, triple O isotopes in clays appear to be water-dominated, being controlled almost exclusively by δ 18 O MW at respective temperature of weathering, with minor effects related to evaporation. Using sand from rivers, correlation of δ 18 O silts with detrital proportions, and estimated surface outcrop of different rock types, we additionally arrive at a+ 11.5‰ estimate for the exposed silicate crust undergoing weathering. Globally-averaged, sediment-flux weighted clay δ 18 O and Δ 17 O values are+ 14.80‰ and− 0.164‰, respectively. These values are significantly skewed toward O isotope signatures for the southeast Asia and western Pacific regions, characterized by very high sediment fluxes to the ocean. Using both clay-and silt-size fractions, the total weighted silicate weathering δ 18 O signature exported to the world's ocean is− 2.59‰, almost 50% higher the previous estimate, yielding an ice-free world hydrosphere estimate of− 0.78‰. Overall, the modern river clays represent a snapshot of modern weathering conditions on continents, and associated first-order climatic signatures related to MAT and δ 18 O of the hydrosphere. This implies that measured increase in δ 18 O and stepwise decrease in Δ 17 O in shales in the geologic record capture: evolving global hydrologic cycle upon continental emergence, decrease in global MAT or diagenetic conditions, and decreasing ocean mass via rehydrating of the mantle by subduction of hydrated low-δ 18 O, high-Δ 17 O slabs.