A global survey of radiogenic strontium isotopes in river sediments
A global survey of radiogenic strontium isotopes in river sediments
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DOI：
10.1016/j.chemgeo.2020.119958
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发表时间：
2021-01-05
期刊：
影响因子：
3.9
通讯作者：
Barrat, Jean-Alix
中科院分区：
文献类型：
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作者：
Bayon, Germain;Freslon, Nicolas;Barrat, Jean-Alix
放射性成因锶同位素通常用于物源研究，但它们在沉积物中的应用往往因各种粒度和风化作用而变得复杂，这些影响可能会影响测量的Sr-87/Sr-86比率。在这里，我们报告了来自世界上最大的河流和其他河流集水区的大量沉积物样本（n = 61）的Sr同位素数据，这些河流排水于特定的地质和气候环境；在富含粘土（20℃）的情况下，长石的强烈风化作用导致非放射性成因的Sr优先结合到次生粘土矿物中，这一过程导致δ Sr-87/Sr-86（粘土-淤泥）值为负。除了气候强迫外，大小依赖的Sr同位素解耦程度也显示出取决于流域风化机制的类型，在低海拔环境（< 2000 m）中更为明显，在低海拔环境（< 2000 m）中，运输限制条件和厚土层序的存在可能与强烈的硅酸盐风化有关，而在高海拔地区（> 4000 m）则主要是动力限制的风化机制。虽然还需要进一步的研究来验证这些结论在局部风化剖面上的有效性，但这些发现表明，对不同粒径组分的组合Sr同位素分析可以作为沉积物记录中新的风化代理，理想地补充了放射性成因Sr同位素作为物源示踪剂的传统使用。最后，我们的结果还用于重新评估每年输出到海洋的悬浮沉积物的平均Sr通量和Sr-87/Sr-86组成，得出全球通量加权平均值为0.7160，与Goldstein和Jacobsen（1988）的开创性研究早期提出的结果相同。
Radiogenic strontium isotopes are routinely used in provenance studies, but their application to sediments is often complicated by various grain size and weathering effects, which can influence measured Sr-87/Sr-86 ratios. Here, we report Sr isotopic data for a large number of sediment samples (n = 61) from the world's largest rivers and other river catchments draining particular geological and climatic settings; using both clay-rich (20 degrees C, intense feldspar weathering leads to the preferential incorporation of unradiogenic Sr into secondary clay minerals; a process which results in negative Delta Sr-87/Sr-86 (Clay-Silt) values. In addition to climate forcing, the degree of size-dependent Sr isotope decoupling is also shown to be dependent on the type of weathering regime in watersheds, being more pronounced in low-elevation environments (< 2000 m), where transport-limited conditions and the presence of thick soil sequences can be associated with intense silicate weathering, than in high mountain regions (> 4000 m) dominated by kinetically-limited weathering regimes.While further studies will be required to test the validity of these conclusions at the local scale of weathering profiles, these findings suggest that combined Sr isotopic analyses of separate size fractions could be used as a new weathering proxy in sediment records, ideally complementing the conventional use of radiogenic Sr isotopes as provenance tracers. Finally, our results are also used to re-assess the mean Sr flux and Sr-87/Sr-86 composition of the suspended sediment exported to the ocean yearly, yielding a global flux-weighted average of 0.7160, identical to that proposed earlier in the seminal study of Goldstein and Jacobsen (1988).