Rare earth element and neodymium isotope tracing of sedimentary rock weathering
Rare earth element and neodymium isotope tracing of sedimentary rock weathering
复制标题
DOI：
10.1016/j.chemgeo.2020.119794
复制
发表时间：
2020-10-20
期刊：
影响因子：
3.9
通讯作者：
Tipper, Edward T.
中科院分区：
文献类型：
--
作者：
Bayon, Germain;Lambert, Thibault;Tipper, Edward T.
化学风化在封存大气CO2方面发挥着重要作用，但其在地质时间尺度上对全球气候的潜在影响仍存在争议。在某种程度上，这种不确定性来自于难以区分沉积和结晶硅酸盐岩石对地质记录中过去风化速率的各自贡献;两种类型的岩石对长期碳循环可能有不同的影响。在这项研究中，我们调查使用的稀土元素（REE）和钕同位素（Nd）在河流沉积物的浸出氧化铁馏分示踪大陆风化岩石的起源。本文提出了一种新的评价地质样品中稀土元素富集程度的指标-使用这个指数，我们表明，在河流沉积物（这里定义为AeNDFeox-Det）成对的铁氧化物和碎屑组分之间的钕（Nd）的差异直接反映了风化过程中沉积与结晶硅酸盐岩石的相对贡献。而排出古老的火山和火山地区的河流显示出接近零的三角洲（Nd）（Feox-Det））值，表明硅酸盐风化作用占主导地位（0.5 +/- 1.1; n = 30），多岩性集水区托管沉积地层产生系统较高的值（2.7 +/- 1.2; n = 44），表明沉积岩风化可以通过与碎屑组分相比具有更多放射成因Nd同位素特征的河流Fe氧化物的出现来追踪。这一假设得到了以下证据的支持：（Feox-Det）值与以往估算的大型河流流域碳酸盐和硅酸盐风化速率吻合较好。（Feox-Det）强烈依赖于低地的温度，这一关系反映了在较温暖的气候条件下硅酸盐岩石的蚀变增强和次生铁氧化物的形成。相比之下，在高海拔集水区，三角洲（Nd）（Feox-Det）定义了与最大盆地海拔高度的显著相关性，我们也将其解释为反映了随着海拔降低，由于较厚的土壤和较温暖的温度的综合影响，硅酸盐风化和相关的氧化铁形成的加剧。我们的新发现与先前的断言一致，即沉积岩的蚀变在高海拔环境中普遍存在，而硅酸盐风化在洪泛平原中占主导地位。这种结合稀土元素和钕同位素的新方法为解开地质记录中沉积和结晶硅酸盐岩石的风化信号开辟了新的视角，可用于未来的研究，以重新评估整个地球历史上山脉隆起，侵蚀和气候之间的因果关系。
Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes (epsilon(Nd)) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called 'concavity index' (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Feoxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the epsilon(Nd) difference between paired Fe-oxide and detrital fractions in river sediments (defined here as AeNd Feox-Det) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero Delta epsilon(Nd) (Feox-Det)) values indicative of dominant silicate weathering (0.5 +/- 1.1; n = 30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 +/- 1.2; n = 44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated Delta epsilon(Nd) (Feox-Det) values agree well with previous estimates for carbonate and silicate weathering rates in large river basins.Examining the influence of climate and tectonics on measured Nd isotopic compositions, we find that Delta epsilon(Nd) (Feox-Det) is strongly dependent on temperature in lowlands, following an Arrhenius-like relationship that reflects enhanced alteration of silicate rocks and formation of secondary Fe oxides in warmer climates. In contrast, in high-elevation catchments, Delta epsilon(Nd) (Feox-Det) defines striking correlation with maximum basin elevation, which we also interpret as reflecting the intensification of silicate weathering and associated Fe oxide formation as elevation decreases, due to the combined effects of thicker soils and warmer temperature.Overall, our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high-elevation environments, while silicate weathering dominates in floodplains. This novel approach combining REE and Nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record, which could be used in future studies to reassess the causal relationships between mountain uplift, erosion and climate throughout Earth's history.