Reconstruction of early Cambrian ocean chemistry from Mo isotopes
Reconstruction of early Cambrian ocean chemistry from Mo isotopes
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DOI：
10.1016/j.gca.2015.05.008
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发表时间：
2015-09
影响因子：
5
通讯作者：
H. Wen;H. Fan;Yuxu Zhang;C. Cloquet;J. Carignan
中科院分区：
文献类型：
--
作者：
H. Wen;H. Fan;Yuxu Zhang;C. Cloquet;J. Carignan
新元古代—寒武纪的过渡是地球历史上一个关键的时间间隔，特别是对海洋和大气化学成分的变化。然而，关于前寒武纪-寒武纪界线的海洋化学性质，存在两种相互矛盾的观点。丰富的地球化学证据表明，在埃迪卡拉纪晚期，海洋盆地是完全含氧的，而其他研究提供了看似矛盾的证据，证明缺氧的深水，含铁条件[Fe（II）富集]一直持续到寒武纪。本文对华南早寒武世两个沉积台地和陆架斜坡剖面进行了Mo同位素示踪。结果表明，早寒武世在缺氧/缺氧条件下沉积的沉积物δ98/95Mo值为- 0.28‰~ 2.29‰，表明早寒武世海水的δ98/95Mo值可能至少为2.29‰，与现代海洋相似。在完全氧化条件下形成的粉砂岩样品中，δ98/95Mo值最重，且相对均匀，可以认为是Mn无氧化的来回迁移导致了δ98/95Mo值较重。此外，结合Fe物种数据和Mo、U的富集程度，两个研究剖面δ98/95Mo值的变化表明，两个研究剖面早期发育了一个氧化还原层状海洋，浅水完全氧化，深水以缺氧（甚至缺氧）为主，最终完全氧化。这表明当时的海洋环流发生了重组，海洋化学的这种变化可能是引发生物多样性“寒武纪大爆发”的原因。
The Neoproterozoic–Cambrian transition was a key time interval in the history of the Earth, especially for variations in oceanic and atmospheric chemical composition. However, two conflicting views exist concerning the nature of ocean chemistry across the Precambrian–Cambrian boundary. Abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late Ediacaran, while other studies provide seemingly conflicting evidence for anoxic deep waters, with ferruginous conditions [Fe(II)-enriched] persisting into the Cambrian. Here, two early Cambrian sedimentary platform and shelf-slope sections in South China were investigated to trace early Cambrian ocean chemistry from Mo isotopes. The results reveal that early Cambrian sediments deposited under oxic to anoxic/euxinic conditions have δ98/95Mo values ranging from −0.28‰ to 2.29‰, which suggests that early Cambrian seawater may have had δ98/95Mo values of at least 2.29‰, similar to modern oceans. The heaviest and relatively homogeneous δ98/95Mo values were recorded in siltstone samples formed under completely oxic conditions, which is considered that Mn oxide-free shuttling was responsible for such heavy δ98/95Mo value. Further, combined with Fe species data and the accumulation extent of Mo and U, the variation of δ98/95Mo values in the two studied sections demonstrate a redox-stratified ocean with completely oxic shallow water and predominantly anoxic (even euxinic) deeper water having developed early on, which eventually became completely oxygenated. This suggests that oceanic circulation at the time became reorganized, and such changes in oceanic chemistry may have been responsible for triggering the “Cambrian Explosion” of biological diversity.