Ocean redox conditions between the snowballs – Geochemical constraints from Arena Formation, East Greenland
Ocean redox conditions between the snowballs – Geochemical constraints from Arena Formation, East Greenland
复制标题
DOI：
10.1016/j.precamres.2017.12.009
复制
发表时间：
2017-12
影响因子：
3.8
通讯作者：
E. Scheller;A. Dickson;D. Canfield;C. Korte;K. Kristiansen;T. Dahl
中科院分区：
文献类型：
--
作者：
E. Scheller;A. Dickson;D. Canfield;C. Korte;K. Kristiansen;T. Dahl
动物生态系统的出现在很大程度上被认为是在斯图特冰期和马里诺冰期结束后，在含氧量增加的海洋中出现的。这种转变导致了对驱动海洋氧合和动物进化机制的几种假设。一种假说认为，风化作用的增强增加了海洋的营养水平、初级生产力和有机碳埋藏，并最终使大气和海洋含氧。另一种假说认为，由动物驱动的海洋生物地球化学循环重组可能为海洋提供了氧气。通过对格陵兰岛东部Arena Fm的钼（Mo）、碳(C)、硫(S)同位素和铁（Fe）物种形成结果的分析，研究了冰川期、Sturtian消冰期后和动物大辐射期前的海洋氧化还原条件。碳和硫同位素地层学用于将Arena Fm与世界各地在Sturtian和Marinoan冰期（~ 720-635 Ma）之间的其他地层联系起来。Arena Fm下部（~ 25 m）由局部缺氧条件下沉积的黑色页岩组成，高比例的高活性铁（FeHR/FeT> 0.38）和黄铁矿（FePY/FeHR> 0.7）证明了这一点。与上覆页岩和显生宙含氧沉积层相比，这些黑色页岩显示出较小的Mo富集（<3 ppm）和较低的Mo/TOC。δ98Mo值在基底区Fm最大（1.5‰）。许多样品的δ98Mo低于典型的海洋输入通量，这可以用δ98Mo ~ 1.3‰的海相Mo池的Mo同位素分馏来解释，这与其他低温富氧盆地的推断相似。低[Mo]和δ98Mo的组合表明，这一时期海洋中普遍存在缺氧。我们的数据与这一时期的大多数其他研究一致，表明海洋氧化与雪球地球脱冰无关，而是延迟到动物有效进入现场。
The emergence of animal ecosystems is largely believed to have occurred in increasingly oxygenated oceans after the termination of the Sturtian and Marinoan glaciations. This transition has led to several hypotheses for the mechanism driving ocean oxygenation and animal evolution. One hypothesis is that enhanced weathering increased oceanic nutrient levels, primary productivity and organic carbon burial, and ultimately oxygenated the atmosphere and oceans. Another hypothesis suggests that an animal-driven reorganization of the marine biogeochemical cycles might have oxygenated the oceans. Through molybdenum (Mo), carbon (C), sulfur (S) isotopes and iron (Fe) speciation results from the Arena Fm, East Greenland, this study constrains ocean redox conditions during the Cryogenian, after the Sturtian deglaciation and before the major radiation of animals. Carbon and sulfur isotope stratigraphy is used to correlate the Arena Fm with other formations worldwide between the Sturtian and Marinoan glaciations (∼720–635 Ma). The lower part of the Arena Fm (∼25 m) consists of black shales deposited under locally euxinic conditions as evidenced by high proportions of highly reactive iron (FeHR/FeT> 0.38) and pyrite (FePY/FeHR> 0.7). These black shales display small Mo enrichments (<3 ppm) and low Mo/TOC compared to overlying shales and Phanerozoic euxinic sediments. The maximum δ98Mo value is observed in the basal Arena Fm (1.5‰). Many samples display lower δ98Mo than typical oceanic input fluxes, which can be explained by Mo isotope fractionation from a marine Mo pool with δ98Mo ∼ 1.3‰, similar to that inferred from other Cryogenic euxinic basins. The combination of low [Mo] and δ98Mo suggests that widespread anoxia prevailed in the oceans at this time. Our data are consistent with most other studies from this time suggesting that ocean oxygenation was not linked to Snowball Earth deglaciation, but was delayed until animals effectively entered the scene.