Isotopic evidence for a sizeable seawater sulfate reservoir at 2.1 Ga
Isotopic evidence for a sizeable seawater sulfate reservoir at 2.1 Ga
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DOI：
10.1016/j.precamres.2011.10.013
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发表时间：
2012
影响因子：
3.8
通讯作者：
M. Reuschel;V. Melezhik;M. Whitehouse;A. Lepland;A. Fallick;H. Strauss
中科院分区：
文献类型：
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作者：
M. Reuschel;V. Melezhik;M. Whitehouse;A. Lepland;A. Fallick;H. Strauss
Tulomozero组，沉积在芬诺斯坎迪亚地盾，由浅海相碳酸盐岩和硅质岩，溶解角砾岩和广泛的同沉积海相碳酸盐岩，现在主要是作为假像后钙硫酸盐。本研究调查了三个不同代理中的硫酸盐，以限制海水硫酸盐的硫同位素组成，承认存在不同类型的硫酸盐（碳酸盐相关硫酸盐（CAS），角砾岩相关硫酸盐（BAS）和硫酸盐残余物（RS））在Tulomozero形成的各种岩性。广泛使用和接受的环境海水硫酸盐硫同位素组成的替代物是从整个Tulomozero组的碳酸盐样品中提取的碳酸盐相关硫酸盐。此外，硫酸盐提取自硅质溶角砾岩，这是建议后形成的硫酸盐岩溶解。对两种岩性的硫酸盐硫同位素组成进行了非原位测定（δ 34 SCAS =10.9±2.7‰，δ 34 SBAS =9.0±1.1‰）。Tulomozero组中普遍存在钙硫酸盐后的假晶，含有μ m大小的硬石膏和重晶石残余物。用二次离子质谱（西姆斯）原位测定了它们的硫同位素组成（δ 34硬石膏=9.6±1.0‰; δ 34辉锑矿=11.0±3.1‰）。碳酸盐岩、角砾岩和硫酸盐残体的硫同位素数据与当时海水硫酸盐的δ 34 S记录相匹配，没有显示出明显的地层变化，表明海水硫酸盐硫同位素组成在Tulomozero组沉积期间不受环境变化的影响。广泛沉淀硫酸盐（而不是丰富的氯化物）的物理证据表明，现代风格的蒸发岩序列，碳酸盐，硫酸盐，然后氯化物。然而，这需要环境海水中的最低硫酸盐浓度为2.5mM。因此，在Tulomozero组中持续的碳酸盐硫酸盐沉积的证据意味着在2.1Ga存在大量的海水硫酸盐储层。根据地层碳酸盐岩厚度估算的最短沉积时间，海水硫酸盐的停留时间预计至少为5 Ma。现有的数据支持在中古元古代海洋中存在相当大的海水硫酸盐储层，事实上超过现代硫酸盐储层的10%。
The 2.1Ga Tulomozero Formation, deposited on the Fennoscandian Shield, consists of shallow marine carbonate and siliciclastic rocks, dissolution breccias and widespread syndepositional marine evaporites, now mostly present as pseudomorphs after Ca-sulfates. This study investigates sulfates hosted in three different proxies to constrain the sulfur isotopic composition of seawater sulfate, acknowledging the presence of different types of sulfates (carbonate-associated sulfate (CAS), breccia-associated sulfate (BAS) and sulfate relicts (RS)) in a variety of lithologies of the Tulomozero Formation. A widely used and accepted proxy for the ambient seawater sulfate sulfur isotopic composition is carbonate-associated sulfate which was extracted from carbonate samples across the entire Tulomozero Formation. In addition, sulfate was extracted from a siliciclastic dissolution breccia, which is suggested to have formed after dissolution of sulfate evaporites. The sulfate sulfur isotopic composition from both lithologies was measured ex-situ (δ34SCAS=10.9±2.7‰, δ34SBAS=9.0±1.1‰). Ubiquitous pseudomorphs after Ca-sulfates occur in the Tulomozero Formation and contain μm-size anhydrite and barite relicts. Their sulfur isotopic composition was measured in situ by secondary ion mass spectroscopy (SIMS) (δ34Sanhydrite=9.6±1.0‰; δ34Sbarite=11.0±3.1‰). Sulfur isotope data from carbonate rocks, breccias and the sulfate relicts match the existing δ34S record of seawater sulfate during that time and show no distinct stratigraphic variations, indicating that the seawater sulfate sulfur isotopic composition remained unaffected from environmental changes during the time of deposition of the Tulomozero Formation. Physical evidence for widespread precipitation of sulfate (rather than abundant chloride) suggests a modern-style evaporite sequence with carbonates followed by sulfate and then chlorides. However, this requires a minimum sulfate concentration of 2.5mM in the ambient seawater. Consequently, the evidence for persistent evaporitic sulfate deposition in the Tulomozero Formation implies a substantial seawater sulfate reservoir at 2.1Ga. According to the estimated minimum depositional time from the thickness of the carbonate rocks of the formation, the residence time of seawater sulfate is expected to have been at least 5Ma. The available data support the existence of a sizeable seawater sulfate reservoir in the Mid-Paleoproterozoic ocean, in fact more than 10% of the modern sulfate reservoir.