Continuously increasing δ98Mo values in Neoarchean black shales and iron formations from the Hamersley Basin
Continuously increasing δ98Mo values in Neoarchean black shales and iron formations from the Hamersley Basin
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DOI：
10.1016/j.gca.2015.05.009
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发表时间：
2015-09
影响因子：
5
通讯作者：
F. Kurzweil;M. Wille;R. Schoenberg;H. Taubald;M. V. Kranendonk
中科院分区：
文献类型：
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作者：
F. Kurzweil;M. Wille;R. Schoenberg;H. Taubald;M. V. Kranendonk
我们给出了来自澳大利亚西部哈默斯利群的黑色页岩、碳酸盐和氧化相铁地层的Mo、C和o同位素数据，这些地层的年龄范围从26亿年到25亿年不等。数据显示，从最古老的Marra Mamba和Wittenoom组的地壳δ98Mo值约为0.50‰，到最年轻的Brockman铁组样品的δ98Mo值高达1.51‰。因此，碳酸盐岩相铁组和黑色页岩均表现出δ98Mo值增大的趋势。考虑到Mo浓度与总有机碳之间的正相关关系，我们认为这种均匀性最好的解释是钼酸盐在碳酸盐铁层中的吸附和硫钼酸盐在黑色页岩中对硫化有机质的清除。在这两个Mo去除过程中，海水δ98Mo在2.6 ~ 2.5 Ga期间出现了时间上的增加。与近同时期黑色页岩相比，氧化相铁组Mo浓度最低，总有机碳最低，δ98Mo略低。这可能表明，在有机质埋藏率非常低的铁地层环境中，轻Mo同位素在铁（氧）氧化物上的优先吸附变得更加相关。此前在南非Griqualand West盆地同时期的黑色页岩和碳酸盐岩中也发现了类似的mo同位素模式。25.4亿年后δ98Mo持续增加，表明在Hamersley盆地和Griqualand West盆地不同沉积背景下，海水钼酸盐的分布更为均匀，同位素组成也较为一致。海洋Mo储量与Mo流入和流出量的关系模型表明，同位素重海水Mo储层的长期积累需要同位素轻Mo的沉积汇。寻找这种汇（即在含氧良好的表层海洋和/或陆地环境中吸附到mn -氧化物上，或在弱硫化物环境中不完全硫钼酸盐形成）仍存在争议。但它的相关性在接近大氧化事件时变得更加重要，甚至可能与2.5 Ga之前已经弱氧化的条件有关。
We present Mo-, C- and O-isotope data from black shales, carbonate- and oxide facies iron formations from the Hamersley Group, Western Australia, that range in age from 2.6 to 2.5 billion years. The data show a continuous increase from near crustal δ98Mo values of around 0.50‰ for the oldest Marra Mamba and Wittenoom formations towards higher values of up to 1.51‰ for the youngest sample of the Brockman Iron Formation. Thereby, the trend in increasing δ98Mo values is portrayed by both carbonate facies iron formations and black shales. Considering the positive correlation between Mo concentration and total organic carbon, we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales. A temporal increase in the seawater δ98Mo over the period 2.6–2.5 Ga is observed assuming an overall low Mo isotope fractionation during both Mo removal processes. Oxide facies iron formations show lowest Mo concentrations, lowest total organic carbon and slightly lower δ98Mo compared to nearly contemporaneous black shales. This may indicate that in iron formation settings with very low organic matter burial rates, the preferential adsorption of light Mo isotopes onto Fe-(oxyhydr)oxides becomes more relevant.A similar Mo-isotope pattern was previously found in contemporaneous black shales and carbonates of the Griqualand West Basin, South Africa. The consistent and concomitant increase in δ98Mo after 2.54 billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the Hamersley Basin and the Griqualand West Basin. The modeling of the oceanic Mo inventory in relation to the Mo in- and outflux suggests that the long-term build-up of an isotopically heavy seawater Mo reservoir requires a sedimentary sink for isotopically light Mo. The search for this sink (i.e. adsorption onto Mn-oxides in well oxygenated surface oceans and/or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings) remains debated, but its relevance becomes more important closer to the Great Oxidation Event and is probably related to already weakly oxidizing conditions even prior to the 2.5 Ga “whiff of oxygen”.