Thallium isotopes in early diagenetic pyrite – a paleoredox proxy?
Thallium isotopes in early diagenetic pyrite – a paleoredox proxy?
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DOI：
10.1016/j.gca.2011.07.047
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发表时间：
2011-11
影响因子：
5
通讯作者：
S. Nielsen;M. Goff;S. Hesselbo;H. Jenkyns;D. LaRowe;Cin-Ty A. Lee
中科院分区：
文献类型：
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作者：
S. Nielsen;M. Goff;S. Hesselbo;H. Jenkyns;D. LaRowe;Cin-Ty A. Lee
本文首次对早期成岩黄铁矿进行了铊同位素研究。本文比较了两个剖面在Toarcian海洋缺氧事件（T-OAE，183 Ma）期间的测量结果与ODP第165和167航次晚第三纪晚期（<10 Ma）沉积于相对好氧海洋环境中的黄铁矿样品的测量结果。晚第三纪黄铁矿的铊同位素组成都明显比海水重，这最有可能表明成岩黄铁矿中的铊部分来源于已知显示相对重的铊同位素特征的铁锰氧氢氧化物。来自葡萄牙佩尼切的T-OAE剖面之一显示出与晚第三纪样品难以区分的黄铁矿铊同位素组成，而来自英国约克郡的样品则缺乏铊的重同位素。这些较轻的组合物是最好的解释缺乏铁锰沉淀在沉积物-水界面由于硫化物（富氧）的条件被认为是普遍存在于克利夫兰盆地的约克郡部分沉积。佩尼切样品中较重的特征似乎是由一个氧化水柱造成的，该水柱使铁锰氧氢氧化物在沉积物-水界面处沉淀。从约克郡的铊同位素配置文件也与以前公布的钼同位素比值确定相同的沉积序列进行了比较。有一个建议，这两个同位素系统之间的反相关性，这是一致的预期发生在海水中的同位素变化时，海洋氧化（铁锰矿物）通量波动。这里概述的结果代表的第一个证据表明，铊同位素在早期成岩黄铁矿有可能揭示过去的海洋氧化在局部范围内的变化，也可能为全球海洋。然而，更多的信息在不同的海洋环境中，特别是在缺氧/缺氧盆地的铊同位素，需要之前，铊同位素可以放心地用作古氧化还原示踪剂。
This paper presents the first study of Tl isotopes in early diagenetic pyrite. Measurements from two sections deposited during the Toarcian Ocean Anoxic Event (T-OAE, ∼183Ma) are compared with data from Late Neogene (<10Ma) pyrite samples from ODP legs 165 and 167 that were deposited in relatively oxic marine environments. The Tl isotope compositions of Late Neogene pyrites are all significantly heavier than seawater, which most likely indicates that Tl in diagenetic pyrite is partially sourced from ferromanganese oxy-hydroxides that are known to display relatively heavy Tl isotope signatures. One of the T-OAE sections from Peniche in Portugal displays pyrite thallium isotope compositions indistinguishable from Late Neogene samples, whereas samples from Yorkshire in the UK are depleted in the heavy isotope of Tl. These lighter compositions are best explained by the lack of ferromanganese precipitation at the sediment–water interface due to the sulfidic (euxinic) conditions thought to be prevalent in the Cleveland Basin where the Yorkshire section was deposited. The heavier signatures in the Peniche samples appear to result from an oxic water column that enabled precipitation of ferromanganese oxy-hydroxides at the sediment–water interface. The Tl isotope profile from Yorkshire is also compared with previously published molybdenum isotope ratios determined on the same sedimentary succession. There is a suggestion of an anti-correlation between these two isotope systems, which is consistent with the expected isotope shifts that occur in seawater when marine oxic (ferromanganese minerals) fluxes fluctuate. The results outlined here represent the first evidence that Tl isotopes in early diagenetic pyrite have potential to reveal variations in past ocean oxygenation on a local scale and potentially also for global oceans. However, much more information about Tl isotopes in different marine environments, especially in anoxic/euxinic basins, is needed before Tl isotopes can be confidently utilized as a paleo-redox tracer.