Thallium isotope evidence for a permanent increase in marine organic carbon export in the early Eocene
Thallium isotope evidence for a permanent increase in marine organic carbon export in the early Eocene
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DOI：
10.1016/j.epsl.2008.12.010
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发表时间：
2009-02
影响因子：
5.3
通讯作者：
S. Nielsen;Sarah Mar-Gerrison;A. Gannoun;D. LaRowe;V. Klemm;A. Halliday;K. Burton;J. Hein
中科院分区：
文献类型：
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作者：
S. Nielsen;Sarah Mar-Gerrison;A. Gannoun;D. LaRowe;V. Klemm;A. Halliday;K. Burton;J. Hein
本文首次在太平洋的CD29和D11两个锰铁地壳（Fe-Mn地壳）中记录了高分辨率铊同位素。地壳在205tl /203Tl中记录了明显但系统的变化，不太可能反映成岩叠印或海水与铁锰地壳之间同位素分异的变化。随着时间的推移，铁锰地壳似乎更有可能追踪海水的Tl同位素组成。现今Tl在海洋中的停留时间估计约为2万年，因此其同位素组成应反映整个海洋的事件。利用新的和已发表的Os同位素数据构建了这些地壳的年龄模型，这些模型彼此一致，但与以前的年龄模型有很大不同。这些年龄模型的应用表明，海水的Tl同位素组成在~55 ~ ~45 Ma之间发生了系统的变化。用一个简单的箱形模型表明，目前海水的Tl同位素组成几乎完全取决于海洋Tl的两种主要输出通量之间的比率。这两种通量是自生铁锰氢氧沉淀从海水中清除Tl的速率和海洋地壳在低温蚀变过程中吸收Tl的速率。后者发生巨大变化的可能性极小。因此，假设海洋Tl收支在新生代期间也没有显著变化，古新世期间低205tl /203Tl最好的解释是Fe-Mn氧氢氧化物对Tl的吸收比现在高4倍以上。计算得到的新生代Tl同位素海水曲线与S的曲线具有惊人的相似性，这表明这两个体系可能对海洋环境的相同变化作出了反应。一个合理的解释是，从~55 Ma到~45 Ma，有机碳输出显著而永久地增加，这导致了更高的黄铁矿埋藏率和铁锰氧化物去除通量的显著降低，这是铁锰生物吸收增加的结果。
The first high resolution thallium (Tl) isotope records in two ferromanganese crusts (Fe–Mn crusts), CD29 and D11 from the Pacific Ocean are presented. The crusts record pronounced but systematic changes in205Tl/203Tl that are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and Fe–Mn crusts. It appears more likely that the Fe–Mn crusts track the Tl isotope composition of seawater over time. The present-day oceanic residence time of Tl is estimated to be about 20,000 yr, such that the isotopic composition should reflect ocean-wide events. New and published Os isotope data are used to construct age models for these crusts that are consistent with each other and significantly different from previous age models. Application of these age models reveals that the Tl isotope composition of seawater changed systematically between ~55 Ma and ~45 Ma. Using a simple box model it is shown that the present day Tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine Tl. These fluxes are the rate of removal of Tl from seawater via scavenging by authigenic Fe–Mn oxyhydroxide precipitation and the uptake rate of Tl during low temperature alteration of oceanic crust. It is highly unlikely that the latter has changed greatly. Therefore, assuming that the marine Tl budget has also not changed significantly during the Cenozoic, the low205Tl/203Tl during the Paleocene is best explained by a more than four-fold higher sequestration of Tl by Fe–Mn oxyhydroxides compared with at the present day. The calculated Cenozoic Tl isotopic seawater curve displays a striking similarity to that of S, providing evidence that both systems may have responded to the same change in the marine environment. A plausible explanation is a marked and permanent increase in organic carbon export from ~55 Ma to ~45 Ma, which led to higher pyrite burial rates and a significantly reduced flux of Fe–Mn oxide removal as a result of increased biological uptake of Fe and Mn.