The large decline in carbonate δ238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia
The large decline in carbonate δ238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia
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DOI：
10.1016/j.epsl.2023.118164
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发表时间：
2023-06
影响因子：
5.3
通讯作者：
Qinghai Zhang;L. Ding;Xinming Chen;G. Brennecka;Yali Sun;Xuesong Ma;Bintao Gao;Qian Zhang;H. Willems
中科院分区：
文献类型：
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作者：
Qinghai Zhang;L. Ding;Xinming Chen;G. Brennecka;Yali Sun;Xuesong Ma;Bintao Gao;Qian Zhang;H. Willems
碳酸盐岩中的铀同位素组成(δ238U)和Th/U作为研究全球海洋缺氧的古氧化还原指标正在被探索。然而，局部沉积环境的变化可能导致δ238U的下降和Th/U的增加的影响尚未得到彻底的研究。在西藏南部定日古新世-始新世热盛期，浅海相碳酸盐岩的δ238U下降(从+0.1Th/U增加到−0.3U)，Th/U增加。δ238U和Th/U的变化没有完全耦合。δ238U的下降与局部海平面下降相一致，而Th/U的上升主要发生在海平面上升期间。δ238U的下降可能是由于孔隙水的氧化还原条件和原生碳酸盐矿物学的变化所致。Th/U的增加归因于自生U(IV)积累的减少。我们的结果表明，局部沉积环境的变化可以导致δ238U和Th/U的行为类似于全球海洋缺氧扩大所引起的行为。这一发现呼吁在应用δ238U和Th/U作为全球古氧化还原替代物之前，应谨慎考虑局部因素。
Uranium isotope compositions (δ 238 U) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in δ 238 U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in δ 238 U (from+ 0.1 to− 0.3‰) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of δ 238 U and Th/U are not fully coupled. The decline in δ 238 U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in δ 238 U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U (IV) accumulation. Our results suggest that changes in local depositional environments can cause behavior of δ 238 U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying δ 238 U and Th/U as global paleoredox proxies.