Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes
Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes
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DOI：
10.1130/g30652.1
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发表时间：
2010-04-01
期刊：
影响因子：
5.8
通讯作者：
Arz, Helge W.
中科院分区：
文献类型：
--
作者：
Montoya-Pino, Carolina;Weyer, Stefan;Arz, Helge W.
在中生代温室世界期间，海洋经历了几次海洋缺氧事件（oae），其特征是富含有机物的黑色页岩沉积的间隔，表明海洋领域存在强烈的氧气消耗。Cenomanian-Turonian OAE2（约93 Ma）是白垩纪最突出的事件之一，对全球碳循环产生了重大干扰。虽然OAE2可能达到全球规模，但该OAE期间海水缺氧的空间范围约束较差。研究表明，新发现的古氧化还原指标U-238/U-235同位素比值（δ U-238）的变化可用于量化海洋缺氧程度。对于中白垩纪OAE2的黑色页岩，我们发现与来自黑海的现代等量富有机质沉积物相比，它们有系统地向较轻的三角洲U-238和较低的U浓度转变。这一转变转化为OAE2期间全球海洋缺氧程度的增加，与今天或OAE2前后的时期相比，至少增加了三倍。与现代黑海沉积物相比，整个OAE2中黑色页岩的U浓度和同位素组成的恒定偏移表明，在OAE2开始之前，海洋缺氧条件已经增强。
During the Mesozoic greenhouse world, the oceans underwent several oceanic anoxic events (OAEs) characterized by intervals during which organic-rich black shales were deposited, indicating strong oxygen depletion in the marine realm. The Cenomanian-Turonian OAE2 (ca. 93 Ma) represents one of the most prominent events of the Cretaceous, with significant perturbations of the global carbon cycle. Although OAE2 likely reached a global scale, the spatial extent of seawater anoxia during this OAE is poorly constrained. Here we demonstrate that variations in the U-238/U-235 isotope ratio (delta U-238), a newly developed paleoredox proxy, can be used to quantify the extent of marine anoxia. For black shales from the mid-Cretaceous OAE2 we find a systematic shift toward lighter delta U-238 and lower U concentrations as compared to modern equivalent organic-rich sediments from the Black Sea. This shift translates to a global increase of oceanic anoxia during OAE2 by at least a factor of three as compared to the present day or to periods before and after OAE2. The constant offset in U concentrations and isotope compositions of black shales throughout OAE2 compared to modern Black Sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of OAE2.