Integrated chemostratigraphy (δ13C-δ34S-δ15N) constrains Cretaceous lacustrine anoxic events triggered by marine sulfate input
Integrated chemostratigraphy (δ13C-δ34S-δ15N) constrains Cretaceous lacustrine anoxic events triggered by marine sulfate input
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综合化学地层学（δ13C-δ34S-δ15N）限制了海洋硫酸盐输入引发的白垩纪湖泊缺氧事件
DOI：
10.1016/j.chemgeo.2020.119912
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发表时间：
2020
期刊：
影响因子：
3.9
通讯作者：
Xiaoqiang Pu
中科院分区：
文献类型：
--
作者：
Hansheng Cao;Wentong He;Fajin Chen;Xuanlong Shan;Deming Kong;Qinghua Hou;Xiaoqiang Pu
在晚白垩世时期（约1.005亿 - 6500万年前），中国东北松辽盆地（面积约为苏必利尔湖的3倍）发生了两次大规模的湖泊缺氧事件，每次持续时间均不到100万年。这些被称为湖相缺氧事件（LAEs）的事件是中国大多数高产油田主要烃源岩发育的驱动因素。然而，支撑湖相缺氧事件起始的细节仍然存在问题。为了探究这一科学问题，我们展示了松辽盆地土仑阶 - 康尼亚克阶青山口组的总有机碳同位素、黄铁矿硫同位素、氮同位素以及微量元素浓度的时间序列。当盆地达到最大扩张时，青山口组底部记录到了显著的δ34S偏移（约 - 10‰）以及黄铁矿硫含量的峰值。我们将这些变化归因于海侵，海侵会给湖泊带来高浓度的硫酸盐，从而促进微生物硫酸盐还原作用（MSR）。随后，由于MSR和黄铁矿埋藏导致的硫酸盐受限以及湖泊与海水隔离，驱动了δ34S正向偏移20‰。需要注意的是，我们所描述的湖相缺氧事件中δ34S波动的模式在大洋缺氧事件2期间的海洋黑色页岩中也被识别出来，这意味着缺氧存在一种共同的特征。碳同位素数据表明，青山口组一段中发现的最大分馏可能是湖水中[CO2]aq增加以及化能自养生物输入的结果。青山口组一段区间内沉积的δ15N增加意味着在最小含氧层中反硝化作用增强，这降低了水柱的N/P比值，从而刺激了青山口组二 - 三段的细菌固氮作用。通过对青山口组和嫩江组沉积期间湖相缺氧事件的硫、碳和氮循环进行比较和总结，我们认为湖相缺氧事件背后的驱动因素是海侵导致的硫酸盐水平升高，这可能加强了营养物质的循环，并进一步调节了湖相碳和氮循环。
During the Late Cretaceous period (~100.5–65 million years ago), two episodes of expansive lake anoxia .occurred in the Songliao Basin (~3× the size of Lake Superior), NE China, either lasting less than one million .years. These events, known as lacustrine anoxic events (LAEs), were the factors driving the development of the .main source rocks for the most productive oilfields in China. The details underpinning the initiation of the LAEs, .however, remain problematic. As a test of this scientific issue, we present time series of bulk organic carbon .isotopes, pyrite sulfur isotopes, nitrogen isotopes and trace element concentrations from the Turonian–Coniacian .Qingshankou Formation in the Songliao Basin. A notable δ34S excursion (~−10‰) and a spike in pyrite sulfur .content are recorded in the basal Qingshankou Formation when the basin reached the maximum expansion. We .attributed these changes to marine transgression, which would have brought high sulfate concentrations to the .lake, thereby promoting microbial sulfate reduction (MSR). Subsequent sulfate limited by MSR and pyrite burial .associated with isolation of the lake from the seawater drove a 20‰ positive δ34S shift. Note that the modes of .the δ34S that fluctuations we describe from the LAEs have also been recognized in marine black shale during .Oceanic Anoxic Event (OAE) 2, implying a common signature of anoxia. The carbon isotope data suggest that the .maximal fractionation found in Qingshankou Member I was likely a consequence of the increased [CO2]aq in lake .water and inputs from chemoautotrophs. Increased sedimentary δ15N during the interval of Qingshankou .Member I implies intensified denitrification in the oxygen minimum zone, which lowered the water-column N/P .ratios, thereby stimulating bacterial nitrogen fixation in Qingshankou Members II-III. By comparing and sum-.marizing the sulfur, carbon and nitrogen cycles of the LAEs during deposition of the Qingshankou and Nenjiang .formations, we argue that the forcing function underlying LAEs was enhanced sulfate levels from marine .transgression, which might have intensified nutrient recycling and further regulated the lacustrine carbon and .nitrogen cycles.