Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2
Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2
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DOI：
10.1016/j.epsl.2022.117910
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发表时间：
2023-01
影响因子：
5.3
通讯作者：
Hongjin Chen;G. Bayon;Zhaokai Xu;Tiegang Li
中科院分区：
文献类型：
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作者：
Hongjin Chen;G. Bayon;Zhaokai Xu;Tiegang Li
摘要 海洋缺氧事件2（OAE 2；ca.∼ 94 Ma）代表了显生宙最极端的碳循环扰动之一，它与陆地和海洋领域的重大环境和气候重组同时发生。大陆硅酸盐岩石的化学风化被认为在 OAE 2 期间发挥了至关重要的作用，通过增加生物必需营养物质向海洋的释放，促进了海洋初级生产和有机碳埋藏的高速率，而且还由于其对大气 CO 2 下降的影响，这可能共同推动了 OAE 2 的终止。然而，OAE 2 期间大陆化学风化的演化仍不清楚，特别是在高纬度地区。在本研究中，我们对白垩纪晚期位于南部高纬度地区（南纬 62°）的澳大利亚西南边缘塞诺曼纪晚期到土伦纪早期沉积物的粘土尺寸碎屑部分（△ε Hfclay）进行了铪-钕同位素联合研究（国际海洋发现计划 U1516）。通过分析从海洋边缘经历海洋硅酸盐风化的产甲烷沉积物中提取的一组样品，评估了△ε Hfclay作为古代缺氧海洋沉积物中大陆化学风化的代理的可靠性，表明逆风化对铪-钕同位素组成的影响可以忽略不计。在U1516站点，OAE 2的早期阶段的特点是相对较低的△εHfclay值（−5.9±2），这是附近大陆地区化学风化减少的典型特征。在最显着的碳同位素偏移开始时，△ε Hfclay 的突然下降表明，由于澳大利亚西南部河流系统的突然重新激活，风化不良沉积物的加速输出。此时期之后，Δε Hfclay 显着向正值转变，表明在 OAE 2 间隔期间化学风化条件加剧，显示出最高的 δ 13 C 异常。基于这些结果，我们认为，水文循环的增强（很可能是由西风带南移引起的）导致 OAE 2 温暖峰值期间南部高纬度地区的耐候性大幅增加。这一发现为高纬度风化系统在驱动 OAE 2 终止中发挥的潜在作用提供了经验支持，通过风化驱动的大气 CO 2 消耗和加速的河流养分通量导致海洋沉积物中有机碳埋藏增强。
Abstract The Oceanic Anoxic Event 2 (OAE 2; ca.∼ 94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO 2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△ ε Hfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼ 62° S) during the late Cretaceous. The reliability of△ ε Hfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low△ ε Hfclay values (− 5.9±2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in△ ε Hfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced△ ε Hfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ 13 C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO 2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments.