The widespread expansion of the oxygen minimum zone onto shelves has been commonly regarded as a primary cause of benthos extinction in epicratonic sea ecosystems during the Cenomanian–Turonian boundary event (CTBE). However, neither lithology, geochemical proxies, nor micropaleontological data support this hypothesis. Instead, our integrated foraminiferal and dinoflagellate cyst study, corroborated by δ¹³C_org and δ¹⁵N_org data, indicate that the biota were impacted by an abrupt shift to well oxygenated oligotrophic conditions and a collapse of primary productivity in the epicontinental Central European Basin. Because the event was concurrent with the development of extensive and extreme oceanic bottom water anoxia that reached the photic zone in oceanic settings, we infer that the biotic crisis in the shelf seas during Oceanic Anoxic Event 2 (OAE2), and possibly during other OAEs, was triggered by this anomalous nutrient cycling in Earth's oceans. This phenomenon was presumably associated with intensive denitrification combined with anammox activity in the deep “ammonium oceans,” which caused a significant loss of biologically reactive nitrogen from the ocean system. Impingement of ammonium-rich anoxic waters on the photic zone resulted in primary productivity based primarily on ammonium assimilation, as recorded by strongly ¹⁵N-depleted organic matter deposited in the oceans during the CTBE. We propose that, unlike in the oceanic settings, productivity in the well-oxygenated, oligotrophic epicontinental seas was nitrate-based, as evidenced by strongly ¹⁵N-enriched organic matter deposited in the contemporaneous epicontinental sea. These very high δ¹⁵N_org values (>+5‰) were related to the spreading of shallow oceanic waters carrying ¹⁵N-enriched nitrate onto epicontinental settings.

中文翻译：

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海洋缺氧事件 2 期间大陆架上贫营养的生物和同位素遗迹

在 Cenomanian-Turonian 边界事件 (CTBE) 期间，氧最低区广泛扩展到大陆架上通常被认为是导致外周海洋生态系统底栖生物灭绝的主要原因。然而，岩性、地球化学指标和微古生物学数据均不支持这一假设。相反，我们的综合有孔虫和甲藻囊肿研究得到了 δ ¹³ C_组织和 δ ¹⁵ N_组织的证实数据表明，生物群受到中欧大陆表陆盆地突然转变为氧充足的贫营养条件和初级生产力崩溃的影响。由于该事件与大洋环境中达到透光区的广泛和极端的大洋底水缺氧的发展同时发生，我们推断在大洋缺氧事件 2 (OAE2) 期间以及可能在其他 OAE 期间，大陆架海域的生物危机，是由地球海洋中这种异常的营养循环引发的。这种现象可能与深部“铵海洋”中强烈的反硝化作用和厌氧氨氧化活性有关，导致海洋系统中生物活性氮的大量流失。CTBE 期间沉积在海洋中的¹⁵种贫氮有机物。我们提出，与海洋环境不同，氧充足、贫营养的陆上海的生产力是基于硝酸盐的，这一点可以通过沉积在同期陆上海中的富含¹⁵ N 的有机物质来证明。这些非常高的 δ ¹⁵ N _org值 (>+5‰) 与携带富含¹⁵ N 的硝酸盐的浅海水域扩散到陆缘环境有关。