Worldwide, a growing number of modern coastal marine ecosystems are increasingly exposed to suboxic- or even anoxic conditions. Low seawater oxygen levels trigger significant ecosystem changes and may result in mass mortality of oxygen-sensitive biota. The applicability of observations from recent (anthropogenically influenced) suboxic coastal settings to fossil anoxic shallow-marine environments is, however, as yet poorly explored. The test case documented here are upper Barremian to lower Aptian strata in the Lusitanian Basin (Ericeira section, Portugal). These are characterized by the transient demise of rudist–coral communities and the rapid establishment of microencruster facies in the vacant ecological niches. The hypothesis is tested that the temporal expansion of the microencrusting organism Lithocodium aggregatum took place in response to platform-top seawater oxygen depletion. We critically discuss the outcome of a multi-proxy palaeoseawater redox approach (e.g. Rare Earth Elements (REEs), U isotopes and palaeoecology) and put the robustness of the proxies applied here to the test. This is done by considering issues with these methods in general but also emphasizing the significance of terrigenous contamination and fractionation effects. Data shown here document that evidence for coastal seawater oxygen depletion in the prelude of Oceanic Anoxic Event (OAE) 1a is lacking, and hence, anoxia was not the driving mechanism for the demise of rudist–coral ecosystems in the proto-North Atlantic platform setting studied here. In contrast, well-oxygenated early Aptian platform-top water masses are proposed for this site. Geologically short (decades to millennia) fluctuations in seawater oxygen levels cannot be excluded, however. But even if these took place, they offer no explanation for the Kyr to Myr-scale patterns discussed here. The present paper is relevant as it sheds light on the complexity of mechanisms that drive punctuated Early Cretaceous coral–rudist ecosystem turnover, and assess strengths and weaknesses of redox proxies applied to ancient shallow-marine platform carbonates.

中文翻译：

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评估沿海缺氧对早期阿普蒂安微结壳层段瞬时扩张的作用

在世界范围内，越来越多的现代沿海海洋生态系统越来越多地暴露在缺氧甚至缺氧条件下。海水含氧量低会引发显着的生态系统变化，并可能导致对氧敏感的生物群的大量死亡。然而，从最近的（人为影响的）低氧沿海环境到化石缺氧浅海环境的观察的适用性尚未得到很好的探索。此处记录的测试案例是 Lusitanian 盆地（葡萄牙 Ericeira 部分）的上 Barremian 到下 Aptian 地层。这些特征的特点是原始珊瑚群落的短暂消亡以及在空置的生态位中迅速建立微壳相。假设被检验为微结壳生物Lithocodium agregatum的时间膨胀发生在平台顶部海水缺氧的情况下。我们批判性地讨论了多代理古海水氧化还原方法（例如稀土元素 (REE)、U 同位素和古生态学）的结果，并将此处应用的代理的稳健性用于测试。这是通过总体考虑这些方法的问题，但也强调陆源污染和分馏效应的重要性来完成的。此处显示的数据表明，缺乏海洋缺氧事件 (OAE) 1a 前奏中沿海海水氧气消耗的证据，因此，缺氧并不是原始北大西洋平台环境中原始珊瑚生态系统消亡的驱动机制在这里学习。相比之下，建议为该地点提供充氧良好的早期 Aptian 平台顶部水团。然而，不能排除海水含氧量在地质上的短期（几十年到几千年）波动。但即使这些发生了，它们也无法解释此处讨论的 Kyr 到 Myr 尺度模式。本论文具有相关性，因为它阐明了驱动间断性早白垩世珊瑚礁生态系统更新的机制的复杂性，并评估了应用于古代浅海平台碳酸盐的氧化还原代理的优势和劣势。