Abstract One of the outstanding issues in the study of nutrient dynamics during Oceanic Anoxic Events (OAEs) is the preservation of 15N-depleted nitrogen isotope signals, which are not observed in analogous modern environments. The favored mechanism for δ 15 N values below 0‰ is utilization of excess ammonium derived from underlying anoxic waters by primary producers. However, there are few modern examples available for testing this hypothesis, due to the overall well-oxygenated state of modern oceans. Here, we present sedimentary δ 13 Corg and δ 15 Nbulk data, as well as pigment biomarker ratios, from the sediments of Lake Kivu, a meromictic lake in East Africa. This system serves as a potential analogue for the development of strongly depleted δ 15 Nbulk in OAE black shales. We focus on intervals of organic carbon enrichment (sapropels) that are interpreted to have been the result of lake water column mixing events. Sediment δ 13 Corg and δ 15 Nbulk decrease by up to 6 and 4‰ respectively near the base of sapropel layers. These negative excursions provide evidence for the mixing of 13C-depleted dissolved inorganic carbon and ammonium in excess of photosynthetic need, derived from the deeper anoxic basin of the lake. Support of this model is also provided by photosynthetic pigment biomarkers which show significant fluctuations in phototrophic ecology consistent with water column mixing and changes in nutrient status. Further evidence for the shallowing of the chemocline during sapropel deposition is found in the presence of bacteriochlorophyll derivatives produced by phototrophic sulfide oxidizers.

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基伍湖趋化层坍塌作为海洋缺氧事件期间氮循环的类似物

摘要 海洋缺氧事件 (OAE) 期间养分动态研究的突出问题之一是保存 15N 耗尽的氮同位素信号，这在类似的现代环境中未观察到。δ 15 N 值低于 0‰ 的有利机制是初级生产者利用来自潜在缺氧水域的过量铵。然而，由于现代海洋的整体充氧状态，很少有现代例子可用于测试这一假设。在这里，我们展示了来自东非分界湖基伍湖沉积物的沉积 δ 13 Corg 和 δ 15 Nbulk 数据，以及色素生物标志物比率。该系统可作为在 OAE 黑色页岩中开发强烈消耗的 δ 15 Nbulk 的潜在类似物。我们专注于被解释为湖水柱混合事件结果的有机碳富集区间（腐泥）。沉积物 δ 13 Corg 和 δ 15 Nbulk 在腐泥层底部附近分别减少了 6 和 4‰。这些负偏移为 13C 耗尽的溶解无机碳和铵的混合提供了证据，这些碳和铵超过了光合作用的需要，来自更深的湖泊缺氧盆地。光合色素生物标志物也提供了对该模型的支持，这些生物标志物显示出与水柱混合和营养状况变化一致的光养生态学的显着波动。在存在光养硫化物氧化剂产生的细菌叶绿素衍生物的情况下，发现腐泥沉积过程中趋化层变浅的进一步证据。