Abstract. The California Current system (CCS) hosts one of the largest oxygen minimum zones (OMZs) in the world: the eastern North Pacific (ENP) OMZ, which is dissociated into subtropical and tropical regions (i.e. the ESTNP and ETNP). In the modern ENP OMZ, bioavailable nitrogen (N) is lost via denitrification and anaerobic ammonium oxidation (anammox). Even so, paleo-reconstructions of N loss have focused solely on denitrification. Fluctuations in bulk sedimentary δ15N over glacial–interglacial cycles have been interpreted to reflect variations in denitrification rates in response to ETNP OMZ intensity changes. This δ15N signal is thought to be transported northwards to the ESTNP OMZ. Here, we present the first CCS sedimentary record of ladderane lipids, biomarkers for anammox, located within the ESTNP OMZ (32° N, 118° W). Over the last two glacial terminations (∼ 160 kyr cal BP), ladderane concentrations were analysed in combination with the index of ladderanes with five cyclobutane moieties (NL5), short-chain (SC) ladderane degradation products, and productivity proxies. This shows that (1) ladderanes were derived from anammox bacteria living within the ESTNP OMZ water column; (2) ladderanes were continuously present, with relatively high concentrations during both glacial and interglacial periods, showcasing that the ESTNP OMZ must have retained an anoxic core in which N loss occurred; and (3) anammox abundance appears to have been driven by both organic matter (OM) remineralization and advection changes, which regulated nutrient and oxygen levels. Our study shows that anammox was an important feature in the CCS, and it provides a more holistic picture of N-loss dynamics and the development of the ESTNP OMZ over glacial–interglacial cycles. Lastly, ladderanes and their SC products were also detected in 160–500 kyr cal BP sediments (15.7–37.5 m b.s.f., metres below sea floor; analysed at a low temporal resolution), highlighting their potential as anammox biomarkers in relatively deeper buried sediments for future studies.

中文翻译：

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第四纪晚期加州洋流系统中通过厌氧氨氧化（anammox）造成的氮损失

摘要。加州洋流系统 (CCS) 拥有世界上最大的最低氧气区 (OMZ) 之一：北太平洋东部 (ENP) OMZ，该 OMZ 分为亚热带和热带区域（即 ESTNP 和 ETNP）。在现代 ENP OMZ 中，生物可利用氮 (N) 通过反硝化和厌氧氨氧化 (anammox) 损失。即便如此，氮损失的古重建仅集中于反硝化作用。冰期-间冰期循环期间沉积物 δ15N 的波动被解释为反映了反硝化速率随 ETNP OMZ 强度变化的变化。该 δ15N 信号被认为向北传输至 ESTNP OMZ。在这里，我们展示了位于 ESTNP OMZ（北纬 32°，西经 118°）内第一个梯烷脂质（厌氧氨氧化生物标志物）的 CCS 沉积记录。在最后两次冰川终止（~ 160 kyr cal BP）中，结合具有五个环丁烷部分的梯烷指数（NL5）、短链（SC）梯烷降解产物和生产力指标，对梯烷浓度进行了分析。这表明 (1) 梯烷源自生活在 ESTNP OMZ 水柱内的厌氧氨氧化细菌； （2）梯烷持续存在，在冰期和间冰期均具有相对较高的浓度，表明ESTNP OMZ一定保留了发生氮损失的缺氧核心； （3）厌氧氨氧化丰度似乎是由有机物（OM）再矿化和平流变化驱动的，这调节了营养物和氧气水平。我们的研究表明，厌氧氨氧化是 CCS 的一个重要特征，它提供了更全面的 N 损失动态和 ESTNP OMZ 在冰期-间冰期循环中的发展情况。最后，在 160-500 kyr cal BP 沉积物（15.7-37.5 m bsf，海底以下米；以低时间分辨率进行分析）中也检测到了梯烷及其 SC 产物，突出了它们在相对较深的埋藏沉积物中作为厌氧氨氧化生物标志物的潜力。未来的研究。