The Paleocene-Eocene Thermal Maximum (PETM, ∼56 Ma) presents a past analog for future global warming. Previous studies provided evidence for major loss of dissolved oxygen during the PETM, although understanding the degree and distribution of oxygen loss poses challenges. Magnetofossils produced by magnetotactic bacteria are sensitive to redox conditions in sediments and water columns, and have been used to reconstruct paleoredox conditions over a range of geological settings. Here, we present records of well-preserved magnetofossils from cores along Walvis Ridge in the Southeast Atlantic that span the PETM across a depth transect (∼1,500–3,600 m paleodepth). Hysteresis, isothermal remanent magnetization curves, first-order reversal curve diagrams, and low-temperature magnetic measurements document large variations in magnetic properties of magnetofossils, which relate to time and water depth. Abundant magnetofossil grains are present within the studied sediments, and their morphologies change with paleodepth, as shown by transmission electron microscope observations. Magnetofossils from samples within the PETM onset at the deeper sites (∼2,600–3,600 m paleodepth) have lower coercivity values, a higher oxidation degree, and smaller grain sizes than those from shallower sites (∼1,500–1,800 m paleodepth), likely reflecting changes in paleoredox conditions at different paleodepths. We use the magnetofossil records to reconstruct relative changes in dissolved oxygen content at different water depths through the PETM, and suggest that ocean deoxygenation likely expanded downwards in the early stages of the PETM. We thus demonstrate the value of magnetofossil records for paleoenvironmental reconstructions over time and space, particularly for sediments that lack carbonate fossils.

中文翻译：

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古新世 - 始新世热最大值期间海洋脱氧的深度横断面：来自东南大西洋的沉积物核心中的磁化石

古新世-始新世热最大值（PETM，~56 Ma）为未来全球变暖提供了过去的模拟。先前的研究为 PETM 期间溶解氧的主要损失提供了证据，尽管了解氧损失的程度和分布带来了挑战。趋磁细菌产生的磁化石对沉积物和水柱中的氧化还原条件很敏感，并已被用于在一系列地质环境中重建古氧化还原条件。在这里，我们展示了保存完好的磁化石记录，这些磁化石来自东南大西洋沃尔维斯海岭沿线的岩心，这些岩心跨越了 PETM 的深度横断面（~1,500-3,600 m 古深度）。磁滞、等温剩磁曲线、一阶反转曲线图、和低温磁测量记录了磁化石的磁特性的巨大变化，这些变化与时间和水深有关。透射电镜观察表明，研究沉积物中存在丰富的磁化石颗粒，其形态随古深度而变化。来自更深地点（~2,600-3,600 m 古深度）的 PETM 起始样本中的磁化石比来自较浅地点（~1,500-1,800 m 古深度）的样本具有更低的矫顽力值、更高的氧化程度和更小的晶粒尺寸，可能反映了变化在不同古深度的古氧化还原条件下。我们使用磁化石记录通过PETM重建不同水深的溶解氧含量的相对变化，并表明海洋脱氧可能在 PETM 的早期阶段向下扩展。因此，我们证明了磁化石记录在时间和空间上对古环境重建的价值，特别是对于缺乏碳酸盐化石的沉积物。