The Nama Group in southern Namibia captures one of the best-preserved records of the last ∼10 million years of the Ediacaran Period, recording pivotal changes to Earth's biosphere and oceans. Within the Nama Group, the lower Schwarzrand Subgroup preserves early complex bilaterian trace fossils, but uncertainties surrounding the ages of their first occurrences hinder global correlation and the understanding of the rates at which critical biological changes occurred. This study presents magnetic susceptibility cyclostratigraphy of a recently acquired drill core that covers the Nudaus Formation and Nasep Member within the lower Schwarzrand Subgroup. Spectral analysis reveals a hierarchy of orbital cycles that could be responsible for the repeating depositional sequences at various length scales in the Schwarzrand Subgroup. Matching the wavelengths of the orbital cycles to their expected periodicities, we established sedimentation rates throughout the stratigraphy. Utilizing available UPb ages from correlative outcrop sections and the sedimentation rates from cyclostratigraphy, we refine the age-depth model for the lower Schwarzrand Subgroup. Our new age-depth model places tighter temporal constraints on the first appearances of complex trace fossils, thus constraining the earliest major radiation of bilaterian metazoans and contributing to a better-calibrated chronostratigraphy of the terminal Ediacaran Period.

中文翻译：

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纳米比亚南部下 Schwarzrand 亚群的磁化率回流学改进了埃迪卡拉纪晚期双侧辐射的时间校准


纳米比亚南部的 Nama 群捕捉了埃迪卡拉纪最后 ∼1000 万年保存最完好的记录之一，记录了地球生物圈和海洋的关键变化。在 Nama 群中，下 Schwarzrand 亚群保存了早期复杂的双侧痕迹化石，但围绕其首次出现年龄的不确定性阻碍了全球相关性和对关键生物变化发生速率的理解。本研究介绍了最近获得的钻芯的磁化率回流地层学，该岩芯覆盖了下 Schwarzrand 亚群内的 Nudaus 组和 Nasep 成员。光谱分析揭示了轨道周期的层次结构，这可能是 Schwarzrand 亚群中不同长度尺度上重复沉积序列的原因。将轨道周期的波长与其预期的周期性相匹配，我们确定了整个地层学的沉积速率。利用相关露头切片的可用 UPb 年龄和旋回地层学的沉积速率，我们改进了下 Schwarzrand 亚群的年龄-深度模型。我们新的年龄深度模型对复杂痕迹化石的首次出现施加了更严格的时间限制，从而限制了双侧后生动物的最早主要辐射，并有助于更好地校准埃迪卡拉纪末期的年代地层学。