Pelagic sediments from the subtropical South Atlantic Ocean contain geographically extensive Oligocene ooze and chalk layers that consist almost entirely of the calcareous nannofossil Braarudosphaera. Poor recovery and the lack of precise dating of these horizons in previous studies has limited the understanding of the number of acmes, their timing and durations, and therefore their likely cause. Here we present a high‐resolution, astronomically tuned stratigraphy of Braarudosphaera oozes (29.5–27.9 Ma) from Ocean Drilling Program Site 1264 in the southeastern Atlantic Ocean. We identify seven episodes with highly abundant Braarudosphaera. Four of these acme events coincide with maxima and three with minima in the ~110 and 405‐kyr paced eccentricity cycles. The longest lasting acme event corresponds to a pronounced minimum in the ~2.4‐Myr eccentricity cycle. In the modern ocean, Braarudosphaera occurrences are limited to shallow marine and neritic settings, and the calcified coccospheres of Braarudosphaera are probably produced during a resting stage in the algal life cycle. Therefore, we hypothesize that the Oligocene acmes point to extensive and episodic (hyper) stratified surface water conditions, with a shallow pycnocline that may have served as a virtual seafloor and (partially/temporarily) prevented the coccospheres from sinking in the pelagic realm. We speculate that hyperstratification was either extended across large areas of the South Atlantic basin, through the formation of relatively hyposaline surface waters, or eddy contained through strong isopycnals at the base of eddies. Astronomical forcing of atmospheric and/or oceanic circulation could have triggered these conditions through either sustained rainfall over the open ocean and adjacent land masses or increased Agulhas Leakage.

中文翻译：

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渐新世南大西洋的轨道强迫超层化。

来自亚热带南大西洋的远洋沉积物含有分布广泛的渐新世软泥和白垩层，几乎全部由钙质超微化石Braarudosphaera组成。在之前的研究中，恢复不佳以及缺乏对这些视野的精确测定，限制了对顶点数量、它们的时间和持续时间以及它们可能的原因的理解。在这里，我们展示了来自大西洋东南部海洋钻探计划站点 1264 的Braarudosphaera软泥（29.5-27.9 Ma）的高分辨率、天文学调整的地层学。我们确定了七个具有高度丰富的Braarudosphaera 的事件。其中四个极限事件与约 110 和 405 kyr 节奏偏心率周期中的最大值一致，三个与最小值一致。持续时间最长的 acme 事件对应于~2.4-Myr 偏心率周期中的明显最小值。在现代海洋中，Braarudosphaera 的出现仅限于浅海和浅海环境，并且Braarudosphaera的钙化球菌可能是在藻类生命周期的静止阶段产生的。因此，我们假设渐新世尖端指向广泛的、偶发的（超）​​分层的地表水条件，具有浅密重斜层，可能充当虚拟海底并（部分/暂时）阻止球藻在中上层区域下沉。我们推测，超层化作用要么通过形成相对低盐度的地表水而扩展到南大西洋盆地的大片区域，要么通过涡旋底部的强等密度来包含涡流。大气和/或海洋环流的天文强迫可能通过公海和邻近陆地上的持续降雨或厄加勒斯泄漏的增加而引发这些条件。