Onset of long-term cooling of Greenland near the Eocene-Oligocene boundary as revealed by branched tetraether lipids

The Eocene-Oligocene (E-O) boundary interval is considered to be one of the major transitions in Earth's climate, witnessing the first major expansion of the East Antarctic Ice Sheet. However, the extent of the associated climatic cooling, especially for high northern latitude continental landmasses, is poorly constrained. In this study we reconstruct the first mean annual air temperature (MAAT) for the Greenland landmass during the late Eocene and early Oligocene by applying a new proxy based on the distribution of branched tetraether lipids derived from soil bacteria preserved in a marine sediment core from the Greenland Basin. The temperature estimates are compared with a composite continental temperature record based on bio-climatic analysis of pollen assemblages. Both proxies reveal comparable late Eocene MAATs of ~13-15 °C and a gradual long-term cooling of ~3-5 °C starting near the E-O boundary. These data are in agreement with other MAAT reconstructions from northern midlatitude continents and suggest a general cooling of the Northern Hemisphere during the E-O transition.始新世-渐新世（Eocene-Oligocene, 简称E-O）界线层段被视作地球气候演化的重大转型事件之一，见证了东南极冰盖的首次大规模扩张。然而，此次伴随而来的气候降温幅度，尤其是针对北半球高纬度大陆地块的降温幅度，尚未得到充分限定。本研究借助格陵兰盆地海洋沉积物岩芯（marine sediment core）中保存的土壤细菌源支链四醚脂类（branched tetraether lipids）的分布特征构建的新型代用指标，重建了晚始新世至早渐新世格陵兰陆块的首个年平均气温（mean annual air temperature, MAAT）序列。研究将该气温估算结果与基于孢粉组合生物气候分析得到的综合大陆气温记录进行对比。两类代用指标均显示，晚始新世的年平均气温约为13~15℃，且自E-O界线附近开始出现逐步的长期降温，降温幅度约为3~5℃。该数据与北半球中纬度大陆的其他年平均气温重建结果一致，表明E-O转型期北半球整体存在普遍性的气候降温过程。