Abstract. Between 14.5 and 5 ka, the Sahara was vegetated owing to a wet climate during the African humid period. However, the climatic factors sustaining the “green Sahara” are still a matter of debate. Particularly the role of winter precipitation is poorly understood. Using the stable hydrogen isotopic composition (δD, where D stands for deuterium) of high molecular weight (HMW) n-alkanoic acids in a marine sediment core from the eastern Mediterranean, we provide a continuous record for winter precipitation in the Nile river delta spanning the past 18 kyr. Pairing the data with δD records from HMW n-alkanes from the same core, we show that HMW n-alkanoic acids constantly derived from the delta, while the HMW n-alkanes also received significant contributions from the headwaters between ∼ 15–1 ka when fluvial runoff enhanced. This enables us to reconstruct the evolution of Mediterranean (winter) and monsoonal (summer) rainfall in the Nile river watershed in parallel. In the delta, the Heinrich stadial 1 (HS1) evolved in two phases, with a dry episode between ∼ 17.5–16.0 ka, followed by wet conditions between ∼ 16–14.5 ka. Winter rainfall enhanced substantially between 11–6 ka, lagging behind the intensification of the summer monsoon by ca. 3 kyr. Heavy winter rainfall resulted from a southern position of the Atlantic storm track combined with elevated sea surface temperatures in the eastern Mediterranean, reinforcing local cyclogenesis. We show that during the green Sahara, monsoon precipitation and Mediterranean winter rainfall were both enhanced and infer that the winter rainfall zone extended southwards, delivering moisture to the Sahara. Our findings corroborate recent hypotheses suggesting that winter rains that extended southward were a crucial addition to the northward displacement of the summer monsoon in helping to sustain a green Sahara.

中文翻译：

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末次冰期以来尼罗河流域冬季降水演变

摘要。14.5至5ka之间，由于非洲湿润期气候湿润，撒哈拉沙漠植被茂盛。然而，维持“绿色撒哈拉”的气候因素仍然存在争议。尤其是对冬季降水的作用知之甚少。利用地中海东部海洋沉积物岩心中高分子量 (HMW) 正链烷酸的稳定氢同位素组成（δD，其中 D 代表氘），我们提供了跨越尼罗河三角洲的冬季降水的连续记录过去 18 公里。将数据与来自同一岩心的 HMW 正构烷烃的 δD 记录配对，我们发现 HMW 正构烷酸不断源自三角洲，而 HMW 正构烷烃也从大约 15-1 ka 之间的水源中获得了显着的贡献。河流径流增强。这使我们能够同时重建尼罗河流域地中海（冬季）和季风（夏季）降雨的演变。在三角洲，海因里希斯塔迪尔 1 (HS1) 分两个阶段演化，约 17.5–16.0 ka 期间出现干燥阶段，随后约 16–14.5 ka 期间出现湿润状况。冬季降雨在 11-6 ka 期间大幅增加，落后于夏季季风的增强约 11-6 ka。3 基里尔。大西洋风暴路径位于南部，加上地中海东部海面温度升高，导致冬季强降雨，加强了当地的气旋作用。我们发现，在绿色撒哈拉期间，季风降水和地中海冬季降雨均有所增强，并推断冬季降雨带向南延伸，向撒哈拉输送了水分。我们的研究结果证实了最近的假设，即冬季降雨向南延伸是夏季风向北移动的重要补充，有助于维持绿色的撒哈拉沙漠。