Abstract To better understand the reported paleoclimate features and underlying mechanisms in the lowest-latitude region (~28°N to 29°30′N), where 12 major river gorges cross the Himalayas, of the southern Tibetan Plateau, we aim to elucidate the major sources and behavior of modern monsoon air masses in the region based on spatial variations in the δ18O and δ2H of stream water and precipitation, and vegetation during the summer monsoon season. Our main findings were following: The river gorges, high mountain ranges bearing extensive dense vegetation, and stronger solar insolation characteristic of the lowest-latitude region may have functioned for an extended period as the essential geological settings for strongly drawing significant amounts of moisture from the Indian Ocean toward the region without passing through the Yarlung Zangbo basin. The combination of such geological settings specific to this region may have caused its earlier response to the climate amelioration after a cold/dry event, such as the Younger Dryas and Heinrich-1, compared to other regions on the Plateau. Furthermore, gorge-derived monsoon air masses that blew through the lowest-latitude region are highly likely to have more significantly influenced the onset of the climate amelioration after a cold/dry event even in the southwestern, south-central, and southeastern regions, rather than the Yarlung Zangbo-derived monsoon air masses. Therefore, this caused the subsequent expansion of the Indian summer monsoon (ISM) over the Plateau. Thus, with its unique geological setting, the lowest-latitude region has presumably played a critical role as the key region, i.e. the initiator, for the early evolution of the ISM on the Tibetan Plateau by responding to climate amelioration after cold/dry events more rapidly than any other region and drawing gorge-derived monsoon air masses strongly toward the region.

中文翻译：

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青藏高原最低纬度地区季风气团的来源、行为及其古气候意义

摘要 为了更好地了解青藏高原南部 12 条主要河流峡谷穿越喜马拉雅山脉的最低纬度地区（~28°N 至 29°30'N）的古气候特征和潜在机制，我们旨在阐明基于夏季季风季节河流水和降水的δ18O和δ2H的空间变化以及该地区现代季风气团的主要来源和行为。我们的主要发现如下： 最低纬度地区的河流峡谷、高山覆盖着茂密的植被和较强的日照特征，可能在很长一段时间内作为重要的地质环境发挥作用，强烈地从地表中吸收了大量的水分。印度洋向该地区不经过雅鲁藏布江盆地。与高原上的其他地区相比，该地区特有的这种地质环境的组合可能导致其在寒冷/干燥事件后对气候改善做出更早的反应，例如年轻的仙女木和海因里希-1。此外，吹过最低纬度地区的峡谷季风气团极有可能对寒冷/干燥事件后气候改善的开始产生更显着的影响，甚至在西南、中南部和东南部地区，来自雅鲁藏布江的季风气团。因此，这导致了印度夏季风 (ISM) 随后在高原上空的扩张。因此，凭借其独特的地质背景，最低纬度地区作为关键区域，即发起者，大概发挥了关键作用，