Sources and behavior of monsoon air masses in the lowest-latitude region on the Tibetan Plateau, and their paleoclimatic implications

摘要

To better understand the reported paleoclimate features and underlying mechanisms in the lowest-latitude region (similar to 28 degrees N to 29 degrees 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 delta O-18 and delta H-2 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.