Appraisal of Asian monsoon variability in the Indian subcontinent and East Asia through the Quaternary using diatom records.

Diatom records in tropical settings have been used to appraise paleomonsoons throughout the Quaternary period due to their ubiquitous presence, rapid response, and preservation in sediments. Despite this, diatoms have been rarely used to assess the Asian monsoonal climate. As the Asian monsoon comprises the Indian and East Asian subsystems, it plays a significant role in global hydrological and energy cycles. Keeping global climate in mind, we reviewed the dynamics and causes of Asian monsoon variability (covering major records exclusive from the tropics) on different spatial and temporal scales using diatoms as a proxy record. The data have been compiled from 50 papers spanning fresh and marine sediment archives, covering entire monsoon regimes, predominantly from Southeast Asia. This study classified the data based on various geological periods ranging from the Pleistocene Epoch to the Holocene Meghalayan age (600 ka −300 a BP). Diatom proxies indicate fluctuating hydrological conditions and varying monsoon intensity throughout the Quaternary period and show regional scale coherence for specific climatic events. Diatom records from both inland and marine sediment archives indicate a wet and warm period during the mid–Pleistocene (590–300 kyr) and a cold climate and weakened East Asian monsoon during 300–140 kyr. Through the late Pleistocene period (75 to 15 kyr), monsoonal conditions were intense, wet and warm during the interglacial stages and weaker and dry during glacial stages across South and East Asia. During the Last Glacial Maximum (24.5–18 kyr), the monsoon significantly weakened with cold climatic conditions in southern Asia, and increased precipitation across East Asia. A majority of the published records witnessed an enhanced Asian monsoon and a warm and humid period towards the early to mid–Holocene, with a decline in the monsoon after the mid–Holocene accompanying intermittent intense wetter conditions. Diatom records from the sediment archives at many geographic locations hold signatures of extreme events such as the Younger Dryas cold event around 13.3–11.3 kyr BP, the Medieval Warm Period around 1.3 to 0.8 kyr, and the Little Ice Age in between 0.6 and 0.3 kyr. Further, we indicate that several sites from different monsoon-dominated geographic-climatic zones should be re-investigated with high resolution across spatial and temporal palaeoclimatic periods (including existing databases) with careful diatom taxonomy to strengthen a climatic reconstruction of this monsoon-dominated region of Asia. [ABSTRACT FROM AUTHOR]