Holocene Asian monsoon evolution revealed by a pollen record from an alpine lake on the southeastern margin of the Qinghai–Tibetan Plateau, China

Abstract

Abstract. We present the results of pollen analyses from a 1105 cm long sediment core from Wuxu Lake in southwestern China, which depict the variations of the East Asian winter monsoon (EAWM) and the Indian summer monsoon (ISM) during the last 12.3 ka. During the period of 12.3 to 11.3 cal ka BP, the dominance of Betula forest and open alpine shrub and meadow around Wuxu Lake indicates a climate with relatively cold winters and dry summers, corresponding to the Younger Dryas event. Between 11.3 and 10.4 cal ka BP, further expansion of Betula forest and the retreat of alpine shrubs and meadows reflect a greater seasonality with cold winters and gradually increasing summer precipitation. From 10.4 to 4.9 cal ka BP, the dense forest understory, together with the gradual decrease in Betula forest and increase in Tsuga forest, suggest that the winters became warmer and summer precipitation was at a maximum, corresponding to the Holocene climatic optimum. Between 4.9 and 2.6 cal ka BP, Tsuga forest and alpine shrubs and meadows expanded significantly, reflecting relatively warm winters and decreased summer precipitation. Since 2.6 cal ka BP, reforestation around Wuxu Lake indicates a renewed humid period in the late Holocene; however, the vegetation in the catchment may also have been affected by grazing activity during this period. The results of our study are generally consistent with previous findings; however, the timing and duration of the Holocene climatic optimum from different records are inconsistent, reflecting real contrast in local rainfall response to the ISM. Overall, the EAWM is broadly inphase with the ISM on the orbital timescale, and both monsoons exhibit a trend of decreasing strength from the early to late Holocene, reflecting the interplay of solar insolation receipt between the winter and summer seasons and El Niño-Southern Oscillation strength in the tropical Pacific.

Variations of the Indian summer monsoon over the last 30 000 years inferred from a pyrogenic carbon record from south‐west China

“…In the last few decades, a wide variety of geological archives have been used to study long‐term changes in the ISM, including speleothem records, and marine and lacustrine sediments (e.g. Fleitmann et al ., ; Cook et al ., ; Contreras‐Rosales et al ., ; Dutt et al ., ; Zhang et al ., ; Sun et al ., ). While numerous studies have reconstructed ISM variations in the Holocene (Dykoski et al ., ; Shakun et al ., ; Cook et al ., ; Wang et al ., ; Xiao et al ., , ; Dutt et al ., ; Zhang et al ., ), high‐resolution and continuous long‐term ISM records that span the whole of the Last Glacial Maximum (LGM) and the deglaciation are scarce (Hodell et al ., ; Wang et al ., ; Zhang et al ., ).…”

Holocene evolution of the Indian Summer Monsoon inferred from a lacustrine record of Lake Wuxu, south‐east Tibetan Plateau

“…The algae ( Pediastrum ) and aquatic plant pollen ( Myriophyllum ) were identified and counted along with the fossil pollen analysis at 4‐cm intervals after pretreatment with HCl and HF (see details in Zhang et al ., ). Subsequently, concentrations were calculated based on the number of counted Lycopodium spores, in order to express abundance in the sediment samples.…”

“…Instead, lake sediment sequences have played an important role in providing palaeoenvironmental information during the Lateglacial and the Holocene period (e.g. Jarvis, ; Kramer et al ., , ; Wischnewski et al ., ; Li et al ., ; Zhang et al ., ). However, records covering the whole Holocene period are still sparse despite some recent efforts (e.g.…”

“…Xiao et al ., ; Zhang et al ), among which discrepancies still exist. For instance, a rather longer Holocene monsoon maximum lasting for over 6000 years was inferred from fossil pollen assemblages in Lake Naleng compared to about 5500 years from the fossil pollen record in Lake Wuxu (Kramer et al ., ; Zhang et al ., ), whereas a significantly shorter strong monsoon phase between 9.2 and 6.2 cal ka bp was reported from Lake Muge Co (Sun et al ., ). In addition, various driving factors of the summer monsoon were also recognized, such as the traditionally recognized solar forcing mechanism on its evolution (e.g.…”

“…Recently, a well‐dated continuous fossil pollen record covering the past 12 000 years was recovered from Lake Wuxu (SETP), which has been shown to reflect Holocene variation of the Asian Summer/Winter Monsoons (Zhang et al ., ). Here we present an organic geochemical record from the same core to provide a better understanding of the lake's evolution history and regional climate change, focusing on determination of: (i) the main source of the organic material preserved in Lake Wuxu; (ii) the evolution history of Lake Wuxu during the Holocene; and (iii) Holocene variation of the ASM and potential forcing mechanism.…”