Holocene hydroclimatic changes in mid-latitude Asia and implications for westerlies and monsoon behavior

Abstract

Due to complex interactions of the westerlies and East Asian monsoon along with relatively dry conditions, mid-latitude Asia is susceptible to hydroclimatic changes. Contrasting moisture evolutions between westerlies-dominated Central Asia and monsoonal Asia since the mid-Holocene has been widely recognized. Yet, inconsistent hydrological and temperature records over the early Holocene from both regions have been reported, and moisture variations at the boundary between two circulations remain controversial. This thesis presents lacustrine records from mid-latitude Asia to investigate Holocene hydroclimatic changes and infer associated mechanisms.

Firstly, water depth control on n-alkane distribution and organic carbon isotope in mid-latitude Asian lakes was investigated using surface sediments from 55 lakes. Variations in relative proportion of mid-chain to long chain n-alkane homologues (Paq) and isotopic compositions of total organic carbon (δ13Corg) resemble arched patterns with water depth, with relatively high Paq and δ13Corg values corresponding to the depth of ~1–10 m. The results suggest that Paq and δ13Corg can be used to infer lake-level changes in mid-latitude Asia, and combined utilization of both indicators could improve the reliability of reconstructions. Further, updated n-fatty acid δD records from Lake Hurleg were presented to assess the relation between precipitation isotopes and moisture levels in arid Central Asia. The results indicate that isotopic differences between lake water and terrestrial water largely reflect isotopic enrichment due to lake water evaporation and thus aridity changes in arid Central Asia since the mid-Holocene, which also shed light on Holocene moisture variations in marginal monsoon regions.

Secondly, isotopic and biomarker records from Lake Sayram, Yanhaizi, and Qinghai were presented. The occurrence of particularly depleted isotopic values of lake water associated with lake freshening within the deglaciation and early Holocene, together with anomalously old 14C dates documented in many Asian lakes, strongly indicate substantial meltwater contribution to lake water budget, along with the climatic warming. Hence, the meltwater effect should be considered when inferring Asian summer monsoon and westerlies behavior over the early Holocene.

Thirdly, alkenone-based (U_37^K') Holocene temperature records from Lake Yihesariwusu, Kuchuk, and Maloye Yarovoye show contrasting trends. Temperature disparity over mid-high latitude Eurasia during the early to mid-Holocene and its reduction towards the late Holocene appear to be driven by wavy westerlies associated with remnant ice sheets and low solar irradiance during the early to mid-Holocene. Wavy westerlies presumably induce temperature disparity over other mid-high latitude northern continents.

Lastly, biomarker and elemental records from Lake Eastern Juyanze, Anguli-nuur, Badain-W, and Sumujilin-S show wet conditions during the Medieval Warm Period (MWP) and dry conditions during the Little Ice Age, following the temperature-moisture association in monsoonal region. The results indicate northwestward migration of the monsoonal limit during the MWP, possibly associated with strengthened monsoon. Moreover, decoupled temperature-moisture records from Lake Eastern Juyanze appear to be associated with abrupt meltwater input to the lake.

Findings in this thesis highlight that Holocene hydroclimatic conditions in mid-latitude Asia appear to be affected by meltwater and atmospheric circulation patterns, with important implications for our understanding of westerlies and monsoon behavior.