Holocene climate change in inland Asia inferred from paleolimnological indicators

Abstract

Lacustrine sediment preserves various compounds which serve as excellent indicators of the past climate. Thus, inland Asia, interspersed with numerous lakes, becomes an ideal place to study Holocene climate which is influenced by the mid-latitude westerlies, the East Asian summer monsoon (EASM) or their interactions. Although intensively investigated, besides their possible spatial patterns, both millennial variability and the overall Holocene trend in temperature and hydrological changes exhibit controversies, largely contributed by uncertainty in indicator interpretations and lacustrine chronology. The thesis presents lacustrine records spreading from Inner Mongolia to Western Siberia to investigate the climatic variability over the Holocene and addresses chronological uncertainty. Firstly, association between lake level and radiocarbon reservoir age (RA) was assessed using sediment cores covering the past ~1500 years at Lake Qinghai and Manas. Substantial downcore RA variation displayed high values over high lake level periods at Lake Qinghai but at low lake level periods at Lake Manas, closely associated with terrestrial old carbon input. These results suggest that selection of samples at discrete depths with similar lake status is curial to avoid large RA uncertainty. Alkenone-based moisture and temperature over the past millennium were retrieved from Lake Daihai. The results corroborate multi-centennial-scale warm-wet/cold-dry association at EASM margin. In contrast, enhanced moisture but lowered temperature over transitional period suggests different hydrological process related to runoff input. The results also imply a remarkable humid Medieval Warm Period, as a result of the strengthened EASM. Further, multi-proxy records from Bayan Nuur unveil the environmental changes since ~7000 cal. BP. Millennial-scale hydroclimate variations were identified superimposing on a drying trend due to the attenuation of EASM. These variations, specifically the wet climate at ~6500-5600 and 4200-2800 cal. BP alongside the dry climate at ~5400, 4600, 2700 and 2400 cal. BP, hint the influence of solar irradiance in monsoon fringe over the investigated period. Lastly, long-chain alkenone records in the westerlies-controlled region were generated at Lake Sayram and Ebeyty. Over the Holocene, moisture records display similar pattern with dry condition before ~6000 cal. BP and humid climate afterwards. By contrast, distinct temperature patterns are characterized by an overall cooling trend tracking the boreal summer insolation at Lake Sayram and low temperature before ~6000 cal. BP at Lake Ebeyty, located at a higher latitude, as perhaps depressed by remnant ice sheet. The higher latitude probably also contributes to the millennial-scale warm-wet/cool-dry association at Lake Ebeyty. Further, exceptional warmth at ~4200-2800 cal. BP clearly expressed at Lake Sayram possibly linked to high solar activity, does not appear to have penetrated to higher latitudes. Overall, this thesis identified clear linkages between climate and solar activity with chronology uncertainty carefully considered. Over the Holocene, in lower latitudes, enhanced humidity in the westerlies-controlled region whilst declined moisture at EASM margin confirmed the distinct patterns of hydrological changes in these two regions which, combined with the Holocene cooling trend, result in warm-dry/cool-wet and warm-wet/cool-dry association respectively. However, at higher latitudes, where the westerlies solely dominate, Holocene wetting and warming collectively promote a warm-wet/cool-dry pattern.