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China University (2010)

Moisture Variability Over The Last Millennium Recorded By Chironomids From Lacustrine Sediments In Arid Northwestern China

Chen J H

Titre : Moisture Variability Over The Last Millennium Recorded By Chironomids From Lacustrine Sediments In Arid Northwestern China

Auteur : J H Chen

Grade : Doctoral Dissertation 2010

Université : China University

Résumé partiel
Knowledge of past climate variability over the last millennium (late Holocene)provides perspective for understanding and modeling current and future states ofregional and global climate. Hydrological changes are especially relevant toecosystem dynamics and human well-being in arid regions. However, obvious lack ofhigh-resolution unambiguous proxy records with reliable chronology in aridNorthwestern China not only hamper attempts to investigate moisture variability andits possible mechanisms in the last millennium, but also prevent us from exploring therelationship between moisture variability in westerly dominated arid Central Asia andAsian summer monsoon history on a (multi-)centennial time scale.The annually laminated sediments of Sugan Lake, a hydrologically-closed salinelake in the northern part of the Qaidam Basin, northem Tibetan Plateau, afforded us aunique opportunity to reconstruct the moisture changes in arid Northwestern Chinaover the last 1000 years at a relatively high resolution, using sub-fossil Chironomidae(Insecta : Diptera) as an indicator of past water salinity. Then, based on ourreconstruction and previous proxy records, moisture variability in westerly-dominatedarid Northwestern China and central Asia was systematically studied, allowing acomparison with the Asian monsoon history during the last millennium.In addition, chironomid analysis was also carried out in a sediment coreBSTC001 retrieved from Bosten Lake, Xinjiang, climatically controlled by westerlies,and the late Holocene lake level fluctuations and climate changes were preliminarilyreconstructed.Main conclusions of this dissertation are outlined as follows:1. Moisture variability in Sugan Lake over the last millennium reconstructedusing subfossil chironomid remains(1) Salinity. optima of taxa in the Tibetan Plateau chironomid training set, spatialpatterns of chironomid assemblage within Sugan Lake, and relevant ecological knowledge from the literature together suggest that the four major chironomid taxaidentified from SG03I can be divided into two groups, Pb and Oc indicative ofrelatively high-saline-waters, and Pr and Ps indicative of relatively low-saline-waters.Therefore, the changes in percentage abundances of high-saline-water taxa (HSWch)and low-saline-water taxa (LSWch) throughout the sequence are suitable forpalaeosalinity reconstruction from Sugan Lake.(2) Chironomid-based salinity records are consistent with other palaeo-moistureproxy data from the same core in terms of general trends and the magnitude andfrequency of the variability, and in broad agreement with the ostracod-shell Sr/Caratio inferred salinity variability derived from parallel core SG00C, confirming thereliability of the effective moisture reconstruction by chironomid analysis for theSugan Lake catchment.(3) The chironomid-based salinity records revealed the effective moistureevolution in the catchment of Sugan Lake during the past millennium. From 990 to1550 AD, the climate was generally arid, as suggested by the. predominance ofhigh-saline-water taxa and the high value of chironomid-inferred salinity (Salch).During the period 1550~1840 AD, LSWch considerably increased and Salchdramatically declined, indicating relatively humid conditions in the Little Ice Age(LIA). After 1840 AD, the HSWch and Salch both rose to a high level, implying thatthe dry climate prevailed in this area once again.(4) On the decadal to centennial time scale, the chironomid-based salinityrecords document two remarkable climatic events.

Mots clés : Sugan Lake, Bosten Lake, arid Northwestern China, subfossil chironomid, effective moisture, last millennium, late Holocene

Présentation (Globethesis)

Page publiée le 19 avril 2013, mise à jour le 4 mai 2018