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

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 (HSW_ch)and low-saline-water taxa (LSW_ch) 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 (Sal_ch).During the period 1550～1840 AD, LSW_ch considerably increased and Sal_chdramatically declined, indicating relatively humid conditions in the Little Ice Age(LIA). After 1840 AD, the HSW_ch and Sal_ch 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. A relatively wet oscillationoccurred in 1200～1230 AD, interrupting the generally dry period (990～1550 AD).And a comparatively dry oscillation occurred in 1590～1700 AD, punctuating thegenerally humid LIA. Regional climate comparison shows that the decadal tocentennial wet and dry events revealed by our chironomid-based salinityreconstruction are just of local climatic significance, and could be related tomoisture anomalies in the headwater area of Sugan Lake drainage basin.(5) As far as the trends in the inferred salinity time series are concerned, it isevident that the magnitude of salinity fluctuation was greater, and the frequencyhigher, in the relatively humid LIA (1550～1840AD) than in the preceding andfollowing dry periods, implying a highly unstable LIA climate in the catchmentarea of Sugan Lake. The highly unstable LIA climate had its fingerprint in a lot ofproxy records from arid Northwestern China, monsoonal China and other parts of the world, and probably represented a shift of regional climatic regimes duringtimes characterized by large-scale climatic anomaly.2. Moisture evolution in westerly-dominated arid Northwestern China andarid central Asia over the last millenniumThe 1000-year moisture variability recorded by chironomids in Sugan Lakewas generally consistent with palaeo-moisture data from other proxy sites in aridNorthwestern China, demonstrating a dry Medieval Warm Period (MWP), arelatively humid LIA, and a dry period during the last 150 years. Moisture recordsynthesized by combining five high-resolution proxy time series from arid centralAsia shows that climate was drier than average from 1000 AD to ca. 1350 AD,moderate from 1350 to 1500 AD, wetter than average from 1500 to 1850 AD, anddrier than average once again after 1850 AD with a slightly wetting trend since themid-twentieth century.3. Comparison of millennium moisture variability in westerly-dominated aridcentral Asia with Asian summer monsoon history(1) On multi-centennial time scale, moisture evolution in westerly-dominatedarid central Asia has an out-of-phase relationship with Asian summer monsoonhistory. Most proxy records from arid central Asia, especially in arid NorthwesternChina, show that there was a relatively dry MWP, contrasting to the moderate tostrong Asian summer monsoon during that period. In LIA, climate of arid centralAsia was generally humid, whereas a weaker or moderate monsoon prevailed from1400 to 1900 AD.(2) On decadal time scale, two records of tree-ring reconstructed PalmerDrought Severity Index (PDSI) and three records of ice core net snowaccumulation indicate that there is an out-of-phase or anti-phase relationship ineffective-moisture or precipitation changes between arid Northwestern China andmonsoonal China during the recent 200 years. Moisture decreased from earlynineteenth to early twentieth century, in arid Northwestern China, and has anincreasing trend since 1920s, especially since 1950s. On the other hand, climate ofmonsoonal China has become progressively drier in the recent fifty years.4. Possible mechanismsThe relatively humid LIA in the westerly-dominated arid central Asia mayhave resulted from combining effects of decreased evapotranspiration caused bythe cooling of arid central Asia, and increased precipitation caused by cooling of the Northern Hemisphere and persistently negative North Atlantic Oscillation(NAO), all of which are plausibly associated with a generally weak solar activity(radiation) and strong volcanic activity during that time. In contrast, on the decadalto (sub-)millennium time scale, the relationship between solar activity (radiation)and Asian monsoon can be generalized as follows: weak summer monsoon broadlycorrelated with weak solar activity (radiation), and vice versa. Therefore, thedifferent response of moisture in arid central Asia from that of Asian summermonsoon to the external forcings, particularly solar forcing, probably results in theout-of-phase or anti-phase relationship between moisture evolution in arid centralAsia and Asian summer monsoon history on the multi-centennial time scale.5. Late Holocene lake-level and climate changes recorded by chironomids inBosten Lake(1) Based on variations in percentage abundance of littoral (Po, Cr, Ps, Dt)and profundal (Ch, Mi, Cc) taxa in the upper 383 cm of a sediment core BSTC001from Bosten Lake, late Holocene lake level fluctuations and thus climate changeswere preliminarily recovered. From 4100 to 2700 cat a BP, littoral chironomid taxadominated the assemblage, indicating a low lake level and dry climate. From 2700to 990 cal a BP, the generally increased profundal taxa demonstrates a relativelyhigh lake-level period. However, this ca. 2500-years-long highstand wasinterrupted by a regression during 1700～990 cal a BP. From 190 cal a BP onward,chironomid communities of Bosten Lake experienced remarkable changes andprobably reflected the eutrophication process in the recent past as a result ofintensified human activity.(2) The chironomid-based lake level reconstruction correlates well with thediatom-based salinity reconstruction from the same core, high (low) lake levelscorresponding to low (high) water salinities and thus indicating wet (dry) climateconditions. The late Holocene lake level and climate evoluation recorded bychironomids at Bosten Lake was in broad agreement with other proxy records fromarid central Asia, showing a regional palaeoclimatic significance.