Sedimentary Evolution Of Da Qaidam Salt Lake Associated With Westerlies Paleoclimate Change And Constraints On The Mg-borate Deposit Formation

Da Qaidam Salt Lake?37°46?³7°55?N,95°02?⁹5°22?E?is a closed-basin lake,located in a subbasin of the northern Qaidam Basin.Da Qaidam basin is in a cold and arid climate with westerlies prevailing.The fluctuations of the saline water level and chemistry are largely controlled by precipitation-dominated runoff from the large mountain catchment.A 100-m drill-core was recovered from the center of the Da Qaidam Salt Lake,which was used for the study of the sedimentary evolusion of the closed-basin lake associated with westerlies paleoclimate change.A multi-proxy approach was applied in the thesis study in order to gain as much environmental information stored in the sedimentary sequence as possible.The aim of the study is to establish the history of sedimentary evolution of Da Qaidam Salt Lake associated with paleoclimate change since the last Deglaciation and using it as a key-site record for the study of the mechanisms of climate change in the Asian continent and of the interplay between westerlies and east Asian monsoons.Meanwhile,Da Qaidam Salt Lake is known for its solid-phase boron deposits,which is economically important as natural resource and scientifically attractive for the study of borate deposit formation related to the sedimentary evolution of Da Qaidam Salt Lake.Multidisciplinary investigations on the DCD100 A sediment core recovered from the center of the salt lake and on the D3 section excavated from the central mining area of the Da Qaidam salt lake were carried out with field lithologic logging,15 dating samples for AMS ¹⁴Cmeasurements,4200 samples for geochemical analyses,431 samples for grain-size,LOI,and total nitrogen analyses,520 samples for X-ray diffraction analysis,30 samples for SEM-EDX analysis.Based on results from the above-mentioned measurements and environmental proxy data,conclusions are drawn as follows:1.Based on our well-established chronological framework for DCD100 A core,the beginning of the last Deglaciation was determined at 18.0 cal.¹⁴Cka BP and the onset of the Holocene was at 11.8 cal.¹⁴Cka BP on the northern Tibet-Qinghai Plateau.2.The beginning of evaporite deposition in Da Qaidam Salt Lake is dated at 11.4 cal.¹⁴Cka BP.The Holocene warmer climate has facilitated evaporite formation in the Da Qaidam Salt lake,as manifested by both radiometric dating and sediment evidence.The results show that:?1?for the area of the Da Qaidam watershed where dry westerlies are prevailing,temperature played an important role on the sedimentation in the salt lake because of the contrasting response to temperature change between the mountain catchment and the terminal lake.Much more precipitation in the mountain catchment occurred during the Early-to-mid Holocene because warmer surface enhances the vertical air flow resulting in condensation under the mountainous topography;Meanwhile,warmer climate resulted in intensified evaporation,facilitating evaporite deposition in the salt lake.?2?The reason that evaporite deposition did not occur in the periods of LGM and last Deglaciation is because cold climate caused limitation of precipitation in the mountain catchment and weak evaporation in the paleo-lake with longer ice-cover of a year.3.Four lacustrine environmental stages were divided based on radiocarbon dated multiple stratigraphic records:?1?the lake water was basically fresh at 19.2 to 18.0 cal.ka BP,as implied by silty clay clastic sediment,including 4% detrital carbonate.?2?a slight increase in salinity of the paleo-lake during the period of 18.0 and 14.4 cal.ka BP,as implied by a substantial increase in carbonate content with variations in TCC as a result of unstable hydroclimatic conditions.?3?the occurrence of gypsum and bassanite?averaging 10% and 6%,respectively?suggests that the paleo-lake became a brackish water body at 14.4¹0.0 cal.ka BP.?4?the salt lake stage began from 10.0 cal.ka BP to the present as indicated by halite-dominated evaporite deposited in the central area of the Da Qaidam salt flat.4.Paleoenvironmental conditions associated with climatic changes in the northern Tibet-Qinghai Plateau are indicated by a series of stratigraphic variables,such as TN,OM,Rb/Sr ratio,carbonate and gypsum contents.:?1?a severely cold and arid LGM?19.2¹8.0 cal.ka BP?climate is implied by the lowermost TN and OM values?organic productivity proxy?and high Rb/Sr ratios?physical weathering index?.?2?climate variability during the last Deglaciation?18.0¹1.4 cal.ka BP?is implied by the increased organic productivity and summer evaporation?gradually increased carbonate content and gypsum precipitation?.?3?The abrupt onset of halite-dominated evaporite deposition was resulted from the early-Holocene?11.4⁰ cal.ka BP?aridification in the Da Qaidam basin due to the temperature increase of the region,which brought about the intense evaporation of the paleo-lake.The trend of temperature increase with fluctuations at Da Qaidam is in general agreement with the global record.5.Da Qaidam Salt Lake is at the location out of touch of Asian monsoon moisture.Therefore,the reconstructed paleoclimate history represents a key-site record for the study of westerlies paleoclimate evolution.Correlation of the Da Qaidam proxy record with Lake Qinghai and Huangqihai,other two closed-basin lakes located on the westerly belt,indicates that the general pattern of sedimentary evolution since the LGM is overall consistent in all three sites,under the circumstances that the latter have been partially influenced by the Asian monsoon moist air.This is an interesting phenomenon valuable for the study of the coupling mechanism between westerlies and Asian monsoon circulations.6.Cemented boron orebody with light-dark lamination couplets reflects a shallow to near desiccation conditions with seasonal fluctuation in water chemistry,facilitating chemical precipitation of pinnoite.SEM-EDX examination and microanalysis of the pinnoite crystals further confirmed that the euhedral pinnoite crystals are primarily nucleated and precipitated directly from the salt lake.7.The thesis study provide solid evidence for the geochemical and sedimentological conditions favorable for pinnoite deposit formation,which may shed new light on the causal mechanism of pinnoite deposit formation in salt lakes.The results show that the abrupt onset of the evaporite deposition was brought about by warming-induced enhancement of both precipitation in the mountain catchment and evaporation in the salt lake.And substantially increased supply of groundwater by the infiltration of enhanced precipitation resulted in the initiation of the Recent pattern of hydrothermal circulation.As a result,a large amount of boron-bearing water flowed into the salt lake and the consequent evaporative concentration in the early stage of evaporite deposition resulted in the favorable geochemical conditions for the Mg-borate deposit formation.Based on the fact that the Mg-borate minerals were not detected in the sediments over the past 1000 years,combined with the lithology characteristics and mineral composition of the D3 section,the sedimentary environment of the paleo-lake when the pinnoite deposited was inferred as the following conditions: the evaporation was stronger in summer;the p H value of the paleo-lake was higher than 9.3,much more alkaline than today.In addition,the paleo-lake was shallower than that of today with Mg/Ca ratio >39 and boron concentration >600 mg/L.