Situation Of Study On Global Salt Lakes And Paleo-climatic And Environmental Evolution Of The Typical Region

In view of the salt lake science's rise and development and the comparative scientific research of the world,in the seventies of twentieth century, it briefly introduced the salt lake around the world and promote the salt lake scientific progress and development. in the nineties of twentieth century,it introduced a more systematic introduction to the resources environment and integrated development of the salt lake around the world and triggered paleo-environmental research and utilization of the comprehensive development to the salt lake in China,in the twenty-first century the need from the global change and regional development with harmonious angle of salt lake resources in a comprehensive study of environmental science and frontiers fields.Based on the forefront of the development of this discipline and national strategic requirements give an overview work of the world salt lake system analysis,provide a rich basis comparative data to earth resources and environment for the current development of science and global change research.This paper includes three core parts as follows:Firstly, Situation of study on global salt lake was illustrated particularly and systematically from the angles of environment and resources.Secondly, research on paleo-climatic and environmental evolution in the past 200ka.B.P.was conducted by choosing representative region-Owens Lake. Because climatic change is a result of comprehensive effect of interior factors and external factors, such as magmatic exhalation, air circulation, topographical change, and so on, this leads to diversified choices of climate proxies, including algae, terrestrial plant sporopollen, isotopes, magnetic susceptibility, and so on, in order to reflect the objective fact. A comparative study was conducted among marine rock cores, Antarctica ice cores and loess plateau of China in order to reveal the teleconnection between regional climates under global climatic change setting.Thirdly, this paper did research on lake surface process, including shallow ground water evaporation, surface crust salt flux, elements geochemistry and so on.Through the research on paleo-climatic and environmental evolution, we could get large amount of climatic and environmental evolution information on Owens Lake in the past 200ka.B.P.(1) Between 200 and 180ka.B.P,There are no diatoms existing in the lake.From 180 to 120ka.B.P,72 to 65ka.B.P.,50ka.B.P. to the early Holocene, the freshwater diatoms were abundant in the lake;From 120 to 72ka.B.P.,65 to 50ka.B.P.,the saline diatoms were abundant in the lake.(2) Between 155 and 140ka.B.P. during the penultimate glaciation, the climate was very cold, and Owens Lake was fresh and overflowing. Between 140 and 123 ka.B.P,the lake became closed with high biological productivity, and salinity climbed to reach a maximum around 126.5ka.B.P. After 126 ka.B.P, climate turned to wet and cold.A rise in level level occurred, and the lake had remained closed and saline until 122.7ka.B.P. Between 122.7 and 121ka.B.P., Owens Lake became a fresh and overflowing lake.Between 121 and 115ka B.P.,a cold/wet episode was sandwiched by two warm/dry climate.And lake level oscillated and the lake alternated between open and closed conditions.Between 115 and 113 ka B.P., the lake became a fast through-flow lake under cool and wet climatic conditions before entering another long-term closed state between 113 and 91ka.B.P, and the lake reached a second maximum ca 105-102 ka B.P.The lowest lake level occurred around 100ka.Between 91 and 84 ka B.P.,climatic conditions were wet and relatively cold.Owens Lake was a freshwater, overflowing lake with relatively low productivity.Between 84 and 72.3ka.B.P., the lake was closed again. Between 72 and 62.6 ka B.P., Owens Lake became a freshwater, overflowing lake again.(3) There are three major advances during last global-glacial cycle.The ages of the three correspond rather well to dates reported for Sierra Nevada moraines.The beginning of the last glaciation was marked by a significant glacier advance beginning at 78ka.B.P.and while Owens Lake was closed and highly saline.The advance continued until the lake began to overflow vigorously at 73 ka.B.P.The last glacial maximum happened around in 1.8ka.B.P.(4) From 16.2 to 15.5ka.B.P.,Juniper woodland dominated the Owens Valley and the climate was much wetter than today. From 15.5 to 13.1 ka.B.P., the climate became fairly warm and dry, with woodland being replaced by shrubs.Next, Chenopodiaceae increased, woody species declined, and lake levels fell-all evidence for a brief drought about 13 ka.B.P., The climate continued to oscillate between wet and dry from 13 to 11 ka.B.P., After 11 ka.B.P., low lake levels and the increased dominance of desert shrubs indicate the beginning of warm, dry Holocene conditions.(5) Between 27 and 15.3ka.B.P.,the lake regressed from 1160 to 1145m.By 11.6ka.B.P, the lake had dropped 45m from the 1145m.This lowstand was followed by an early Holocene transgression that attained a highstand near 1135m before dropping to 1120m at 7.86-7.65ka.B.P.The lake lowered another 30m to shallow and near desiccation levels between 6.85-4.3ka.B.P.There was a minor lake-level rise after 4.3 ka.B.P.,followed by shallow conditions during the latest Holocene.(6) The climate history of the past 1000yr as follows:Between A.D.950 and 1760yr, the climate can be divided into three intervals at a 270-yr duration for each, with the first interval (A.D.950-1220yr) and the third interval (A.D.1480-1760yr) corresponding to the Medieval Climatic Anomaly and Little Ice Age, respectively. During the first interval, three dry periods sandwiched two wet ones, with an 50yr duration for each period.The climate of the second interval (A.D.1220-1480yr) was wet, with a short dry spell about A.D.1395yr.The third interval saw the return of dry climate, but it was wetter and perhaps colder than the first interval, particularly in the period prior to A.D.1620yr.It also was characterized by the presence of dry/wet cycles with a 30-to 50-yr periodicity. Diversions since about A.D.1880yr have converted Owens Lake a playa.Based on the research of the surface process for Owens Lake playa, we could properly draw conclusion as follows:(1) The solute profiling technique has provided a long-term average analysis of evaporation,and is relatively insensitive to short-term fluctuations.The eddy correlation and microlysimeter methods are real-time measurements of evaporation flux.The chloride profile estimates be considered a lower bound of the evaporative flux,and other independent methods be used to provide a more accurate representation of the actual flux.(2) Pyrite is the predominant sulfur-bearing phase in the anoxic capillary fringe and ground-water zones.The availability of reactive Fe limits formation of iron sulfides.There is evidence of bacterial sulfate reduction in the surface sediments and shallow groundwaters.The groundwater and sediments are an open system with respect to the availability of sulfate.The vertical mixing initiated by instability of water density due to evaporation between upper and lower sedimentary layers is likely to promote convective mixing in the sediment and provides an additional mechanism to diffusion to ensure adequate sulfate supply in the sediment. (3) Ground waters in Owens Lake are relatively depleted in Ca with respect to alkalinity,and evolve into high alkalinity, high PH brines as they become concentrated near the lakebed surface.Precipitation of carbonate minerals is followed by precipitation of halite,and finally mirabilite or thenardite.The most important factor that contributes to the mobility of As and F in shallow ground waters of Owens Lake is the low concentration of Ca relative to carbonate alkalinity in the source ground waters.(4) Arsenic distribution, speciation and redox chemistry in the shallow groundwater of Owens Lake were strongly affected by groundwater inflows and evaporation.Total dissolved As concentrations appear to increase conservatively as evaporation concentrates solutes.As a result,dissolved arsenic concentrations increase away from the historic shorelines toward the center of the lakebed and decrease with depth in the upper 3 m of the groundwater column.Inorganic arsenic species represent the majority of total dissolved As.Eh regulates the distribution of As (â…¢) versus As (â…¤).Redox chemistry over the majority of the dry lakebed is regulated by sulfate reduction.As (â…¢) is the dominant species of arsenic in the Owens Lake shallow groundwater.The low apparent sorption capacity coupled with the high solubility of As (â…¢) minerals results in the accumulation of arsenic primarily in the solution-phase.Only orpiment was likely to be over-saturated under highly reducing conditions.(5) Visual appearance and sedimentary structure of wind-erodible Owens Lake playa crusts do not correlate with elemental composition. Potentially toxic elements and high concentrations of salts may be present in any type of crust at a site.The surface sediments are geochemically well mixed, no matter their visual form.No clear trends of element concentrations consistently increasing or decreasing.(6) The 1872 earthquake produced a sedimentary record recognizable over most of the Owens Lake area.Seismic shaking fluidized and deformed bedding and created small fault offsets at several sites on the lake floor.A seismically induced wave eroded much of the lake bed surface and deposited a poorly sorted graded pebby sand.Offsets of the Owens Valley fault elsewhere in the valley indicate that at least two additional large earthquakes occurred during the Holocene,which is consistent with the observations in Owens Lake records.