西藏纳木错沉积物单水方解石出现前后的环境变化

2005年在西藏纳木错水下60m处钻取一支332cm的湖芯,沉积物皆为灰黑-黑色碳酸盐粘土。对湖芯1cm间隔取样并进行X射线、扫描电镜、Sr/Ca和碳酸盐含量的分析。研究发现,纳木错湖底0～258cm沉积物中出现了单水方解石,扫描电镜下该矿物晶形完好,这是个亚稳定矿物,具有重要环境意义。利用碳酸盐含量（24.12%～54.52%）、Sr/Ca比值（<0.006）、方解石中Mg含量（MgCO₃mol%<3.325%）、石膏、粘土矿物（伊利石和镁绿泥石）、单水方解石成因和沉积速率讨论了单水方解石形成前后湖泊环境的变化。2.1cal.kaB.P.单水方解石开始出现,此时纳木错湖水性质推断为pH>8.6,mol Mg/Ca>6.5,Ca²⁺和SO^2-₄离子浓度足以沉淀少量石膏,演化至现代,表层湖水性质为pH=9.4,mol Mg/Ca=10.03～15.03,SO^2-₄浓度较高,Ca²⁺含量低,不足以沉淀石膏。单水方解石出现之前的3.0～2.1cal.kaB.P.时期,沉积速率低（0.134mm/a）,蒸发作用强度不稳定,湖水温度低,矿化度呈上升趋势。该矿物出现后的2.1～1.7cal.kaB.P.时期,沉积速率快（1.639mm/a）,矿化度稳定,气温低,1.8cal.kaB.P.温度达到最低值,为气候冷事件的表现。较快的沉积速率（>1.168mm/a）是纳木错单水方解石形成的重要原因之一,碳酸盐沉积加快和入湖碎屑物质增加是沉积速率加快的主要原因。

LATE-HOLOCENE LAKE ENVIRONMENT REFLECTED BY THE OCCURRENCE OF MONOHYDROCALCITE IN NAM CO,CENTRAL TIBET

A 332cm-long core was obtained in 2005 at a water depth of 60m from the Nam Co,Central Tibet.The core reveals continuous lacustrine accumulation of grey-black grey carbonate-clays.Samples were collected at 1cm interval and examined to identify the types of minerals by an X-ray diffractometer and scanning electron microscope.Sr/Ca and CaCO₃ content in sediments were analyzed by chemical methods.Monohydrocalcite(MHC),a rare and meta-stable carbonate mineral with important environmental significance,was found at 0～258cm(0～2.1cal.kaB.P.)in the core.This paper is aimed to elucidate the environment changes with the occurrence of MHC in this area.The mineralogy of MHC,gypsum and clays(illite and Mg-chlorite),MgCO3 mol% in low-Mg calcite(<3.325%),carbonate content(24.12%～54.52%),Sr/Ca(<0.006)in lacustrine sediments,and sedimentation rate in the core provide important information and can be used to reconstruct ancient chemical and physical conditions in the area.MHC with distinct crystals was first observed at the depth of 258cm(2.1cal.kaB.P.),indicating a condition of pH>8.6 and mol Mg/Ca>6.5,and with activities of microorganisms such as algae and bacteria in the lake water.The concentrations of Ca²⁺ and SO^2-₄ were high enough to precipitate gypsum at about 2.1cal.kaB.P.As a meta-stable mineral,MHC is developed chemically in modern waters(1.7g/L)with high Mg/Ca molar ratio(10.03～15.03),high pH(9.4),sedimentation rate of more than 1.168mm/a,and the presence of bacteria,algae,and diatom.The concentrations of Ca²⁺ and SO^2-₄ are too low to precipitate gypsum in modern lake water.During 3.0～2.1cal.kaB.P.,there was no MHC,and the sedimentation rate was 0.134mm/a with variable evaporation,low temperature of lake water,and increasing alkalinity.During 2.1～1.7cal.kaB.P.the sedimentation rate was 1.639mm/a along with stable evaporation,low temperature of lake water,and a cold event occurred at 1.8cal.kaB.P.As an important reason of the occurrence of MHC,the possible high sedimentation rate were likely caused by the high precipitation of carbonate and inflow of detrital materials.