岱海沉积物元素地球化学特征反映的末次冰期以来季风/干旱过渡区的水热条件变迁

HYDROTHERMAL STATUS IN THE MONSOON/ARID TRANSITION BELT OF CHINA SINCE THE LAST GLACIATION INFERRED FROM GEOCHEMICAL CHARACTERISTICS OF THE SEDIMENT CORES AT DAIHAI LAKE

对季风/干旱过渡区封闭湖泊岱海两支沉积物岩芯进行了元素地球化学特征分析。结果揭示,沉积物中易(溶)迁移元素和难(溶)迁移的相对含量及其与铝元素的比值可以很好地表征流域化学风化强度的变化,进而指示盆地的水热状态。过去大约16.2kaB.P.以来,易迁移元素含量以及与铝元素比值较高的几个阶段,如大约14.5～11.7kaB.P.,10.3～9.0kaB.P.,6.7～3.5kaB.P.,3.2～2.6kaB.P.和1.7～1.2kaB.P.代表了岱海盆地化学风化较强、水热条件较好的时期,其中以大约6.7～3.5kaB.P.期间的水热条件最为优越,可以认为是流域气候的适宜期。而沉积物中难(溶)迁移元素在大约14.5kaB.P.前,11.7～10.3ka B.P.,3.5～3.2kaB.P.和2.6～1.7kaB.P.期间以及1.2kaB.P.以来的相对高含量则代表了流域相对较弱的化学风化强度和较为低下的水热状态。大约9.0～6.7kaB.P.期间,沉积物较高的CaO/MgO比值与降低的易迁移元素含量指示了流域较高温度条件以及由此导致的高蒸发率。综合岱海沉积物已有的有机质、碳酸盐、花粉组合、湖面变化等方面的数据,认为岱海盆地末次冰期以来,水热同期的模式涵盖了岱海除大约9.0～6.7kaB.P.外的各个阶段。而大约9.0～6.7kaB.P.期间,季风有效降水与温度变化的不一致现象可能更多地反映了岱海自身的自然环境特点对区域大尺度气候变化的反馈。

Two sediment cores retrieved from the enclosed Lake Daihai in the monsoon/arid transition belt of north- Central China were analyzed for major and trace elements and their ratios normalized to AI to investigate the history of changes in the hydrothermal status of the study area based on the chemical weathering intensity（CWI） in the lake basin. The data indicate that weak CWI, which is characterized by high contents of insoluble, resistant elements （ Al2O3, TFe2O3, SiO2, K2O, Ti, Cr, Mn, Ni, Cu, Zn and Rb） and large ratios of the elements to Al （ （ CaO ＋ MgO ＋ Na2O）/Al2O3） , occurred prior to ca. 14.5kaB. P. and during the intervals of 11.7 - 10. 3kaB. P., 3.5 - 3.2kaB. P. and 2.6 - 1.7kaB. P. and after 1.2kaB. P., denoting declines in the hydrothermal condition. Strong CWI as suggested by high contents of mobile, soluble elements （CaO, MgO, Na2O and Sr）and low ratios of the elements to AI occurred during the intervening episodes of ca. 14.5 -11.7kaB. P., 10. 3 -9. 0kaB. P.,6.7 - 3.5kaB. P.,3.2 -2.6kaB. P. and 1.7 - 1.2kaB. P.,implying ameliorations of the hydrothermal status in the lake basin. During the period of ca. 9.0 -6.7kaB. P., CWI was dramatically low, and CaO/MgO ratio was relatively higher, presumably suggesting warm temperatures and the consequent, increased evaporation rates in the lake region. As a whole, CWI displays in-phase variations with organic matter （TOC, TN）concentration, carbonate （ CaCO3 ） content and pollen assemblage that are related to effective monsoon precipitations. Thus high CWI would be associated with strong monsoon-induced precipitation and low CWI with weakened precipitation. The interval between ca. 6.7kaB. P. and 3.5kaB. P. ,which is marked by the highest CWI and maximum monsoon precipitation, could be defined as the Holocene Climate Optimum in the Lake Daihai region. Changes in the hydrothermal status in the study area during the most part of the last 16.2ka coincide with the pattern of climate changes in Eastern China. The discrepancies during the interval of ca. 9.0 - 6.7kaB. P. might be related to the feedback of topography of the lake basin to regional climate changes.