短时间尺度上湖泊沉积物粒度指示的气候意义--以朝那湫为例

PALEOCLIMATIC IMPLICATIONS OF LACUSTRINE SEDIMENTS GRAIN SIZE IN SHORT TIME SCALES:A CASE STUDY OF LAKE CHAONAQIU

湖泊沉积物粒度在短时间尺度、高分辨率(年际-十年尺度)古气候研究中的指示意义通常被解释为:粒度偏粗指示降水较多、气候湿润,粒度偏细指示降水较少、气候干旱。本文以六盘山朝那湫湖泊73 cm沉积物岩芯粒度指标为研究对象,采用137Cs定年法,重建了朝那湫流域过去268年的高分辨率气候变化序列,结合同一岩芯其他环境代用指标(TOC、C/N值、化学元素等)指示的环境变化信息,对朝那湫湖泊沉积物粒度指示的气候意义进行综合判别。结果表明:朝那湫湖泊沉积物粒度与其他环境代用指标有较好的相关关系,粒度指标能够指示流域降水变化历史,并记录了4次典型的干旱气候事件(1770 A.D.前后、1835 A.D.前后、1875 A.D.前后和1970 A.D.前后),这与邻近地区树轮等气候记录有良好的对应关系,表明本文重建的结果可靠。同时,本文揭示了湖泊沉积物粒度在短时间尺度研究中的特殊指示意义:粒度偏粗指示降水较少、气候干旱,粒度偏细指示降水较多、气候湿润,这主要是由流域降水特点和植被覆盖程度决定的。由于沉积环境复杂,湖泊沉积物粒度的环境指示意义具有多解性,因此在不同的湖泊研究中应充分考虑影响沉积物粒度的各种因素,并结合其他指标进行综合辨识。

In short time scales and high resolution(annual or decadal scales)studies,the variations of grain size in lacustrine sediments are often used to indicate the changes of paleoclimate,namely,coarse-grained sediments reflect elevated precipitation and thus wetter climatic conditions;while fine-grained sediments indicate lower precipitation and thus drier climatic conditions.However,the paleoclimatic implications of grain size in lacustrine sediment reveal a complex spatial pattern in different lakes due to site-specific conditions,such as lake/catchment ratio,vegetation cover,and other factors.Thus,it should limit our understanding of long-term paleoclimate variability.Here,the paleoclimatic implications in short time scales of grain-sizes of Lake Chaonaqiu sediments were discussed,based on the characteristics,the geographical location and the significances of multiple proxy indices from lake sediments of Lake Chaonaqiu.Lake Chaonaqiu,a small open alpine lake in the Liupan Mt.(the lake area is ca.0.02 km2and the catchment area is ca.0.2 km2,altitude in 2430 m a.s.l.),is located on the western Loess Plateau(LP),China.A 73-cm long sediment core was obtained in the centre of the lake(35°15'53.08"N,106°18'35.99"E)using a 60-mm UWTTEC gravity corer,in September 2012.The lithology is mainly consistent of lacustrine black and brown silt.The chronology was well established by 137Cs activities for the past 268 years.A total of 73 samples were taken at 1 cm intervals for grain-size,TOC,TN,C/N ratio andδ13Corg analysis,and 37 samples were collected at 2 cm intervals for XRF analysis.The sediments was mainly composed of silt(4~63μm),fine silt(4~16μm)and coarse silt(16~63μm)accounted for 40.20%~58.00%and 19.20%~37.00%respectively,the clay(<4μm)accounted for 17.10%~30.90%,and the content of sand component(>63μm)was the lowest,ranging from 0~4.80%.Based on the well correlation of grain-size and other proxy indices of Lake Chaonaqiu sediments,we suggested that the variations of grain-size of Lake Chaonaqiu sediments could effectively indicate paleo-precipitation history in the study area.Fine-grained size indicates humid climate conditions,because the densely vegetation cover in the catchment during wetter period not only stabilizes the surface soil,but also weakens the load capacity of surface runoff,which result in fine-grained size entering into the lake.However,coarse-grained size indicates arid climate conditions,because the sparsely vegetation cover during drier periods exposes more soil surface area,and produces a less cohesive soil,even relatively low-intensity rainfall could create a stronger surface runoff load capacity,resulting in coarse-grained size entering into the lake.Therefore,four drought climate events at Lake Chaonaqiu area were identified(e.g.,ca.1770 A.D.,1835 A.D.,1875 A.D.,and 1970 A.D.),which correlate well with the records of tree rings and historical documents in neighboring regions.