东北哈尼泥炭记录的早全新世长白山火山喷发及其古气候意义

EARLY HOLOCENE VOLCANIC ERUPTION DISCOVERED IN HANI PEAT BOG OF NE CHINA AND ITS PALEOCLIMATE IMPLICATION

泥炭作为古气候与环境载体,对火山活动及其气候效应有着良好记录.文章采用酸化法成功提取出东北哈尼泥炭沉积样品600～625cm的火山灰层.将制成的薄片在偏光显微镜和扫描电镜下观察,鉴定发现火山灰颗粒的粒径都小于2mm,主要由玻屑组成,含有长石和辉石等矿物,其颗粒呈气孔、海绵状,结构松散,形态极不规则.使用电子探针技术对火山灰化学成分进行测试,数据显示SiO2含量为62％ ～ 66％ （wt）,Al2O3含量为19.13％ ～24.13％ （wt）,Na2O含量为4.91％ ～6.68％ （wt）,K2O含量为2.16％ ～4.39％ （wt）,呈现高Na2O低K2O的特点,元素组成与长白山天池火山岩成分相似,为粗面岩.火山灰层位边界的泥炭纤维素AMS 14C年龄为9337～ 10745cal.aB.P.,该层火山灰可能源于早全新世长白山火山喷发;对比哈尼泥炭中δ18O和腐殖化度等气候代用指标曲线,发现该次火山喷发事件导致气温降低、降水减少,对区域的早全新世气候环境有着重要的影响.

The Hani peat bog （42°13＇N, 126°31＇E） is situated in the Hani Village of Liuhe County, Jilin Province, Northeast China, and the height is about 900 meters above sea level. The peatland accumulated in the Holocene, with its sectional maximum thickness of 9.6m. It is the largest peat deposit in Northeast China, whose development is through Holocene. The sampling site is near the center of the Hani peatland and the drill core is 900cm long. Tephra were discovered in the 600-625cm sandy peat layer of Hani peat core by extraction and identification. Combined with the AMS 14C dating data of peat cellulose, used the age-depth modeling, the age of tephra layers had been calculated to 9337- 10745 cal.a B.P. Observed the sample sheets with polarizing microscopy and environmental scanning electron microscope, identification shows that volcano ash is mainly composed of vitric fragment and concludes other minerals, such as feldspar and pyroxene. The particles have very irregular pores and spongy shape with loose structure. Chemical composition of volcano ash is tested by electron microprobe techniques. Statistic shows that content ofSiO2 is 62% -66% （wt）, content of A1203 is 19.13% -24.13% （wt）, content of Na20 is 4.91%- 6.68% （wt）, and content of K20 is 2.16% -4.39% （wt） which exhibits a character that Na20 content is high while K/O content is low. The elemental composition is similar to Tianchi volcano of Changbaishan Mountain, which is trachyte. The conclusion reveals that volcano eruption is in 9337-10745cal.a B.P., from Early Holocene. The result implies that the volcano ash layer is situ deposit from explosive eruption of proximal volcano, which originated from eruption of Changbaishan Mountain Tianchi volcano in Early Holocene; comparison of δ18O and humification degree curves shows that the volcano eruption cause temperature decline, precipitation reduction and has an important effect on climate environment in Early Holocene.