北极巴罗Elson湖过去450年气候与环境变化记录

北极阿拉斯加巴罗地区Ｅｌｓｏｎ湖长６０ｃｍＡＢ—６７钻孔岩芯提供了过去４５０年连续的高分辨环境变化记录。综合分析²¹⁰Ｐｂ测年、沉积物粒度、有机质、化学元素和微体古生物化石等资料，一致反映出巴罗地区过去４５０年的气候、环境变化过程存在如下３个阶段：１)大约１５４０～１７４０年，为低海面寒冷时期；２)１７４０～１８２７年，为气候转暖－海侵过渡时期；３)１８２７年至今，为继续海侵－气候波动变暖时期。其中１８００年前后和１９４０年前后，气温较高。１９２１～１９９３年巴罗的气温记录说明，北极地区气温不仅有明显的１０ａ和６０ａ周期变化，而且年平均温度变幅极大，达４．５℃。显示极地气候变化特别强烈并不反映大幅度升温。近２０年来的变暖趋势是１８２７年以来自然的气候波动变暖的继续。

CLIMATIC AND ENVIRONMENTAL CHANGES OVER THE PAST 450 YEARS RECORDED IN ELSON LAGOON AT BARROW,ALASKAN ARCTIC

Under cooperative arrangement between Chinese Academy of Sciences and the North Slope Borough, Alaska, Zhang Qingsong and Hou Shugui, two of the authors of this paper made a four-week field investigation at Barrow from August to September, 1994. Barrow is located in the northeast of Alaskan coastal plain (71 ° 20′ N, 156°40′W), which is encircled by the Chukchi Sea in the west and the Beaufort Sea in the north. Thus, it is considered to be a key region for studying the land-ocean interaction related to climatic and environmental changes.Oimatically, Barrow is a quite cold area with the annual mean temperature of-12.4℃ and the mean winter and summer temperatures ranges between-29℃ and 2℃. Annual mean precipitation is less than 250 mm /a.For understanding the climatic and environmental changes in Barrow area, an unfrozen 60 cm deep lake deposit core (AB-67) was recovered at the water depth of 2m (2km to the bank of the Elson Lagoon) by using of Piston Corer. Based upon the analyses and measurements on 210Pb, grain size, orgahic materials, environmental geochemical elements and micro-fossils, etc., climatic and environmental records for the past 450 years at Barrow were reconstructed. In addition, some events related to sea level changes and future climatic changing tendency are also discussed.According to the measurements on 210Pb of the upper sector (0～20cm) and on the grain size of the whole core, the sedimentary time span of the whole core (60cm) is estimated to be about 450 years, i.e. 1540 ～1990 AD. It can be stratigraphically divided into tWo sectors. The upper sector (0～32cm) mainly consists of silt and clay with relatively high contents of organic materials while and the lower sector (32～60cm) mainly consists of fine sands and silts with small gravels. Grain size data showed that the driving force of sedimentation is stronger in the lower sector and weaker in the upper one. It probably indicated that the water depth increased with the time, going at the drilling site by the simpler carrying medium in the lower sector and complex one in the upper one. It is probably indicated to be the terrestial deposits from the depth of 50～60cm by the 46.5 × 10-6 contents of Boron while the sector above 50 cm depth Of the core is the marine Ones by the Boron contents of 50 to 75×10-6.In the Sectors below and above the 20cm depth of the core, the contents of major elements including SiO2, Fe2O3 and of trace elements, such as Ni, Cu, Zn, Pb,show clearly variations while the varying tendency of the trace elements is consistent with those of clay (smaller than 63 μp and 2μm in grain size) and of organic materials. Ths indicated the intensifying of chemical weathering in relation with climate warming.Assemblages of foraminifera, ostracoda and diatom in the core all showed that both marine and non-marine fossil lives appeared in the lower sector (30～60cm)of the core. This indicated that it was the deltaic /coastal depositional environment with frequent salinity varying and more fresh water feeding. Instead of all marine lives,the non-marine lives disappeared in the upper sector (0 ～30cm) of the core which reflected the transgresion to the lagoons followed the sea level rising.All data mentioned above showed that three stages of climatic and environmental changes occurred in Barrow area over the past 450 years: 1) Form 1540 to 1740 AD,it was a relatively cold stage with the low sea level which might be 2.30 ～ 2.60cm lower than that of present; 2) From 1740 to 1827 AD, it was a transitional stage of warming and transgresion; 3) Since 1827 AD, it was the fluctuated warming stage followed by sea level rising. These records from the Elson Lagoon are coincided to the ice core record of Greenland and the tree ring record of northern America which indicated that the Arctic area was being in the fluctuated warming stage since the mid-18th century after the Little Ice Age of 16～18th centuries. The basic reasons of these climatic changes is probably related to the solar activities.