小喀湖有机碳记录的慕士塔格地区过去200年温度变化

摘要

青藏高原气候变化存在显著空间差异,相对于东部和南部地区,高原西北部的气候变化研究较为薄弱.本文利用慕士塔格地区小喀湖77 cm沉积总有机碳(TOC)序列,高分辨率(平均约3年)地重建了该区1812—2012年的温度变化历史.结果表明:小喀湖沉积物中的TOC主要来自受湖水温度控制的水生生物,其含量指示了流域温度变化.近200年来,慕士塔格地区总体呈现升温特征,在1812—1890年和1970—1980年气候相对寒冷;1980—2004年升温最为剧烈,其次为1864—1914年,分别对应于全球快速变暖和小冰期结束后的升温时期.上述历史有别于我国大陆和北半球的温度变化,即升温幅度最大时期为全球快速变暖期而非小冰期结束后时期(19世纪50年代至20世纪40年代);该区近十年来呈降温趋势,可能是气候由暖转冷的提早表现.慕士塔格温度变化历史既体现了高寒山区对全球变化的敏感响应,又表征了其在全球变化中的区域独特性和超前性.%Background,aim,and scope Significant spatial differences of (paleo-) climate change on the Tibetan Plateau (TP) has been revealed by researches. However,there are scarcer paleoclimate records in the northern than the southern and eastern regions,which hinders our fully understanding of the regional differences in climatic change on the TP. Little Kalakul Lake (LKL,38°27.67′ —38°28.62′N,75°01.77′ —75°03.02′E,3661 m.a.s.l) is located in the Muztag Ata region. LKL is a hydrologically open freshwater lake mainly fed by rainfall,connects to the upstream lake in the southwest,and has a lake outlet in the northeast. The area is dominated by westerly winds and has a dry and cold climate. Owing to be sensitive response to climate changes and little human activity,lake sediment in Little Kalakul Lake is ideal for reconstructing regional climate and environment change. To fill the gap about temperature change in northwestern TP,we reconstructed a temperature-proxy record with ～3-year resolution during AD 1812—2012,based on the total organic carbon (TOC) variation of lake sediment from Little Kalakul Lake in the Muztag Ata. Materials and methods In June 2013,a lake sediment core (6.28 m long) was collected from the eastern basin of LKL. The water depth of the sampling site was about 22 m. The core was sliced in the field at intervals of 1 cm. In this study,we analyzed samples of the upper 77 cm. Meanwhile,we collected 7 dominant terrestrial plant species,7 submerged plant species,1 emergent plant species and 1 algae sample (all above plant were unidentified species). The sediment chronology was constructed by measuring radionuclides (210Pb and 137Cs). The contents of TOC were obtained through total carbon contents subtract inorganic carbon contents,and Kjeldahl method was employed to determine the contents of TN. Results According to the results of 210Pb and 137Cs specific activity measurement,the average sedimentation rate of the upper 77 cm samples is 0.38 cm?a?1,and the sediment record covers about 200 years,reaching back to AD 1812. TOC contents of LKL sediment vary between 5.1％ and 12.2％ with an average of 7.4％. TN contents range from 0.4％ to 1.8％ with an average of 0.8％. In addition,the average C/N ratios of sediment samples,terrestrial plants,aquatic plants and algae are 9.1,29.4,14.3 and 1.9,respectively. Over the past 200 years,TOC contents of sediment increased generally in fluctuation,but grew remarkably during AD 1980—2004 and AD 1864—1914 (to 12.2％ from 6.2％ and to 8.9％ from 5.1％—the lowest,respectively),and decreased during AD 1812—1864,AD 1914—1980 and since 2004. TN contents had a consistent trend with TOC contents while smaller amplitude,resulting in the decreasing C/N ratios. Discussion C/N ratio has been commonly used to identify the source of organic matter. Normally,C/N ratios in algae are less than 10; C/N ratios in terrestrial plants are more than 20. The results showed that the C/N ratios of terrestrial plants were much higher than those of aquatic plants (P=0.009),C/N ratios of sediment were lower than those of terrestrial plants and similar to those of aquatic plants,which indicated that TOC of LKL sediment mainly originated from aquatic organisms. This coincided with LKL catchment conditions such as cold and dry climate,poor development of terrestrial plants. The growth of aquatic organisms in lakes is mainly controlled by the lake nutrient conditions and temperature changes. The relatively high TN contents demonstrated that Little Kalakul Lake was in good nutritional status. The TN contents had a consistent trend with the TOC contents while smaller amplitude,which demonstrated that temperature was the main controlling factor in the variation of the TOC contents of the LKL sediments. The consistent trend of the TOC contents of the Little Kalakul Lake sediments,Taxkorgan temperature record (1957—2012) and the δ18O record in Muztag Ata ice core also agreed with this conclusion. According to the sediment age and the variations of TOC contents,the relatively warm periods occurred in AD 1890—1970 and since AD 1980. Furthermore,temperature increased dramatically during AD 1980 —2004 and AD 1864 —1914 in this region,corresponding to the rapid global warming and the warming since the end of the Little Ice Age,respectively. However,there was a cooling trend in recent ten years,which might account for the relatively stability of the glaciers in the Muztag Ata. The temperature variation pattern in this region was comparable to those in Chinese mainland and the Northern Hemisphere,but responded more sensitively to global change. Moreover,in this region: (1) temperature increased fastest since 1980s,but not the end of the Little Ice Age (during 1850s—1940s); (2) the cooling tendency over the past decade may be an early signal of the tendency to climatic cooling. Conclusions In this paper,it was proved that TOC in LKL sediment originated from aquatic organism and was controlled by water temperature. Based on the historical variations of TOC,temperature had an overall increasing trend during the past 200 years. Specially,this region has experienced two cold periods: AD 1812 —1890 and AD 1970 —1980,and the warm periods had appeared after each cold event: AD 1890—1970 and since AD 1980. Although the temperature variation pattern in this region was comparable to those in Chinese mainland and the Northern Hemisphere,there were some differences. All above differences of temperature variation demonstrated the regional uniqueness and foresight ability of the climate change on the TP under the background of global change. Recommendations and perspectives Total organic carbon of Little Kalakul Lake sediment from Muztag Ata is a good proxy of temperature,more studies based on other variables (like hydrogen isotopes of leaf wax,nitrogen content) from Little Kalakul Lake sediment are needed to deepen our fully understanding of the climatic change in the Muztag Ata.