云南湖泊碳库效应

The radiocarbon reservoir effects of lakes in Yunnan

具有高精度年代序列的湖泊沉积物是研究古气候的理想载体,而碳库效应是制约精准年代序列建立的主要障碍之一。选取云南地区13个湖泊的年代数据,通过现代校正法、线性回归法和地层关联法等碳库校正方法,探讨了云南地区可能存在的湖泊碳库效应及其影响因素。结果表明,泸沽湖、程海、洱海、大理西湖、云龙天池、腾冲青海、星云湖、杞麓湖、阳宗海和滇池10个湖泊均存在较大的碳库效应,勐梭龙潭、异龙湖和抚仙湖的碳库效应则极小或不存在;云南地区湖泊的平均碳库年龄为1010年,碳库效应介于我国青藏高原、西北地区和北方地区、南方地区之间,这与云南地区碳酸盐岩基岩分布广泛、植被密集且湖泊生产力较高有关。从空间上看,云南地区的湖泊碳库效应有明显的差异,滇中的阳宗海碳库年龄高达2642年,滇西各湖的碳库年龄均低于2000年,滇南的勐梭龙潭碳库年龄仅有105年。在碳酸盐岩广布的云南地区,基岩性质可能是碳库效应形成的主导因素;此外,湖泊汇水面积和补给水源等也影响了抚仙湖和腾冲青海等湖泊的碳库效应。从时间上看,湖泊水文状况及流域植被覆盖度的变化可能导致了星云湖、腾冲青海、程海、泸沽湖和云龙天池等湖泊碳库效应的时间变化,比如泸沽湖深部岩芯的碳库年龄与现代碳库年龄相差较大,云龙天池的碳库年龄随着深度的增加而增大。研究表明,云南地区碳库效应时空变化的复杂性对建立精确定年有较大影响,因此厘清湖泊碳库效应大小及时空变化特征对建立可靠的湖泊沉积物年代序列十分重要。

Yunnan(21°08'32″~29°15'08″N,97°31'39″~106°11'47″E),located in the Southwest China under the influence of the South Asian and East Asian monsoons,is one of the regions sensitive to climate change in China.A lot of high-quality paleoclimatic and paleoenvironmental records have been obtained from studies on lake sediments in Yunnan.Most of those studies rely on 14C dating data to establish their chronological sequences,however,the lake radiocarbon reservoir effect affects the accuracy of 14C dating and thus challenges the establishment of high-precision chronological sequences.In this study,we selected the chronological data of 13 lakes in Yunnan to determine possible radiocarbon reservoir effects in those lakes using the calibration methods of radiocarbon reservoir effects such as the modern calibration approach,linear regression,and stratigraphic correlation.The results showed that ten lakes,including Lugu,Chenghai,Erhai,Dali Xihu,Yunlong Tianchi,Tengchong Qinghai,Xingyun,Qilu,Yangzonghai and Dianchi Lakes,have significant radiocarbon reservoir effects,whereas Mengsuolongtan,Yilong,and Fuxian Lakes have minor or no radiocarbon reservoir effects.The average radiocarbon reservoir age in Yunnan is 1010 years,indicating that its radiocarbon reservoir effects lie between the Tibetan Plateau/Northwest China(large)and North/South China(small),probably due to the wide distribution of carbonate bedrock,dense vegetation,and high lake productivity in Yunnan.Spatially,significant differences are observed in lake radiocarbon reservoir effects among different regions in Yunnan.The radiocarbon reservoir age in Yangzonghai Lake of central Yunnan is as high as 2642 years,whereas it is below 2000 years in all lakes of western Yunnan,and it is just 105 years in Mengsuolongtan Lake of southern Yunnan.The nature of the bedrock may be the dominant factor responsible for this difference.The radiocarbon reservoir effects are large in Lugu,Chenghai,Erhai,Dali Xihu,Xingyun,Qilu and Yangzonghai Lakes where the bedrock is carbonate,while the radiocarbon reservoir effects are minimal in Mengsuolongtan Lake where the bedrock is not carbonate.Furthermore,the water catchment area and recharge water sources also affect the radiocarbon reservoir effects of some lakes such as Fuxian Lake and Tengchong Qinghai Lake.The bedrock around the watershed may lead to differences in the ages of radiocarbon reservoir effects at different locations in the same lake,for instance,the different locations in the Erhai Lake have 1282 and 300 years in reservoir ages respectively.Temporally,the hydrologic conditions of the Xingyun Lake resulted in reservoir ages from 960 to 2200 years over the last 8500 years,while Tengchong Qinghai Lake has reservoir ages from 180 to 870 years over the past 15000,probably due to the influence of dense vegetation cover in the catchment of the lake.The reservoir effects in Chenghai,Lugu and Yunlong Tianchi Lakes are also not constant in the time sequence,for example,the reservoir ages in Yunlong Tianchi Lake increase with depth.The complexity in the spatial and temporal variations of reservoir effects significantly affects the establishment of accurate dating,it is thus critical to clarify the ranges and spatio-temporal variations of lake reservoir effects before establishing a reliable chronological sequence of lake sediments.