巢湖碳氮地球化学沉积记录揭示全新世以来的环境演化

Environmental evolutions derived from carbon and nitrogen geochemistry of lacustrine sediments since the Holocene in the Chaohu Lake,East China

通过对巢湖湖泊沉积物的总有机碳(TOC)、总氮(TN)、有机碳同位素(δ^(13)Corg)、氮同位素(δ^(15)N)和粒度参数的研究,重建了全新世以来湖泊初级生产力的变化,从而揭示了巢湖地区全新世环境变化历史。早全新世时期(10100—8200 cal a BP),较高的TOC和TN、在波动中逐渐升高的δ^(13)Corg值以及高δ^(15)N值、高黏土(<4μm)含量指示了较高的湖泊初级生产力,反映气候温暖较湿;中全新世时期(8200—4200 cal a BP),TOC、TN、δ^(13)Corg值和黏土(<4μm)含量总体上较高但呈波动降低趋势,粉砂(4—64μm)含量升高,δ^(15)N值总体较低但变化幅度较大,指示初级生产力由较高水平开始逐渐降低,气候由温暖湿润逐渐向凉干方向发展;7300—5800 cal a BP和4300—3800 cal a BP的低δ^(15)N和高TN值可能受到固氮蓝藻的影响,分别对应着7500—7000 cal a BP和4200—4000 cal a BP气候事件时期夏季风强度的减弱;晚全新世前期(4200—2000 cal a BP),TOC、TN、δ^(13)Corg值、黏土(<4μm)含量的低值和较高的δ^(15)N值、高粉砂(4—64μm)含量代表初级生产力继续降低,气候环境进一步向冷干转变。全新世以来湖泊初级生产力的逐渐下降反映了季风的逐步减弱,而逐渐减少的30oN夏季太阳辐射是驱动季风减弱的主要因素。晚全新世后期(2000 cal a BP至今),各环境代用指标的变化表现出不同于全新世环境演化趋势的异常,说明湖泊环境及沉积物受人类活动的影响加剧。

Study of total organic carbon(TOC),total nitrogen(TN),organic carbon isotope(δ^(13)Corg),nitrogen isotope(δ^(15)N)and grain size parameters in the lacustrine sediments of Chaohu Lake reconstructed the changes in the primary productivity of the lake since the Holocene,thus revealing the history of Holocene environmental changes in the Chaohu Lake area.During the early Holocene(10100-8200 cal a BP),higher TOC and TN,gradually increasingδ^(13)Corg values amid fluctuations,as well as highδ^(15)N values and high clay(<4μm)content indicated higher lake primary productivity,reflecting a warm and wetter climate.During the middle Holocene(8200-4200 cal a BP),the TOC,TN,δ^(13)Corg values and clay(<4μm)content were generally higher but showed a fluctuated decreasing trend;silt(4-64μm)content increased,andδ^(15)N values were generally lower but with a large variation,indicating that the primary productivity gradually decreased from a higher level and the climate gradually changed from warm and wet to cool and dry;and the lowδ^(15)N and high TN values of 7300-5800 cal a BP and 4300-3800 cal a BP might be influenced by nitrogen-fixing cyanobacteria,corresponding to the weakening of summer monsoon strength during the climatic events of 7500-7000 cal a BP and 4200-4000 cal a BP,respectively.In the earlier stage of late Holocene(4200-2000 cal a BP),the low values of TOC,TN,δ^(13)Corg,clay(<4μm)content and higherδ^(15)N values,high silt(4-64μm)content represented the continued decrease of primary productivity and further shift of climate to cold and dry.The gradual decrease in primary productivity of the lake since the Holocene reflected the gradual weakening of the monsoon,and the gradually decreasing 30°N summer solar radiation was the main factor driving the weakening of the monsoon.In the later stage of late Holocene(2000 cal a BP to present),changes in various environmental proxies showed anomalies that differed from the Holocene environmental evolutionary trends,suggesting that the lake environment and sediments have been influenced by intensified human activities.