Chronology And Environment Change Since The Last Deglaciation Recorded By Lake Sediments From Gonghai Lake, Shanxi, China

Lake sediments are the ideal materials for Quaternary paleoclimate study and chronology is the important basis and prerequisite for the reconstruction of paleoclimate. However, there are still some difficulties and shortages to use the radioacarbon chronology in lake sediments dating study for lack of ideal dating material in lake sediments. Terrestrial plant macrofossil are usually regarded as ideal material for dating in lake sediments and the dating results of submerged plant macrofossil and bulk organic samples are usually older for the influence by radiocarbon reservoir. For numerous lakes sediment samples, the amount of terrestrial plant macrofossils are hard to meet the needs of high resolution dating framework reconstruction and other dating materials like bulk organic samples should be added. Therefore, the study of reservoir effect in lake sediments plays an important role on establishing age framework.The study site Gonghai Lake, a typical alpine fresh lake, is located in the northwest margin of north China, the north trail of Lvliang Mountains. There is no in-washed river and the main water source is from precipitation and groundwater. Sandstone is the main bedrock in the small drainage basin and the origin of lake sediments is simple, therefore the lake is an ideal place to study the cause and evolution of reservoir effect. In addition, the lake lies in the convergence zone between present Asian summer monsoon region and inland arid area. The reconstruction of paleoclimate change since the last deglaciation has important significance for understanding the mechanism and regional differences of climate change in the margin of Asian summer monsoon.The present paper focuses on the dating problems in lake sediments. By using the chronology methods of radioacarbon,210Pb and 137Cs to sediments from Gonghai Lake, we heavily debate the influence of reservoir effect on lake sediments dating. We establish the reservoir that changes with time by comparing the dating results of terrestrial plant macrofossils and bulk organic samples in the same layer. After calculating the reservoir age, we establish high resolution, high quality age framework for the lake sediments cores. With the support of grain size, magnetic susceptibility, carbonate content, total organic carbon content and tree pollen, we reconstruct the evolution history of environment since the last deglaciation and get the following conclusions and cognition:(1)The basin of Gonghai Lake was formed during the Cenozoic tectonic movement and there are no robust evidences on ice erosion effect. Water injected the lake basin at about 14.7ka BP and the main lake water source is groundwater. Then the lake experienced a high lake level during the early and middle Holocene (11.5-3.0 ka BP). The continuous sediments provide ideal material to study paleoclimate and palaeoenvironment in the study area.(2)Through analysis and comparison of radiocarbon specific activity of different terrestrial and aquatic plants and lake water, we establish the carbon reservoir of the lacustrine surface sediments and propose that emergent aquatic plants are also affected by carbon reservoir effect, with the same radiocarbon specific activity as submerged plants.(3) Based on the changing characteristics of reservoir effect and combined with carbonate content index, we propose that the characteristic of the reservoir effect in Gonghai Lake is mainly showed as hard water influence and the old carbon are originated from the recharge of groundwater and atmospheric dust, carbonate content and terrestrial organic matter input (C/N).(4) We reconstruct the evolution history of environment since the last deglaciation in the study area through multiply environmental proxies.Supported by sediments palynology and environmental magnetism evidence, we reconstruct vegetation evolution history in the catchment since the last deglaciation. From 14.7 to 13.1 ka BP, a period of climate transformation, it was marked with a shift from rare vegetation to dense vegetation coverage. During the YD events (13.1-12.0 ka), the sedimentary environment maintained deterioration and lasted for 1.1 ka. During the early and middle Holocene (12.0-3.0 ka), lake sedimentary environment experienced a rapid transformation and presented a good state with high lake level and high vegetation coverage. During the late Holocene (3.0 ka so far), lake sedimentary environment appeared larger fluctuation due to the monsoon depression and increasing human activities.