江汉平原湖相沉积物元素地球化学记录揭示的12.8 cal. ka BP 以来气候环境演变

Evolution of climate in the Jianghan Plain: evidence from elemental geochemical records of lacustrine sediments since 12.8 cal. ka BP

关键词:  江汉平原  湖相沉积  元素地球化学  环境演变

Key words:  Jianghan Plain  lacustrine sediment  element geochemistry  environmental evolution

摘要:

以江汉平原JZ-2010湖相沉积剖面为研究对象，AMS ¹⁴C年代数据为框架，通过对湖相沉积物的元素地球化学记录进行综合分析与对比研究，重建了江汉平原冰消期末至全新世以来的古环境演化过程，并探讨了区域气候变化的可能驱动因素。结果表明：12.8—9.1 cal. ka BP，研究区气候总体上温凉偏干，但有逐渐向暖湿变化的趋势；9.1—6.3 cal. ka BP，气候温暖湿润，水热配置均匀；6.3—4.4 cal. ka BP，气候前期较为暖湿，后期有逐渐向干旱转变的趋势；4.4—1.3 cal. ka BP，气候经历4.2 ka冷干事件后逐渐向温暖湿润转变。硅铝率、CIA和Rb/Sr指标的变化趋势与太阳辐射量、北大西洋赤铁矿染色颗粒及底栖有孔虫壳体δ¹³C记录之间存在较好的对应关系，揭示出江汉平原12.8 cal. ka BP以来的气候变化受控于北大西洋深水流的变化，同时，中晚全新世以来气候变化趋势与北半球夏季太阳辐射变化趋势较为吻合，太阳辐射主要通过季风环流间接影响区域气候变化。

Abstract:

Background, aim, and scope The Jianghan Plain is a typical flood plain composed of inter-river depressions, with alternating fluvial and lacustrine sediments that are sensitive to climate change and contain a large amount of paleoclimate information. Therefore, it is an ideal place to study paleoclimate. Previous studies have mostly concentrated on magnetic susceptibility, particle size, and pollen assemblages. However, it is insufficient to reconstruct the climatic and environmental evolution of Jianghan Plain from the end of the deglacial to the Holocene through the combination of regular trace elements in lacustrine sediments and geochemical analysis methods on a longer time scale. Based on the analysis of lacustrine sedimentary geochemical elements, we aimed to reconstruct the climatic and environmental evolution of the Jianghan Plain from the end of the deglacial to the Holocene and discuss the possible driving factors of this change. Materials and methods A lacustrine sedimentary section (numbered JZ-2010) in Jingzhou, with a thickness of 637 cm was selected for sampling, which was composed of silty clay rich in organic matter. Furthermore, only the lacustrine sedimentary section in the middle and lower parts was sampled, and that in the upper part was not discussed due to flood disturbances. The lacustrine sedimentary section in the middle and lower parts was further subdivided into nine layers, and 206 samples were collected from bottom to top at 2 cm intervals. X-ray fluorescence spectroscopy (XRF) was used to determine the composition of geochemical elements. To establish the sedimentary chronosequence, six samples were collected from the middle and lower parts of the section for accelerator mass spectrometry radiocarbon (AMS ¹⁴C) dating. The samples ware jointly performed by the AMS ¹⁴C Sample Preparation Laboratory of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and the State Key Laboratory of Nuclear Physics and Technology, Peking University. Results The results showed that from 12.8 to 9.1 cal. ka BP, the climate of the study area was generally cool and dry, with a trend of gradually changing to warm and humid. From 9.1 to 6.3 cal. ka BP, the climate was warm and humid, which should be the beginning of the Holocene Optimum. From 6.3 to 4.4 cal. ka BP, the environmental proxies changed obviously, indicating unstable environmental fluctuations in the early stage, and there was a trend that the climate changed to drought gradually. From 4.4 to 1.3 cal. ka BP, the climate gradually changed to warm and humid after the 4.2 ka cold-dry event. Discussion We found a significant relationship between SiO₂/Al₂O₃ ratios, chemical index of alteration (CIA), Rb/Sr ratios, solar radiation, a record of hematite-stained particles in North Atlantic, and δ¹³C of benthic foraminifera shells, revealing that the climate change in the Jianghan Plain since 12.8 cal. ka BP has been driven by the variation of North Atlantic Deep Water (NADW). The millennial cold-dry event might have established an ocean-atmosphere linkage mechanism affecting climate change with the Atlantic through the low-latitude Pacific and its upper atmosphere. At the same time, the trend of climate change was consistent with the reduction of the northern hemisphere summer solar radiation since the Middle and Late Holocene, and the solar radiation indirectly affected regional climate change mainly through monsoon circulation. In addition, the climate in the Jianghan Plain was strongly suppressed by the Laurentide Ice Sheet in the high latitude of the northern hemisphere, resulting in a more severe climate during the Early Holocene. Conclusions The five environmental proxies of Ti, CIA, Rb/Sr, SiO₂/Al₂O₃, and SiO₂/Fe₂O₃ ratios in the JZ-2010 section of the Jianghan Plain significantly recorded the abrupt climate events, such as YD, 9.5 ka BP, 5.3 ka BP, and 4.2 ka BP events, while there was no obvious record of the 8.2 ka BP event. By comparing the solar radiation record, the North Atlantic hematite staining particles and benthic δ¹³C records with the variation characteristics of environmental indicators in the Jianghan Plain, we found that solar radiation might be one of the main driving factors of the millennial-scale climate change in the Middle and Late Holocene. There was a certain correlation between the NADW, North Atlantic drift ice events, and SiO₂/Al₂O₃, Rb/Sr, and CIA, indicating that the air-sea coupling process would cause regional climate change by influencing the atmospheric circulation model. Therefore, in the Jianghan Plain, the Early Holocene climate change was dominated by NADW, and the Middle and Late Holocene climate change was driven by NADW and the northern hemisphere solar radiation. Recommendations and perspectives The results of this study provide possible driving mechanism of climate change in the Jianghan Plain. Furthermore, more research should be conducted to deepen understanding of the possible driving factors of paleoenvironmental evolution from the end of the last deglaciation to the Holocene.