| 摘要: |
| 盐度的重建对于理解历史时期的湖泊环境演变，以及预测未来的湖泊动态都具有重要价值，但目前对淡水湖泊盐度的重建还比较缺乏，限制了对淡水湖泊盐度演化的认识。文章对采自我国安徽巢湖1根总长度为143 cm的沉积柱（CHX）进行了210Pb年代学、粒度、磁化率、主微量元素等指标的分析，并尝试利用Sr/Ba重点重建了过去210 a（表层28 cm样品）盐度变化。结果表明：采样点的沉积速率约1.3 mm/a，样品中Sr元素的含量变化范围在104—147 mg/kg（平均值121 mg/kg）；Ba的含量变化范围为510—740 mg/kg，平均值605 mg/kg；Sr/Ba比值分布在0.16—0.26，平均值为0.20。巢湖沉积物中Ba含量与长江中下游地区湖泊沉积物相当，Sr含量明显小于海相沉积，Sr/Ba比值与文献中报道的淡水系统沉积物的Sr/Ba值接近，具有明显的陆相沉积特征。尽管巢湖是典型的淡水湖泊，盐度变化相对较小，但巢湖CHX沉积柱Sr/Ba比变化仍十分明显，这说明该指标对淡水湖泊的盐度变化也相当敏感。结合210Pb定年结果和Sr/Ba比，重建了该湖泊过去210 a的盐度记录，巢湖盐度在过去210 a下降明显。降水可能是历史时期巢湖水体盐度变化的主要控制因素：较强的降水导致了湖泊盐度的下降。研究结果为重建淡水湖泊的盐度提供了新的视角。 |
| 关键词: 盐度 湖泊沉积 Sr/Ba 降水 巢湖 |
| DOI：10.7515/JEE2024080 |
| CSTR：32259.14.JEE2024080 |
| 分类号: |
| 基金项目:国家自然科学基金项目（42373007）；上海交通大学海洋学院省部级科研平台开放课题项目（SOO2024-18） |
| 英文基金项目: |
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| A reconstruction of salinity in Chaohu Lake by Sr/Ba over the past 210 a |
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XU Liqiang1，2，LIU Shiyan1
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1. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009 , China ； 2. Key Laboratory of Polar Ecosystem and Climate Change, Ministry of Education, Shanghai 200030 , China
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| Abstract: |
| Background, aim, and scope Salinity is a key factor affecting population structure, biocenosis, and biodiversity of lake ecosystems. Reconstruction of paleosalinity is crucial for understanding the historical evolution of lake environment, and also provides critical information for predicting future lake dynamics. However, most previous studies have focused on oceans or saline lakes, and thus our understanding of salinity changes in freshwater lakes in the past, especially over the past 200 a, is extremely insufficient. The Sr/Ba ratio in sediments has been widely accepted as an ideal proxy for paleosalinity. This study aims to evaluate whether Sr/Ba is available for reconstructing salinity of freshwater lakes. Materials and methods A sediment core (CHX) was collected from west part of the Chaohu Lake, the fifth largest freshwater lake in China. The core is 143 cm in length, and can be divided into two sedimentary units. The upper part (0—20 cm) of the core is greyish black, while the lower part (below 20 cm) is dark grey. The samples of the core mainly consist of fine-grained particles. The core was sectioned at intervals of 1 cm, and a total of 143 samples were obtained, among which 70 even-numbered samples were analyzed. This study focused on the sediments above the 28 cm depth (the past ca. 210 a) of the core. Grain size compositions, low-frequency magnetic susceptibility, and concentrations of some major and trace elements (including Sr, Ba, Al, Ca, and Na) of the well-preserved sequential sediments were measured. To examine its sedimentary history, chronology of the core was established based on techniques of 210Pb and 137Cs dating. Results Based on the dating results, sedimentation rate at the sampling site is approximately 1.3 mm/a. The core dates back approximately 1100 a, and age of the the botton sample of the core is around AD 910. It is found that concentrations of Sr in samples of the core vary between 104 mg/kg and 147 mg/kg, with a mean of 121 mg/kg, which is significantly lower than that of marine sediments. The range of Ba concentration in the core is 510—740 mg/kg, with an average of 605 mg/kg. The mean concentration generally falls within the range of Ba concentrations in terrestrial detritus samples (300—750 mg/kg), and similar to those reported for lake sediments in the lower reaches of the Yangtze River. The Sr/Ba ratios in samples of the core CHX range from 0.16 to 0.26 (mean=0.20), much lower than threshold value of 1 for saline environment, indicating clear terrestrial characteristics. These values are consistent with previously reported Sr/Ba ratios in freshwater sediments. Discussion Although Chaohu Lake is a freshwater lake with small salinity variations, the vertical profile of Sr/Ba shows considerable fluctuations, suggesting that Sr/Ba is sensitive to salinity changes even in freshwater environment. Therefore, it can serve as a useful proxy for reconstructing freshwater salinity. The profile of Na/Ca follows a pattern similar to that of Sr/Ba, indicating that Na/Ca may also have a potential to be an ideal indicator for salinity of freshwater lakes; however, more data in future research are needed to confirm its applicability. In combination with 210Pb and 137Cs dating techniques, a 210-a record of paleosalinity of the Chaohu Lake based on Sr/Ba was reconstructed and the record shows significant variability. Based on the reconstructed record of paleosalinity, the salinity shows an overall rapid decline over the past 210 a. A strong correlation is observed between salinity and precipitation, suggesting that precipitation is a major controlling factor. Increased rainfall results in raises of lake water level, which further increase volume of the lake basin. This dilutes dissolved salts in lake waters and ultimately reduces lake salinity. Conclusions Based on geochemical analyses of the CHX core from Chaohu Lake, it is found that Sr/Ba ratios of sediments show significant variations. These results demonstrates that Sr/Ba ratio could be an effective proxy for salinity in freshwater lakes. And precipitation is a possible control on historical salinity of the Chaohu Lake. Recommendations and perspectives Reconstructing salinity in freshwater systems remains challenging. This study provides a new perspective on reconstruction of paleosalinity in freshwater lakes. The Sr/Ba ratio has strong potential for application in other freshwater lakes. |
| Key words: salinity lake sediments Sr/Ba precipitation Chaohu Lake |
