Exploring and optimizing characterization of radionuclide fingerprint for tracking sediment source: Comparison of mangrove, seagrass,and coral reef ecosystems
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摘要: 物质来源是沉积学研究的关键问题。沉积地球化学是沉积学的重要研究手段，放射性核素在沉积地球化学中的应用探索仍较为匮乏。本研究利用高纯锗γ谱仪同时测量红树林、海草床、珊瑚礁三大典型生态系统的沉积物中天然放射性核素（238U、226Ra、228Ra、40K），分析3种生态系统的沉积物中放射性核素分布特征，探索基于放射性核素的一维视角下活度特征、二维视角下核素对与活度比值、三维视角下三角图解的指纹表征技术，识别沉积物中不同物质的来源，并尝试推广至中国不同海域。本研究强调三维指纹表征技术—基于核素活度的238U-232Th(228Ra)-40K/10三角图解可以综合展示3种放射性核素的活度大小、组成比例、空间分布等信息，指出238U-232Th(228Ra)-40K/10三角图解可能是沉积物中物源识别的一种优化方法，可为沉积地球化学研究中的现有代用指标工具库提供有益补充。Abstract: Sediment source is a key issue in sedimentology. Although sediment geochemistry is an important tool in sedimentology, the explorations and applications of radionuclides in sediment geochemistry are still limited. In this study, naturally occurring radionuclides (238U, 226Ra, 228Ra, and 40K) in sediments were simultaneously measured using a high-purity germanium γ-spectrometer in three typical ecosystems of mangrove, seagrass, and coral reefs. The distribution characteristics of radionuclides in sediments from the these ecosystems were discussed in one-dimensional view of activity level, two-dimensional view of radionuclide pairs and activity ratio, and three-dimensional view of ternary diagram of radionuclides to identify distinct sources of sediments. Ternary diagram of 238U-232Th(228Ra)-40K/10 in sediments was applied in the China different seas for tracking distinct sediment sources. This study emphasized that the three-dimensional view of ternary diagram of 238U-232Th(228Ra)-40K/10 comprehensively exhibited the information of activity level, composition, and spatial distribution area of radionuclides. In conclusion, ternary diagram of 238U-232Th(228Ra)-40K/10 may be an optimized method for source identification in sediments and provide a supplement to the existing tools of geochemical proxies.
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Key words:
- radionuclide /
- geochemical proxy /
- sediment source /
- mangrove /
- coral reefs /
- seagrass
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图 1 红树林、海草床、珊瑚礁生态系统的沉积物采样站位
Fig. 1 Stations of sediments collected from mangrove, seagrass and coral reef ecosystems
图 2 红树林、海草床、珊瑚礁生态系统沉积物中238U、226Ra、228Ra、40K的活度箱线图
虚线代表所有沉积物中放射性核素的活度均值；箱线图内部横线代表中位数；箱线图上、中、下3个数值分别代表不同生态系统沉积物中放射性核素的最大活度、活度均值和最小活度
Fig. 2 Boxplots of 238U, 226Ra, 228Ra and 40K activities in sediments of mangrove, seagrass and coral reef ecosystems
The dotted lines represent the average activities of radionuclides in all sediments. The inner line of the boxplot depicts the median value. The upper, middle and lower values of the boxplot refer to the maximum activity, average activity, and minimum activity of radionuclides in sediments, respectively
图 3 红树林、海草床、珊瑚礁系统的沉积物中232Th（228Ra）-238U（a）、226Ra-238U（b）、40K-226Ra/238U（c）的关系
黄色十字代表中国土壤[30]；k代表放射性核素活度比值；图c十字星号分别代表在不同生态系统沉积物中40K活度均值和226Ra/238U活度比值均值
Fig. 3 Correlation analysis of 232Th(228Ra)-238U (a), 226Ra-238U (b) and 40K-226Ra/238U (c) in sediments of mangrove, seagrass, and coral reef ecosystems
The yellow cross represents the average activities of radionuclides in China soil[30]; k means the activity ratio. The cross asterisk represents the average activities of 40K and average activity ratio of 226Ra/238U in sediments from these ecosystems in Figure c
图 4 红树林、海草床、珊瑚礁中沉积物的238U-232Th(228Ra)-40K/10三角图解（a，b，c）和226Ra-232Th(228Ra)-40K/10三角图解（e，f，g）以及3个生态系统的空间分布特征区域(d，h)
十字星号代表不同生态系统沉积物的活度均值；图a, b, c中括号内数值依次代表238U、232Th(228Ra)、40K/10的活度均值；图e, f, g中括号内数值依次代表226Ra、232Th(228Ra)、40K/10的活度均值。颜色棒代表3个核素端元的活度之和（Bq/kg）
Fig. 4 Ternary diagrams of 238U-232Th(228Ra)-40K/10 (a, b, c) and 226Ra-232Th(228Ra)-40K/10 (e, f, g) in sediments from mangrove, seagrass, and coral reef ecosystems and their spatial distribution areas (d, h)
The cross asterisks exhibit the average activities from different ecosystems. The values in the brackets refer to the average activities of 238U, 232Th (228Ra) and 40K/10 in Figures a, b, c. The values in the brackets exhibit the average activities of 226Ra, 232Th (228Ra) and 40K/10 in Figures e, f, g. The color bar quantitatively exhibits the sum of activities of three radionuclides’ endmembers (Bq/kg)
图 5 花岗岩[31]、玄武岩[31]、全球土壤[32]、地壳[33]的238U-232Th(228Ra)-40K/10三角图解
颜色棒代表3个核素端元的活度之和（Bq/kg）
Fig. 5 Ternary diagram of 238U-232Th(228Ra)-40K/10 of granite[31], basalt[31], global soil[32] and crust[33]
The color bar quantitatively exhibits the sum of activities of three radionuclides’ endmembers (Bq/kg)
图 6 渤海（a）[34–35]、黄海（b）[36–37]、东海（c）[29, 38]、南海（d）[12, 39–44]沉积物的238U-232Th(228Ra)-40K/10三角图解
虚线指示不同海域沉积物的空间分布特征区域；颜色棒代表核素端元的活度之和（Bq/kg）；黄色十字代表中国土壤[30]；十字星号代表中国不同海域沉积物的活度均值；图中括号内数值依次代表238U、232Th(228Ra)、40K/10的活度均值
Fig. 6 Ternary diagram of 238U-232Th(228Ra)-40K/10 in sediments from Bohai Sea (a)[34–35], Yellow Sea (b)[36–37], East China Sea (c)[29, 38], and South China Sea (d)[12, 39–44]
Dashed lines indicate the spatial distribution characteristics of sediments in different sea areas. The color bars represent the sum of the activities of 238U, 232Th(228Ra), and 40K/10 (Bq/kg). The yellow cross represents the average activities of radionuclides in China soil[30]. The cross asterisks exhibit the average activities from different sea areas. The values in the brackets refer to the average activities of 238U, 232Th (228Ra) and 40K/10
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