植硅体释光信号特征及其对测年应用的启示

doi:10.3969/j.issn.0253-4967.2024.01.007

摘要/Abstract

摘要：

中国西部青藏高原等地区往往缺乏可供AMS ¹⁴C测年的炭屑、植物残体等物质, 开展传统的¹⁴C测年有一定难度。然而, 这些地区的现代地表常生长着大量草本植物, 在其第四纪沉积物中均发现了丰富的植硅体。探索新的植硅体测年技术流程, 可为中国西部地区的第四纪沉积物提供新的有效测年途径。植硅体以其自身的广泛分布性、结构稳定性和保存完整性, 在测年应用中具有特殊优势, 尤其是植硅体碳可用于¹⁴C测年。然而, 植硅体碳的¹⁴C年龄存在高估问题, 这可能是由于前处理过程中的快速氧化作用和高温作用破坏了其结构的稳定性和封闭性, 需依据其化学组成和某些物理性质变化识别其结构在高温环境下的细微改变。植硅体作为特殊的非晶质含水SiO₂, 其热释光(Thermoluminescence, TL)与光释光(Optically stimulated luminescence, OSL)信号具有测年潜力, 可作为直接比对和替代植硅体¹⁴C测年法的另一测年手段, 同时其释光性质变化可能对识别其物理结构变化具有参考价值。文中以从干稻草中提取的现代植硅体样品为例, 对其进行了一系列条件实验, 从而确定了植硅体OSL与ITL₁₆₅ 信号的具体测试流程, 并进行了不同给定剂量的剂量恢复实验, 以探讨其剂量-信号的响应关系及测试流程的可行性和可靠性。结果表明, 天然植硅体颗粒存在稳定且易晒退的OSL信号, 在较高给定剂量(850Gy)的条件下植硅体存在较显著且稳定的165℃ TL峰, OSL信号与ITL₁₆₅ 信号均有潜力应用于测年研究, 但需要已知年龄的样品进行检验。与此同时, 不同退火温度加热后的ITL₅₀₀ 曲线具有识别植硅体释光性质和物理结构变化的潜力, 退火温度升高至300~350℃时植硅体的结构开始发生改变, 至约600℃时已发生不可逆的结构改变且逐渐敏化。这也意味着在提取植硅体的过程中应尽可能使用湿式氧化法而非干灰化法, 以避免植硅体的结构封闭性遭受破坏, 从而造成植硅体碳的AMS ¹⁴C年龄被高估。

关键词: 植硅体, 光释光信号, 等温热释光, 植硅体碳, 退火实验

Abstract:

Phytoliths have shown a lot of advantages in dating due to their widespread distribution, structural stability, and preservation integrity, especially since phytolith carbon was used in radiocarbon dating. However, there is a problem of overestimation of phytolith carbon ¹⁴C ages, which may be due to its structure destruction during pre-treatment processes of the rapid oxidation and high temperature. It is necessary to identify the subtle changes in its structure under high-temperature conditions based on its chemical composition and certain physical properties. As a special amorphous hydrated SiO₂, using their luminescence signals for thermoluminescence(TL)or optically stimulated luminescence(OSL)dating can be directly compared with their ¹⁴C ages even as an alternative dating method, and the luminescence property changes of phytoliths may have a reference value for identifying the physical structure changes. In this paper, modern phytolith samples extracted from dry rice straw were taken as an example to study the stability of OSL and isothermal TL(ITL)signals. We conducted a series of conditional experiments to determine the specific experimental process of OSL and ITL₁₆₅ signals, and discussed the feasibility and reliability of the dose-signal response relationship and test process under different given doses of dose recovery experiments. Moreover, the OSL signal were tested by a conventional single-aliquot regenerative-dose(SAR)protocol which firstly preheated at 200℃/180℃ to remove the instability signal and then blue excited at 125℃. The OSL signal of phytolith sample rapidly decays under blue light and is basically reset within 5s of excitation. The sensitivity corrected OSL signal intensity and regeneration dose can respond one by one and a growth curve can be established, indicating that there is also a trap structure within the phytolith particles that can generate relatively stable OSL signals. However, there are still some problems which need further to be solved such as relatively weak signal intensity and poor sensitivity. The results of bleaching experiment show that the OSL signal intensity of phytolith particles decays exponentially under sunlight, and the OSL signal irradiated by given dose of 85Gy can be bleached about 90%within 100s and bleached completely around 800s, which suggests that the phytolith samples have good sunlight bleaching susceptibility. There is a stable and easily bleached OSL signal in natural phytolith particles, however, the experimental procedure is only suitable for samples with lower given doses(below about 200Gy)and the dose recovery rate of older samples is lower. On the other hand, 425℃ TL peaks were found in the ITL₅₀₀ curves of phytolith at both a higher given dose(850Gy)and a lower given dose(85Gy), but they could not be used for dating because of instability. There was a remarkble and stable TL peak at 165℃ under a higher given dose(850Gy). The relatively stable ITL₁₆₅ signal has the potential to be used in dating research and its experimental procedure is also by SAR method. The dose recovery rate of ITL₁₆₅ signal under different given doses(above about 50Gy)was in the range of 0.8-1.2. Both OSL and ITL₁₆₅ signals have the potential to be used in dating studies but need to be tested with samples of known-age. The characteristic dose D₀ of OSL and ITL₁₆₅ signals of phytoliths are(326.8±19.5)Gy and(504.9±49.9)Gy, which implys that phytoliths have a greater saturation level than quartzes. Meanwhile, the ITL₅₀₀ curve heated at different annealing temperatures has the potential to identify changes in phytolith luminescence properties and physical structure. The structure of phytoliths begins to change at around 300-350℃, and irreversible structural changes have occurred at around 600℃ and gradually become sensitized. This also means that the extraction process of phytoliths using wet ashing rather than dry ashing may destroy the structural integrity of phytoliths and resulting in an overestimation of phytolith carbon AMS ¹⁴C ages. Whether it is by using AMS ¹⁴C dating method or OSL/TL dating method which phytoliths as the main dating material, phytolith particles should not be placed in a high temperature environment above 300℃ at any stage during the experiment in order to avoid irreversible damage to its structure. The luminescence age obtained by the OSL signal and ITL₁₆₅ signal of phytoliths can be compared with the ¹⁴C age to determine whether there exists an overestimation, and if the two ages can be verified, there is no need to use other dating minerals which is of great significance for the dating of precious archaeological materials.

Key words: phytolith, optically stimulated luminescence signal, isothermal thermoluminescence, phytolith occluded carbon, annealing experiment

李兆宁, 尹金辉, 杨会丽, 石文芳, 郑勇刚. 植硅体释光信号特征及其对测年应用的启示[J]. 地震地质, 2024, 46(1): 101-116.

LI Zhao-ning, YIN Jin-hui, YANG Hui-li, SHI Wen-fang, ZHENG Yong-gang. THE LUMINESCENCE SIGNAL CHARACTERISTICS OF PHYTOLITH AND ITS APPLICATION IN DATING[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(1): 101-116.

图/表 10

图1 植硅体的释光测片示例(a)与显微镜照片(b—d)

Fig. 1 The sample aliquots for luminescence experiment(a)and the microscope photographs of phytolith(b—d).

图2 不同预热温度(a)与不同给定剂量(b)的剂量恢复率

Fig. 2 The distribution of dose recovery ratio of samples at different preheating temperatures(a) and irradiated with different given doses(b).

表1 植硅体样品OSL SAR法测试程序(Murray et al., 2000, 2003)

Table1 The OSL SAR procedure for phytolith samples(Murray et al., 2000, 2003)

步骤	实验过程、条件	备注
1	天然或再生剂量
2	预热(200℃, 10s)
3	蓝光激发(125℃, 100s)	L_n, L_i
4	实验剂量
5	预热(180℃, 10s)
6	蓝光激发(125℃, 100s)	T_n, T_i
返回步骤1		(n代表天然信号,i为循环数)

图3 植硅体蓝光激发衰减曲线(a)与生长曲线(b)

Fig. 3 The OSL decay curve(a)and growth curve(b)of phytolith samples.

图4 给定剂量85Gy的植硅体样品OSL信号晒退实验

Fig. 4 The OSL signal bleaching experiment of phytolith samples with given dose of 85Gy.

图5 85Gy(a, b)和850Gy(c, d)给定剂量的ITL500 曲线 a、 c 加热到500℃所记录的TL信号; b、 d 稳定在500℃持续加热600s所获得的ITL曲线

Fig. 5 The ITL500 curves for given doses of 85Gy(a, b)and 850Gy(c, d).

表2 植硅体样品ITL165 SAR法测试程序(Jain et al., 2007; Vandenberghe et al., 2009)

Table2 The ITL165 SAR procedure for phytolith samples(Jain et al., 2007; Vandenberghe et al., 2009)

步骤	实验过程、条件	备注
1	天然或再生剂量
2	预热(180℃, 10s)
3	TL激发(165℃, 600s)	L_n, L_i
4	实验剂量
5	预热(180℃, 10s)
6	TL激发(165℃, 600s)	T_n, T_i
返回步骤1		(n代表天然信号,i为循环数)

图6 植硅体ITL165 激发衰减曲线(a)与生长曲线(b)

Fig. 6 The ITL165 decay curve(a)and growth curve(b)of phytolith samples.

图7 给定剂量为42.5Gy、 85Gy、 425Gy、 850Gy的剂量恢复率

Fig. 7 The distribution of dose recovery ratio of samples irradiated with different given doses of 42.5Gy, 85Gy, 425Gy and 850Gy.

图8 不同退火温度加热后植硅体的ITL500 曲线(a, b)及重复试验(c, d)

Fig. 8 The ITL500 curves of phytolith samples heated at different annealing temperatures(a, b) and results of repeated tests(c, d).

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