光释光技术在我国海岸晚第四纪沉积测年中的应用

doi:10.11928/j.issn.1001-7410.2018.03.03

文章编号：1001-7410(2018)03-573-14

年小美, 张卫国

( 华东师范大学河口海岸学国家重点实验室, 上海 200062)

摘要：位于陆地与海洋交界的海岸系统，对环境变化反应敏感，海岸沉积是记录区域及全球环境变化的重要载体。但海岸沉积动力环境复杂，多存在沉积物的侵蚀、搬运与再沉积现象，或缺乏¹⁴C测年所需的有机质材料，因此年代问题是海岸第四纪地层研究的难点。光释光测年技术（OSL）主要通过石英或长石矿物的释光信号，确定沉积物的埋藏年龄，测年范围从近百年到几十万年。OSL的快速发展为海岸第四纪地层年代确定提供了有利条件，特别是单片再生剂量法的提出，提高了光释光测年结果的准确度和精度。OSL测年需要根据样品的年龄与性质，进行测年矿物、测量程序条件及粒级的选择。文章基于近年来石英和长石光释光测年技术的发展，结合我国海岸第四纪地层断代中已发表的500多个光释光年代数据，探讨光释光测年技术在海岸第四纪沉积物定年中遇到的常见问题及应对策略，包括测年矿物的选择（石英vs.长石）、样品的晒退、剂量率、石英信号的组分、长石包裹体等。对这些问题的认识有助于对光释光测年结果准确度的评估，可以更好地服务于海岸晚第四纪沉积研究。

主题词：光释光测年不完全晒退剂量率石英长石石英信号组分长石包裹体

中图分类号 P597⁺.3 文献标识码 A

第一作者简介: 年小美, 女, 34岁, 助理研究员, 第四纪地质学, E-mail:xmnian@sklec.ecnu.edu.cn

^*国家自然科学基金项目（批准号：41771009、41302135和41271223）、中国博士后科学基金特别资助项目（批准号：2017T100284）和中国博士后科学基金面上项目（批准号：2015M571521）共同资助

2018-02-08 收稿，2018-03-22 收修改稿

1 前言

光释光测年技术(OSL)于1985年由Huntley等^[1]首次提出，它是测量样品最后一次完全曝光至埋藏以来的年龄，近年来得到了快速的发展，无论是测年准确度还是精度都有了很大的提高^[2]。光释光的微观机制比较复杂，现普遍采用能带理论对其进行解释，其物理原理可参见Aitken^[3]的早期专著。光释光测年的一般原理是：沉积物的矿物颗粒在搬运过程中经过光的晒退，使其以前积累的光信号归零，在后期埋藏过程中，沉积物中放射性核素(U、Th、K等)衰变发射出具有一定能量的α、β、γ射线，宇宙射线对其也有少量贡献，这些射线与矿物颗粒相互作用，将能量从周围环境中转移到矿物颗粒中进行储存，而其储存信号的多少正比于矿物颗粒最后一次见光以来的时间^[3]。光释光样品在实验室的测量包括等效剂量(D_e)和剂量率(dose rate)两部分^[3]。样品最后一次曝光以来所吸收的总辐射剂量为古剂量(palaeodose)，在实验室中用等效剂量的值代替古剂量的值，等效剂量为实验室中产生的与自然剂量光释光信号强度相同的辐射剂量，样品每年所吸收的辐射剂量为剂量率，样品的年龄由古剂量/等效剂量除以剂量率得到^[3]。

近年来，光释光技术在海岸沉积物定年中得到了广泛的应用，并取得了丰硕的成果^[4~8]。如Jacobs^[4]发表于2008年的文章，当时光释光技术在我国海岸沉积物定年中应用较少，因而涉及我国海岸沉积物年代研究案例较少；尽管Lamothe^[7]的文章发表于2016年，但其综述主要侧重释光技术在较老沉积物(末次间冰期- 1 Ma)中的应用，也很少涉及我国的研究。本文主要回顾近年来光释光技术在我国海岸晚第四纪地层年代测定中的应用，并探讨存在的问题及其应对策略，以便使光释光技术更好地服务于我国海岸第四纪地层的年代研究。

2 光释光技术在我国海岸晚第四纪地层年代研究中的应用及面临的问题与挑战 2.1 光释光测年技术在我国海岸晚第四纪地层年代研究中的应用现状

对过去环境变化过程的研究，需要高分辨率地层年代学的支撑。海岸带处于陆地与海洋环境的过渡地带，沉积相变化复杂，往往相距几百米的地层岩性便无法直接对比，且可能经历过多次的侵蚀、搬运与再沉积，因此准确而大量的测年数据，是建立海岸地区高分辨率地层年代学的基础^[9]。海岸晚第四纪碎屑沉积物年代的确定，目前主要手段包括放射性¹⁴C、²¹⁰Pb、¹³⁷Cs、^239+240Pu等测年技术。¹⁴C测年技术主要利用沉积物中的有机质、植物残体、生物壳体的碳酸盐等进行测年(测年极限＜5万年)。但是，一些沉积物中往往缺乏适合测年所需含碳材料(如河口沙坝)，从而导致无法进行¹⁴C定年^[10~14]；此外，由于海岸地区沉积物来源的多样性及沉积环境的复杂性，如沉积物的改造与再沉积、碳库效应等，多出现上、下层位年龄的倒转或与海岸演化历史框架无法匹配的年代^[13~22]。²¹⁰Pb、¹³⁷Cs和^239+240Pu目前主要用于测量近百年的沉积物^[23~25]。对于较老的沉积物，古地磁也有应用^[26~28]，但是它是一种相对测年技术，需要与其他测年方法配合，才能更为可靠地确定层位所处的绝对年代。

光释光技术在海岸沉积年代确定中的应用，主要涉及海岸沙丘^[29~33]以及三角洲演变等领域^[34~38]。近年来，光释光技术在我国海岸第四纪地层年代分析中也逐渐得到应用，从图 1和表 1总结的数据可以看出，光释光测年技术应用区域集中在我国大河三角洲及毗邻陆架以及台湾海峡的沙质海岸，测年范围主要集中在全新世和晚更新世晚期，少数几个较老沉积物的年龄(＞70~80 ka)主要是采用传统的石英光释光信号或未经异常衰退校正的长石红外释光信号进行测定，这可能导致年龄的低估，这个问题将在本文的2.2.1节进行讨论。本文统计的我国海岸沉积光释光年代测年的40余篇文章中，约有一半的文章主要是利用光释光技术提供年代数据，未重点介绍测年的细节。光释光测年技术需要根据样品的性质选择合适的测试方法、测试矿物与粒级以及数据处理方法等，这对保证年代数据的准确性具有重要意义。综合分析近年来国内外海岸第四纪地层光释光测年技术的进展文章，可以发现测年矿物的选择、样品的晒退、剂量率、石英信号的组分、矿物中存在的长石包裹体等，是光释光测年需要主要关注的问题。下面就以上几个问题进行讨论。

图 1 光释光技术在我国海岸晚第四纪地层年代分析中应用的主要地点图中钻孔/剖面的数字编号与表 1中的数字编号(第二列)一一对应 Fig. 1 Late Quaternary coastal deposits dated using optically stimulated luminescence(OSL)technique in China. The sites of dated cores/sections(solid circles)are detailed in Table 1, with the numbers corresponding to the second column in Table 1

表 1 光释光技术在我国海岸晚第四纪地层年代分析中的应用 Table 1 Application of optically stimulated luminescence dating(OSL)to Late Quaternary coastal deposits in China

钻孔位置	数字编号^a	钻孔编号^b	钻孔长度 (m)	经纬度与海拔^c	矿物^d	粒级 (μm)	测试方法	年龄 (ka)^e	样品数目	其他测年技术对比^f	参考文献
长江三角洲古河谷	1	TZ	60.9	32°23.8317′N， 119°54.845′E； 4.72m	Q	45~63； 90~125	SAR^g	3.2~8.8	7	AMS ¹⁴C	^[13]
长江三角洲古河谷	2	NT	60.9	32°3.9417′N， 120°51.4′E； 3.99m	Q	45~63； 90~125	SAR	1.07~14	9	AMS ¹⁴C	^[13]
长江水下三角洲	3	A6-6	1.8	30°48′12″N， 122°48′27″E；水下26m	Q	4~11	SAR	0.1~0.26	4	²¹⁰Pb；¹³⁷Cs； ^239+240Pu；微塑料	^[25]
长江三角洲古河谷	4	SD	50.8	32°20.3′N， 120°46.75′E； 4.87m	Q	45~63； 90~125； 150~180	SAR (单颗粒与单片)	1.5~62	8	AMS ¹⁴C	^[14]
长江水下三角洲	5	H5	58.7	31°39.6′N， 122°09′E；6.11m	Q	粘土	—	4.3~7.6	2	AMS ¹⁴C	^[39]
南黄海西海岸	6	YZ07 (0~50m)	150	32°05′2.40″N， 121°35′49.20″E； -5m	Q； KF； P	4~11； 63~90； 90~200	SAR；post-IR blue OSL^h； pulsed OSLⁱ； pIRIR^j	0.13~11	18	AMS ¹⁴C	^[40]
南黄海西海岸	6	YZ07 (0~50m)	150	32°05′2.40″N， 121°35′49.20″E；-5m	Q	90~200	SAR	0.9~26.2	28	AMS ¹⁴C	^[41]
东海内陆架	7	ECS-DZ1 (0~50m)	153.6	30°29′29.76″N， 122°03′0.12″E；水下12m	Q	4~11； 100~200	SAR	1.3~16.6	20	AMS ¹⁴C	^[42]
东海内陆架	7	ECS-DZ1 (58.0~153.6m)	153.6	30°29′29.76″N， 122°03′0.12″E；水下12m	Q	4~11	SAR	11.3~136	6	古地磁	^[27]
长江水下三角洲	8	YD13-G3	3.23	31°02′ 58″N， 122°50′ 5″E；水下38.5m	Q；P	4~11	SAR；pIRIR	1.92~2.30	9	N/A	^[43]
	9	YD12-H1	39.5	31°02′ 60″N， 122°50′0.3″E；水下35.6m	Q；P	4~11	SAR；pIRIR	1.93~6.77	35	N/A
	10	现代悬浮颗粒	0	31°57′13″N， 118°37′50″E	Q；P	4~11	SAR； pIRIR	D_e=0.18(Q)， 7.36(P)	1	N/A
长江三角洲平原	11	QP88	105	31°7.3′N， 120°53.2′E	Q	4~11	SAR	27~78	6	U系	^[26]
长江三角洲平原	12	WJ	51.2	31°13′N， 120°39′E	Q	粘土和细砂	SAR	48~100	3	AMS ¹⁴C；古地磁	^[26]
南黄海辐射沙脊群	13	11DT02	20.62	32°57.188′N， 121°31.623′E；水下5.7m	—	—	—	3.2~57.5	2	AMS ¹⁴C	^[44]
南黄海西部	14	SYS-0701	70.2	34°39.7535′N， 121°27.0000′E；水下33m	—	砂	—	41~158	18	AMS ¹⁴C	^[45]
南黄海西部	15	SYS-0702	70.25	34°18.0919′N， 122°05.7459′E；水下32m	—	砂	—	79~129	8	AMS ¹⁴C	^[45]
长江三角洲平原	16	FX	102	31°12′N， 121°15′E	Q	—	—	1.89~130	9	N/A	^[46]
	17	MFC	112	31°14.6′N， 121°27.5′E	Q	—	—	8.4~116	8	N/A	^[46]
	18	SG7	101	30°55.206′N， 121°53.6193′E；2.3m	Q；KF； P	粗样；细样 (2~8；4~11)	—	3.8~148	9	U系	^[47]
渤海北部	19	LZK06 (0~18m)	100	40°54.44′N， 121°37.77′E；4.51m	Q； KF	4~11； 163~100； 75~100	SAR； pIRIR	0.24~32	10	AMS ¹⁴C	^[48]
松辽平原	20	PJ沙丘	5.5	—	Q； KF	63~100； 100~150	SAR； pIRIR	0.08~5.8	8	N/A	^[49]
渤海	21	YRD-1101 (0~50m)	200.3	38°02′08.97″N 118°36′25.88″E； 1.84m	—	—	—	9~99	20	AMS ¹⁴C；古地磁	^[28]
渤海海岸带	22	MDS	6.6	37°56′ 31.9″N， 120°40′35.9″E；12m	Q	—	—	19.4~136	9	AMS ¹⁴C	^[50]
渤海西岸	23	QX02	32	38°38′ 24.2″N， 116°57′ 24.2″E；3.57m	Q	63~90	SAR	7.6~84	15	AMS ¹⁴C	^[51]
黄河三角洲	24	YRD-1402	40.3	37°54′32.82″N， 18°20′42.48″E；4.237m	Q	38~63	SAR	2.3~100.5	5	AMS ¹⁴C	^[52]
山东半岛南部水域	25	QDZ03 (0~12m)	40.2	36°16′03.21″N， 120°56′58.77″E；水下15.5m	Q	—	—	63~140；＞140	13	AMS ¹⁴C	^[53]
渤海南部	26	BH1 (0~40m)	198.8	37°17′N， 119°06′E；水下4m	Q	4~11； 38~63；63-90	SAR； MAR^k	0.27~186	16	AMS ¹⁴C	^[54]
渤海南部	27	BH2 (0~66m)	228.2	37°10′N， 119°04′E；3m	Q	4~11；38~63； 63~90；90~125	SAR； MAR	1.04~128	12	AMS ¹⁴C	^[54]
渤海	28	Lz908 (0~54m)	101.3	37°09′N， 118°58′E；6m	Q	4~11	SAR；MAR	2.1~100	9	AMS ¹⁴C	^[55]
渤海湾西岸	29	BT113	80	38°44′53.8″N， 117°31′07.3″E；1.44m	Q；P	45~63	SAR；IRSL	1.17~265	12	AMS ¹⁴C	^[56]
渤海湾西岸	30	BT114	80	38°42′32.8″N， 117°34′54.6″E；2.26m	Q；P	45~63	SAR；IRSL	0.22~200	9	N/A	^[56]
渤海湾	31	Dawuzhuang	12	39°25.10′N， 117°52.25′E	Q；P	4~11	SAR；IRSL； post-IR blue OSL	0.23~6.37	7	N/A	^[57]
天津大直沽	32	ZH2	30.5	39°06′47″N， 117°14′10″E	Q；P	4~11；粗颗粒	多测片	2~50	12	¹⁴C	^[58]
福建平潭岛	33	PT01	28.3	25°35′13″N， 119°45′22″E；8m	Q	4~11； 90~125	SAR	＞600 Gy； 121~124	4	AMS ¹⁴C	^[59]
福建海岸沙丘	34	东山(DS)沙丘	—	25°52′44.09″N， 119°34′32.64″E	Q	4~11	SAR	0.2~5.6	9	AMS ¹⁴C	^[60]
福建晋江海岸沙丘	35	SHA	4	24°36′32.7″N， 118°39′20.3″E	Q	90~125	SAR	0.1~1	11	N/A	^[61]
福建东南沿海“老红砂”	36	KR	14.8	24°34′56.3″N， 118°39′14.1″	Q	90~125	SAR	12~117	26	N/A	^[62]
福建晋江 “老红砂”	37	JT	1	—	Q	150~250	SAR	0.07~3.5	2	N/A	^[63]
	38	SZ	1	—	Q	150~250	SAR	0.32	1	N/A
	39	XZ	6	24°46′13.0″N， 118°45′43.9″E	Q	150~250	SAR	50~74	3	N/A
	40	SH	10	24°36′53.6″N， 118°40′25.8″E	Q	150~250	SAR	32~77	5	N/A
	41	SHW(现代海岸沙丘、海滩砂)	—	—	Q	150~250	SAR	0.08~0.18； D_e=0.004~ 0.027 Gy	4	N/A
珠江三角洲	42	QZK6	39.40	22°41′53.88″N， 113°32′08.65″E； 0.123m	Q	—	SAR	67.48	1	AMS ¹⁴C	^[64]
珠江三角洲	43	珠海淇澳岛小沙澳湾剖面T01	2.4	22°24′54.288″N， 113°39′13.572″E	Q	4~11	SAR	7.09~7. 71	3	N/A	^[65]
	44	澳门黑沙湾剖面AT27	5.2	22°07′08.796″N， 113°33′59.112″E	Q	4~11	SAR	3.86~15.24	4	N/A	^[65]
	45	Xiaohushan	—	—	Q	4~11	SAR	5~26.2	6	N/A	^[66]
	46	QZK4	58.50	22°43′13.98″N， 113°23′44.63″E；0.76m	Q	—	SAR	39.38~43.41	4	AMS ¹⁴C	^[67]
	47	MZ04	50	—	—	—	—	1.3~8.8	4	N/A	^[68]
	48	Xilingang	5	—	Q	—	SAR	17~51	9	AMS ¹⁴C	^[69]
	49	西淋岗	—	—	Q	4~11	SAR	5.62~51	14	AMS ¹⁴C	^[70]
	50	番禺石楼	—	—	Q	4~11	SAR	27~33	3	AMS ¹⁴C
	51	眉山	—	—	Q	4~11	SAR	5.67~57.63	7	AMS ¹⁴C
香港浅海湾	52	BH6 (Tai O Bay)	65.53	—	Q； P	4~11； 100~200	SAR；MAR	28~＞105	2	N/A	^[71]
香港浅海湾	53	BH9 (Tai O Bay)	57.55	—	Q； P	4~11； 100~200	SAR；MAR	32~＞107	4	N/A	^[71]
海南岛	54	TLG01	49.01	18°16′57″N， 109°42′38″E；3m	Q	4~11；90~125	SAR	70~125	4	AMS ¹⁴C	^[72]
台湾西南海岸平原	55	SF-Shihfen	250	—	Q	90~112	SAR	9.1~44	5	AMS ¹⁴C	^[73]
	56	TW-Tawen	250	—	Q	90~112	SAR	9.3~56	4	AMS ¹⁴C
	57	TY-Tsungyeh	250	—	Q	90~112	SAR	13.1~61	4	AMS ¹⁴C
	58	MCH-Machouhou	250	—	Q	90~112	SAR	9.6~97	5	AMS ¹⁴C
	59	HK-Hsinkang	250	—	Q	90~112	SAR	13.5~88	11	AMS ¹⁴C
a：数字编号与图 1钻孔/剖面的编号一一对应； b：标注的深度为光释光测量的钻孔深度范围； c：“—”表示没有找到相关信息； d：Q为石英；KF为钾长石；P为混合矿物； e：表中标注等效剂量(D_e)值的，表示文章中没有给出年龄，只有等效剂量值； f：“N/A”没有其他测年技术对比 g：SAR：单片再生剂量法^[74] h：post-IR blue OSL：红外后蓝光释光^[75~78] i：pulsed OSL：脉冲光释光^[79~80] j：pIRIR：单片钾长石红外激发后高温红外激发释光方法^[81~82] k：MAR：经灵敏度校正的多片再生剂量法^[83~84]

表 1 光释光技术在我国海岸晚第四纪地层年代分析中的应用 Table 1 Application of optically stimulated luminescence dating(OSL)to Late Quaternary coastal deposits in China

2.2 面临的问题与挑战 2.2.1 光释光测年矿物的选择：石英vs.长石

光释光测年技术主要利用石英和长石两种矿物，它们在自然界中分布十分广泛，因此光释光测年技术可用于多种第四纪沉积物的年代测定^{[2, 85~91]}。在光释光定年中，石英的光释光信号较易晒退，信号比较稳定，方法发展相对比较成熟，特别是石英单片再生剂量法(SAR)的提出，大大提高了光释光测年结果的准确性和精度^[74]。但是，一些样品的石英光释光信号较弱，且饱和剂量较低，往往出现70~80 ka附近的“极限年龄”^[92~94]，如采自于明显老于70~80 ka地层的沉积物样品，石英光释光测年测量结果仍然为70~80 ka，限制了石英矿物对老样品的测量。Wang等^[95~96]发展了回授光释光(TT-OSL)测年方法，扩大了石英光释光的测年范围，最老可以测量近百万年的样品^[97]。但是，石英TT-OSL信号并不适合所有样品^[98]，特别是对于一些年轻的样品，往往存在不可忽视的光释光信号晒退问题^[99~100]。

长石矿物的释光信号强度比石英强，且饱和剂量较高，具有测量较老样品的巨大潜力。但是，长石信号由于其陷阱电荷的量子力学隧道效应^[101]，即陷阱电子不需要晶格振动获得能量，就可以直接贯穿势垒与发光中心结合，这会导致长石信号的异常衰退^[102~105]，异常衰退会引起长石样品光释光年龄的低估，限制了长石矿物在光释光测年中的应用。目前针对长石的异常衰退已提出了多种校正方法，但是每种校正方法都有其局限性，大部分校正方法适合于线性增长区的生长曲线，即较年轻的样品^[106~110]；此外，长石异常衰退的校正需要多次的反复测量，比较耗时耗力。近年来为了拓展光释光的测年范围，研究者们不断努力寻找减少甚至消除红外释光信号异常衰退的方法，从而获得更加稳定的信号，目前在这方面已取得了很多进展，如钾长石的等时线年龄法^[111]、单片钾长石红外激发后高温红外激发释光方法^[81~82](pIRIR)、钾长石多步升温的单片红外激发后高温红外激发释光方法^[112~1113](MET-pIRIR)、单颗粒钾长石红外激发后高温红外激发释光方法^[114]等，但这些方法都有自身的前提条件或者局限性，如钾长石的等时线年龄法要求一个样品具有多个粒级，并且都要晒退完全；在应用钾长石pIRIR方法时，发现了长石的热转移信号，这个热转移信号可能影响等效剂量的测定^[114~118]，而大的检测剂量可以减少甚至消除这个热转移信号的干扰^{[114, 116, 118~121]}；在利用MET-pIRIR测量程序时，样品要经历多次较高温度条件下的激发，要求样品具有很强的光释光信号和良好的晒退性。目前，长石pIRIR方法在长江三角洲和渤海湾已有应用^{[43, 48~49]}，但是主要是针对全新世沉积物，对于更老的样品研究还很少，本文2.2.2节将对该方法的晒退问题进行讨论。

综上，根据样品的年龄与释光性质，选择合适的矿物及测量程序对沉积物年代的准确测定至关重要。石英的光释光信号较长石信号更容易晒退，因此对于较年轻样品，如果可以提取出纯净的石英，且石英信号足够强，通常选择信号较易晒退、技术发展相对比较成熟的石英矿物。对于较老样品(如黄土沉积，＞70~80 ka)^[92~94]，传统石英光释光信号得到的年龄为最小年龄。因此要确定“较老样品”沉积物的年代要选择其他信号进行测量，如石英的热转移信号或者长石信号。

2.2.2 晒退问题

在利用光释光技术测量沉积物的年龄时，通常假设样品在埋藏之前曝光完全^[3]。但是，对于很多沉积物很难达到这个理想条件，往往存在晒退问题(特别对于年轻样品不可忽视)，且不同颗粒晒退程度不同，在测量样品的等效剂量时，要对样品的晒退性进行评定，特别是对于水成沉积物^[122]。

通过对长江三角洲深切古河谷内的双甸(SD)、泰州(TZ)、双甸(NT)3个钻孔的中粒和粗粒石英矿物研究发现，多数粗粒石英样品存在显著的晒退问题(高估可达80 %)，中粒石英整体上比粗粒石英晒退要好，但是少数中粒石英样品也存在晒退问题^[13~14]。长江三角洲南部平原的WJ和QP88钻孔细颗粒石英^[26]及东海内陆架ECS-DZ1钻孔粗粒石英^[42]测年结果也显示了部分样品存在晒退问题。但是，南黄海西岸YZ07钻孔的细颗粒与粗颗粒石英^[40~41]和长江水下三角洲YD13-G3、YD12-H1^[43]钻孔的细粒石英却表现出了良好的晒退性。恒河三角洲不同粒级石英表现出了不同的性质，细颗粒石英光释光信号较强且晒退完全，适合光释光测年，而粗颗粒石英光释光信号弱且存在晒退问题，不适合用于光释光测年^[38]。而密西西比河三角洲的细颗粒和粗颗粒石英(近百年尺度的沉积物存在约100 a的残余年龄)^[35~36]、湄公河三角洲的粗颗粒石英^[34]、洛东江三角洲细颗粒和粗颗粒石英^[37]都表现出了较好的晒退性。这可能与沉积物不同的物质来源及经历的不同搬运历史相关。

对于较老的样品，晒退情况对样品的测年结果影响较小或者可以忽略，但是对于较年轻的样品，晒退影响却不可忽视。如Sugisaki等^[43]对长江口现代悬浮颗粒石英SAR测年结果约0.18 Gy，长石pIRIR的测年结果约7.36 Gy，如果剂量率按照3 Gy/ka计算，那么石英和长石的年龄分别为60 a和250 a；Wang等^[25]对长江水下三角洲A6-6钻孔近百年沉积进行了OSL、²¹⁰Pb、¹³⁷Cs、^239+240Pu和微塑料等测年方法的比较研究，发现细颗粒石英的OSL年龄存在约60 a的高估；Li等^[48~49]对渤海湾沉积物长石pIRIR的研究发现，pIRIR存在一定的残余年龄，特别是对于小于1 ka的较年轻样品，长石高估更明显。通常情况下，＜100 a的“残余年龄”被认为是晒退比较“理想”的样品，对于较老的样品，考虑到测年误差基本可以忽略不计，但是这样的“残余年龄”对于近百年或近千年的样品影响还是很大。

对于判断、解决由于样品晒退问题造成的高估，目前主要有如下方法：1)单颗粒或者小片技术^[123~124]；2)多粒级多矿物多种光释光测年方法的比较^{[13~14, 49, 98]}；3)与其他测年技术、地层或者环境信息进行对比综合分析^{[13~14, 40]}；4)通过统计模型分析，选择合适的统计分析模型^[125~126]；5)D_e (t)图，即等效剂量随不同激发时间段的变化^[127~128]，但是在应用D_e(t)图时应考虑到中组分的热不稳定性的影响^[129]；6)石英或者长石信号不同的晒退速率^[130]。前2种方法是针对样品的测量，后4种方法是关于数据的处理和分析(不受测试结果影响，根据样品性质决定采用哪种分析方法)，这里主要对前两种样品的测量进行讨论。

多颗粒单片技术是对若干颗粒进行同时测量，得到的测量结果是所有颗粒的平均值，如果样品包含晒退不完全的颗粒，那么所得到的等效剂量由于受到不完全晒退颗粒残余剂量的影响会偏高，即年龄的高估。为解决样品的晒退问题，单颗粒测量技术是近些年来发展的一个重要方法，它对每一个颗粒分别测量，可以判断样品的晒退情况，根据测量结果挑选出晒退较好的颗粒^{[123, 131~133]}，再通过统计模型分析，可以显著改善或者避免由于晒退不完全导致的高估。通过石英单颗粒的光释光测年发现，不同颗粒间的发光性质有很大的变化，通常情况下只有约5 % ~10 %的颗粒发出的光释光信号可以用于年代的测定^[133~134]，我们对长江三角洲深切古河谷全新世沉积物的研究发现，只有0.5 % ~0.8 %的石英颗粒能用于年代的测定^[14]，因此小片技术往往也可以达到单颗粒的效果，用于判断样品的晒退情况。

样品粒级的选择对年代的确定也至关重要，同一样品不同粒级的矿物晒退情况可能不同，因此采用多粒级多矿物的对比分析可以提高数据的准确性。多粒级多矿物对比分析目前已逐渐应用到我国海岸晚第四纪地层光释光年代测定中，提高了数据的可信度^{[13~14, 40, 48~49]}。例如，对长江三角洲全新世沉积物光释光测年研究中发现，不同粒级的石英矿物具有不同的晒退性^[13~14]。

2.2.3 剂量率

剂量率作为释光测年的两个重要参数(等效剂量和剂量率)之一，对最终测年结果准确性的影响显而易见^[3]。用于剂量率计算的放射性元素主要为U、Th、K，宇宙射线也有少量贡献。放射性元素的含量可以通过中子活化法(NAA)、γ能谱仪、原子吸收光谱法(AAS)或电感耦合等离子体质谱(ICP- MS)等方法进行测定。在计算钾长石的剂量率时，涉及到钾长石的内部剂量率，可以通过扫描电镜(SEM)或电子探针显微分析(EPMA)等方法进行测量。

在沉积物中，放射性核素衰变产生的能量主要由沉积物颗粒及周围的水吸收，因此水含量的多少会直接影响沉积物颗粒吸收能量的多少，即含水率影响沉积物剂量率的大小^[3]。剂量率计算时通常假设沉积物为均一的，且在埋藏历史时期放射性核素的活度及含水率没有发生改变，因此可以通过测量沉积物现有放射性元素及含水率确定样品的剂量率。但是，这种理想的环境很难实现，特别是对于海岸比较复杂的沉积环境，可能存在含水率的变化及U系不平衡等问题。例如，Zhang等^[63]在对福建晋江“老红砂”的释光年代研究中发现，沉积物在发生化学风化作用后导致U和Th的富集，得到的剂量率偏大，即年龄的低估。γ能谱法可以检测沉积物的U系不平衡状态，对于剂量率可能存在问题的沉积物，可以通过数学建模方法，模拟自然沉积过程，确定样品的剂量率，提高测年结果的准确度^{[31, 135]}。

2.2.4 石英信号的组分

石英信号具有不同的组分，通常被分解成3个组分，分别被定义为“快组分”(fast component)、“中组分”(medium component)和“慢组分”(slow component)^[136]，它们具有不同的晒退性和生长特性，石英的快组分最容易被晒退，用它得到的年代最可靠^[137]，光释光信号组分的不同可以导致等效剂量的变化。关于石英信号的组分，不同的研究者提出了不同的组分数目^[138~140]。

可以通过测量程序直接得到石英的快组分信号^[141~143]，但是这样的程序一般消耗机时较多。此外，在测量之后，也可以判断石英信号的组成及从中提取需要的信号组分，方法主要有：1)曲线拟合分解^[144~145]；2)fast ratio^[146](只限于判断是否以快组分为主)；3)分析数据时初始信号减前背景积分信号，从而计算时尽可能多的利用快组分信号^[147]。我们对比了由石英前背景和后背景得到的长江三角洲深切古河谷钻孔的光释光年代，发现通过前后背景得到的年代在误差范围内一致(未发表数据)，表明样品光释光信号可能以快组分为主。但是对密西西比河沉积物研究发现，沉积物的年代受到前后背景值选择的影响^[35]。因此，要通过地质地貌背景来判断沉积物中石英信号的特征，从而选择合适的测试和分析方法，从而确保计算年代的信号为快组分。

2.2.5 长石包裹体

通过化学提纯的石英矿物可能存在长石包裹体(OSL IR depletion ratio＜0.9^[148])，因此在后期测量中光释光信号是由石英和长石共同发出的，长石信号由于异常晒退会导致最终年龄的低估。如Gao等^[40]对南黄海西海岸沉积物的研究发现粗颗粒石英样品的年龄低估可能是由于长石包裹体所致。如果一个样品中只有个别石英矿物颗粒中包含无法去除的长石信号，在测量时可以对每一个石英测片或矿物进行长石信号检测^[148]，从而剔除混有长石的个别测片。对于石英矿物中普遍存在长石包裹体而无法去除的样品，可以通过测量方法剔除长石信号影响，提取出源于石英的释光信号，提高测年结果的准确性，如红外激发后蓝光释光(post-IR blue OSL)^[75~78]和脉冲释光技术(pulsed OSL)^[79~80]。

3 结论与展望

光释光技术的快速发展，为海岸带晚第四纪地层年代的确定提供了新机遇。通过对已发表的500多个光释光年代数据分析，可见目前我国海岸沉积的年代确定主要是集中在晚更新世晚期以来的样品，石英是主要的测量矿物。样品的晒退程序、剂量率、石英纯度、粒级和测量程序的选择，都会影响光释光测年结果的准确性。因此，应该根据沉积物的性质，选择合适的粒级、矿物、测量程序进行测量，在分析数据时应考虑可能存在的晒退问题、剂量率及释光信号组分特征等，从而得到样品的准确年代。目前对于我国较老的(＞70~80 ka)海岸沉积光释光定年的研究还非常有限(现有少数几个较老年龄是采用传统的石英光释光信号和未经异常衰退的长石红外释光信号进行测定，可能导致年龄的低估)。对于较老沉积物年龄的测定，长石矿物应为优先选择。由于海岸带晚第四纪地层的复杂性，对于中晚更新世沉积物缺少合适的断代技术，而长石光释光年代学研究在海岸沉积定年中具有巨大的潜力。鉴于以上情况，各种测年技术手段的对比研究(包括不同粒级、矿物、测量程序的光释光技术内部对比)，并结合地层层序、地质地貌特征的综合分析，可以提高测年结果的可靠性。

致谢: 感谢邱凤钺在文章制图过程中的帮助及有益的讨论；感谢匿名审稿专家和编辑部杨美芳老师提出的宝贵修改意见。

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Application of optically stimulated luminescence dating to Late Quaternary coastal deposits in China

Nian Xiaomei, Zhang Weiguo

( State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062)

Abstract

The coastal system, lying at the interface between the land and the ocean, is sensitive to environmental changes. Coastal deposits are archives of regional and global environment changes. However, due to the complex sedimentary processes, such as sediment reworking and redeposition, and lack of suitable material for ¹⁴C dating, accurate age determination remains a challenge for Quaternary stratigraphic study in coastal system. Optically stimulated luminescence(OSL) dating, which normally uses quartz and feldspar as the dating minerals, can be applied to obtain depositional age or burial age of the sediments in a variety of sedimentary environments. The technique provides a wide age range from nearly a hundred years to several hundred thousand years. Recent development in the luminescence methodology has led to great improvement in the applicability and precision of luminescence dating, especially the improved single-aliquot regenerative-dose(SAR) protocol for OSL, and its rapid development has offered an invaluable technique for dating late Quaternary coastal deposits. Here we compiled more than forty papers published since 2002 in China, twenty-seven of which are written in English and contain more than five hundred OSL data. From the statistical analysis, we can found that OSL dating technique is mainly applied to the Huanghe(Yellow)/Changjiang(Yangtze)/Zhujiang(Pearl) River deltas and their adjacent continental shelfs, and the coastal plains on both side of Taiwan Strait. Based on a review of recent progress of OSL dating technique and its application in coastal regions of China, this paper discusses the common issues and coping strategies in establishing reliable chronology for coastal sediments using OSL method, including selection of dating minerals(quartz vs. feldspar), incomplete bleaching, dose rate, components of quartz OSL signal and feldspar inclusion. An understanding of these issues is critical to the assessment of OSL dating accuracy, which would provide good support to Quaternary environment change study in coastal region. It has been found that OSL technique is mainly applied to sediments deposited during the second half of Late Pleistocene and Holocene in China, and quartz is the main choice for measurement. Quartz from different coastal areas show different bleaching characterizes, which may be connected with various sediment provenance and transport history in the coastal zones. Through the investigation of the samples from the Yangtze River delta, we found that medium-grained quartz samples generally appeared to be well bleached than coarse-grained quartz. Incomplete bleaching, dose rate, purity of quartz, different grain-size fractions(quartz or feldspar), different protocols can affect the accuracy of OSL dating results. Therefore, it is needed to choose suitable grain-size fractions, minerals, and protocols according to the characteristics of the sediment. Meanwhile, when analyzing the data, we should consider the possibility of incomplete bleaching, dose rate and characteristics of quartz OSL signal components to obtain accurate ages for the samples. Quartz is normally an ideal choice for dating relative young sample. However, there has been an apparent age limit around 70~80 ka, i.e. samples from obviously older layers tend to yield luminescence ages around 70~80 ka. Feldspar should be the first choice for determining the age of relative old samples. At present, there are limited studies on feldspar OSL dating for old(>70~80 ka) coastal sediments in China. A few relative old OSL ages were mainly obtained using the traditional quartz OSL signal and feldspar infrared stimulated luminescence(IRSL) signal without anomalous fading correction, which may underestimate the OSL ages. According to the above considerations, an integrated study of various dating techniques(including the internal comparison of OSL results obtained from different particle-size fractions, different minerals and different protocols), as well as the consideration of stratigraphic sequence and geomorphological evolution, will improve the reliability of dating results.

Key words: optically stimulated luminescence(OSL)dating incomplete bleaching dose rate quartz feldspar components of quartz OSL signal feldspar inclusion


第四纪研究 2018, Vol.38 Issue (3): 573-586	PDF