Magnetic characteristics and early diagenesis of Holocene sediments in the Zhujiang River Delta
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摘要: 磁性矿物的早期成岩作用是沉积物埋藏后的重要过程,辨别早期成岩作用,才能更好地解释地层的矿物磁性变化。本研究对珠江三角洲顺德平原全新世钻孔MZ孔进行沉积相和室温磁性分析,并辅以热磁分析鉴定磁性矿物,以探讨钻孔不同深度和沉积相的早期成岩作用阶段。结果表明,MZ孔全新世地层自下而上包括感潮河道、河口湾和三角洲相。室温磁性特征与沉积相缺乏明显关联,表现出强烈的早期成岩作用。此外,全新世晚期岩芯磁性特征还受人类活动影响。该孔早期成岩作用以磁性矿物溶解和形成自生黄铁矿为主。在三角洲前缘相的上部和河口湾相底部保存了硫复铁矿。根据矿物组合推测以4.51~4.56 m和30.4~30.5 m两个深度为代表的硫复铁矿形成机制不同,即三角洲前缘相中硫复铁矿可能形成于早期成岩作用的硫酸盐还原阶段,而河口湾相的硫复铁矿形成于甲烷厌氧氧化阶段,后者的含量随着深度增加逐渐增多。上述现象说明,沉积环境可以通过影响有机质和硫酸根离子的供应量,决定磁性矿物所达到的早期成岩作用阶段。Abstract: Magnetic mineral diagenesis is an important early diagenetic process after the burial of sediments and its proper identification is the precondition of interpretations for the mineral magnetic properties in the sediments. This study carried out analyses of sedimentary facies, room temperature magnetic and thermomagnetic properties in a Holocene Core MZ collected in the Shunde Plain of the Zhujiang River Delta to identify the vertical changes in the assemblage of magnetic minerals, so as to explore the early diagenetic stages and possible linkage to the sedimentary facies. The results show that the Holocene sedimentary sequence of Core MZ includes tidal channel, embayment, and deltaic successions from bottom upward. The magnetic properties at room temperature lack correlation with sedimentary facies and demonstrate features of strong early diagenesis. In addition, the magnetic properties of the late Holocene sediments were strongly influenced by the human activities. The early diagenesis mainly includes the dissolution of magnetic minerals and the formation of authigenic pyrite. Greigite was also identified in the upper section of the delta-front succession and the bottom of embayment succession. The concentration of greigite increases with depth in the embayment succession. According to the magnetic mineral assemblages, we infer different formation mechanism of greigite in the two successions. We suggest that the greigite in the delta-front facies was formed in the sulfate reduction stage of early diagenesis, whilst it was formed in the anaerobic oxidation stage of methane in the embayment facies. These phenomena indicate that sedimentary environment has impacts on the early diagenetic stage of magnetic minerals by controlling the availability of organic matter and sulfate.
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图 1 稳态早期成岩作用分带及磁性矿物浓度变化的概念模型图(修改自文献[26-27])
红色为磁铁矿,灰色为硫复铁矿。SD、PSD、MD、SP分别为单畴态、假单畴态、多畴态以及超顺磁态颗粒;SMTZ为硫酸盐-甲烷转换带
Fig. 1 Steady mtate redox zones of early diagenesis and associated changes in the concentrations of magnetic minerals (refer to references [26-27])
Magnetite is red, greigite is gray. SD, PSD, MD and SP are single-domain, pseudosingle-domain, multi-domain and superparamagnetic particles. SMTZ is sulfate-methane transition zone
表 1 珠江三角洲顺德平原MZ孔测年结果及校正(校正年龄取概率大于0.8的区间)
Tab. 1 AMS14C ages and calibrations for Core MZ in the Shunde Plain of Zhujiang River Delta (the calibrated ages are selected with a probability of more than 0.8)
深度/m 标高/m 测年材料 常规年龄/a BP 校正年龄/cal. a BP 实验室编号 2σ 中值 概率 6.52 −6.1 植物 380±30 318~503 445 1.000 Beta-548917 7.78 −7.36 植物 1 100±30 955~1 063 1 000 0.961 Beta-548918 10.39 −9.97 木头 2 880±30 2 921~3 078 3 010 0.899 Beta-548919 11.80 −11.38 贝壳 2 590±30 1 993~2 388 2 205 1.000 Beta-548920 17.43 −17.01 贝壳 2 690±30 2 117~2 565 2 335 1.000 Beta-548921 18.44 −18.02 贝壳 3 010±30 2 516~2 918 2 730 1.000 Beta-548923 21.74 −21.32 贝壳 5 420±30 5 524~5 891 5 695 1.000 Beta-548924 21.88 −21.46 植物 7 820±30 8 534~8 649 8 595 0.968 Beta-548925 22.14 −21.72 植物 4 950±30 5 598~5 732 5 660 1.000 Beta-548926 30.06 −29.64 植物 8 780±30 9 658~9 908 9 785 0.963 Beta-548927 31.30 −30.88 木头 8 850±30 9 766~10 154 9 970 1.000 Beta-548928 42.73 −42.31 贝壳 10 980±30 12 813~12 998 12 880 0.915 Beta-548929 44.20 −43.78 贝壳 11 180±30 13 081~13 162 13 110 1.000 Beta-548930 表 2 MZ孔磁性单元的磁参数特征值
Tab. 2 Characteristic values of magnetic parameters in each unit of Core MZ
磁性参数 磁性单元及沉积相类型 A单元
(三角洲前缘)B单元
(三角洲
前缘−斜坡)C单元
(斜坡−前三
角洲)D单元
(前三角洲)E单元
(河口湾)F单元
(河口湾)G单元
(感潮河道−河床)χ/
(10−8 m3·kg−1)范围 119.7~395.4 11.5~71.2 7.1~26.4 17.5~28.0 13.0~30.4 16.1~93.2 3.9~73.5 平均值 202.1 52.2 13.8 21.9 20.8 34.1 22.2 标准差 73.3 15.4 7.6 2.3 3.9 18.0 11.0 SIRM/
(10−6 Am2·kg−1)范围 10 461.3~30 448.1 905.2~28 471.9 341.0~3 542.4 1 636.0~4 355.8 1 711.9~4 217.1 1 955.8~18 601.5 1 007.5~27 547.1 平均值 17 272.3 6 909.8 1 466.5 2 607.6 2 792.0 7 210.8 2 959.0 标准差 5 966.5 5 625.4 1 295.6 690.5 623.9 5 291.8 3 697.6 χfd /% 范围 5~9.71 0~7.0 0~1.9 0~1.6 0~9.72 0~3.7 0~2.7 平均值 6.8 4.2 0.3 0.2 0.9 0.7 0.5 标准差 1.2 2.1 0.7 0.4 2.0 0.9 0.7 χarm/
(10−8 m3·kg−1)范围 467.3~1 842.4 11.9~1 756.8 1.5~20.9 33.0~404.0 200.6~521.2 60.3~338.2 22.8~266.8 平均值 940.6 356.9 5.8 145.6 346.1 198.6 83.5 标准差 320.6 403.5 6.8 101.2 83.5 86.7 49.7 χarm/χ 范围 3.3~7.2 0.2~67.1 0.1~3.0 1.6~18.5 11.2~21.9 1.1~13.4 1.7~14.0 平均值 4.8 8.4 0.7 6.6 16.7 6.8 4.0 标准差 1.0 15.0 1.0 4.5 2.7 3.2 2.3 (χarm/SIRM)/
(10−5 mA−1)范围 33.1~88.4 2.1~594.3 0.9~61.4 16.1~134.8 85.9~162.6 10.8~70.5 9.7~91.9 平均值 56.5 74.9 12.5 52.5 124.6 35.3 32.2 标准差 14.7 133.4 21.9 27.5 15.9 17.3 12.6 SIRM/χ 范围 7.7~10.0 7.8~50.7 4.8~13.6 9.1~19.2 8.4~14.7 5.0~60.9 6.6~37.5 平均值 8.6 13.0 8.7 11.9 13.4 24.4 12.7 标准差 0.8 9.5 3.6 2.7 1.1 16.2 5.8 HIRM/
(10−6 Am2·kg−1)范围 66.1~1 241.4 121.0~698.4 25.9~256.2 129.5~231.5 73.6~236.0 6.5~721.2 9.3~372.1 平均值 595.6 318.5 129.6 196.2 165.3 195.8 131.7 标准差 319.5 126.2 84.9 21.0 40.4 155.5 69.8 S−20/% 范围 53.3~75.7 50.8~80.1 61.1~73.8 56.5~81.5 81.1~568.7 59.8~98.8 53.6~97.8 平均值 58.7 57.1 69.3 62.3 63.0 73.8 66.4 标准差 7.5 7.8 4.2 6.4 4.2 11.0 8.7 S−40/% 范围 75.1~86.9 50.5~76.7 53.0~68.4 52.6~73.9 59.8~82.8 51.9~98.8 53.5~98.5 平均值 79.2 70.1 58.1 63.8 64.6 63.6 62.0 标准差 3.4 5.1 6.0 4.1 4.6 10.4 7.5 S−100/% 范围 90.0~95.3 64.3~91.3 69.0~88.4 80.5~88.6 86.2~95.8 81.1~99.6 75.3~100.0 平均值 92.2 87.7 77.9 84.4 87.8 88.3 86.3 标准差 1.4 6.0 7.6 2.1 1.5 3.5 3.5 S−300/% 范围 95.4~99.6 81.7~97.6 83.3~94.1 89.2~95.4 92.5~97.7 91.2~99.9 88.7~99.7 平均值 96.6 94.2 89.2 92.1 94.0 96.1 94.5 标准差 1.2 3.3 3.8 1.6 0.9 2.5 2.1 注:磁参数特征值包括最大值、最小值、平均值和标准差。 -
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