Magnetic variations of sediments from a drilling core in the Lingdingyang Bay, Zhujiang River Estuary, and their responses to marine transgression and regression
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摘要: 华南珠江三角洲河口地区受到珠江流域和近海海洋环境的共同影响,其沉积体系中承载的古地质记录对于揭示海陆交互作用下的地质地貌过程演化具有独特意义。本研究对珠江伶仃洋湾口的一条钻孔岩芯进行了环境磁学研究。研究发现,在晚更新世以来海水入侵的相对高海平面时期,岩芯沉积物中磁性矿物的组成或高矫顽力组分的来源更稳定,反映了沉积区环境的稳定性或沉积物源区的相对固定;在海水退出的风化剥蚀期,沉积序列中磁性矿物组合出现大幅度旋回变化,这表明同时期碎屑物质源区可能经历了显著的环境更替。多重磁学参数包括低频磁化率、非磁滞剩磁、饱和等温剩磁以及HIRM参数,在全岩芯中指示了比较一致的磁性变化特征;相较之下,S-ratio参数的变化更灵敏地响应了伶仃洋湾口地区海水入侵和退出背景下的沉积演化历史。Abstract: The estuary area of the Zhujiang River Delta is significantly influenced by its drainage and offshore environment. Since being under the interaction of sea and land, the depositional system within the estuary area could be of great potential to reveal the simultaneous succession of geological and geomorphic processes. This paper presents a magnetic investigation report of a sedimentary core drilled from the Lingdingyang Bay at the Zhujiang River Estuary. Results of this study show that, the concentration of magnetic minerals or the source area of the high-coercivity magnetic minerals within the core sediments is relatively constant during the two periods of relatively high sea-level (i.e., under marine transgression) since late Pleistocene, revealing a stable sedimentation environment or a relatively fixed sediment source, respectively; however, during the period of marine regression, when the weathering and denudation processes dominate, the magnetic mineral composition in sediments is characterized by significant large-scale fluctuations, possibly indicating that the source areas of detrital materials have undergone a dramatic climate evolution. Multiple environmental magnetic parameters, including low-frequency magnetic susceptibility, anhysterestic remanent magnetization, saturation isothermal remanent magnetization, and HIRM (the ‘hard’ isothermal remanent magnetization), reveal a synchronous pattern of magnetic variations through the core. In comparison, the S-ratio record of the core sediments is more sensitive to the sedimentation evolution history during the marine transgression and regression in the Lingdingyang Bay at the Zhujiang River Estuary.
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Key words:
- Zhujiang River Estuary /
- sediment /
- magnetic variation
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图 1 NZ2钻孔位置及地层信息
背景底图由GeoMapApp (3.6.6版)提供,NZ2站位和地层信息等根据文献[23]改绘,邻近站位L2和L16位置信息来自文献[24]
Fig. 1 Location of Core NZ2 and its stratigraphic information
Base-map was downloaded from GeoMapApp (Version 3.6.6). Stratigraphic and chronological framework information of the study Core NZ2 were modified from reference [23], and location information of two additional cores L2 and L16 from reference [24]
图 3 归一化的等温剩磁获得曲线(a)与剩磁矫顽力曲线(b)
a中虚线对应300 mT磁场时的等温剩磁获得情况(参见表1)
Fig. 3 Normalized results for saturation isothermal remanent magnetization acquisition (a) and back-field demagnetization (b) curves for representative samples
The gray dash line in a indicates the magnetic field is 300 mT (see details in Table 1)
图 4 钻孔NZ2沉积序列的磁性变化特征
灰色曲线展示了对数坐标下各参数的变化,用于与线性坐标下的结果进行对照。灰色水平线用于标记磁性变化剧烈的界限,符号“X”标记了岩芯中的6处未采样的层位
Fig. 4 Down-core magnetic variations for Core NZ2
Down-core variations for each parameter are also shown on a log scale for comparison with their counterpart curves on the linear scale. Gray horizontal lines mark the positions of dramatic shifts on the curves. The symbol "X" represents an interval with no samples collected for analysis
图 5 钻孔NZ2沉积序列的磁性矿物含量变化特征
L-ratio用于评价参数HIRM和S-ratio作为反铁磁性矿物含量指标的有效性,根据文献[28]计算得到。灰色虚线将全岩芯记录分为4个区段,符号“X”标记了岩芯中的6处未采样的层位
Fig. 5 Down-core variations of magnetic mineral contents for Core NZ2
L-ratio, calculated according to reference [28], is used to evaluate the validity of HIRM and S-ratio as content indices of antiferromagnetic minerals. Gray dashed lines divide the entire section into four phases. The symbol "X" represents an interval with no samples collected for analysis
表 1 样品等温剩磁获得特征的信息
Tab. 1 Isothermal remanent magnetization acquisition data for representative samples
取样位置/m 300 mT时的IRM/SIRM 剩磁矫顽力/mT 0~0.20 94.34% 47.2 0.75~0.95 89.52% 42.2 8.30~8.50 85.34% 45.3 11.30~11.50 84.15% 61.9 13.90~14.20 91.61% 21.8 18.70~18.90 95.50% 24.3 24.70~24.90 93.41% 39.5 表 2 伶仃洋岩芯地层划分及与珠江三角洲晚第四纪地层划分方案对比
Tab. 2 Stratigraphy divisions on core sediments from Lingdingyang Bay and their comparison with the stratigraphy framework of the Zhujiang River Delta
伶仃洋NZ2孔
(本研究)伶仃洋L2和L16孔
文献[15]珠江三角洲多钻孔资料*
文献[17−18]地层单元(深度/m) 沉积相 沉积相 年代/ka BP 地层单元(编号) 沉积相 年代/ka BP Ⅳ(0~8.3) 粉砂质黏土和黏土质砂,含贝壳、有孔虫化石及碎屑(海相层) 海相层(桂州海侵,或全新世海进沉积层) 约9.56~0 M1b 三角洲相(海相层) 8~0 (MIS 1) M1a 河流−河口通道相 10.5~8 (MIS 1) Ⅲ (8.3~16.5) 粉砂质黏土以及粗砂−细砂−粗砂分层(埋藏风化壳,陆相层) 陆相层(新会风化期,或风化与冲洪积层) 约20~9.56 T1 陆相层 120~10.5 (MIS 5a−d, 4−2) Ⅱ (16.5~25.3) 黏土质粉砂(海相层) 海相层(礼乐海侵) 约36~20 M2 浅海相层 126~120 (MIS 5e) Ⅰ(25.3~27.9) 粗砂(风化层,陆相层) 陆相层(风化壳,或珠江冲积层) 超过40~36 T2 陆相层 超过126 (MIS 6) 基底 基岩 基岩 — 基岩 — 注:*列MIS指根据氧同位素记录划分的气候阶段,奇数和偶数期分别对应间冰期和冰期气候阶段。—表示无数据。 -
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