Luo Yi (1982-), female, Ph.D. candidate, mainly engaged in marine geological research, email: wulude @163.com.

1. School of Oceanography, China Geo University, Beijing 100083.

2. Guangzhou Marine Geological Survey, Guangzhou 5 10760.

Sedimentology and magnetic analysis of DSH- 1C core samples taken from the northern slope of the South China Sea were carried out. Combined with relevant data, the vertical variation of magnetic characteristics of core sediments in this area and its relationship with the change of sedimentary environment are discussed. The results show that DSH- 1C core samples are divided into three lithologic units from top to bottom, and the surface sediments are Holocene MIS 1 deep-sea and semi-deep-sea sediments dominated by clayey silt. The middle part contains several layers of gravity flow sedimentary interlayer, which belongs to late Pleistocene MIS2 deposition; The bottom is clay silt of late Pleistocene MIS3 stage. The average x value of the columnar sample is 1.72× 10-7m3/kg. The IRM of all samples reached more than 80% of SIRM, and the minimum value of S300 was 0.605. There are few magnetic minerals in columnar sediments, mainly low coercivity minerals; During the interglacial period (MIS 1 and MIS3), the magnetic characteristics of the columnar samples were higher when more terrestrial materials were input. On the contrary, the magnetic parameters in MIS2 are low, which may be related to the decrease of terrestrial materials in this area during the ice age. In addition, the low magnetic parameters of gravity flow layer sediments rich in foraminifera shells or bivalves in the core column are related to the increase of these diamagnetic carbonate components.

Keywords: magnetic characteristics; Particle size analysis; Late Pleistocene; Dongsha; the South China Sea

Magnetic characteristics and environmental significance of late Pleistocene sediments from DSH- 1 C core in the northern South China Sea

Luo Yi 1, Su Xin 1, Chen Fang 2, Huang Yongyang 2

1. School of Marine Science, China Geo University, Beijing 100083.

2. Guangzhou Marine Geological Survey, Guangzhou 5 10760, China.

Abstract: The magnetism of DSH- 1C columnar sediments in Dongsha deep-sea area of South China Sea was studied. The 626 cm core is subdivided into three lithologic units: Holocene clay silt (unit I, MISl) in the top interval of the core; Late Pleistocene turbidite sequence, characterized by 3-4 main sand layers in the middle section (unit ⅱ, MIS 2); Then the lowest sequence (unit ⅲ, MIS 3) is composed of clayey silty sand and silty sand or thin layers of silty sand ... The average x value of sediments is 1.72× 10-7m3/kg, and the IRM value of all samples is very high, exceeding 80‰ of the SIRM of sediments, while the minimum s 300 value of all samples is 0.605. The lowest magnetic values (X, NRM and SIRM) appear in the interval of Unit II, where there is a turbid layer rich in calcareous foraminiferal shells, indicating that carbonate in these sediments is diluted. On the other hand, during the interglacial period (MIS 1 and MIS3), the higher values of these parameters appeared, which may be due to more terrigenous debris input in the warm period in this area.

Keywords: magnetism; Particle size; Late Pleistocene; Dongsha area; the South China Sea

Introduction to 0

Since its establishment in 1980s, environmental magnetism has gradually formed a new interdisciplinary subject, which takes magnetic measurement as the core means and magnetic minerals as the carrier, and studies environmental effects, environmental processes and environmental problems by magnetic methods [1-3]. The study of environmental magnetism of marine sediments has also become a hot spot in recent years. In this field, scholars at home and abroad have studied the magnetic characteristics of deep-sea core sediments or surface sediments, combined with chronology, sedimentology and geochemistry, studied the source of sediments and the changes of sedimentary environment, and reconstructed the paleoclimate and paleoenvironment [1-6].

At present, in the study of magnetic characteristics of marine sediments, the change of magnetic susceptibility of sediments can reflect the change of material source and environment, which has been widely recognized and applied. Other magnetic parameters (such as natural remanence, isothermal remanence, non-hysteresis remanence, etc.). ) gradually introduced into the study of mineralogy, paleomagnetism, secondary changes and diagenesis of marine sediments [4- 12]. Moreover, in recent years, in the study of marine natural gas hydrate, foreign scholars have discussed the magnetic parameters (mainly represented by magnetic susceptibility) of sediments in hydrate occurrence areas and their relationship with authigenic minerals (mainly represented by pyrite) [13- 15].

In this paper, the magnetism of columnar core sediments in hydrate occurrence area of South China Sea is studied for the first time. By using the methods of environmental magnetism and sedimentology, the magnetic characteristics and sedimentary environment of DSH- 1C gravity columnar sediments in the hydrate occurrence area in the northern South China Sea were compared, and the factors of the change of magnetic parameters of surface sediments in this study area and their relationship with the change of sedimentary environment were discussed. It is hoped that through the above research, the magnetic characteristics and environmental significance of the surface sediments in this study area can be obtained.

1 samples and methods

1. 1 sample source

The total column length of DSH- 1C pressure-keeping gravity piston is 626 cm, which was collected by "Haiyang No.4" scientific research ship in 2006 in the gas hydrate investigation area of "Haiyang No.4 sedimentary body" in Dongsha sea area on the northern slope of the South China Sea, with a water depth of 3 000 m. The evidence of cold spring activity in this area was first discovered by "Haiyang No.4" scientific research ship. In 2004, the Sino-German cooperative voyage SO177 "Sousaphone" obtained more evidence in the investigation of "Study on the Distribution, Formation and Environmental Impact of Methane and Natural Gas Hydrates on the North Slope of the South China Sea" and named it "Haiyang No.4" sedimentary body [16-65438+.

This area is located in the eastern part of the continental slope in the northern South China Sea, on the north bank of the Taiwan Province Strait, structurally belonging to the passive continental margin, adjacent to the offshore accretionary wedge in the southwest of Taiwan Province Island. The water depth is between 1 500 and 3 000 m, and the average water depth is more than 2 500 m[ 16] (Figure1).

The seabed in the study area has strong seismic reflection characteristics similar to BSR. This bivalve and fungus mat with deep-water cold spring were found in submarine TV survey. According to the core description of SO 177 voyage GC 10 [16] (figure 1), there are fracture structures formed by methane gas filling in the sediments in this area. The results of pore water geochemical analysis also show that there are geochemical characteristics such as abnormal chloride ions in some depths of pore water, and it is inferred from methane flux that there is methane source in the deep part of the station.

Figure 1 bathymetric map of "Haiyang No.4 Sedimentary Body" on the northern slope of the South China Sea and schematic diagrams of DSH- 1C and SO 177-GC 10 stations.

1.2 research methods

After describing the lithologic characteristics of DSH- 1C columnar samples and taking photos, 63 sediment samples were obtained with a spacing of 10 cm and a sampling thickness of 2 cm, and their magnetism, particle size and carbonate content were tested.

1.2. 1 rock magnetic method

The magnetic parameters of DSH- 1C cylindrical samples were tested, including magnetic susceptibility (X), natural remanence (NRM), non-hysteresis remanence (ARM), isothermal remanence (IRM) and saturated isothermal remanence (SIRM).

The adjacent SO 177-GC 10 columnar sample has been tested for the age of foraminifera AMS 14C [16], and its bottom age is 50 ~ 60 ka, which belongs to the positive polarity period, so the magnetic dip direction of DSH- 1C columnar sample is not considered. Environmental magnetic samples are directly packed in non-magnetic cubic boxes, and all samples are dried at low temperature (below 40℃).

(1) The magnetic susceptibility was measured in the Earth Science Experimental Center of China Geo University (Beijing), and the mass magnetic susceptibility of all samples was measured by Suli -4S Kappa bridge magnetic susceptibility meter.

(2) The remanence and demagnetization parameters of the samples were measured in the Paleomagnetic Laboratory of Institute of Geology and Geophysics, China Academy of Sciences. Completed on 2G-755R rock superconducting magnetometer, the natural remanence of all samples was measured and then demagnetized. The measuring range of the instrument is 2.0×10-12 ~ 2.0×10-4am2; The sensitivity is1.0×10-12am2. Except that the measured value of the sample at 490cm is 2.37× 10-4Am2, which is out of range for reference only, and the measured value at 320cm is not saved due to computer failure, the minimum value of the other 610-6am2 is 1.65438. Generally, the characteristic remanence can be obtained by alternately demagnetizing marine sediment samples in the range of 15 ~ 25mt, so the demagnetizing step size is selected as 0,5, 10, 15,20,25,30,40,50,60,70mt. The characteristic remanence of 6 1 sample was obtained at 240 cm and 320 cm, because the computer fault measurement value was not saved.

(3) Using 2G-760 superconducting magnetometer, the hysteresis-free remanence of the sample was measured under the DC magnetic field of 0. 1 m T and the alternating magnetic field of 90 mt. The measuring range of the instrument is1.0×10-7 ~1.0×10-2mm2; The sensitivity is 2.0× 10- 12Am2. The minimum value of all samples is 3.96× 10-5Am2, and the maximum value is 2.68× 10-3Am2, which is a reliable value.

(4) In order to ensure that the saturation isothermal remanence of the sample is within the measurement range of 2G-760 superconducting magnetometer, the quality of the measured sample is reduced. Magnetize with 660 pulse magnetizer in 1.7 T magnetic field, and then measure saturated isothermal remanence with 2G-760 superconducting magnetometer. The minimum value of all samples is 1.38× 10-4Am2, and the maximum value is 9.08× 10-3Am2, which is a reliable value. Samples were magnetized in reverse magnetic fields of 100 and 300 m T to obtain isothermal remanence (IRM- 100, IRM-300) of all samples.

Define S300 = (-IRM-300)/SIRM and calculate S300.

1.2.2 particle size analysis

Particle size measurement was carried out in China Geo University (Beijing) College of Oceanography, using Mastersize2000 laser particle size analyzer of Malvern Company in England. In this paper, the organic matter and calcium components in the sample were not removed, and the particle size characteristics of all the debris in the sediment were predicted, so the full particle size analysis was carried out. The method is as follows: take about 2 g of the sample to be tested, put it in a beaker of 20 m L, soak it in distilled water, and make it disperse in the natural state. Before the test, 0.5 mol/L sodium hexametaphosphate solution was added for chemical dispersion, and ultrasonic treatment was not carried out during the test.

1.2.3 carbonate content test

China Geo University (Beijing) College of Oceanography also determined the carbonate content by volumetric method. Because some samples contain more calcareous biological shells, in order to ensure the accuracy of sample determination, at least 3 samples from each sample should be taken for parallel determination.

2 Results and discussion

2. 1 Lithology and granularity characteristics

The main lithology of DSH- 1C columnar sediments is gray-green clayey silty sand, with several layers of coarse-grained silty sandstones rich in foraminifera and biological debris in the middle, and some layers are sandwiched with gray-yellow or gray-black fine-grained layers, which are highly viscous, chapped at the lower part and expanded in pore structure. According to the change of lithology and grain size, the core can be divided into three lithologic units (I-III) from top to bottom (Figure 2).

Fig. 2 results of particle size analysis of dsh-1c columnar sediments

Lithologic unit I (0 ~ about 152 cm) is silty sand containing foraminifera, and there are many foraminifera in the sand body composition, so it corresponds to the change of carbonate content. Lithologic unit ⅱ (about 152 ~ 470 cm) is mainly characterized by a large number of biological debris (bivalves, gastropods and other shells) and clayey silt mixed with foraminiferal sand. Sand layer and clayey silt layer appear alternately. Lithologic unit ⅲ (about 470~620 cm) is clayey silty sand with dark gray-black silty sand interlayer. The calcium content in sediments is relatively low and stable.

2.2 years determination

Table1so177-GC10 Age data of planktonic foraminifera AMS14c [16-17]

Fig. 3 Lithology, particle size analysis and contrast curves of DSH-1c and SO 177-GC 10 columnar samples (as shown in the left file [17]).

Age data of planktonic foraminifera AMS 14C obtained from the surface columnar sediment samples of SO 177 voyage GC 10 (table 1)[ 16- 17]. According to the research of Zhang et al. [17], the upper part of the three lithologic units of GC 10 core sample (Figure 3, left) is Holocene deposit, and the middle and lower part is Pleistocene top deposit. The boundary between them is marked by the last stratum rich in foraminifera and biological debris. Compared with GC 10, it can be considered that the first appearance of DSH- 1C columnar sediment rich in foraminifera and bioclastic sand in the lower part of about 152 cm depth is a sign. More than 152 cm is Holocene deposit, and below it is Pleistocene top deposit (Figure 3). Among them, lithologic unit ⅱ was deposited in MIS2 stage of the last glacial period, and lithologic unit ⅲ was deposited in MIS3 stage.

2.3 magnetic results

Fig. 4 is a graph showing the variation of magnetic parameters of DSH- 1C columnar samples with depth, in which the magnetic variation of natural substances recorded by X, NRM, ARM and SIRM is related to the content, type and particle size of magnetic minerals in sediments. Generally speaking, the size of S300 is directly proportional to the relative contents of low and medium coercivity magnetic minerals and high coercivity magnetic minerals in sediments [18]. This paper mainly discusses the content change of magnetic minerals in DSH- 1C columnar sediments.

According to the test results, combined with its lithological characteristics, the magnetic parameter characteristics of DSH- 1C cylindrical samples can be divided into three sections: section I (0 ~ 152cm), section II (152 ~ 470cm) and section III (470 ~ 626cm).

Fig. 4 Variation of magnetic parameters (X, NRM, ARM, SIRM, S300) of DSH-1C columnar samples with depth.

The first section (0 ~ 152cm): the variation range of this depth section x is (2.37 ~ 4.84) × 10-7m3/kg, which fluctuates greatly and decreases with the increase of depth. The characteristics of numerical curves of NRM, ARM and SIRM are consistent with the changing trend of X. The S300 of samples in this depth ranges from 0.925 to 1.00.

Section Ⅱ (152 ~ 470 cm): The average values of X, NRM, ARM and SIRM in this depth section decreased obviously, and the overall values tended to be stable. The average value of x is1.10×10-7m3/kg. The average value of ARM is1.17×10-7am2/kg, which is 87.6% lower than before. The average SIRM is 6.54× 10-6Am2/kg, which is 57.5% lower than that of the previous year. S300 of this section fluctuates greatly, and the minimum value of 0.605 in the whole column appears at 330cm.

The third section (470 ~ 626 cm): the values of X, NRM, ARM and SIRM are all higher than the previous section, with obvious fluctuation. The maximum value of the whole column appeared at 490 cm, and its X, NRM, ARM and SIRM all appeared maximum values. S300 is obviously different from the last two paragraphs, with a small change range and a stable trend.

Because the magnetic susceptibility of natural materials mainly depends on the content of magnetic minerals, if the content of ferrimagnetic minerals is small, the magnetic susceptibility is very weak. It is mainly the actual contribution of paramagnetic minerals and even diamagnetic minerals to magnetic susceptibility [1-2]. From the three depth profiles, the maximum X value of DSH- 1C columnar sample is only 6.02× 10-7m3/kg, and the average value is 1.72× 10-7m3/kg. It can be seen that the content of magnetic minerals in columnar sediments is very small.

In the natural sample S300, the value of low coercivity magnetic minerals (such as magnetite) is close to 1, while that of high coercivity magnetic minerals (such as hematite) is lower than 0.5[9, 18]. The minimum S300 of DSH- 1C cylindrical samples is 0.605, and the IRM obtained by all samples under the external magnetic field of 300 T is more than 80% of SIRM. Therefore, soft magnetic minerals with low coercivity mainly exist in columnar sediments.

In addition, the variation trend of X, NRM, ARM and SIRM of columnar sediments with depth is consistent, indicating that the content of magnetic minerals is the main influencing factor of the magnetic characteristics of sediments in the study area.

2.4 Magnetic characteristics and its environmental significance

In this paper, the magnetic parameters X and S300 are selected and compared with the obtained sedimentary characteristics and paleoceanographic results (Figure 5).

Fig. 5 Changes of X, S300, clay volume fraction and carbonate volume fraction with depth of DSH-1C columnar samples.

Variation of magnetic parameters

The magnetic parameters of marine sediments represented by magnetic susceptibility are influenced by many factors. As we all know, the magnetic characteristics of sediments in this study area are mainly influenced by the content of magnetic minerals. Generally speaking, the content of magnetic minerals in sediments between lithologic units I and III is higher than that in sediments within lithologic unit II. In addition, the magnetic susceptibility of sediments is relatively high in the range of high clay particle size (volume) percentage. This change trend is similar to the research results of the correlation between the magnetic susceptibility and grain size of the sediments from hole NS93-5 in the southern South China Sea [19], core MD98-2 172 in the East China Sea [12] and hole EC2005 in the inland shelf of the East China Sea. In the study of the magnetic susceptibility of the surface sediments [2 1] in the western waters of the Taiwan Province Strait and the surface sediments [15] on the bay slope, it is also found that the finer the grain size of the sediments, the higher the magnetic susceptibility value.

The magnetism of sediments in the same lithologic unit is mainly influenced by carbonate content and clastic mineral content. Take the sediments in Lithology Unit II as an example: First, in the interval with high carbonate content, the x value is relatively low (shaded part in Figure 5), because carbonate is an diamagnetic mineral, which makes little contribution to magnetic parameters such as magnetic susceptibility, and the large increase of carbonate content dilutes the clay particle content in the sediments, making the sediments in the corresponding interval relatively low in magnetism. Secondly, in the interval where the particle size content of clay is relatively low, the x value is relatively high (dashed box in Figure 5). In the depth section with this feature, through the observation of sediment lithology, sediment picture observation and particle size analysis, it is concluded that the silt content in these depth sections is high and contains relatively more clastic minerals. It can be considered that the content of detrital minerals in this depth interval has an important contribution to the magnetic parameters of sediments.

S300 ratio of columnar samples is smaller in the gravity flow sedimentary section with high carbonate content and larger in the section with high detrital mineral content. This feature still shows the relationship between magnetic parameters and the content of clastic minerals.

2.4.2 Changes of magnetic parameters and deposition environment

By comparing with the results of sedimentology and paleoceanography at SO 177 [16-17] GC10, the sedimentary records of three lithologic units from bottom to top can be obtained respectively. During this sedimentary period, according to previous studies [17, 22-23], MIS 1 (post-glacial period) was the Holocene high sea surface warm period, MIS2 was the last glacial period, and MIS3 was the last interglacial period. As can be seen from Figure 5, the magnetic parameter value is the highest in the warmest climate, the lowest in the last glacial period and the higher in the last interglacial period.

The sources of magnetic minerals in marine sediments are not only brought about by submarine volcanoes and hydrothermal diagenesis, but also transported by wind, rivers, glaciers and coastal erosion. Others are authigenic magnetic minerals formed by biological action and diagenesis. At present, it is considered that the magnetic minerals in marine sediments of continental slope are mainly from terrestrial sources, and their magnetic parameters (such as magnetic susceptibility) are related to the abundance of terrestrial substances in sediments [1-2,4,9, 19]. In the study of paleoclimate change revealed by magnetic minerals in loess [24], predecessors suggested that warm and humid climate promoted the chemical weathering of loess to form strong magnetic paleosoil, while the magnetism of loess in cold period was weak.

Therefore, it can be inferred that the magnetic difference caused by the weathering process of clastic minerals in the provenance due to climate change may also lead to the strong magnetism of marine sediments in the warm period, and vice versa.

Therefore, in the study area, the fresh water input of rivers in the warm interglacial period was relatively large, which brought more terrestrial materials [23] and showed relatively high magnetic parameters. This feature is especially obvious in the interval with high detrital mineral content (such as the interval with a depth of 490 cm): the sediment not only contains more detrital minerals, but also contains a small amount of sawdust, which has the characteristics of terrigenous detrital and is also highly magnetic. In the cold MIS2-2 period, the fresh water input decreased, the sea level dropped, and the distance from the land increased. The lack of terrigenous input leads to the low content of magnetic minerals in sediments, and the magnetic parameters of this core are the lowest in this period. In addition, the sea level was the lowest in this geological period, and there were several layers of gravity flow deposits from the continental shelf [16-17,23,25-28], which contained a large number of foraminifera and biological debris [26-27]. Their existence increases the carbonate content of sediments in these layers, which is also the reason why the magnetism in these gravity flow layers is the lowest.

3 Conclusion

By analyzing the grain size analysis results, magnetic analysis results and carbonate content of DSH- 1C core samples on the northern slope of the South China Sea, and comparing them with the sediment cores of SO 177-GC 10 in the neighboring station, the following understandings are obtained:

1) The core sample DSH- 1C is a deep-sea semi-deep-sea deposit from late Pleistocene to Holocene, and its main lithology is clayey silt with several layers of gravity flow deposits in the middle.

2) The variation of magnetic characteristics of DSH-1C columnar sediments with depth shows that it is mainly influenced by the content of magnetic minerals in the sediments; The content of magnetic minerals in sediments is very small, mainly soft magnetic minerals with low coercivity; The vertical variation of sediment magnetism is similar to the variation of clay particle size content, and is influenced by carbonate dilution and clastic mineral content.

3) The core sample DSH- 1C has the highest sea level, the largest land input and the highest magnetic parameters during the warm Miocene Glaciation (0 ~ about 152 cm). In the last glacial period of MIS2 (about 152 ~ 470 cm depth), the sea level was the lowest, the land source input was insufficient and the magnetic parameters were the lowest. During the last interglacial period of MIS3 (about 470~626 cm depth), the climate was relatively warm, the sea level was high and the magnetic parameters were high.

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