Institute of Marine Geological Science Development Strategy, Guangzhou Marine Geological Survey; Guangzhou 510760; Key Laboratory of Marine Geology, Ministry of Education, Tongji University, Shanghai 200094)
According to the analysis of SiO _ 2 and Al _ 2O _ 3 of 33 surface samples and 1 columnar samples from the shelf to the slope in the southeast of Qionghai, the identification of planktonic foraminifera, δ 18O, δ 13C analysis and 14C dating, the results show that the content of SiO _ 2 decreases from the shelf to the slope, and the change of Al _ 2O _ 3 content is complicated, and the overall trend is as follows. On the continental shelf and slope, the correlation between SiO _ 2 and Al _ 2O _ 3 is completely different. In the shelf area with water depth less than 150 m, there is a negative correlation between SiO _ 2 and Al _ 2O _ 3, while in the slope area with water depth greater than 300 m, the correlation between SiO _ 2 and Al _ 2O _ 3 is not obvious. The ratio of SiO _ 2/Al _ 2O _ 3 of columnar samples in continental slope area is obviously related to the surface water temperature calculated by the transfer function of planktonic foraminifera. The ratio of SiO _ 2/Al _ 2O _ 3 is higher at oxygen isotope 1 and 3, and lower at the second stage, which reflects the different weathering and leaching intensities of terrestrial materials under different climatic conditions, so the ratio of SiO _ 2/Al _ 2O _ 3 may become another index reflecting paleoclimate characteristics.
Geochemical paleoclimate in southeastern Qionghai during the last glacial period.
The South China Sea is the largest marginal sea in the western Pacific Ocean, with a wide continental margin in the north and a high sedimentation rate. At the same time, the development of various calcareous and siliceous microfossils has become one of the hot spots in paleooceanography research. There are many studies on sedimentary geochemistry of the South China Sea from 12 to 16, but little on paleoclimate and paleomarine environment.
The distribution of SiO _ 2 and Al _ 2O _ 3 on the continental shelf of China presents a negative correlation of 16 ~ 17, but the relationship between them is more complicated in the continental slope area. Generally speaking, SiO _ 2/Al _ 2O _ 3 increases with the increase of water depth, then decreases and then increases. Through the study of SiO _ 2/Al _ 2O _ 3 since the Late Glaciation, it is found that it is related to the surface temperature of seawater, which may provide some useful information for paleoclimate research.
1 materials and methods
Geological samples were taken from 34 stations in the southeast of Qionghai (Figure 1), of which 1 was a columnar sample and the rest were surface samples. Judging from the water depth of sampling stations, 5 stations are less than 150m, 4 stations are 150 ~ 300m, and 24 stations are more than 300m. Column sample S 19, 664cm long, sampled at the interval of 10cm, and 64 * * * samples were taken. All samples were analyzed by silicate and identified by planktonic foraminifera. The δ 18O and δ 13C of the upper 49 samples were analyzed, and three 14C ages were determined, among which SiO2 _ 2 and Al _ 2O _ 3 were determined by atomic absorption spectrometry.
Figure 1 sampling station bitmap
Figure 1 sampling point
The surface sediments in the study area generally become thinner from northwest to southeast, and the northwest is dominated by terrigenous debris, mainly fine sand and clayey silt. The southeast is dominated by biological debris, including calcareous biological sandy silt, calcareous biological clay-sand-silt and siliceous calcareous biological clay silt.
Distribution characteristics of silica and alumina in surface samples
2. Distribution characteristics of1silica
Generally speaking, from the northwest shelf area to the southeast slope, the content of SiO2 gradually decreased from 70% to 34% (Figure 2). On the shelf, the isoline is basically parallel to the bathymetric isobath, and goes to the south slope of the Xisha trough in the southwest. Although the water depth becomes shallow, the content continues to decrease, and the change gradient is basically the same. Obviously, it is mainly controlled by offshore distance and water depth, and the land-derived materials mainly come from Hainan Island in the northwest. With the increase of offshore distance and water depth, the terrestrial materials decrease, and the content of its main component SiO2 _ 2 decreases.
Distribution map of silica content in surface sediments of southeast Hainan.
Fig. 2 distribution of silicon dioxide content in seabed surface sediments in southeastern Hainan
2.2 Al2O3 distribution characteristics
The distribution of Al2O3 content is more complicated than that of SiO2 content, showing a block distribution (Figure 3). The northwest shelf area has the lowest content, which is 8.0%- 1 1.0%. The outer continental shelf-upper continental slope has the highest content of 10.0%- 14.5%. On the south slope of Xisha trough in the southeast, the content decreased to 10.5% ~ 12.5%. It is not completely consistent with the isobath. Obviously, it has more influencing factors. But generally speaking, from northwest to southeast, from continental shelf to uphill, it increases with the increase of water depth and offshore distance, and decreases with the increase of offshore distance in continental slope area.
Fig. 3 Distribution map of Al2O3 content in surface sediments in southeastern Qionghai.
Fig. 3 Distribution of Al2O3 content in seabed surface sediments in southeastern Hainan.
2.3 SiO2/Al2O3 distribution characteristics
The distribution of SiO _ 2/Al _ 2O _ 3 ratio has obvious regularity (Figure 4), decreasing from northwest to southeast, from 8.5 to 3.2. The change gradient of inland shelf area is large, and the shallower the water depth, the greater the change gradient, which is roughly parallel to the isobath. However, the content from the outer continental shelf to the continental slope is relatively stable, especially in the trough area, which is 3.2 ~ 3.6, but it is still controlled by the offshore distance, and the southeast is the lowest.
Distribution map of SiO _ 2/Al _ 2O _ 3 ratio of surface sediments in southeastern Hainan.
Fig. 4 SiO _ 2/Al _ 2O _ 3 ratio in surface sediments of southeast Hainan.
2.4 Correlation analysis
It is generally believed that the distribution of SiO2 _ 2 and Al _ 2O _ 3 contents in sediments in the shelf area is negatively correlated with 16 and 17. The detailed study of surface samples in this sea area shows that the relationship between SiO2 _ 2 and Al _ 2O _ 3 is different in different water depths, and it is negatively correlated in the inland shelf with water depths less than 150m, which is consistent with previous studies. However, there is no obvious correlation between the continental shelf and the upper continental slope beyond the water depth of 150 ~ 300 m; In the land slope area where the water depth is more than 300 meters, there is an obvious positive correlation between them (Figure 5).
Fig. 5 correlation diagram of silica and alumina in surface sediments in southeastern Qionghai.
Fig. 5 Relationship between silica and alumina content in seabed surface sediments in southeast Hainan.
A: water depth <150m, b: water depth > > 300m.
Stratigraphic age of 3 S 19 columnar samples
The results of oxygen isotope determination of S 19 columnar samples (Figure 6) show that the section from 0 to 0~ 140cm is oxygen isotope 1, the section from 140 ~ 420cm is oxygen isotope 2, and the section below 420cm is oxygen isotope 3, which has not bottomed out. At the same time, the 14C dating results of this columnar sample show that 143 ~ 146cm is (10860 240) ab.p, and 246 ~ 249cm is (14000350).
Changes of SiO _ 2/Al _ 2O _ 3 in 4 S 19 columnar samples and paleowater temperature
4. 1 Distribution characteristics of silica and alumina
There is an obvious positive correlation between SiO _ 2 and Al _ 2O _ 3 in core sample S 19, which is consistent with that in surface sediments of continental slope area > 300 m in this area (Figure 6), but contrary to the distribution in shallow shelf area. Generally speaking, the contents of SiO2 _ 2 and Al _ 2O _ 3 in columnar samples are higher in oxygen isotope phase 2, but lower in 1 and 3. However, at the end of the second period, that is, the deglaciation period of the last glacial period, all decreased significantly and reached the lowest value. The contents of silica and alumina are 465,438 0.26% ~ 43.56% and 22.58% ~ 65,438 0.03% respectively. It rose in the early Holocene, with contents of 45.08% ~ 46.76%, 65,438+03.58% ~ 65,438+04.58% respectively. In the middle and late stages, it decreased by 43.6 1% ~ 44.67% and12.72% ~15.15% respectively (Figure 6).
Fig. 6 the relationship between the distribution of silicon and aluminum in the s19 columnar sample and the surface water temperature.
Fig. 6 Relationship between SST of piston core and Si and Al contents S 19
14C dating: 1: 10860 240 absolute pressure, 2: 14000 350 absolute pressure, 3: ≥ 17000 absolute pressure.
4.2 SiO _ 2/Al2O3 and paleowater temperature
The change of SiO _ 2/Al _ 2O _ 3 ratio in columnar samples is completely different from the change of SiO _ 2 and Al _ 2O _ 3 contents (Figure 6), and it is negatively correlated as a whole. However, it has obvious positive correlation with the surface water temperature 18 calculated by the transfer function of planktonic foraminifera, which can be roughly divided into three sections: 0 ~ 0~ 140cm, that is, the oxygen isotope is 1, the ratio of SiO _ 2/Al2O3 is relatively high, ranging from 3.08 to 3.55, and the surface water temperature is also relatively high. 140 ~ 460 cm, which is roughly equivalent to oxygen isotope 2 phase. The ratio of SiO _ 2/Al _ 2O _ 3 is 3.04 ~ 3.47, and the surface water temperature is also low, which is 2 1.4 ~ 24.4℃ in winter and 28. 1 ~ 29.0℃ in summer. 460 ~ 664 cm, oxygen isotope 3, SiO _ 2/Al _ 2O _ 3 value is higher, which is 3.27 ~ 3.68, and the surface water temperature also increases obviously, which is 2 1.9 ~ 25.2℃ in winter and 28.5 ~ 29.3℃ in summer.
From the positive correlation distribution of SiO _ 2 and Al _ 2O _ 3 in low slope area, it is obviously different from the "particle size distribution rate" of inland shelf 16. The terrigenous materials on the lower slope are mainly clay minerals, so the change of SiO _ 2/Al _ 2O _ 3 ratio is related to the chemical composition of clay minerals. Under different climatic conditions, the intensity of weathering and leaching on land is different, which makes the content and composition of clay minerals brought to continental slope different. During the warm and humid period, the weathering and leaching of land is strong, and the siliceous components are leached into the sea first, while aluminum remains relatively more on land. Weathering leaching is weak in dry and wet periods, and the difference between silicon and aluminum caused by leaching is small, which may be the reason why the ratio of SiO _ 2/Al _ 2O _ 3 is higher in warm and wet periods and lower in dry and wet periods.
5 conclusion
(1) From the continental shelf to the continental slope, the content of SiO2 _ 2 decreases, and the content of Al _ 2O _ 3 changes in a complicated way. Generally, it first decreases and then increases, and it is the highest from the outer shelf to the uphill.
(2) On the continental shelf and the continental slope, the correlation between SiO _ 2 and Al2O3 is completely different. In the shelf area with water depth < 150 m, there is a negative correlation between SiO _ 2 and Al _ 2O _ 3, while in the slope area with water depth > 300 m, the correlation between SiO _ 2 and Al _ 2O _ 3 is not obvious.
(3) The SiO _ 2/Al _ 2O _ 3 ratio of core samples in continental slope area is obviously related to the surface water temperature calculated by the transfer function of planktonic foraminifera, and the oxygen isotope 1 and 3 are higher, and the second is lower, which may be another indicator reflecting paleoclimate characteristics.
refer to
[1] Wang, Bian Yunhua, et al. 19955. South China Sea in the past150,000 years. Shanghai: Tongji University Press, 46 ~ 82.
Qian jianxing. 1999. Paleooceanographic research in the South China Sea since the Late Quaternary. Beijing: Science Press.
[3] Schoenfeld, J, Kudras H. 1993. Accumulation rate of semi-pelagic sediments in the South China Sea related to sea level change in late Quaternary. Quaternary research 40:368~379
[4]Sarnthein M, Pflaumann U, Wang P X, et al., 1994. Preliminary report on "Monson monitoring" of Sonne-95 cruise to the South China Sea. Report from Institute of Geology and Paleontology, Kiel University, 68 years old.
Li, Tang, etc. Study on HY4-90 1 pore oxygen isotope and paleoclimate in the northern South China Sea. Science bulletin, 41(10): 911-965438.
Li, Duan, Wei, etc. Carbonate cycles in the northern and western South China Sea in recent 20,000 years and their paleooceanographic significance. Marine geology and quaternary geology, 17: 9 ~ 20.
[7] Wang L, Sarnthein M, Eelenkeuser H, et al., 1999. Pleistocene East Asian monsoon climate: high-resolution sediment records from the South China Sea. Marine geology, 156:245~248
, Li, Wei Gangjian, etc. Material source of high-speed deposition on continental slope of South China Sea. Zhongke (Series D), 3 1 (10): 828 ~ 833.
High-resolution paleooceanographic records of the northern slope of the South China Sea in recent 40,000 years. Quaternary studies, (1): 27 ~ 3 1.
Tu Xia, Chen. 2000 Paleoceanographic Characteristics since Late Quaternary Revealed by NS93-5 Borehole in Southern South China Sea. Tropical Ocean, 19 (4): 38 ~ 43.
[1 1] Xu Dongyu, Liu Xiqing, Zhang Xunhua, etc. 1997. offshore geology of China. Beijing: Geological Publishing House.
[12] Yao Bochu, Lan Xianhong, Qiu Yan. 1998. Geochemical characteristics of surface sediments in the southwest of Xisha. Marine Geology and Quaternary Geology, 18 (1): 23 ~ 35.
[13], Dong Wan, et al. 1999. Analysis of rare earth trace elements in sediments of South China Sea. Journal of Tongji University (Natural Science Edition), 27 (Supplement): 55 ~ 60.
[14] Fuyuan Zhang. 199 1. Elemental geochemistry of surface sediments in the central South China Sea. China Ocean and Lakes, 22 (3): 253 ~ 263.
Gu, Chen Shaomou, Wu Bihao, etc. Geochemistry of rare earth elements in surface sediments of the South China Sea. Tropical Ocean, 8 (2): 93 ~ 10 1.
[16] Zhao Yiyang, Yan. 1994. Geochemistry of sediments in China shallow sea. Beijing: Science Press.
[17] Zhang Hucai. 1997. Supergene geochemical characteristics of elements and their theoretical basis. Lanzhou: Lanzhou University Press.
Li, Jiang. 2002. planktonic foraminifera and surface water temperature since the last glacial period in Xisha trough, South China Sea. China Science (Ⅰ) 32 (5): 423 ~ 429.
Distribution and significance of silicon and aluminum since the last glacial period in the northern South China Sea
plum
(GMGS Institute of Marine Environmental Geology and Engineering Geology, Guangzhou 510760; China Marine Geology Laboratory of Tongji University, Shanghai 200092.
Abstract: Thirty-three seabed sediment samples and 1 piston core samples collected from Qiongdongnan Island in the northern South China Sea were analyzed for their silica, alumina, planktonic foraminifera, δ 18O, δ 13 C and 14C dating. With the increase of water depth and away from the mainland, the content of SiO2 _ 2 gradually decreases from the continental crust to the continental slope, while the distribution of Al _ 2O _ 3 is more complicated, reaching the highest from the crust to the uphill, and gradually decreasing to the coast and deep-sea basin. The relationship between SiO2 _ 2 and Al _ 2O _ 3 changes with the change of water depth, which is negative in the area with water depth less than 150m and positive in the area with water depth greater than 300m, and the relationship is not obvious in the area with water depth of 150m to 300m. According to Planck's foraminifera transfer function EP- 12E, there is a good correlation between SiO _ 2/Al _ 2O _ 3 in piston core S 19 collected from slope and sea surface temperature (SST), which is higher in δ 18O stage 1 3 and lower in stage 2. It reflects different weathering and leaching under different climatic conditions, and may be another proxy index of paleoclimate in slope areas. Key words: geochemistry, last glacial period, southeast Hainan, paleoclimate