(1. Guangzhou Marine Geological Survey Guangzhou 510760; 2. Department of Earth Sciences, Zhejiang University, Hangzhou 3 10027)

The SZ2 large gravity piston columnar sediment on the northern slope of the South China Sea provides a 50,000-year sedimentary record. The results of a hundred-year scale study on the phosphorus content of sediments in this columnar sample show that before 5000aB. P, the phosphorus content in sediments shows a gradual change trend with depth, indicating that the transportation of terrestrial phosphorus to the ocean is basically constant under natural environmental conditions, and the fluctuation of phosphorus content in sediments at different depths corresponds to the climate events that occurred in geological periods, reflecting that the record of phosphorus accumulation in sediments is sensitive to climate and environmental changes. Start with 5000aB. P, the phosphorus content in sediments showed a sudden increase trend, which was due to the dual effects of natural processes and human factors.

Keywords environmental changes of sedimentary phosphorus on the northern slope of the South China Sea

1 preface

In recent ten years, through the study of ice cores, loess, seabed sediments, lake sediments, tree rings, cave carbonates and biological remains (Hafildson et al.,1995; GRIR Menbers， 1993； Ding Zhongli et al.,1996; Jian Zhimin et al.,1996; Wang Sumin et al.,1996; Fex et al.,1994; Liu Dongsheng et al.,1997; Froval et al., 1995), constantly revealing the records of climate and environmental changes in geological period and human development period. When we further explore the causes and mechanisms of climate and environmental changes, people increasingly realize the important role of biogeochemical processes of phosphorus in global changes, because it is directly or indirectly related to the contents of O2 and CO2 in the atmosphere. Mcelroy( 1983) and Holland( 1994) believe that the marine carbon cycle, which plays a major role in controlling atmospheric O2 and c O2, strongly restricts the nutrient element-phosphorus, because phosphorus is an important limiting factor for marine production; Broeceker and Stewart also pointed out that the change of CO2 is restricted by the change of phosphorus to some extent, which is related to the short-term (104a) exchange of phosphorus in marine sediments (Malone et al., 1990).

Phosphorus in the ocean mainly comes from phosphorus in solution, suspended matter, inorganic and organic components in surface runoff. Most of the terrestrial phosphorus entering the ocean can be deposited on the continental margin through biological, chemical and physical processes (Weng Huanxin, 1999). Because phosphorus is influenced by the background of environmental system in the whole deposition process, the information of paleoclimate and paleoenvironment changes recorded by carbon and oxygen isotopes of biological remains in marine sediments will also be recorded in sedimentary phosphorus. Levin et al. (1994) found that the accumulation rate of deposited phosphorus is similar to that of oxygen on the time scale of ten thousand years.

In order to reveal the history of phosphorus accumulation in marine sediments and its relationship with climate and environmental changes with high resolution, we studied the changes of phosphorus content in sediments with depth since 50,000 years on the northern slope of the South China Sea, and then discussed the effects of climate and environmental changes on phosphorus accumulation on a hundred-year scale, which has important theoretical significance and scientific value for understanding the environmental and biogeochemical behavior of phosphorus and revealing the mutual verification of geochemical indicators and past global changes.

2 samples and analysis

The large gravity piston columnar sediment core SZ2 in this study was collected from the northern slope of the South China Sea, with a water depth of 2,903 meters ... The geographical location is 20 29 ′ 43.8 ″ north latitude,119 07 ′ 04.3 ″ east longitude, and the columnar sediment core is 8.26 meters long, providing a sedimentary record of about 50,000 years. The sediment cores have the same color tone, except for yellow silty sand between 7.54 and 7.75 m, the rest are gray silty clay.

The sediment column samples were divided at intervals of 3 cm, and 275 analytical samples with a resolution of 100 years were collected. Samples for sediment analysis were dried and ground in an incubator at 40℃. Put the 1g sample into a beaker, add10ml of 70% HClO4, and digest it at 120℃ until the solution fades, strong HClO4 smoke appears, and the silica turns white. After cooling, the volume was adjusted to 250mL with deionized water, and then the total phosphorus was determined by spectrophotometry. The minimum detection limit is 0.01m. Using repeated samples and standard samples CRMMESS- 1, the analysis error is less than 5%.

The age of 14C measured by Guangzhou Institute of Geochemistry, China Academy of Sciences is (7500150) a. At 798 ~ 80 1 cm, the thermoluminescence age is (51100 4000) a.

3 Results and discussion

3. Variation trend of phosphorus content with depth in sediments with slope of1.

The quintic polynomial is used to fit the vertical variation of total phosphorus content in the SZ2 sediment column on the northern slope of the South China Sea, and the curve is shown in figure 1.

Figure 1 Variation curve of phosphorus content with depth on continental slope in northern South China Sea (five-segment fitting)

Figure 1 Variation curve of phosphorus content with depth in borehole SZ2 on the northern slope of the South China Sea (after fitting by quintic polynomial)

As can be seen from the figure, around 5000aB. P, the change curve of phosphorus content in sediments appears inflection point, and the change of total phosphorus content in sediments on the northern slope of the South China Sea is divided into two different trends. Before 5000aB. P, although the content of deposited phosphorus varies greatly at different depths, the maximum amplitude can reach 222.63mg/kg, but the fitting curve changes gently with depth, and the content of deposited phosphorus is only between 420 and 450 mg/kg. Such a small range of content changes shows that the amount of phosphorus supplied by land sources to the ocean is basically in the natural environment without human factors. It is not difficult to understand that the phenomenon that the phosphorus content in sediments at different depths deviates from the change curve is the inevitable result of the change of paleoclimate and paleoenvironment, which leads to the increase of the amount of terrestrial phosphorus transported to the ocean during the interglacial period and the obvious decrease of the amount transported to the ocean during the glacial period, which leads to the change of the biogeochemical process of phosphorus, thus changing the accumulation of sedimentary phosphorus. Therefore, the obvious increase or decrease of phosphorus content in marine sediments with depth is related to climate and environment, and the change range of phosphorus content in each period reflects the influence of climate and environmental changes on phosphorus accumulation at that time.

Start with 5000aB. P, the change curve of phosphorus content in sediments on the northern slope of the South China Sea shows a sudden increase trend, which on the one hand shows that the natural warming of climate increases the transport of terrestrial phosphorus to the ocean, and on the other hand shows that the discharge of phosphorus to the ocean has increased due to human activities. Therefore, the substantial increase of phosphorus content in modern land wave sediments in the northern South China Sea is the result of both natural processes and human factors.

Fig. 2 shows the frequency distribution of phosphorus content in sediments on the northern slope of the South China Sea, in which fig. 2a shows the distribution types of phosphorus content in modern sediments since 5000aB. P, Figure 2b shows the distribution type of phosphorus content in sediments before 5000aB. P. Comparing the two pictures, it can be seen that the factors controlling the accumulation and distribution of phosphorus in sediments are obviously human factors and pure natural factors, and the two types of phosphorus content distribution have obviously different characteristics.

Fig. 2 Frequency distribution of phosphorus content in sediments of continental slope in the northern South China Sea.

Fig. 5 Frequency distribution of phosphorus content in sediments on the northern slope of the South China Sea since 5000aB. Before P(a) and 5000aB. P(b)

In the process of phosphorus deposition, in addition to natural precipitation, a very important biogeochemical process is that phosphorus is absorbed by marine organisms and then precipitated in the form of excrement or remains. Therefore, the accumulation of phosphorus in sediments not only directly reflects the input of terrestrial phosphorus, but also indirectly reflects the primary productivity of the ocean. That is to say, the increase of terrestrial phosphorus transport to the ocean will promote the enhancement of marine primary productivity, and the resulting positive effects will play a role in regulating atmospheric CO2 and O2 (Holland,1994; Mcelroy, 1983), but the negative effect of excessive phosphorus discharged into the ocean by modern human activities is to lead to eutrophication of the ocean, especially in recent years, the global red tide events are frequent, reflecting that this negative effect has brought adverse effects to our environment.

3.2 The relationship between the accumulation of sedimentary phosphorus and climate and environmental changes

The phosphorus content in the sediments on the northern slope of the South China Sea changes with depth, showing periodic fluctuation characteristics. Compared with the δ18o curve of Greenland ice core (Bond et al., 1993), it shows that this periodic fluctuation has a good corresponding relationship with global climate events (Figure 3). It can be seen from the figure that the global YD and H 1-H5 events are recorded in the phosphorus deposited on the northern slope of the South China Sea in China. This is consistent with the Heinrich event and the existence characteristics of H 1-H4 discovered by Wang et al. (1996) after the high-resolution chronological analysis of 17940 column in the northern slope area. Looking at the changes of phosphorus content in sediments on the northern slope of the South China Sea since 50,000 years, the change depth of sedimentary phosphorus content from high to low is obviously greater than that from low to high, that is, the time for the change of sedimentary phosphorus content from the peak value to the low value during the interglacial period is much shorter than that from the low value to the high value during the interglacial period. This change of phosphorus content on the northern slope of the South China Sea completely conforms to the law of global climate change (Figure 3). The variation curve of Greenland ice core δ 18O reflects the global climate change, and the time from cold to warm is shorter, while the time from warm to cold is obviously prolonged. The above results show that in the process of climate change from interglacial period to glacial period, it not only created climatic conditions (including temperature, precipitation, weathering conditions, etc. ) It provides sufficient time for land-based sources to input more phosphorus into the ocean and for the accumulation of marine sedimentary phosphorus. As can be seen from Figure 3, the sedimentary phosphorus changes obviously at different depths on the northern slope of the South China Sea, which reflects the accumulation of sedimentary phosphorus in different periods and is sensitive to the record of climate and environmental impacts.

Wang et al. (1995) once analyzed isotopes and microfossils on the north slope, revealed the existence of YD events, corrected the inference that there was no YD event in the Pacific Ocean put forward by Broecker et al. (1988), and measured the age of YD events at1kloc-0/390 ~ 65438+. According to the deposition rate, this time period is equivalent to the depth of SZ2 core sample of 227 ~ 206 cm. As can be seen from Figure 3, the content of sedimentary phosphorus decreased obviously in this depth period, which not only reflected the influence of YD event on the accumulation of sedimentary phosphorus, but also provided evidence for the existence of YD event in the South China Sea.

According to AMS65438+240cm dating, the last glacial period began at14900 ~14800 ab.p (Broecker et al., 1988), and the deglaciation period in the South China Sea began at 13600aB. P (Wang Lvjiang et al., 65438). As can be seen from Figure 3, above this depth, the content of sedimentary phosphorus began to increase gradually, reflecting that the climate has obviously warmed since the last deglaciation period, and the amount of terrestrial phosphorus input into the ocean has increased.

Fig. 3 Comparison between vertical variation of sedimentary phosphorus on the northern slope of the South China Sea and ice core GRIPδ 18O curve.

Fig. 3 Correlation between phosphorus content curve of borehole SZ2 sediment on the northern slope of South China Sea and δ 18O of Greenland ice core.

Huang Lin (1988) pointed out that there have been five drought events since Holocene, with the time ranging from 8000 to 75000 ab. p，5400 ~ 5 100 ab。 P and 4700 ~ 465438+, according to the dry and wet degree reflected by the change of organic matter content in drilling hole Q- 14A in Qinghai Lake. 2800 ~ 2200 ab。 P and 1600 ~ 100 ab. P, the occurrence periods of these drought events are respectively equivalent to the depths of SZ2 borehole samples on the northern slope of the South China Sea of 32 ~ 2 cm, 56 ~ 44 cm, 94 ~ 82 cm,108 ~108. As can be seen from figure 3, starting from 7500aB. P, the content of sedimentary phosphorus on the northern slope of the South China Sea showed a sharp increase in general, but the content of sedimentary phosphorus in the corresponding time period showed a relatively low level, which indicated that the transportation of terrestrial phosphorus to the ocean was obviously reduced during the drought event, and also indicated that these low peaks of sedimentary phosphorus recorded the drought events since Holocene.

Guo et al. (1996) studied Weinan loess profile with chemical weathering index (CIW), and revealed dry and cold climate events according to the low peak of weathering intensity. The ages of these events correspond to the depth of the low content of sedimentary phosphorus in the corresponding period of the column, which further shows that the vertical change of sedimentary phosphorus content in the northern slope of the South China Sea is closely related to every climate event in geological period.

To sum up, the fluctuation of phosphorus content in marine sediments with depth is the direct and indirect result of climate and environmental changes, that is to say, marine sedimentary phosphorus records the information of paleoclimate and paleoenvironment changes, so marine sedimentary phosphorus can be used as an indicator of paleoclimate and paleoenvironment changes.

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Preliminary study on the accumulation of phosphorus in sediments of the northern continental slope of the South China Sea since 50 ka and its relationship with environmental changes

Wu Nengyou (1) Weng Huanxin (2) Zhang Xingmao (2)

(1) China marine geological survey Guangzhou marine geological survey, Guangzhou 5 10760.

(2) Department of Earth Sciences, Zhejiang University, Hangzhou 3 10027.

The piston column on the northern slope of the South China Sea (20 29 ′ 43.8 ″ north latitude, 2903m west longitude and119 07 ′ 04.3 ″ east longitude) is 8.26m long, which provides the sedimentary records since 50 ka. This paper discusses the accumulation of phosphorus in SZ2 sediments since 100 and its relationship with environmental changes on the northern slope of the South China Sea. 5,000 years ago. The phosphorus content in sediments changes gently with the increase of core depth. The results show that the input and output of phosphorus from the mainland to the ocean are basically constant under natural environmental conditions. The change of phosphorus content at different depths is consistent with the climatic events in geological history, which reveals the sensitivity of phosphorus accumulation to climate and environmental changes. After 5000 BP. The phosphorus content in sediments is on the rise, which reflects the comprehensive effect of natural processes and human factors.

Key words: North slope of South China Sea, sedimentary phosphorus, environmental change