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Geochemical record of the climate effect in sediments of the China Shelf Sea
Chemical Geology, 107 (1993) 267-269 Elsevier Science Publishers B.V., Amsterdam
267
Geochemical record of the climate effect in sediments of the China Shelf Sea Y.-Y. Zhao a and M.-C. Yan b alnstitute of Oceanology, Academia Sinica, Qingdao, People'sRepublic of China blnstitute of Exploration of Geophysics and Geochemistry, Ministry of Geology and Mineral Resources, Langfang, People's Republic of China (Received March 1, 1993; revised and accepted March 30, 1993 )
The China Sea is well known for its broad continental shelf. It is commonly divided into four parts, that is, from north to south, the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea. The Bohai Sea and Yellow Sea are located in northern China, and the East China Sea and South China Sea in southern China. Based on many studies, the sediments of the China Shelf Sea are assessed to be mainly derived from the China continent, which are transported by the three largest Chinese rivers, the Huanghe (Yellow), Changjiang (Yangtze) and Zhujiang (Pearl) Rivers. The Huanghe River flows into the Bohai Sea; the Changjiang River, in the East China Sea; and the Zhujiang River, in the South China Sea. Their sediment discharge is 1000-106 t (Huanghe River), n e a r l y 5 0 0 - 1 0 6 t (Changjiang River) and ~ 100- 106 t (Zhujiang River), which together is > 90% of the total suspended load from the main Chinese rivers. The geochemistry of some elements (A1, Na, K, Rb, Mg, Ca, Sr, Ba, Ga, Cu and Si) in sediments from the China Shelf Sea has been studied. 286 sediment samples were collected from the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea, where the water depth is < 2 0 0 m (Fig. 1 ). Quantitative analysis of the elements was made by X-ray fluorescence spectrometry. The relative error of the analyses is less than _+5%.
Detailed studies show that the variation of abundance, grain-size effect and correlation of some elements in sediments of the China Shelf Sea are obviously controled by the climate. It is well known that southern China lies in a tropical or subtropical climate zone, where the temperature is high, rainfall abundant, and chemical weathering very strong. In its seawater, there are more marine organisms because of the warm water temperature. Northern China, in contrast, is located in a temperature zone which has relatively dry and chill weather, low precipitation and the prevailing less intense physical weathering. Due to the climatic difference, geochemical variations of some elements developed, and it can be called the "climate effect". This effect may be briefly summarized as follows: ( 1 ) The abundances of some elements such as A1, Na, K, Rb, Mg, Ba, Ga and Cu decrease from north to south, i.e. from the Bohai Sea, the Yellow Sea and the East China Sea to the South China Sea, or their abundances are markedly higher in the northern area of the China Sea (Bohai Sea and Yellow Sea) compared to those in the southern sea area (East China Sea and South China Sea). This fact indicates that some of the easily migrating elements have been, more or less, leached and lost due to strong chemical weathering in southern China. Based on the regional distribution of elements in sediments of the China Shelf Sea,
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
268
Y.-Y. Zhao AND M.-C. Yan
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Fig. 1. Sampling stations in the China Shelf Sea.
the elements mentioned above are always enriched in muds, but the element contents are lower in the muds distributed on the inner shelf o f the South China Sea. A typical example is low K and Mg in the muds and undoubtedly, this results in, to some extent, loss o f K and Mg in southern China. Therefore, the key factor
effecting the "north-south variations" o f these elements is climate. (2) A number o f elements, such as Ca, Sr and Si, regularly increases in abundance southward, or the average contents o f these elements are lower in the northern sea area than in the southern sea area. Ca and Sr in sedi-
GEOCHEMICAL RECORD OF THE CLIMATE EFFECT IN SEDIMENTS OF THE CHINA SHELF SEA
ments from the China Sea are mainly present in biogenic detritals (e.g., shells of mollusks and foraminifera). In the southern part of the China Sea, biological activity is higher because of warmer weather conditions, and the "excess" Ca and Sr are contributed by marine organisms. Based on our study, Ca and Sr abundances in sediments from the Huanghe, Changjiang and Zhujiang Rivers obviously decrease from north to south, so under normal conditions, those in the China Shelf Sea sediments should decrease in the same way. However, the fact is just the opposite and indicates that the "climate effect" contributes more than the "source effect" for Ca and Sr. As to Si enrichment in the South China Sea, on the one hand, it may be attributed to the enrichment of Si's host mineral quartz which has strong a weathering-resistant character. On the other hand, the loss of most elements makes Si relatively abundant. If only the climate-weathering factor is considered, Si in the South China Sea should be to more enriched, but in fact the Si is just a little more than that in other sea areas. This is due to the abundant organic fragments in the sea bed sediments, and the biogenic fraction has a lot of Ca and Sr which dilutes most elements (like Si ). (3) In various grain-size types of terrigenous detrital sediments, the average content of most elements increases gradually with the decrease of grain size, and that of some elements decreases with decreasing grain size. This is the "law of grain-size control of elements" proposed by Zhao ( 1985 ). Because of the climate effect, the grain-size control of elements can be enhanced, i.e. the difference of elemental content between coarse sand and fine m u d is more distinct. For example, the ratios of A1 in m u d to AI in sand decrease from the Bohai Sea, the Yellow Sea and the East China Sea to the South China Sea ( 1.56--, 1.67---,2.08--,2.43 ). This indicates that under the strong chemical weathering conditions in the South China Sea, A1 in sands is more lost, and the clay mineral in muds is characterized by dominance of Al-rich ka-
269
olinite. Therefore, the "climate effect" enhances the "grain-size effect". In addition, the differences of Rb concentration in various grain-size types of sediments are smaller in the northern sea area, but greater in the southern sea area. In the Bohai Sea, for example, the content of Rb in m u d is ~>30% higher than in sand, whereas the Rb content in m u d is >/74% higher than that in sand in the South China Sea This also shows that the grain-size effect of Rb increases with increasing chemical weathering. (4) The correlation of elements is also influenced by the climate. In northern China, physical weathering is dominant, and the sediments have good correlation between most elements, indicating the proper relationship of elements in the continental source rocks. Contrarily, in southern China where chemical weathering is strong, the loss of many elements and the addition of biophile elements may interfere with the proper correlations between elements. Thus, the correlations between elements are generally better in the Bohai Sea, but poor in the South China Sea. For instance, in the Bohai Sea, the correlation coefficient between Si and A1 is r(s~-Al>= --0.84, in the South China Sea, r(Si-Al)= -- 0.51 ; other correlation coefficients in the Bohai Sea are r(A~-Mg)= 0.88 and r(Cu_Ga)=0.91 compared to those in the South China Sea, r(A~_Mg)=0.71 and r~cu_ Ga) =0.83. References Zhao, Y.-Y., 1985. Some geochemical patterns of shelf sediments of the China Seas. Chin. J. Oceanol. Limnol., 2:200-211. Zhao, Y.-Y. and Yan, M.-C., 1989. Gold abundance in sediments of the China Seas. Chin. Sci. Bull., 19:16211624. Zhao, Y.-Y. and Yan, M.-C., 1992. Abundance of chemical elements in sediments from the Huanghe River, the Chan~iang River and the Continental Shelf of China. Chin. Sci. Bull., 23: 1991-1994. Zhao, Y.-Y., Wang, J.-T., Qin, z.-Y., Chen, Y.-W., Wang, X.-J. and Wu, M.-Q., 1989. Rare earth elements in sediments of the China continental shelf. Chin. J. Oceanol. Limnol., 1: 70-78.
267
Geochemical record of the climate effect in sediments of the China Shelf Sea Y.-Y. Zhao a and M.-C. Yan b alnstitute of Oceanology, Academia Sinica, Qingdao, People'sRepublic of China blnstitute of Exploration of Geophysics and Geochemistry, Ministry of Geology and Mineral Resources, Langfang, People's Republic of China (Received March 1, 1993; revised and accepted March 30, 1993 )
The China Sea is well known for its broad continental shelf. It is commonly divided into four parts, that is, from north to south, the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea. The Bohai Sea and Yellow Sea are located in northern China, and the East China Sea and South China Sea in southern China. Based on many studies, the sediments of the China Shelf Sea are assessed to be mainly derived from the China continent, which are transported by the three largest Chinese rivers, the Huanghe (Yellow), Changjiang (Yangtze) and Zhujiang (Pearl) Rivers. The Huanghe River flows into the Bohai Sea; the Changjiang River, in the East China Sea; and the Zhujiang River, in the South China Sea. Their sediment discharge is 1000-106 t (Huanghe River), n e a r l y 5 0 0 - 1 0 6 t (Changjiang River) and ~ 100- 106 t (Zhujiang River), which together is > 90% of the total suspended load from the main Chinese rivers. The geochemistry of some elements (A1, Na, K, Rb, Mg, Ca, Sr, Ba, Ga, Cu and Si) in sediments from the China Shelf Sea has been studied. 286 sediment samples were collected from the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea, where the water depth is < 2 0 0 m (Fig. 1 ). Quantitative analysis of the elements was made by X-ray fluorescence spectrometry. The relative error of the analyses is less than _+5%.
Detailed studies show that the variation of abundance, grain-size effect and correlation of some elements in sediments of the China Shelf Sea are obviously controled by the climate. It is well known that southern China lies in a tropical or subtropical climate zone, where the temperature is high, rainfall abundant, and chemical weathering very strong. In its seawater, there are more marine organisms because of the warm water temperature. Northern China, in contrast, is located in a temperature zone which has relatively dry and chill weather, low precipitation and the prevailing less intense physical weathering. Due to the climatic difference, geochemical variations of some elements developed, and it can be called the "climate effect". This effect may be briefly summarized as follows: ( 1 ) The abundances of some elements such as A1, Na, K, Rb, Mg, Ba, Ga and Cu decrease from north to south, i.e. from the Bohai Sea, the Yellow Sea and the East China Sea to the South China Sea, or their abundances are markedly higher in the northern area of the China Sea (Bohai Sea and Yellow Sea) compared to those in the southern sea area (East China Sea and South China Sea). This fact indicates that some of the easily migrating elements have been, more or less, leached and lost due to strong chemical weathering in southern China. Based on the regional distribution of elements in sediments of the China Shelf Sea,
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
268
Y.-Y. Zhao AND M.-C. Yan
36"
-
~..
:.:
. : "."
,b.~,--
\ - "•. . ..- . :. k.
t
j 32
i
•
•
"
•
•
•• @ • • •
•
•
•
~28"
•
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/
•
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124"
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Fig. 1. Sampling stations in the China Shelf Sea.
the elements mentioned above are always enriched in muds, but the element contents are lower in the muds distributed on the inner shelf o f the South China Sea. A typical example is low K and Mg in the muds and undoubtedly, this results in, to some extent, loss o f K and Mg in southern China. Therefore, the key factor
effecting the "north-south variations" o f these elements is climate. (2) A number o f elements, such as Ca, Sr and Si, regularly increases in abundance southward, or the average contents o f these elements are lower in the northern sea area than in the southern sea area. Ca and Sr in sedi-
GEOCHEMICAL RECORD OF THE CLIMATE EFFECT IN SEDIMENTS OF THE CHINA SHELF SEA
ments from the China Sea are mainly present in biogenic detritals (e.g., shells of mollusks and foraminifera). In the southern part of the China Sea, biological activity is higher because of warmer weather conditions, and the "excess" Ca and Sr are contributed by marine organisms. Based on our study, Ca and Sr abundances in sediments from the Huanghe, Changjiang and Zhujiang Rivers obviously decrease from north to south, so under normal conditions, those in the China Shelf Sea sediments should decrease in the same way. However, the fact is just the opposite and indicates that the "climate effect" contributes more than the "source effect" for Ca and Sr. As to Si enrichment in the South China Sea, on the one hand, it may be attributed to the enrichment of Si's host mineral quartz which has strong a weathering-resistant character. On the other hand, the loss of most elements makes Si relatively abundant. If only the climate-weathering factor is considered, Si in the South China Sea should be to more enriched, but in fact the Si is just a little more than that in other sea areas. This is due to the abundant organic fragments in the sea bed sediments, and the biogenic fraction has a lot of Ca and Sr which dilutes most elements (like Si ). (3) In various grain-size types of terrigenous detrital sediments, the average content of most elements increases gradually with the decrease of grain size, and that of some elements decreases with decreasing grain size. This is the "law of grain-size control of elements" proposed by Zhao ( 1985 ). Because of the climate effect, the grain-size control of elements can be enhanced, i.e. the difference of elemental content between coarse sand and fine m u d is more distinct. For example, the ratios of A1 in m u d to AI in sand decrease from the Bohai Sea, the Yellow Sea and the East China Sea to the South China Sea ( 1.56--, 1.67---,2.08--,2.43 ). This indicates that under the strong chemical weathering conditions in the South China Sea, A1 in sands is more lost, and the clay mineral in muds is characterized by dominance of Al-rich ka-
269
olinite. Therefore, the "climate effect" enhances the "grain-size effect". In addition, the differences of Rb concentration in various grain-size types of sediments are smaller in the northern sea area, but greater in the southern sea area. In the Bohai Sea, for example, the content of Rb in m u d is ~>30% higher than in sand, whereas the Rb content in m u d is >/74% higher than that in sand in the South China Sea This also shows that the grain-size effect of Rb increases with increasing chemical weathering. (4) The correlation of elements is also influenced by the climate. In northern China, physical weathering is dominant, and the sediments have good correlation between most elements, indicating the proper relationship of elements in the continental source rocks. Contrarily, in southern China where chemical weathering is strong, the loss of many elements and the addition of biophile elements may interfere with the proper correlations between elements. Thus, the correlations between elements are generally better in the Bohai Sea, but poor in the South China Sea. For instance, in the Bohai Sea, the correlation coefficient between Si and A1 is r(s~-Al>= --0.84, in the South China Sea, r(Si-Al)= -- 0.51 ; other correlation coefficients in the Bohai Sea are r(A~-Mg)= 0.88 and r(Cu_Ga)=0.91 compared to those in the South China Sea, r(A~_Mg)=0.71 and r~cu_ Ga) =0.83. References Zhao, Y.-Y., 1985. Some geochemical patterns of shelf sediments of the China Seas. Chin. J. Oceanol. Limnol., 2:200-211. Zhao, Y.-Y. and Yan, M.-C., 1989. Gold abundance in sediments of the China Seas. Chin. Sci. Bull., 19:16211624. Zhao, Y.-Y. and Yan, M.-C., 1992. Abundance of chemical elements in sediments from the Huanghe River, the Chan~iang River and the Continental Shelf of China. Chin. Sci. Bull., 23: 1991-1994. Zhao, Y.-Y., Wang, J.-T., Qin, z.-Y., Chen, Y.-W., Wang, X.-J. and Wu, M.-Q., 1989. Rare earth elements in sediments of the China continental shelf. Chin. J. Oceanol. Limnol., 1: 70-78.