The Mineral-Geochemistry Records Of The Environmental Evolution In South Yellow Sea Since Late Pleistocene Epoch

Based on the research to the features of mineral and element geochemistry for sediment from two representational bores NT1 and NT2 in South Yellow Sea, the substance source and sedimentary environment in South Yellow Sea since Late Pleistocene are discussed in the paper. Bring to bear the mathematical statistic method, the sediment from the two bores and Yangtze River, Yellow River and Korean rivers is compared, and the contribution of the different rivers to substance source in South Yellow Sea is confirmed. The occurrence of two"ancient cold eddies"during the high sea level in South Yellow Sea since Late Pleistocene is demonstrated. The greigite (Fe3S4), manganocalcite and barite crystal, three kinds of marine authigenic minerals with environmental meaning, are first discovered in South Yellow Sea. The paper also expounds the features of authigenic pyrite and its environmental indication significance in the cold eddy sediment from South Yellow Sea.The Bore NT1 with 70 meters deep is located at the central gyre mud area (123o24.29'N,35o26.60'E) in South Yellow Sea, where the fine-grained substance has been deposited since Late Pleistocene. More than 40 kinds of minerals are identified in the core. According to the vertical distributional characteristics of minerals and elements and its sedimentary types, the core NT1 can be divided into five layers or five mineral segments (ten mineral sub-segments) from above to below. Based on the analysis and comparison on mineral– geochemistry characteristics of sediment from the core NT1, Yangtze River, Yellow River, Keum River and Yeongsan River, the paper can get the following results:In the aspect of substance source, the heavy minerals of the core sediment in the 1st Layer, 3rd Layer and 5th Layer (Segment) are mainly consisted of authigenic pyrite. The terrigenous mineral association is consisted of hornblende, epidote, dolomite, ilmenite and schistic minerals, which displays that it is closer to the mineral association in Yangtze River, and the sediment may mainly come from Yangtze River. Its element geochemistry characterizes the rich content of most metallic oxide, organic carbon and most minor elements. By means of the analysis to the elements, enrichment factors and discriminant functions of minor elements, it is shown that all kinds of characteristics are all close to that of Yangtze River, which illustrates that the sediment was mainly from Yangtze River. In the aspect of environment, for the concentration of magnanimous authigenic pyrite in the 1st Layer, 3rd Layer and 5th Layer, it reflects the weak hydrodynamic, reducing environment and marine deposit environment during high sea level period. The ratios of Na/Al and Na/K reflects that the warm and humid climate condition. Therefore, the contribution of Yangtze River to the sediment of Yellow Sea is more than Yellow River, Keum River and Yeongsan River. The heavy mineral associations of the 2nd Layer and 4th Layer (Segment) are mainly consisted of schistic minerals, calcite, epidote, hornblende and garnet, which displays that it is closer to the mineral association of Yellow River. This illustrates the sediment may mainly come from Yellow River. The element analysis reflects that the sediment in the 2nd layer and 4th Layer characterizes the rich Si, Ca, Na, Mn, P, and Sr, deficient Al, K, Ti, Mg, Fe, organic carbon, as well as fourteen kinds of minor elements except for Sr. Based on the analysis to the elements, enrichment factors and discriminant functions of minor elements, it is found that the contribution of substance from Yellow River is more than that from Yangtze River, Keum River and Yeongsan River. In the aspect of environment, the mineral associations and the ratios of Na/Al and Na/K in the 2nd layer and 4th Layer reflect stronger hydrodynamic condition and the sedimentary environment of shore, estuary and terrestrial facies, which represents the low sea level period and arider and cold climate condition. It is to say that the contribution of Yellow River to the sediment of Yellow Sea is more than Yangtze River, Keum River and Yeongsan River under the arider and cold climate condition during the low sea level period. The sediment of the 1st Layer and 3rd Layer separately corresponds to the marine strata HⅠand HⅡduring high sea level period, which characters of mineral and element all prove the existence of"ancient cold eddy"when the two core segments were formed, and the paleospecies, hydrodynamic mechanism, sporo-pollen and sedimentary types all provide the evidence for this. Thus, it can be inferred that the"ancient cold eddy"surely exists so long as the Yellow Sea warm current and Yellow Sea circular current appear during high sea level period.The Bore NT2 with 70 meters deep is located at the northwestern area (122o15.49'N,33o27.53'E) of South Yellow Sea, where the sediment has been formed since late Middle Pleistocene. More than 42 kinds of minerals are found in the core. According to the vertical distributional characteristics of the minerals and its sedimentary types, the core NT2 can be divided into three mineral segments and six mineral sub-segments from above to below. According to the geochemistry character, the sediment of Bore NT2 is also divided into five layers from above to below, which basically correspond to the above mineral segments. The distributional characters of chemistry elements from the 1st layer, 2nd layer and 4th layer are similar each other, which characterizes rich most oxides and minor element, deficient Si, Na and Sr. It reflects the offshore neritic environment and represents the high sea level period of marine strata HI,HII,HIV. The ratios of Na/Al and Na/K reflect the warm and humid climate. The chemistry elements, enrichment factors and discriminant functions all show that the contribution of Yangtze River to the substance source is more than Yellow River. The rare-earth element pattern tends to further approach Yangtze River via the normalization based on the North American shale, supracrust and chondritic meteorite. The chemistry elements of the 3rd layer and 5th layer distribute similarly, which characterizes deficient Al, K, Ti, Mg, Corg, REE and microelement, rich Si, Ca, Na and Sr. The characters of mineral association and elements reflect marine and terrestrial alternation facies, which is consistent with the environment that paleospecies and sporo-pollen also reflect. The ratios of Na/Al and Na/K reflect the arider and cold climate. The elements, discriminant functions and enrichment factors show that the contribution of Yellow River to the substance source is more than Yangtze River.In a word, the data about the minerals, geochemistry, paleospecies, and sporo-pollen in Bores NT1 and NT2 all reflect that the contribution of Yangtze River to the sediment in South Yellow Sea is more than Yellow River in the warm and humid climate condition and high sea level period, and the contribution of Yellow River to the substance source of South Yellow Sea is more than Yangtze River, Keum River and Yeongsan River in the arider and cold climate condition and low sea level period since Late Pleistocene.