Geochemical Characteristics Of Elements In The Kuroshio Water Near Taiwan Island And As The Tracers Of Kuroshio Incursion Into The East China Sea

The studies on geochemical characteristics of elements in seawater are of important scientific significance in illuminating the input and influence from the Kuroshio to the East China Sea?ECS?,and in revealing the regulation role of the Kuroshio on the ecological environment of the ECS.In this study,geochemical characteristics of multiple elements in sea water were studied by investigation of the Kuroshio mainstream east of Taiwan Island and the adjacent ECS from May to June 2014.Systemic studies on geochemical distributions,transport processes and influencing factors of dissolved elements,and the coupling relationships between elements and environmental parameters were presented on the basis of establishing and improving the simultaneous determination method for elements in sea water.The indicator system of dissolved elements for the Kuroshio incursion into the ECS was established.A series of new results and viewpoints were presented as follows:1.Two preprocessing methods,including Mg?OH?₂ co-precipitation and direct dilution,had been studied in the determination of multiple elements in sea water by inductively coupled plasma mass spectrometric?ICP-MS?.Suitable method was defined for the quantitative analysis of corresponding elements.A method of accurately and rapidly determining 28 elements in offshore and open sea water was established by combining the two methods together.Mg?OH?₂ co-precipitation could achieve the separation,enrichment and accurate determination of 19 elements including V,Cr,Mn,Co,Cd,Pb and rare elements?La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu?,while 10 fold direct dilution method could accurately and simultaneously determine 13 major/ trace elements including B,Sr,Li,Rb,I,V,Cr,As,Cd,U,Mo,Cu,Mn,but it was not suitable for Zn,Ni,Co,Pb and low concentration elements like rare earth elements.The two methods were simple and rapid for suitable elements with high accuracy and precision.Combining the two methods together,accurate determination of 28 elements,including Co,Pb,La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu?15 elements,by Mg?OH?₂ co-precipitation?and B,Sr,Li,Rb,I,V,Cr,As,Cd,U,Mo,Cu,Mn?13 elements,by 10 fold direct dilution?could be achieved with about 50 m L of offshore and open sea water.2.Concentration,distribution and controlling factors of conservative elements including Ca,Mg,Sr,K,Br,SO₄^2-,Li,Rb,Mo and nutrient-type elements including I,Cd,Mn,Sb,were revealed in water of the Kuroshio mainstream east of Taiwan Island and the adjacent ECS.The influx of elements from the Kuroshio into the ECS was systematically obtained.The concentration of Ca,Mg,Sr,K,Br,SO₄^2-,Li,Rb,Mo in the Kuroshio mainstream water was in the range 420⁴53 mg/L,1321¹369 mg/L,7.94⁸.38 mg/L,391⁴12 mg/L,65.17⁷1.07 mg/L,2629²834 mg/L,164¹87 ?g/L,110¹31 ?g/L,14.11¹6.10 ?g/L respectively.The vertical distribution of these elements was similar to salinity,showing the trend of lower concentration in surface water,increasing for subsurface water,decreasing in intermediate water,but rising again in deep water,in which Mg,SO₄^2-,and Li was mostly affectd by the change of water layer.The concentration of I,Cd,Mn,Sb in the Kuroshio mainstream water was in the range 49.7⁶4.3 ?g/L,0.02⁰.20 ?g/L,0.30¹.42 ?g/L,0.24⁰.44 ?g/L respectively,all showing the trend of increaseing gradually downward from the surface,in which the vertrical enrichment of Cd,Mn was most noteable.The influx of elemens from the Kuroshio into the ECS during May to October?Rainy season?was calculated to be Ca 8.09×1012 kg,Mg 25.1×1012 kg,Sr 0.152×1012 kg,K 7.53×1012 kg,Br 1.11×1012 kg,SO₄^2-51.0×1012 kg,Li 3.33×109 kg,Rb 2.25×109 kg,Mo 2.8×108 kg,I 11.03×108 kg,Cd 1.59×106 kg,Mn 1.18×107 kg,Sb 5.60×106 kg by a water exchange model.For Ca,Mg,Sr,SO₄^2-,Rb,Mo,I,Mn,Sb,the subsurface layer had the largest portion,followed by surface layer,and the intermediate water had the lowest portion.There were similar influx between subsurface and surface water for K and Br.But for Cd and Li,the surface water had the largest portion,followed by subsurface water,and the intermediate water had the lowest portion.As shown from the distribution of elements,water of surface and 30 m layer at the stations of the outer shelf was mainly controlled by Taiwan Strait water.Water of low Ca,Mg,Sr,K,Br,SO₄^2-,Li,Mo,I,Sb concentration,flowing northward from the north exit of the Taiwan Strait,could reach the south-central ECS shelf.This restricted KSW,which contained higher Ca,Mg,Sr,K,Br,SO₄^2-,Li,Mo,I content,to the southeast part of ECS shelf.In the bottom layer of the outer shelf,the concentration of Ca,Mg,Sr,K,SO₄^2-,Li,I,Cd,Mn,Sb was higher than in the middle shelf water,which showed the characteristics of KSSW and that the Kuroshio had begun to expand its influence by intruding onto the shelf northwestward from station TW0-1.Vertically,the distribution of Ca,Mg,Sr,K,SO₄^2-,Li,I,Sb in the section DH5 and DH7 could refelect the phenomenon of Kuroshio subsurface water climbing northward along shelf bottom to as far as the Qiantang River estuary,and plunging westward into shelf water along bottom from the shelf edge in section DH9.Because of the different gechemical characteristic,some elements could not fully refect the Kuroshio incursion in plane distribution,vertical distribution and the distribution at specific station.In general,Ca,Mg,Sr,K,SO₄^2-,Li,I,Sb could well reflect the regular pattern of Kuroshio incursion into the ECS both in plane distribution,vertical distribution and the distribution at TW0-1 station.3.The indicator system of Ba-Salinity?S?for indicating water mixing between the Kuroshio and the ECS was established by using Ba as a tracer.A similarity analysis method for sea water elements was proposed to explore the incursion of Kuroshio water based on element cluster map and vector similarity analysis method.The indicating results of Ba as a parameter,showed that the Kuroshio surface water had little impact on the ECS,while the Kuroshio subsurface water formed an intrusion current by climbing northwest along the bottom of the middle shelf from the sea area northeast of Taiwan Island into the Qiantang Estuary,of which the volume of Kuroshio water was nearly 65%.Kuroshio water was the predominant part of the water on the outer shelf bottom and its proportion in areas deeper than the 100 m isobath could reach more than 95%.In the DH9 section?north of Taiwan Island?,Kuroshio subsurface water intruded westward along the bottom from the shelf edge and then rose upward?in lower proportion?.Kuroshio water accounted for 95% of the ocean volume could reach as far as 122° E.Ba was able to provide detailed tracing of the Kuroshio incursion into the ECS owing to its geochemical characteristics and became an effective tracer for revealing quantitative interactions between the Kuroshio and the ECS.The element cluster map of Kuroshio subsurface water was established by 6 major elements including Ca,Mg,Sr,K,Br,and SO₄^2-.Similarity analysis method for sea water elements was put forward based on the principle of vector similarity.Results showed,the similairty between bottom layer water of the ECS shelf and Kuroshio subsuface water could reflect the track of Kuroshio incursion into the ECS,which illustrated that the similarity analysis method for sea water elements based on element cluster map and vector similarity method could reveal the pattern of Kuroshio incursion,indicating a need for further study on the application of the similarity analysis method for sea water elements in marine water mixing research.