Abstract
This paper presents some initial results of the (superscript 226)Ra measurements on seawaters from the areas off northern Taiwan where the Kuroshio water encounters the East China Sea water and off eastern Taiwan within the Kuroshio current. Samples were collected from the sea surface and also in the water column during the KEEP-MASS cruise onboard the R/V Vinogradov and three other cruises onboard the R/V Ocean Researcher I. Surface water (superscript 226)Ra varies somewhat randomly between 6.1 and 12.8 dpm/100kg in the study area with a slight tendency to increase northward along the Kuroshio path off eastern Taiwan. Similar but less random variations were observed by Nozaki in the northern East China Sea surface water. The shallow water (superscript 226)Ra profiles in the shelf and upper slope areas show fairly uniform values at about 10 dpm/100kg.The intermediate and deep water profiles in the lower slope and deep basin off the east coast of Taiwan show a monotonous increase from south to north in the layer between the depths of 300m and 1000m.The deep profiles at the south display a pronounced (superscript 226)Ra increase from about 12dpm/100kg at 1000m to about 30dpm/100kg at 2500m and remain relatively constant below. Although the northward (superscript 226)Ra increase in the intermediate depth along the Kuroshio is contradictory to the conventional notion predicting no significant change along the flow, it is nonetheless consistent with the hydrographic data which show clear changes from south to north. Published (superscript 210)Pb profiles in the study area also show significant changes both in shape and activity level. Comparisons between these (superscript 210)Pb profiles with the (superscript 226)Ra profiles measured in adjacent areas in this study indicate that the (superscript 210)Pb excess due to the atmospheric flux may penetrate down to about the depths of 300m to 1000m. As the decay rate of the excess (superscript 210)Pb is small compared to the atmospheric flux, the scavenging residence time for (superscript 210)Pb is controlled mainly by the atmospheric flux and the inventory of excess (superscript 210)Pb. The (superscript 210)Pb scavenging residence time calculated for the excess (superscript 210)Pb based on a box model ranges from 2.3 to 8.8yrs, which is proportional to the depth of the excess (superscript 210)Pb penetration. The (superscript 210)Pb/(superscript 226)Ra activity ratio is about 0.25 below the 1000m depth in the area off northeastern Taiwan, giving a scavenging residence time of about 10yrs for Pb. This value is quite comparable to that obtained from the bottom water in the deep open oceans.
Highlights
226Ra, with a half-life of 1622 years, has been widely used as a tracer in the oceans for mixing and circulation Chung and Craig, 1973).studies(e.g., Large-scale sKysotcezmy,at1ic95m8;eaBsruoreecmkeerntestoafl.2, 2169R6a7; Ku et aL, 1970; were undertaken in the GEOSECS (Geochemical Ocean Sections Study) program which encompassed the Atlantic, Indian, Pacific and Chung and Craig, 1980; KuCircumpolar et aL, 1980·; Oceans (Broecker Chung, 1984; Ku et al, 1976; Ku and Luo, 1994).a2n2d6LRian,st1u9d7ie6s; were extended to the marginal seas including marshes and estuaries (e.g., Elsinger andMoore, 1980; Moore, 1981; Fanning et al, 1982; Elsinger and Moore, 1983; Bollinger andMoore, 1984; Elsinger and Moore, 1984; Harada and Tsunogai, 1986).The Kuroshio is a warm, saline western boundary current flowing northward along the east coast of Taiwan '(Nitani, 1972)
Published 210Pb profiles in the study· area show significant changes both in shape and activity level. Comparisons between these 210Pb profiles with the 226Ra profiles measured in adjacent areas in this study indicate that the 210Pb excess due to ·the atmospheric ftux may penetrate down to about the depths of 300 m to 1000 m
The purpose of this paper is to present for the first time the results of 226Ra measurements made on water samples collected from the Kuroshio off eastern and northeastern Taiwan on the KEEP-MASS (Kuroshio Edge Exchange Processes-Marginal Sea Study) cruise of the
Summary
Upon reaching the continental margin off northeastern Taiwan, it turns northeastward and intrudes into the East China Sea, creating a frontal zone mixing and upwelling of the subsurface water in the area W hoawng2, 2619R8a9;beWhaovnegs et al, 1991; Liu et al, 1992) It and distributes under the influence should of the Kuroshio and its exchan e with the ambient water. The purpose of this paper is to present for the first time the results of 226Ra measurements made on water samples collected from the Kuroshio off eastern and northeastern Taiwan on the KEEP-MASS (Kuroshio Edge Exchange Processes-Marginal Sea Study) cruise of the. :& wR2rRauaetsshReseyriaas,arntabchthCeRls:eSeae/rvduVetIhorioannVlrstsi1hnta9aeol9tsgi22oor2na6radsenR. -odweavvi1dtaih9nal9utsa3aJptu.eielncByti-htaehAelsiiuassdtgtetsauesttunsudtdts,iiyoso1cnfa9unrt9soads2di,tntihhogaeeanct2dhpt1ieu0vobePn2l2ibds6tihheRserxeaedcqeeur2seic1slsr0iubuiPlnirtsbiseussum dinraoftbarfaeec(ltteahwLtaeiieonneRndna/stn2oudV1b0hCsPyOuhbdrcufraeaonancggdne, 1991; Yang and Lin, 1992).·
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