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1. OS41A-03 Ra-226 and Pb-210/Ra-226 Activity Ratio in the Northern South China Sea Chi-Ju Lin, Yu-Chia Chung, Tsung-En Wu Institute of Marine Geology, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. Sample collections and measurements Abstract The surface water distributions and vertical profiles of Ra-226 in the northern South China Sea (SCS) have been measured. Surface water Ra-226 varies between 10 and 16 dpm/100 kg with higher values at stations closer to the landmass of coastal China. Each Ra-226 profile generally shows an increase from the surface toward the bottom. Above 1500m depth Ra-226 is systematically higher in the northern SCS than in the western North Pacific. This difference may be attributed to higher Ra-226 input from the shelf and slope area of the SCS. Below this depth Ra-226 displays large variation in some of the profiles but the mean values are quite comparable to those of the western North Pacific at the corresponding depth. The deep water in the SCS basin probably derives its Ra from the underlying sediments similar to the case in open oceans. The Pb-210/Ra-226 activity ratio ranges between 1.4 and 2.7 in the surface water with higher activity ratio at the stations closer to the Luzon Strait due to lower Ra-226 over there. The Pb-210 in excess over Ra-226 in the surface water due to atmospheric input may penetrate to a depth of about 200 to 500m. Below this depth, Pb-210/Ra-226 activity ratio decreases rapidly with depth and reaches values around 0.7 to 0.5 because Pb-210 is scavenged and removed by settling particulates. Box model calculations within a mixed layer of 50m in the area yield a mean residence time of about 1 yr for Pb-210 if an atmospheric Pb-210 flux of 1.05 dpm/cm2/y is adopted. The activity ratio of about 0.5 to 0.7 in the deep water corresponds to a Pb-210 mean residence time of about 30 to 70 yrs with respect to particulate scavenging. These values are quite comparable to those determined from the Pacific deep water. Fig. 4. Vertical distributions of the Pb-210/Ra-226 activity ratio at Stations C, D and F. Table 1. Pb-210 mean residence time in a mixed layer of 50m as calculated with a box model using an atmospheric Pb-210 input of 1.05 dpm/cm2/y. Fig. 2. Surface water Ra-226 distribution in October , 2002 (a) and July, 2003 (b). Fig. 3. Vertical Ra-226 profiles in October , 2002 (a) and July, 2003 (b). Introduction The South China Sea (SCS) is a large marginal sea with an area of 3.5 × 106km2 and an average depth of about 1350m. Its surface water circulation is strongly influenced by the Southwest Monsoon in summer and the Northeast Monsoon in winter. The central SCS is occupied by a basin generally over 3000m depth. The sill depths that allow exchanges of the SCS seawater with outside waters are 400m at the Mindoro Strait and 2200m at the Luzon Strait. The deep sill at the Luzon Strait allows the SCS deep water to exchange with the western Philippine Sea (WPS) deep water. The Kuroshio flows northward along the east coasts of Luzon Island and Taiwan. A branch of the Kuroshio down to 200m depth may intrude into the SCS through the Luzon Strait with different intensity and pathway in different seasons. Ra-226 in the oceans is derived mostly from the underlying sediments where it is produced by Th-230 decay. Due to its long half-life of 1622 yrs and soluble nature, Ra-226 has been widely used as a tracer for large-scale mixing and circulation studies. This paper presents Ra-226 measurements made on surface seawater as well as vertical profile samples from the northern SCS in different seasons. Due to a scarcity of data and its marginal sea environment, Ra-226 distribution in the SCS was of great interest to us. Its relation to Pb-210 in terms of the Pb-210/Ra-226 activity ratio in both surface waters and vertical profiles is used to evaluate Pb-210 removal by sinking particulates. The Pb-210 data used for this purpose are the total, i.e. the sum of the particulate and dissolved Pb-210, which were measured on samples collected from earlier cruises. This allows a comparison of the SCS with open oceans in terms of these parameters. Seawater samples of 20-liter size were collected during two Ocean Researcher I (ORI) cruises conducted in 2002 and 2003 (Cruise 662 in October, and Cruise 688 in July) for Ra-226 measurements. The sampling stations are shown in Figure 1. The surface water samples were collected by pumping. The profile samples were collected at Stations C, D, F and J with 20-l GoFlo bottles mounted on a CTD rosette. The water depth of these stations ranges from about 2700m to over 4200m. The collected samples were transferred on board to acid-cleaned 20-l plastic containers and returned to the shore-based laboratory where they were transferred into pre-cleaned 20-l thick-wall plastic bottles, stripped of their Rn-222 and other dissolved gases, and then stored for about a month so that Rn-222 may reach equilibrium with Ra-226. The regenerated Rn-222 was stripped again, purified and counted for Rn-222 and its two alpha-decay products in an alpha scintillation counter. The counter calibration was based on the GEOSECS Ra-226 standards which were traceable to the NIST. The overall precisions of the data presented here are better than ± 10﹪based on duplicate measurements. Each Pb-210 sample was first filtered on board the ship through 0.4 μm Nucleopore filter to separate the particulate from the dissolved phases for separate measurements. Pb-210 in each particulate or dissolved sample was spiked with a known quantity of stable Pb, then the Pb was extracted and purified through an anion exchange resin column prepared in AG1-X8 Cl- form. The purified sample in the form of PbSO4 was deposited onto a disc and stored for 1 month in a desiccators prior to counting the energetic Bi-210 activity in a low-background gas-flowing anti-coincidence beta counter. This is because Bi-210, with a half life of 5 days, can reach equilibrium with its parent, Pb-210, in a month. The counter was calibrated with a Pb-210 standard prepared from a standard purchased from the Isotopes Product Laboratory of Burbank, California. In this study, the particulate and soluble Pb-210 data are added so that only the total Pb-210 data are used together with our Ra-226 data to obtain the Pb-210/Ra-226 activity ratio. Fig. 1. Station locations for Ra-226 measurements in the northern SCS. Results and discussion Pb-210/Ra-226 activity ratio is a useful indicator for the scavenging rate or the mean residence time of the particulate matter in the deep oceans. The ratio in profile (Fig. 4) shows a fairly uniform distribution between 0.5 and 0.7 in the deep water (＞1000m). The ratio in the surface water ranges between 1.4 and 2.7 with a high value at Station F closer to the Luzon Strait due to its lower Ra-226. Pb-210 is in excess over Ra-226 from the surface down to a few hundred meters depth due to the atmospheric flux and subsequent penetration. Pb-210 activity decreases with depth and becomes lower than Ra-226 below 200m at station D and below 500m at Stations C and F. The activity ratio in the deep water is less them 1 because Pb-210 is continuously removed by the settling particulates. Assuming steady-state for a “box” of 50m mixed layer and adopting an atmospheric flux of 1.05 dpm/cm2/y, one can estimate the mean residence time of Pb-210 in the mixed layer as given in Table 1. The Pb-210 mean residence time is about 1 yr (~0.8 to 1.2) in the mixed layer. The mean residence time of Pb-210 in the deep water (τPb) is expressed by the equation as follows, assuming at steady-state, τPb = [ R / (1 –R) ] τ where R is the activity ratio of Pb-210 to Ra-226 in the deep water and τ is the mean life of Pb-210 (32 yrs). With R values from 0.5 to 0.7 in the deep water, the corresponding Pb-210 mean residence time is about 30 and 70 yrs. Figure 2 shows surface water Ra-226 distribution in the northern SCS. Ra-226 concentrations vary between 11.8 and 15.1 dpm/100kg (except for Station H where it is 31.9 dpm/100kg) in October, 2002 (Fig.2 a), and between 10.0 and 16.4 dpm/100kg in July, 2003 (Fig.2 b). Higher values are observed at stations closer to the landmass of coastal China. The surface water Ra-226 is higher in the study area than in open oceans where it usually varies between 6 and 8 dpm/100kg. The surface water Ra-226 distribution shows seasonal variation: low Ra-226 of the western Philippine Sea (WPS) surface water enters the SCS and moves mainly northwestward in July, and the flow shifts toward the center in October. This seasonal change probably reflects changes in the monsoonal forcing and the intensity of intrusion from the WPS. The vertical profiles (Fig. 3) show a general increase from the surface toward the bottom for each station. Above 1500m depth, Ra-226 is systematically higher in the northern SCS than in the western North Pacific probably due to strong input from the shelf and slope area of the northern SCS. Below this depth, the mean Ra-226 profile of the July cruise is quite comparable to a typical western North Pacific profile although somewhat lower above 3000m and higher below 3000m depth. However, the October profiles below 1500m depth display two groups in quite different values: one matches fairly well with the western North Pacific profile while the other is systematically higher by at least 5 dpm/100kg. We have no satisfactory explanation for these variations at present since. Ra-226 in the deep water is most likely derived from the underlying sediments. (a) (b) (a) (b) Conclusions The surface water Ra-226 distribution varies seasonally. In summer, the low Ra-226 of the WPS water enters into the northern SCS at higher latitudes and flows northward, reflecting the forcing of the Southwest Monsoon. In autumn, the low Ra-226 water enters with higher intensity, and shifts southward in response to the Northeast Monsoon.Ra-226 in the SCS water above 1500m is higher relative to that in open oceans such as the western North Pacific because of greater input from the shelf and slope area of the SCS. Ra-226 in the SCS deep water may be derived from the underlying sediments, similar to the case in deep open oceans. The Pb-210 mean residence time is about 1 yr in the mixed layer and about 30 to 70 yrs in the deep water in the study area. (b)