Transcript 投影片 1 - National Sun Yat

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.
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.
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.
Sample collections and measurements
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.
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. Pb210 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.
Ra-226 ( dpm / 100kg )
24
Taiwan Strait
China
23
H
22
Taiwan Strait
China
1
A
G
Taiwan
H
M1
5
15
20
25
30
35
40 5
10
15
20
25
30
35
Pb-210 / Ra-226 ratio
Pb-210 / Ra-226 ratio
40
0.0
0.5
1.0
1.5
2.0
2.5
ST. F
3.0 0.0
0.5
1.0
1.5
2.0
2.5
Pb-210 / Ra-226 ratio
3.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
(a)
500
M1
30
10
ST. D
ST. C
0
1
A
G
Taiwan
Fig. 1. Station locations for Ra226 measurements in the northern
SCS.
16
(b)
500
21
F
15
14
15
19
14
18
C
D
13
D
12
13
Luzon
(b)
Luzon
17
1000
1500
1500
Luzon Strait
Luzon Strait
C
1000
12
Depth (m)
20
F
I
16
Depth ( m )
Latitude (oN)
I
2000
2000
2500
2500
11
3000
J
16
J
11
(a)
15
10
(b)
3000
3500
14
112
113
114
115
116
117
118
119
Longitude (oE)
120
121
122
112
113
114
115
116
117
118
119
120
121
122
ST. C
ST. D
ST. F
ST. J
ST. M1
Western North Pacific
ST. C
ST. D
ST. F
ST. J
Western North Pacific
3500
ORI-606
ORI-662
ORI-688
ORI-575
ORI-662
ORI-688
ORI-575
ORI-662
ORI-688
4000
4000
Longitude (oE)
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).
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.
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.