東南亞河川流域及海洋之碳循環---總計畫(II)

1

行政院國家科學委員會補助專題研究計畫

;

成 果 報 告

□期中進度報告

總計畫：東南亞河川流域及海洋之碳循環(II)

計畫類別：

□

個別型計畫

; 整合型計畫

計畫編號：NSC 93－2621－Z－110－004－

執行期間：93 年 8 月 1 日至 94 年 7 月 31 日

計畫主持人：陳鎮東教授

共同主持人：林曉武副教授、楊盛行教授、吳朝榮副教授、莊秉潔教授、彭宗

仁助理教授、王樹倫副教授、鍾竺均副教授、鄭利榮教授、羅建

育助理教授

計畫參與人員：黃國銘、何美、雷佳、羅立章、黃啟書、許筱薇、邢麗玉、候

偉萍、曾筱君、王昱文、林毅杰、張裕彰、王冰潔、黃修儀、

邱萬敦、陳凱暐

成果報告類型(依經費核定清單規定繳交)：□精簡報告 ;完整報告

本成果報告包括以下應繳交之附件：

■赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

■出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式: 除產學合作研究計畫、提升產業技術及人才培育研究計畫、

列管計畫及下列情形者外，得立即公開查詢

□ 涉及專利或其他智慧財產權， □一年□ 二年後可公開查詢

執行單位: 中山大學海洋地質及化學研究所

中 華 民 國 九十四 年 十 月 二十四 日

2

(一) 本計畫自九十三年八月至今，共發表論文如下：

SCI 1.

Chen C.T.A.*, J.T. Wu, B.J. Wang and K.M. Huang., 2004. Acidification and trace

metals of lakes in Taiwan, Aquatic Geochemistry, 10, 33-57 (SCI: 0.720; cited once

until 2005; This paper acknowledged 92–2621-Z-110–005).

SCI 2. (附件1)

Chen, C.T.A* and J.K. Wann., 2004. Factors regulating the distribution of elements in

the sediments of a seasonally anoxic lake in tropical Taiwan. Terrestrial Atmospheric

Oceanic Sciences, 15, 785-811 (SCI: 0.277; cited once until 2005; This paper

acknowledged 92-2611-M-110-014 and NSC 92-2611-Z-110-005).(

EI 3.

Chen, C.T.A., B.J. Wang and L.Y. Hsing., 2004. Upwelling and degree of nutrient

consumption in Nanwan Bay, Southern Taiwan, Journal of Marine Science and

Technology, Vol. 12, No. 5, pp. 442-447 (This paper acknowledged

NSC93-2611-M-110-009; 93-2621-Z-110-004).

SCI 4. (附件2)

Chen, C.T.A., 2005. Tracing tropical and intermediate waters from the South China Sea

to the Okinawa Trough and beyond, Journal of Geophysical Research, Vol. 110,

C05012, doi:10.1029/2004JC002494 (SCI: 2.839; This paper acknowledged NSC

92-2611-Z-110-005; NSC-92-2611-M-110-014)

5. (附件3)

Chen, C.T.A., J. Zhang, T.R. Peng, and T. Hagiwara, 2005. Exploratory sampling of

submarine groundwater discharge in Taiwan, Geochemistry 39, 165-171, 2005. (This

paper acknowledged NSC 93-2621-Z-110-004; NSC 93-2621-Z-005-002)

(二) 自九十三年八月至今，以海研三號研究船 1081 航次為期 6 天，進行珠江

河口採樣工作。

(三) 自九十三年八月至今，已完成工作如下：

總計畫及子計畫一至五目前已完成之採樣區域及測站，包括河川、湖沼與海域等區域

（圖

1）。河川及湖沼採樣包括台灣本島 21 條主要河川、12 條次要河川、24 條小型溪流與

16 個湖泊，共計 73 個測站。初步估算台灣高山湖泊如大鬼湖、嘉明湖、天鑾池、撤退池、

小鬼湖、翠峰湖、鴛鴦湖、夢幻湖等，每年由集水區供應至湖中之碳量至少為

0.016~2.1

gC/m

2

/yr 之間；湖水供碳率介於 1.3~12.9 gC /m

2

/yr 之間。各湖集水區表土剝蝕率介於

0.01~0.15 mm/yr 或 3.6~69.9 g/m

2

/yr 之間，集水區坡度對沖蝕率之影響似乎最為重要。另

外，台灣

39 條河川以懸浮荷重方式約輸出碳酸鹽 6.22x10

4

tC/yr 與有機碳 14.63x10

4

t C/yr；

3

而以河床荷重方式約輸出碳酸鹽

230.1x10

4

t C/yr 與有機碳 56.3 x10

4

t C/yr 入海，合計台灣

河川每年總輸出顆粒態碳量至少為

307t C/yr。

大陸地區於福建九龍江以南之河流，包括九龍江、珠江與各支流，共計

34 個測站。

並於東南亞國家採取將近

80 個測站的水樣，分別是越南 10 個測點、柬埔寨 2 個測點、泰

國

9 個測點、緬甸 11 個測點、印尼(雅加達以及巴厘島)9 個測點、馬來西亞(婆羅洲)7 個測

點、新加坡

6 個測點、汶萊 4 個測點、菲律賓 6 個測點、香港 18 個測點及澳門 7 個測點。

在懸浮顆粒樣本方面於國內河川取得

53 個，東南亞國家 39 個樣品。水中 BOD、生菌數、

氧化還原電位及底泥總氮與

CO

2

通量有正相關；水中氧化還原電位、懸浮有機物、硝酸鹽、

氨氮及底泥之總有機碳及總氮亦與

CH

4

通量正相關。海域部分已參與海研一號

695 航次 (92

年

9 月；表水 CO

2

釋放量

10g/m

2

/yr，CH

4

亦為過飽和，且可能有甲烷水合物釋放

CH

4

之信

號)，海研二號 896 航次(92 年 8 月；表水 CO

2

釋放量

4g/m

2

/yr)，海研三號 983 航次(93 年 7

月)，海研一號 728 航次(93 年 8、9 月)，海研三號 1081 航次(94 年 7 月)等等的採樣，前往

珠江口、九龍江口、湄公河口及巽它陸棚，進行由河口、大陸棚、大陸坡至南海海盆之採

樣共

671 個水樣。並在珠江口外密集採取沈積物樣品，結果顯示珠江為南海北部陸棚最主

要之陸源沉積物來源。沉積物金屬與有機碳之含量皆以珠江口與三角洲處為高值出現之區

域，而其他多數陸棚區域則沉積物金屬與有機碳含量皆偏低，顯示陸源沉積物多未沉降於

絕大多數之陸棚上。多數金屬與有機碳含量自珠江口向南延伸至陸坡區域，顯示珠江正向

南傳輸其沉積物並延伸傳輸到陸坡區域。研究區域沉積物主要來源包括陸源黏土礦物與海

源碳酸鈣殼體。沉積物顆粒大小組成控制了金屬含量的變化。河川沉積物金屬含量異常偏

高可能是受珠江下游區域之城市與工業區污染所導致。

Diagenesis 亦是控制此區域金屬含量

變化之主要因素之一。部份沉積物因有機碳含量偏高，故其金屬含量亦偏高。此研究區域

位於東沙群島附近有金屬異常偏高的情況。此航次並取得

ADCP 資料。

計畫目前已完成測量河川、水庫及海洋之溫度、鹽度、溶氧、CH

4

、N

2

O、pH、TCO

2

及

fCO

2

等參數，其他如

DIC/N/P、DOC/N/P、鹼度、葉綠素甲及懸浮顆粒之 POC/N/P 等各

項參數正積極進行分析，陸續接近完成階段。結果顯示河川、湖沼及海洋表水大體上均是

4

總計畫於

93 年 8 月亦邀請了日本富山大學張勁教授等一行四人，前來協助子計畫六

進行地下水入海之採樣，順利於高雄西子灣、屏東枋山、台北鹽寮灣等地採得地下水入海

之證據。於枋山外海

300m、水深 7.8m 處，甚至採得鹽度 0.2 之淡水（圖 2）。利用 86~91

年

6 年之水文數據及相關水文地質參數，進行初步水平衡模式驗證，初步估算結果顯示枋

山地區地下水年補注水量約為

1,338 萬噸，而地下水年入海量約為 217 萬噸，此枋山地區

之地下水入海估算結果與該區實際採集到純淡水之

SGD 標本的結果互相契合。94 年則自

行於柴山、西子灣、枋山及蘭嶼採得

SGD。

圖

3 為珠江口外由河口、大陸棚、大陸坡至南海海盆表水之溫度、鹽度、溶氧、AOU、

pH、NO

3-

+NO

2-

、PO

43-

、SiO

2

等參數之分布，各參數大多有自河口往外海之梯度變化，顯

示珠江淡水輸出至海洋之影響。其次，珠江口外西南海域發現高溫低鹽之水團訊號，而其

溶氧、AOU、營養鹽等參數亦顯示特殊之分佈，指出該區域可能有來自南海周邊其他淡水

來源(可能為紅河或湄公河)，造成各參數特殊之分佈趨勢。目前數據仍在分析中，故詳情仍

待探討。本年度預定中南半島各國河流與附近海域之研究，將對於各項碳循環參數有關鍵

性之影響。

沈積物中有機碳之氧化分解對於碳循環亦可能有重要影響。一般而言，有機物沈降至

海洋底部沈積體系開始進行其在沈積環境下之氧化作用，有機物藉著不同種類之氧化劑氧

化成

CO

2

，同時增加間隙水內之鹼度，亦同時釋出部份營養鹽，回歸於間隙水而重新循環

入上部水體，或再沈澱。圖

4 為珠江口外陸棚沈積物間隙水中鹼度隨深度之變化，資料顯

示沈積物間隙水中鹼度自表層隨深度增加而增加，而且表層間隙水中鹼度亦較一般陸棚水

體為高。此乃由於沈積體系內有機碳之氧化分解，增加間隙水內之鹼度，這是珠江口外陸

棚地區皆可觀察到的現象，而間隙水內鹼度增加對於碳循環可能有相當大的影響；同時釋

出的營養鹽，可經由間隙水而重新循環，造成沈積物上部水體之高營養鹽，此自沈積體系

釋出營養鹽幅度與範圍，則需要大範圍採樣更深入研究及計算才可得知。尤其河口三角洲

與陸棚是有機碳主要堆積區域，有機碳沈降後，即快速進行氧化分解，而鹼度之增加對於

碳循環更有重要影響。目前資料顯示特別在近河口地區(如 Station 32) 表層間隙水中鹼度明

顯高於較深部之間隙水，可能由於最近中國大陸在南部珠江流域大量之開發，而改變珠江

5

向外傳輸之物質種類與通量。此種在沈積物垂直分佈上明顯之差異，似乎與近年來大陸大

量人為之活動對珠江口陸棚沈積物之分佈有明顯之影響，而切確因素則需陸續完成之參數

互相配合。圖

5 顯示研究區表水之 CH

4

均為過飽和，且甲烷水合物之釋出信號。圖

6 之珠

江口

pCO

2

信號則顯示，有機質之分解在河口造成

CO

2

過飽和，但在河口之外，則

pCO

2

迅

速下降。

由海鮪類資源與漁海況關係之初步結果可看出，79～89 年南中國海鮪延繩釣漁

業的

CPUE 距平值(CPUE Anomaly)、海溫距平值(SST Anomaly)及南方振盪指標值

(SOI)的時系列變動。在南海表面海溫異常高的期間，鮪延繩漁業之 CPUE 也有相對

較高的現象。南海的海溫從

85 年 6 月起即處於較高溫狀態，在 86 年聖嬰現象發生

後，使得本海域之水溫進一步繼續升高，在

87 年底達最高峰，而本海域之鮪延繩漁

業的

CPUE 也在 86 年 8 月至 87 年 10 月間有相對較高的值，顯示似乎 SST 的增高

有利於鮪釣漁業

CPUE 的提高。

就航次報告而言，本計畫共出海五次（ORI-695, ORI -728, ORII-896, ORIII-983,

ORIII-1081），航次報告為：中山大學海洋地質及化學研究所研究報告第 38~40 號)。

Bjwang D:\24-3-NSC\93\02-casa\成果\fig1-6.doc 2005-10-24

6

95

100

105

110

115

120

125

Longitude (oE)

0

5

10

15

20

25

30

Latitude (

o

N)

China

Tai wan Lu zon

Sulu Sea

Sunda Shelf

Thailand

V ie tn am L ao_s

Brunei

Borneo

Celebes

Sea

bjwang d:23-4-nsc 95 01-sampling CASA-9501.srf 2005-10-19

Cambodia

Myanmar

Bjwang D:\24-3-NSC\93\02-casa\成果\fig1-6.doc 2005-10-24

7

8

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 Latitude ( oN) OR-I 695 ( ) ℃ surface θ China lclo:695s-t.srf

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 La titu de ( oN) OR-I 695 Salinity surface China 33.9 <33.6 33.9 >33.9 lclo:695s-s.srf 111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 L ati tu d e ( oN) OR-I 695 DO ( mol/L) surface China <201 μ 205 >203 lclo:695s-do.srf

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 L ati tu d e ( oN) OR-I 695 AOU ( mol/kg) surface China μ >0 0 lclo:695s-aou.srf

9

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 L ati tu d e ( oN) OR-I 695 pH surface China 8.15 8.13 8.11 8.11 >8.11 >8.16 8.15 lclo:695s-ph.srf

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 La titude ( oN) China <0.2 <0.2 OR-I 695 NO +NO ( mol/L) surface 2 3 - - _μ lclo:695s-no3+2.srf 111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 Latitude ( oN) China 0.01 0.01 0.03 μ OR-I 695 PO ( mol/L ) surface4 3-lclo:695s-po4.srf

111 112 113 114 115 116 117 118 Longitude (oE) 17 18 19 20 21 22 23 24 La tit ude ( oN) China 1.8 μ OR-I 695 SiO ( mol/L ) surface <2.0 >2.2 2 lclo:695s-sio2.srf

10

30

15

0

2200

3000

3800

Depth

(cm)

30

15

0

2200

3000

3800

Depth

(cm)

2200

3000

3800

2200

3000

3800

30

15

0

2200

3000

3800

Depth

(cm)

2200

3000

3800

2200

3000

3800

Station 32

Station 40

Station 41

Station 34

Station 39

Station 42

Station 38

圖 4 TA in pore water for OR-695.

11

OR-I 695

CH4 (nM)

0

2

4

6

8

10

12

14

16

Pres.(db)

0

1000

2000

3000

4000

released from sediments or gas

hydrates on the continental slope

12

Sal.

0

2

4

6

8

10 30

32

34

p

CO

2

(μ

at

m

)

0

500

1000

1500

2000

2500

3000

ORI-695

The Pearl river delta

bjwang D:\24-3-NSC\95\fig11.jnb 2005-10-15

13

14

附件 2

Tracing tropical and intermediate waters from the South China Sea to

the Okinawa Trough and beyond

Chen-Tung Arthur Chen

Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan Received 17 May 2004; revised 16 November 2004; accepted 28 December 2004; published 18 May 2005.

[1] Intermediate waters with a salinity minimum from 350 to 1350 m depths have

been reported to flow out of the South China Sea (SCS) through the Luzon Strait. This eastward flowing SCS Intermediate Water (SCSIW) is blocked by the northward flowing Black Stream (Kuroshio) southeast of Taiwan and is forced to turn to the north. The SCSIW subsequently enters and occupies the western half of the Okinawa Trough. Because of strong upwelling in the SCS basin, the SCSIW contains more nutrients than does the Kuroshio Intermediate Water in the Philippine Sea. One of the major purposes of this note is to reconfirm that the SCSIW actually does upwell onto the East China Sea (ECS) shelf and to show that the South China Sea Tropical Water (SCSTW), with a salinity maximum of 50– 150 m in depth, follows the same pathway as the SCSIW and similarly occupies the western half of the Okinawa Trough. Part of these relatively nutrient-rich subsurface waters upwell, thereby supplying nutrients to the ECS shelves. The remaining SCSIW and SCSTW flow along with the Kuroshio and can be traced as far east as 140
E, south of Japan. This is just around the region where the Oyashio joins the Kuroshio to form the North Pacific Intermediate Water.

Citation: Chen, C.-T. A. (2005), Tracing tropical and intermediate waters from the South China Sea to the Okinawa Trough and beyond, J. Geophys. Res., 110, C05012, doi:10.1029/2004JC002494.

1. Introduction

[2] As the mightiest current in the Pacific Ocean, the

Black Stream (Kuroshio) carries an enormous amount of water, salt, heat and nutrients poleward. It is generally believed it begins east of Luzon Island as a continuation of the westward flowing North Equatorial Current. Waters south of about 15
N turn to the south while, conversely, those north of 15
N turn to the north and become the Kuroshio [Nitani, 1972]. Nitani pointed out that a fairly strong intermediate current enters the South China Sea (SCS) along the northern coast of Luzon (around about 18
400 and 19
300N) and that the intermediate waters of the SCS flows out to the West Philippine Sea (WPS) along the southern coast of Taiwan. He also noted that ‘‘This may explain the comparatively higher salinity minimum in the (western) part of the Kuroshio just east of Taiwan’’ [Nitani, 1972, p. 145]. More recently, Chen and Huang [1996] identified a subsurface front at about 122
E. The outflowing modified Kuroshio Intermediate Water (KIW) from the SCS is hereby named the SCS Intermediate Water (SCSIW) (taken to be between 350 and 1350 m deep). It initially flows eastward but only reaches 122
E when it turns to join the northward flowing Kuroshio [C.-T. A. Chen et al., 1994, 1995a; Chen and Huang, 1995].

[3] Chen and Wang [1998] further determined that this

saltier SCSIW is centered at, or slightly shallower than, the 500 m depth and is able to flow into the Okinawa Trough. Accordingly, the SCSIW thus maintains its higher-salinity signature in the southern and western Okinawa Trough. Chen and Wang [1998, 1999] have further drawn our attention to the fact that since the SCSIW is rich in nutrients, when it upwells on the East China Sea (ECS) continental shelf, it helps to propagate high biological productivity, resulting in one of the richest fishing grounds in the world.

[4] It is the major purpose of this note to strengthen and

confirm the arguments of Chen and Wang [1998] in order to prove that indeed the SCSIW actually does upwell onto the ECS shelf. The present research also demonstrates that the modified tropical water (Smax) originated from the WPS,

now named the South China Sea Tropical Water (SCSTW, taken to be between 50 and 150 m in depth), follows the same path as the SCSIW. In fact, remnants of these waters can even be traced to areas south of Japan, with the SCSIW signal only disappearing east of the Izu Ridge at 140
E. Still farther to the east, the warm, salty Kuroshio mixes with the cold, fresh Oyashio to form the North Pacific Intermediate Water (NPIW).

2. Study Area and Experimental Method

[5] The present author occupied a north-south cross

section along 121
430E on the R/V ORI 403 (R/V Ocean Researcher I cruise 403, 5 – 7 October 1994) and an

east-JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, C05012, doi:10.1029/2004JC002494, 2005

Copyright 2005 by the American Geophysical Union. 0148-0227/05/2004JC002494$09.00

west cross section along 21
450N on the R/V ORI 462 (2 – 15 September 1996). Station locations and relevant data obtained from literature are given in Figure 1. Continuous temperature and salinity (conductivity) profiles and discrete water samples were obtained using a Seabird CTD-Rosette (SBE-11) assembly fitted with eleven 2.5 L Niskin bottles. Discrete salinity (S) samples were taken in order to check the accuracy of the conductivity-temperature-depth (CTD) data. Dissolved oxygen (DO) was measured in accordance with the Winkler method with the apparent oxygen utiliza-tion (AOU) calculated from the equautiliza-tion of Chen [1981]. pH was measured at 25 ± 0.05
C using a Radiometer pHM-85 pH meter calibrated with NIST Tris buffer. Nitrate was measured with a flow injection analyzer [Pai et al., 1990].

3. Results Near the Luzon Strait

[6] In line with that previously reported by Chen and

Wang [1998], an intermediate-depth salinity front for the east-west cross section at 21
450N exists between stations 11 and 13 near 122
300E. The proof is found in Figure 2 where vertical sections of salinity and dissolved oxygen concentrations are plotted against potential density (sq)

along 21o45'N (ORI-462 data) plus 20
N and 18
N (both based on the results of Qu et al. [2001]). Salinity at the Smin

layer (sq = 26.7) is consistently lower than 34.3 at the 18
N

cross section east of Luzon, suggesting little influence of the saltier SCSIW. The middepth salinity front appears at 20
N near the southern Luzon Strait where S = 34.4 water is observed, but only in areas east of 121
E. On the other hand, S = 34.4 water, obviously under the influence of the SCSIW, is found as far east as 122
E in the 21
450N cross section.

[7] East of the front the S_minlayer is very low in salinity

(<34.3), whereas west of the front the salinity jumps to 34.4 (Figure 2 (top left)). This is because the Smin layer east of

the front is mainly the KIW which is composed of the North Pacific Intermediate Water (NPIW). The NPIW is formed in the subarctic region of the Northwest Pacific and subse-quently becomes a subtropical gyre [Reid, 1965]. The lowest salinity is less than 34.15, but it approaches 34.2 east of Luzon. West of the front, the Sminlayer is influenced

by the SCSIW, which is higher in salinity on account of vertical mixing and upwelling in the SCS [Chen and Huang, 1995; Chao et al., 1996]. The north-south cross sections for S and DO are plotted againstsqalong 120
430E

(ORI-403 data) in Figure 3. The intermediate water (sq =

26.7) with a S 34.4 can only be found north of 21
N. This is consistent with the notion that the east-west cross section at 21
450N (Figure 2) is under more influence of the SCSIW than the cross section at 20
N.

[8] Theq/S, AOU/S, pH/S and NO₃/S plots based on the

ORI-462 data are given in Figure 4. The data west of the Figure 1. Study area and relevant station locations: ORI-403, 5 – 7 October 1994 (inverted triangles);

ORI-462, 2 – 15 September 1996 (open circles); KEEP-MASS, 10 July – 5 August 1992 (diamonds); TPS-24, 29 March – 4 June 1985 (triangles); data spanning from 1977 to 1988, taken from Sekine et al. [2000] (solid circles).

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Figure 2. Vertical sections of salinity and dissolved oxygen (in mL L1) againstsq along 21o45'N (the

ORI-462 data) as well as 20
and 18
N (these two sections are modified from Qu et al. [1999]).

Figure 3. Vertical sections of salinity and dissolved oxygen (in mL L1) againstsqalong 120
430E (the

ORI-403 data).

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front are in open circles whereas those east of the front are in crosses. The westernmost stations have the highest S (marked SCSIW in Figure 4) whereas the easternmost stations have the lowest S (marked KIW in Figure 4) in the intermediate layer. It is also indicated in Figure 4 that west of the front, the curves between tropical and intermediate waters have a steeper slope, which is indic-ative of enhanced mixing rate in the SCS. On the other hand, these curves have a flatter slope east of the front, which suggests that vertically the waters are not as well mixed in the WPS. Figure 4 is based on data along 21
450N, which is the latitude of the northern limit of the Luzon Strait. The influence of the outflowing SCSIW is also evident based on its higher salinity as well as higher pH but lower AOU and NO3 at the same salinity on the

east-west cross section.

[9] Chen and Wang [1998] noted that the SCS-influenced

waters east of Taiwan contain higher amounts of terrestrially generated226Ra than do waters of the Kuroshio. It is shown in Figure 5 that the water temperatures versus 226Ra for intermediate waters east of Taiwan [Chung and Yin, 1995] (the relevant stations are marked in Figure 1). Obviously, the lower-salinity KIW, which originates in an open ocean contains less than half of the 226Ra than the higher-salinity SCSIW which is influenced by the continents and islands surrounding the SCS. Further, Xie et al. [1995] and Huang et al. [1996] also reported that226Ra concentrations in the northern Luzon Strait are one order of magnitude higher than those in the southern Luzon Strait. These are all strong

indications that the 226Ra-poor Kuroshio waters flow into the SCS through the southern Luzon Strait but that the

226_{Ra-rich SCS waters exit through the northern Luzon}

Strait.

[10] Of particular interest is that waters are all saltier than

S = 34.9 in the Smaxlayer in the 18
N cross section (Figure 2).

This subsurface salinity maximum is formed at the sea surface in winter by strong evaporation in regions within 20
30
N and 160
180
E, 25
N and 165
W, 25
N and 180
E as well as within 22
26
N and 172
E [Wyrtki, 1961; Qu et al., 1998]. This water has a salinity of 35.5 and has been coined as the North Subtropical Lower Water by Wyrtki [1961]. For simplicity here, it is referred to as the Kuroshio Tropical Water (KTW) (taken to be between the 50 and 150 m depths); the Smax signal is reduced to a

salinity below 35.0 east of Luzon. At 20
N, however, there appears to be a subsurface Smax front at about the same

longitude as that where the Smin front is observed. East of

the Smax front, salinity is generally higher than 34.9,

whereas west of the front, salinity quickly drops to below 34.7 (Figure 2). Undoubtedly, the same vertical mixing and upwelling processes that reduce the Sminsignal in the SCS

also reduce the Smaxsignal, making the Smaxlayer less salty

due to its mixing with fresher waters above and below this layer. The fact that the salinity front at the Smax layer also

exists at the 21
450N cross section, but not at 18
N, is another indication that the westernmost part of the Kuroshio is affected by outflowing SCS water, which turns only to the north. Further, the dissolved oxygen section at 18
N Figure 4. Diagrams ofq/S, AOU/S, pH/S, and NO3/S based on the ORI-462 data at 21
450N.

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(Figure 2) shows only a slight shoaling of the contours toward the west. The shoaling of the oxygen contours for waters with a sq value smaller than 26.0, however, is

significant at both 20 and 21
450N. This also reflects the upwelling and vertical mixing of older, lower oxygenated deep waters (sq between 26 and 27) in the SCS. To be

underscored here is that the bottom waters (sq > 27.5) in

the SCS and the WPS alike are relatively richer in oxygen than the deep waters. Hence the upwelling and vertical mixing of the bottom waters to the intermediate depths actually increases the oxygen concentration in the deep waters with sq between 27.0 and 27.5 (Figures 2

and 3). This upwelling at depth in the SCS explains the higher oxygen values west of 122
E for waters denser than sq = 27.0.

4. Okinawa Trough

[11] On the basis of theq, S, and AOU data of Swift et

al. [1990], Chen and Wang [1998] pointed out that the Smin layer in the western Okinawa Trough is more

influenced by the SCSIW than are the Smin waters in

the eastern trough. To illustrate this, the salinity of the intermediate waters from the westernmost stations in the Okinawa Trough are all higher than 34.3, which com-pares to values of below 34.2 for the easternmost stations (Figure 6a) (the relevant station locations of the TPS-24 cruise of Swift et al. [1990] are shown in Figure 1). On the other hand, the intermediate waters on the western

side contain slightly lower AOU (near 4
C in Figure 6b). This is explained by the upwelling of the relatively oxygen-rich bottom water in the SCS, as mentioned above. Particularly noteworthy is that when the Chloro-fluorocarbon (CFC) data of Swift et al. [1990] in the vicinity of the central Okinawa Trough are plotted versus q (Figures 6a – 6j), two distinctive lines emerge: the one with higher CFC concentrations represents data from eastern stations (stations 357 – 374) while the other with lower CFC concentrations represents data from western stations (stations 376 – 380) (Figures 6c and 6d, with closeups in Figures 6e and 6f). A gap, analogous to a front, exists between stations 374 and 376 (there are no data at station 375). This phenomenon is most plausibly be the result of the fact that waters on the western side (higher salinity) are influenced by the SCSIW which is older than the KIW [Chen et al., 2001] because the SCSIW receives a significant fraction of the older, CFC-free deeper waters in the SCS. As a result, CFC concentrations are lower in intermediate waters at stations farther to the west in the Okinawa Trough. Note that waters colder than 3.7
C in Figures 6a – 6j can be found only east of the Okinawa Trough.

[12] The above findings confirm the previous work of

Chen and Wang [1998] which reported that the western portion of the minimum-salinity intermediate waters in the Okinawa Trough are influenced by the SCSIW. This is because the ridge east of Taiwan is mostly shallower than 600 m while the KIW is centered at 600 – 700 m and the Figure 5. Temperature versus Ra-226 in waters east of Taiwan. A front separates the Ra-226-poor

Kuroshio Intermediate Water (KIW) from the Ra-226-rich South China Sea Intermediate Water (SCSIW) (data are taken from Chung and Yin [1995]).

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center of the NPIW farther to the east is even deeper than 700 m (Figure 7). The result is that not until KIW reaches a deep gap in the Ryukyu Ridge further north (near 127
E and 26
N) does it freely enter the trough and subsequently form the eastern portion of the intermediate water in the trough (recently confirmed by Guo and Morinaga [2000], Morinaga et al. [2000], and Li et al. [2000]). On the other hand, the center of the SCSIW is shallower at around 500 m. Therefore it enters the southern Okinawa Trough more freely over the ridge and occupies the southern and western portions of the Trough. Theq/S plot for the ORI-462 station CM6 near the shallowest part of the ridge is plotted in

Figure 6a. It is identical to the SCSIW found both in the western Okinawa Trough and adjacent to the southeastern tip of Taiwan (Figure 4a).

[13] In fact, it is clear that the S_max layer in the western

portion of the Okinawa Trough is also influenced by the outflow of the SCS Tropical Water (SCSTW) which has a lower salinity than the Kuroshio Tropical Water (KTW) (Figure 6g). The SCSTW is older than the KTW since the former has received some older water from below as a result of vertical mixing and upwelling in the SCS. Thus AOU values are higher (Figure 6h) but CFC concentrations are lower (Figures 6i and 6j).

Figure 6. Plots of (a)q/S, (b) q/AOU, (c) q/CFC 11, and (d) q/CFC 12 based on the data of Swift et al. [1990] in and near the central Okinawa Trough. The q/S plot of ORI-462 station CM6 near the ridge northeast of Taiwan is also given in Figure 6a. Theq/CFC 11 and q/CFC 12 data for the intermediate waters are blown up in Figures 6e and 6f, while theq/S, q/AOU, q/CFC 11, and q/CFC 12 data for the tropical waters are blown up in Figures 6g – 6j.

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Figure 6. (continued)

Figure 7. Depth of the core of the salinity minimum layer.

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[14] It has repeatedly been shown that every season and

every year there is permanent upwelling northeast of Taiwan where the Kuroshio impinges upon the ECS continental shelf [Wong et al., 1991; Liu et al., 1992a, 1992b; Hu and Chang, 1992; Chuang and Liang, 1994; Gong et al., 1995; Chen et al., 1995b; Chen, 1996; Tang et al., 2000]. In fact, a similar phenomenon has been reported, also repeatedly, along the ECS continental shelf [e.g., C. S. Chen et al., 1994; Ito et al., 1994; Chen et al., 1996; Wang et al., 2000]. An example of this is given in Figures 8a – 8h where the TPS 24 data of Swift et al. [1990] are shown. The shoaling of q, S, sq, AOU and the nutrient contours (only nitrate is

shown) near the shelf break are obvious (Figures 8a – 8e). [15] These features show a cross-flow structure of a

baroclinic current. The thermal wind balance relates a depth-dependent flow perpendicular to the cross section to the sloping isopycnals and associated water properties. Eddy motions in this baroclinic system exchange water properties isopycnally between the shelf and Kuroshio (J. Toole, personal communication, 2003). However, there is more to it. As early as 1972, Nitani [1972, p. 141] reported that the high-salinity tongue reaches the East China Sea continental shelf ‘‘and creeps up along the bottom.’’ Surface offshore waters clearly upwell onto the shelf. Salt could not be balanced if the saltier bottom water on the shelf were transported offshore. Such an uplifting of these tongue-like

contours is a time honored, and clear-cut indication that these waters upwell onto the ECS shelf [Su and Pan, 1987; Wong et al., 1991; Su et al., 1994], as opposed to downwell as Tsunogai et al. [1999] has claimed. There are indeed reports of downwelled dense water flowing from some parts of the ECS shelf toward the Okinawa Trough in late spring due to winter cooling, negative wind stress in winter and frontal eddies. However, this limited extent of downwelling is confined within the upper water column shallower than 100 m, and the broad feature is still upwelling across the shelf [e.g., Isobe et al., 2004].

[16] Further proof is in the CFCs 11 and 12 as well as in

the tritium plots (Figures 8f – 8h). Without question, older subsurface waters with lower CFC and tritium concentra-tions upwell onto the shelf. Young shelf waters, for instance those at station 389 where CFC values are high, do not sink to the interior of the Okinawa Trough. To put it simply, after first being modified by the South China Sea tropical and intermediate waters, the subsurface waters in the western Okinawa Trough definitively upwell onto the ECS shelf. Shipboard ADCP measurements [e.g., Tang et al., 2000; Valle-Levinson and Matsuno, 2003] also confirm the appar-ent on-shore detided flow. It is important to point out that nutrients provided by the upwelled subsurface waters drive high new productivity and convert inorganic carbon to organic carbon which has a net offshore transport [Chen Figure 8. Cross sections of (a)q, (b) S, (c) st, (d) AOU, (e) NO3, (f) CFC 11, (g) CFC 12, and (h) tritium

in and near the central Okinawa Trough (data are taken from Swift et al. [1990]).

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and Wang, 1999]. Consequently, many shelves, like the ECS, become sinks for atmospheric CO2 [Chen, 2003,

2004].

5. South of Japan

[17] The now northeastward flowing Kuroshio, of course,

moves on after passing the Okinawa Trough and is again

forced to turn toward the east south of Japan. Sekine et al. [2000] have reported that the Izu Ridge at about 140
E confines the movement of the NPIW. Here, I group some of their data into three subsets, namely those between 129
and 133
E, between 133
and 139
E and those between 141
and 142
E. There are S = 34.35 waters at the Smin layer, a

clear signal of the influence of the salty SCSIW, in the first group, but all waters at the Smin layer in the second group

Figure 8. (continued)

Figure 9. Diagrams ofq/S for selected stations south of Japan: (a) KH-84-2 and KH-87-1 data between 129
and 132
400E; (b) KH-77-3, KH-84-2, and KH-88-4 data between 133
and 139
E; and (c) KH-87-1 data between 141
and 142
E (data are taken from Sekine et al. [2000]).

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have a salinity below 34.3. Waters in the Sminlayer of the

easternmost third group have a still lower salinity of 34.2 or less (Figure 9). No trace of either the saltier SCSIW or the fresher SCSTW can be found between 141
and 142
E. This is near the region where the Kuroshio mixes with the Oyashio and where the NPIW is reborn [Qu et al., 2001], perhaps with some contribution from the more saline and nutrient-rich SCSIW.

6. Conclusions

[18] Additional data have been used to attest to the

validity of the findings of Chen and Wang [1998] that the SCSIW occupies the western half of the Okinawa Trough and the findings clearly dismiss the report of Tsunogai et al. [1999] that the subsurface ECS shelf waters are transported away from the shelf and into the deeper layers of the Kuroshio. Conclusive evidence is presented here to sub-stantiate that the SCSIW indeed upwells onto the ECS continental shelf. The data here include those collected by the present author in an east-west cross section southeast of Taiwan along 21
450N between 120
and 130
E. In addi-tion, tracer data in the literature, such as those concerning

226_{Ra, CFCs 11 and 12 as well as tritium east of Taiwan and}

in the Okinawa Trough have been used to support the above findings.

[19] Besides the above, the SCSTW is found to follow the

pathway of the SCSIW and to occupy the western portion of the Okinawa Trough before entering the ECS shelf. Finally, both SCSTW and SCSIW can be traced to about 140
E near the Izu Ridge south of Japan.

[20] Acknowledgments. This work was supported by the National Science Council of the Republic of China (NSC 92-2611-Z-110-005; NSC-92-2611-M-110-014). The assistance provided by J. H. Swift and Y. Sekine is greatly appreciated. J. Toole, J. Richman, and three anonymous reviewers provided constructive comments that strengthened the manuscript.

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C.-T. A. Chen, Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan. ([email protected])

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