Chemical weathering of carbonates and silicates in the Han River basin, South Korea

Chemical weathering of carbonates and silicates in the Han River basin, South Korea

Available online at www.sciencedirect.com Chemical Geology 247 (2008) 66 – 80 www.elsevier.com/locate/chemgeo Chemical weathering of carbonates and ...

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Available online at www.sciencedirect.com

Chemical Geology 247 (2008) 66 – 80 www.elsevier.com/locate/chemgeo

Chemical weathering of carbonates and silicates in the Han River basin, South Korea Jong-Sik Ryu a,b , Kwang-Sik Lee a,⁎, Ho-Wan Chang b , Hyung Seon Shin a a

Division of Isotope Geoscience, Korea Basic Science Institute, 52, Eoeun-dong, Yusung-gu, Daejeon 305-333, South Korea b School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea Received 20 April 2007; received in revised form 27 September 2007; accepted 28 September 2007 Editor: B. Bourdon

Abstract A detailed investigation of the fluvial geochemistry of the Han River system allows to estimate the rates of chemical weathering and the consumption of CO2. The Han River drains approximately 26,000 km2 and is the largest river system in South Korea in terms of both water discharge and total river length. It consists of two major tributaries: the North Han River (NHR) and the South Han River (SHR). Distinct differences in basin lithology (silicate vs. carbonate) between the NHR and SHR provide a good natural laboratory in which to examine weathering processes and the influence of basin geology on water quality. The concentrations of major elements and the Sr isotopic compositions were obtained from 58 samples collected in both summer and winter along the Han River system in both 2000 and 2006. The concentrations of dissolved loads differed considerably between the NHR and SHR; compared with the SHR, the NHR had much lower total dissolved solids (TDS), Sr, and major ion concentrations but a higher Si concentration and 87Sr/86Sr ratio. A forward model showed that the dissolved loads in the NHR came primarily from silicate weathering (55 ± 11%), with a relatively small portion from carbonates (30 ± 14%), whereas the main contribution to the dissolved loads in the SHR was carbonate weathering (82 ± 3%), with only 11 ± 4% from silicates. These results are consistent with the different lithologies of the two drainage basins: silicate rocks in the NHR versus carbonate rocks in the SHR. Sulfuric acid derived from sulfide dissolution in coal-containing sedimentary strata has played an important role in carbonate weathering in the SHR basin, unlike in the NHR basin. The silicate weathering rate (SWR) was similar between the NHR and SHR basins, but the rate of CO2 consumption in the SHR basin was lower than in the NHR basin due to an important role of sulfuric acid derived from pyrite oxidation. © 2007 Elsevier B.V. All rights reserved. Keywords: Chemical weathering; Silicate; Carbonate; CO2 consumption rate; Sulfuric acid; Han River

1. Introduction Two major weathering processes that consume CO2 operate in the inorganic cycle of carbon: the congruent ⁎ Corresponding author. E-mail address: [email protected] (K.-S. Lee). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.09.011

dissolution of carbonates and the incongruent degradation of aluminosilicate minerals, in which silicate rocks are converted to residual clays, dissolved cations, and silica (Roy et al., 1999; Huh, 2003; Mortatti and Probst, 2003). The consumption of atmospheric CO2 during mineral weathering plays a major role in determining the long-term carbon budget (Dalai et al., 2002; Wu et al., 2005) and

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consequently in any associated global air temperature changes (Gaillardet et al., 1999; Roy et al., 1999). It has been established in many watersheds that factors such as temperature, runoff and topographic relief are important in controlling weathering and in determining associated CO2 consumption rates (Walker et al., 1981; Raymo and Ruddiman, 1992; White and Blum, 1995; Gislason et al., 1996; Gaillardet et al., 1999; Oliva et al., 2003; Riebe et al., 2004; West et al., 2005; Gislason et al., 2006; Moon et al., 2007). The Han River is the largest river in South Korea. It supports a variety of industrial and agricultural activities, and supplies freshwater to more than 20 million inhabitants of the central Korean Peninsula. Despite the river's socio-economic value, no information is presently available on weathering rates and the associated CO2 consumption rates in the river's basin. A pronounced difference exists in the silicate vs. carbonate basement lithology of the Han's two major tributaries, the North Han River (NHR) and the South Han River (SHR), and the catchment provides an excellent natural laboratory in which to examine how the intensity of the weathering process and the basin lithology can affect water quality. Specifically, the Han River system affords a first-rate example of the geochemical differences that exist in the weathering of carbonate and silicate rocks under temperate conditions. In this study, we first determined the major element concentrations of the dissolved loads of the Han River waters, considering seasonal and spatial variations, in order to establish the main lithologic types being weathered. These results were used to ascertain the intensity of weathering within the drainage basin. We then employed a forward model to quantify the relative contributions from rain, carbonates, and silicates to the dissolved loads, prior to calculating the rates of silicate weathering and associated CO2 uptake. 2. Study area 2.1. Geography and climate The Han River system consists of two major tributaries (the NHR and SHR) with numerous subsidiary branches (Fig. 1). The river is 5417 km long and drains an area of 26,018 km2, or 27% of South Korea (KOWACO, 1993). The annual discharge varies from 16.0 to 18.9 km3. The river rises at an altitude of more than 1300 meters above sea level in the Taeback Mountains and traverses the mid-west Korean Peninsula before flowing into the Yellow Sea. The NHR and SHR join at the Paldang dam to form the main channel of the

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Han River (Fig. 1). Six major dams are located on the NHR, but only one is on the SHR (Fig. 1). These dams were constructed primarily for flood control and freshwater supply, but they also allow for some hydroelectric generation. The Paldang dam is the first reservoir on the main channel and serves as the primary reference point for the sampling programs undertaken in this study. The climate of the Korean Peninsula is determined by its latitude and geography. It possesses four distinct seasons, with the mean monthly temperature showing clear seasonal variations that range from below freezing in winter to over 25 °C in summer. Wind and precipitation are largely controlled by the Pacific Ocean in the south and the Eurasian landmass in the north. The mean annual rainfall is 1274 mm, of which about 70% occurs during the summer months from June to September. The rainfall is evenly distributed over the Han River basin. 2.2. Geology The Korean Peninsula has been carved from a remnant mass of deformed basement, intruded by granite stocks, and overlain by a succession of sediments and volcanics. The total rock mass records a protracted history of basin formation and crustal deformation (Chough et al., 2000). The Han River basin transects the Precambrian Gyeonggi massif and Ogcheon Fold Belt (Fig. 1). The basement rocks of Gyeonggi massif are composed of Late Archean to Early Proterozoic high-grade gneisses and schists (Gaudette and Hurley, 1973; Hurley et al., 1973; Na and Lee, 1973). Middle to Late Proterozoic supracrustal sequences, consisting mainly of schists, quartzites, marbles, calc-silicates, and amphibolites, unconformably overlie the basement rocks within the massifs. These Precambrian rocks suffered amphibolite- to granulite-facies metamorphism and were intruded by Mesozoic plutons (Lee, 1987). The Ogcheon Fold Belt is divided into two zones, based largely on lithology and metamorphic grade: the Ogcheon Basin in the southwest and the Taebaeksan Basin in the northeast (Fig. 1a). The Ogcheon Basin consists of non-fossiliferous, low- to medium-grade metasedimentary and metavolcanic rocks whose ages are not well constrained. The Taebaeksan Basin comprises mainly the Choson Supergroup (Cambrian–Ordovician) and Pyongan Supergroup (Carboniferous–Triassic). The lower Paleozoic sediments are mostly shallow marine in origin and consist predominantly of carbonates, with lesser amounts of sandstone and shale, whereas the Pyongan

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Fig. 1. Tectonic units of the southern Korean Peninsula (a), and geologic map of the Han River basin showing sampling locations (b). (GM: Gyeonggi Massif, OFB: Ogcheon Fold Belt, OB: Ogcheon Basin, TB: Taebaeksan Basin, YM: Yongnam Massif, GB: Gyeongsang Basin).

Supergroup comprises thick clastic successions of marginal marine to non-marine environments, containing economically important coal measures (Cheong, 1969). The two major tributaries of the Han River system are incised in distinctly different lithologies (Fig. 1b). The NHR basin is dominated by metamorphic gneisses and schists, with associated granites, whereas the SHR basin is composed of metasedimentary and metavolcanic rocks downriver, with sedimentary rocks containing coal seams in its upper reaches. 3. Methods River water samples were collected seasonally during both 2000 and 2006. In 2000, samples were obtained from 9 stations along the NHR and 8 stations along the SHR, in both summer (August) when water was high and winter (December) when water was low. In 2006, samples were collected from 6 stations along the NHR and 7 stations along the SHR in both August

and December. The numbers and locations of samples varied on each sampling campaign and were determined by accessibility and water regime. All sampling sites were selected to avoid anthropogenic point source contamination and direct influences of the minor tributaries. At each site, a sample was taken from a consistent depth. The temperature and pH of water samples were measured in the field. The alkalinity was determined within 12 h of sampling, by Gran titration using 0.1 N HCl. The samples for chemical and Sr isotopic analysis were filtered through 0.45-μM, pre-cleaned membranes and kept refrigerated at approximately 4 °C prior to analysis. The samples for cation analysis were acidified in the field with ultrapure HNO3 to a pH of b 2. Cations were analyzed by ICP-MS (X-7, Thermo Elemental) and ICP-AES (Optima 4300DU, Perkin Elmer) at the Korea Basic Science Institute (KBSI). Anions were analyzed by ion chromatography (Dionex DX-500 IC) at the Korea Institute of Geoscience and Mineral Resources (KIGAM).

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Table 1 The chemical compositions of the dissolved loads of the Han River Sam. #

Date

pH

yyyy-mm

87

Sr/86Sr NICB a CSI b

T

Na

K

Mg

Ca

Cl

NO3

SO4

Alk

Si

TDS

Sr

°C

μM

μM

μM

μM

μM

μM

μM

μeq

μM

mg/l

μM

114 845 100 117 991 65.8 83.5 1202 128 109 1097 59.6 105.9 1034 88.4

107 114 133 126 120

0.99 0.87 0.77 0.92 0.89

0.72154 – 0.72042 0.72164 0.72120

1.0 − 8.0 4.0 8.8 1.4

− 0.4 − 0.5 − 1.3 − 0.9 − 0.8

%

Main channel of the Han River M-1 2000-08 8.05 26.8 M-1 2000-12 8.11 10.0 M-1 2006-08 6.89 23.8 M-1 2006-11 7.47 14.1 Mean 7.63 18.7

237 242 382 351 303

48.3 37.8 36.0 62.2 46.1

138 142 134 148 140

404 432 481 523 460

166 188 107 205 166

117 129 106 82.6 108.5

North Han River NS-1 2000-08 NS-1 2000-12 NS-3 2000-12 NS-3 2006-08 NS-3 2006-11 NS-4 2000-08 NS-4 2000-12 NP-1 2000-12 NP-2 2000-08 NP-2 2000-12 NP-2 2006-08 NP-2 2006-11 NP-3 2000-08 NP-3 2000-12 NP-3 2006-08 NP-3 2006-11 N-5 2000-08 N-5 2000-12 N-5 2006-08 N-5 2006-11 N-6 2000-08 N-6 2000-12 N-6 2006-08 N-6 2006-11 N-6-1 2000-12 N-6-1 2006-08 N-6-1 2006-11 Mean

7.41 7.90 7.99 8.72 7.15 8.96 7.99 7.94 8.62 8.29 7.21 6.73 7.32 8.03 6.68 7.45 7.56 8.23 6.88 7.37 9.33 7.75 7.31 7.26 8.28 8.20 6.68 7.75

20.4 2.5 3.3 30.5 6.8 28.1 12.2 9.8 27.5 12.1 25.8 13.0 24.2 10.2 15.7 11.3 23.1 10.0 21.1 11.0 28.7 8.7 24.3 13.1 4.5 31.3 8.9 16.2

108 120 187 261 184 144 122 127 160 125 295 146 161 129 96.6 102 179 150 401 206 176 158 232 223 370 557 300 200.7

16.1 11.6 23.5 41.8 41.4 32.0 26.8 27.6 34.0 27.2 40.4 36.2 37.9 26.1 26.7 33.8 37.6 27.2 49.8 50.2 36.3 28.5 36.9 48.9 34.4 68.7 66.9 35.9

24.7 25.9 69.6 86.8 78.8 63.3 53.3 135 162 111 112 112 131 104 41.3 43.9 110 70.1 81.8 70.1 82.7 70.2 75.2 76.1 94.1 132 140 87.4

97.3 98.2 198 278 245 187 147 310 379 265 304 284 334 255 122 137 289 183 220 220 234 187 221 239 271 422 469 244.3

54.6 61.2 145 113 114 86.4 73.1 69.4 95.9 70.5 102 67.6 102 71.7 59.2 51.6 116 85.8 158 118 104 107 115 131 188 202 246 107.8

44.4 40.1 166 153 43.0 57.2 42.2 170 151 43.6 102 54.4 397 117 64.0 56.7 50.4 729 112 87.9 91.0 59.8 495 179 80.0 74.1 58.1 402 103 59.6 87.4 52.7 324 108 52.0 43.9 74.5 694 68.1 79.7 49.6 89.8 905 76.5 100 55.3 59.3 694 95.0 79.3 83.2 63.9 767 138 96.8 54.8 55.5 716 86.4 80.7 56.9 76.7 744 80.1 87.7 57.1 61.5 595 95.4 73.1 77.2 44.3 206 139 45.7 72.9 45.7 243 160 51.6 64.6 71.1 704 84.4 83.9 74.0 51.3 397 109 59.3 87.6 72.5 586 163 90.7 92.9 61.7 442 138 71.6 87.1 64.9 552 114 74.6 83.4 57.7 397 101 60.0 95.2 68.8 458 128 72.2 77.0 61.1 526 37.3 65.3 114 82.1 595 49.4 79.6 135 109 978 119 127 140 114 1020 123 128 78.3 64.6 552 112.1 75.5

0.22 0.21 0.79 0.83 0.60 0.55 0.56 0.54 0.68 0.49 0.79 0.48 0.66 0.48 0.34 0.34 0.64 0.47 0.58 0.47 0.64 0.47 0.64 0.55 0.89 1.37 1.34 0.62

0.71843 – – 0.72107 0.72242 0.72447 – – 0.72128 – 0.72639 0.72468 0.72307 – 0.72343 0.72248 0.72338 – 0.72933 0.72432 0.72681 – 0.72606 0.72568 – 0.72359 0.72292 0.72388

6.3 1.7 − 1.0 3.3 6.2 − 0.4 − 7.4 8.5 3.7 − 3.7 7.5 2.6 6.6 3.1 4.2 7.8 − 1.1 3.5 7.4 7.2 − 3.2 − 0.4 6.5 5.0 6.4 11.5 − 3.1 3.3

− 2.3 − 2.1 − 1.4 0.1 − 2.0 − 0.2 − 1.5 − 0.9 0.2 − 0.6 − 1.4 − 2.1 − 1.3 − 1.0 − 3.0 − 2.2 − 1.1 − 1.1 − 2.0 − 1.8 0.3 − 1.6 − 1.6 − 1.8 − 0.8 − 0.1 − 1.9 − 1.3

South Han River S-2 2000-12 S-2 2006-08 S-2 2006-11 S-3 2000-08 S-3 2000-12 S-3 2006-08 S-3 2006-11 S-3-1 2006-08 S-3-1 2006-11 S-4 2000-08 S-4 2000-12 S-5 2000-08 S-5 2000-12 S-5 2006-08 S-5 2006-11 S-5-1 2006-08 S-5-1 2006-11 S-6 2000-12

8.34 8.82 7.07 8.84 8.40 8.45 8.71 8.32 7.42 8.64 8.63 8.68 7.84 8.50 7.53 7.20 7.19 8.48

2.0 26.2 9.0 25.3 5.2 27.4 13.1 28.8 11.2 26.1 4.8 26.2 5.4 28.9 12.0 28.2 12.0 10.7

169 222 174 124 160 196 158 223 255 122 189 125 152 206 190 741 918 122

181 195 170 135 172 150 161 200 272 120 195 122 145 165 169 435 786 124

213 170 167 177 184 154 149 156 165 165 202 170 175 147 130 114 157 146

1.63 1.61 1.37 2.13 3.78 2.56 2.58 1.09 1.31 1.60 2.57 1.44 1.68 1.94 2.06 1.81 1.74 1.17

– 0.71523 0.71564 0.71601 – 0.71461 0.71412 0.71777 0.71721 0.71562 – 0.71609 – 0.71570 0.71484 0.71741 0.71720 –

0.4 − 0.1 7.0 3.7 1.0 − 6.4 4.0 − 7.9 6.7 4.5 1.9 2.7 1.6 − 2.1 2.1 4.8 3.5 3.5

0.0 0.8 − 1.2 0.8 0.4 0.7 0.8 0.4 − 0.5 0.7 0.6 0.7 − 0.3 0.8 − 0.4 − 0.8 − 1.1 0.2

24.1 42.1 40.7 27.9 26.5 34.1 38.4 45.4 62.9 29.2 30.4 31.2 29.2 40.6 42.3 85.2 109 32.6

226 203 174 246 366 293 293 229 314 258 364 260 286 315 296 152 157 205

756 738 738 801 1138 980 1072 712 1041 796 1008 763 941 934 1025 580 618 643

158 150 126 246 436 299 256 106 125 207 302 173 255 231 218 121 123 139

1437 56.0 1483 72.6 1305 111 1358 41.3 1933 55.2 2049 62.5 1984 63.1 1751 44.2 2136 61.7 1458 58.0 1903 37.2 1508 58.7 1765 45.5 2028 57.4 2081 62.5 1388 65.9 1296 135 1239 23.7

156 161 150 157 230 217 213 169 215 161 210 160 191 208 214 166 189 130

(continued on next page) (continued on next page)

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Table 1 (continued) Sam. #

Date yyyy-mm

South Han River S-7 2000-08 S-7 2000-12 S-7 2006-08 S-7 2006-11 S-9 2000-08 S-9 2000-12 S-10 2000-12 S-10 2006-08 S-10 2006-11 Mean a b

pH

8.93 8.23 7.45 6.63 8.12 7.93 8.29 7.35 6.99 8.04

T

Na

K

Mg

Ca

Cl

NO3

SO4

Alk

Si

TDS

Sr

°C

μM

μM

μM

μM

μM

μM

μM

μeq

μM

mg/l

μM

29.5 10.7 20.2 15.4 25.5 9.2 8.9 20.6 11.7 16.8

152 137 176 252 207 256 308 241 456 246

34.0 120 30.8 48.0 43.2 39.9 41.6 36.5 72.8 45.9

245 232 158 199 200 219 221 174 202 240

134 206 97.7 133 160 216 235 152 345 206.5

109 156 113 98.1 130 166 171 124 128 153.2

198 159 93.4 140 158 166 177 102 179 186.7

1433 10.7 1487 27.0 1409 118 1570 102 1358 76.2 1338 30.8 1437 34.4 1330 124 1489 95.8 1591 64.1

150 157 145 168 149 148 157 148 178 174

1.45 1.21 0.87 1.03 1.21 1.22 1.25 0.94 1.19 1.65

699 704 589 737 644 674 674 642 728 792

87

Sr/86Sr NICB a CSI b %

0.71616 – 0.71716 0.71681 0.71698 – – 0.71784 0.71774 0.71632

0.1 − 1.9 − 6.3 4.3 − 1.3 1.5 − 2.7 5.2 2.9 1.2

0.9 0.0 − 0.7 − 1.4 0.1 − 0.4 0.0 − 0.8 − 1.2 0.0

NICB (Normalized Inorganic Charge Balance) = (TZ+ − TZ−)/TZ+ × 100%. CSI: Calcite Saturation Index.

Approximately 60 ml of each water sample was prepared for Sr isotopic analysis by first evaporating it to dryness in an ultra-clean Teflon vessel. The residue was then dissolved in distilled 2.5 N HCl, and the strontium in the solution was separated from the other ions using a cation exchange resin (BioRad® AG 50 W X8, 200–400 mesh) in a quartz column. A total blank contained negligible Sr. The Sr isotopic composition was determined using a MC-ICPMS (Axiom, VG Elemental) at the KBSI. The 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194. The mean 87Sr/86Sr ratio of the NBS987 standard obtained to establish analytical reproducibility was 0.710247 ± 0.000008 (2σ, n = 24). 4. Results and discussion 4.1. Hydrogeochemistry and chemical weathering 4.1.1. Major elements and Sr isotopes The pH of Han River waters ranged from 6.63 to 9.33, with an average of 7.90 (Table 1). The waters were more alkaline during summer than during winter, with the exception of a few samples believed to be contaminated by biological activity. The water temperature ranged from 2.0 to 31.3 °C and showed a clear seasonal variation. With one exception, the total dissolved cations (TZ+ = Na+ + K+ + 2Mg2+ + 2Ca2+) and total dissolved anions (TZ− = Cl− + 2SO42− + HCO3−) were balanced to within ± 9% of the normalized inorganic charge balance (NICB) (Table 1). The total dissolved solids (TDS) concentration showed large differences between the NHR and SHR. When compared with other world rivers, the average TDS of the SHR draining its carbonate basin (174 mg/l) was very similar to that of the Ganges (187 mg/l) and the

Indus Rivers (164 mg/l) draining the Himalayas (Karim and Veizer, 2000; Dalai et al., 2002) and to that of the Mackenzie (160 mg/l) draining the Rockies (Millot et al., 2003), whereas it was much lower than that of the Upper Huang He (Yellow) River (274 mg/l) draining the eastern Qinghai-Tibet Plateau (Wu et al., 2005). In contrast, the average TDS (75.5 mg/l) of the NHR, which drains an area that has experienced numerous orogenic events, was much lower than that of the SHR but was comparable to that of other rivers draining orogenic zones, e.g., the Brahmaputra (71 mg/l) draining the Himalayas and Tibet Plateau (Galy and France-Lanord, 1999); the Amazon (41 mg/l) and the Orinoco (82 mg/l) draining the Andes (Edmond et al., 1996; Dosseto et al., 2006); and rivers of Eastern Siberia (70 mg/l) draining a collision zone (Huh et al., 1998). The major element compositions indicated that the weathering of silicates is important in the NHR basin, whereas carbonate weathering predominates in the SHR basin (Fig. 2a). Most of the samples collected from the NHR were dominated by Ca with Ca N Na N Mg N K, except for one Na-dominant sample (NS-1) taken from a tributary located in granite. Calcium also was the dominant cation in the SHR but with Ca N Mg N Na N K, except for one sample (S-5-1), collected from another small tributary draining granites, in which Na N Ca N Mg N K. Little seasonal difference was found in river water geochemistry (Fig. 2b). Bicarbonate (HCO3) was the dominant anion in both tributaries, accounting for approximately 60–70% of TZ−, except in sample NS-1. The concentration of Si varied from 10.7 to 179 μM, with an average of 88 μM. It differed significantly between the NHR and SHR; the average Si in the NHR (112.1 μM) was nearly twice that in the SHR (64.1 μM). On the other hand, the concentrations of Cl and SO4 in

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Fig. 2. Ternary diagrams of cations and anions for water samples from the Han River, with ions expressed as charge equivalent units (a), and cation and anion seasonal variations (b). Ca and alkalinity dominate the major element composition of the SHR, whereas the NHR shows enrichment in Si as a result of intensive silicate weathering.

the SHR were about twice those in the NHR, reflecting a greater influence of agricultural fertilizer and/or sulfide oxidation during weathering in the SHR basin, or the presence of some minor evaporites in the sedimentary sequences in this basin because even a tiny amount (too small to appear on geological maps) could significantly affect the water chemistry. The water samples from the Han River are plotted between their silicate and carbonate end-members in Fig. 3. The NHR water samples clustered near the silicate end-member, whereas those from the SHR were near the carbonate end-member. Clearly silicate weathering predominates in the NHR basin, but carbonate weathering is the norm in the SHR. The Sr isotopic ratio (range, 0.71412–0.71784) slightly increased downstream in the SHR (Fig. 4). The lithology of this basin passes from carbonates in the upper reaches to metamorphic rocks in the lower. Although the carbonate rocks in the upper reaches are of marine origin, the water samples had Sr isotopic ratios higher than those typical of Phanerozoic marine carbonates (Faure, 1986). This may reflect the presence

of radiogenic metamorphic carbonates in the sedimentary strata of the SHR basin. Park and Cheong (1998) reported that some marbles in the SHR basin have Sr isotopic ratios markedly higher than the Phanerozoic marine carbonates. Similar results have been reported from the Lesser Himalayas (Edmond, 1992) and from the Highlands of the Mackenzie River (Millot et al., 2003). Hence, the radiogenic signature of Sr isotopic composition of the SHR could be due to the dissolution of metamorphosed carbonates, as in the Himalayan Rivers and the Mackenzie River. The Sr concentration ranged from 0.21 to 1.37 μM, and the isotopic composition ratio ranged from 0.71843 to 0.72933 in the NHR. Sample NS-1, taken from a major tributary of the NHR basin draining Precambrian banded gneisses and Mesozoic granites, was characterized by a low Sr concentration and low Sr isotopic ratio. This sample diverged significantly from the other NHR samples (Fig. 5), perhaps indicating a relative increase in its rainwater contribution from surface reservoirs or peculiar geological characteristics of this specific area.

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Fig. 4. Seasonal and spatial variations in the 87Sr/86Sr ratio in the Han River. The Sr isotopic ratio increases slightly downstream in the SHR.

Fig. 3. Plots of (a) HCO3/Na vs. Ca/Na and (b) Mg/Na vs. Ca/Na for the Han River waters, showing mixing between silicate and carbonate end-members. Data from other world rivers, and silicate and carbonate end-members are from Gaillardet et al. (1999).

Further detailed investigations of Sr isotopic composition and geochemistry of this area would appear to be warranted. The differences in Sr isotopic data between the NHR and SHR are indicative of the differences in the lithologies of their drainage basins and demonstrate that Sr isotopes provide useful tools for tracing source rocks within the Han River basin. In summary, the waters of the NHR and SHR exhibited distinct differences in their chemical and isotopic compositions. By comparison with other rivers (Gaillardet et al., 1999), the NHR has lower TDS concentration (∼75 mg/l), higher Si concentration (∼20% of total anions), and more radiogenic 87Sr/86Sr ratios (0.718– 0.729) than global average values, whereas the SHR values resemble the global averages (Fig. 6).

4.1.2. Atmospheric inputs Several sources such as atmospheric Cl and SO4, lithologic Ca, and anthropogenic NO3 have contributed to the chemistry of river waters (Roy et al., 1999). It is important to evaluate the atmospheric and anthropogenic

Fig. 5. Plot of 87Sr/86Sr vs. Sr/Na for the Han River and for other world rivers, and indicating the three possible end-members: rain, silicate, and carbonate (data from Roy et al., 1999). Note that sample NS-1 deviates significantly from the major grouping for the NHR. This indicates a relative increase in rainwater contribution or peculiar geological characteristics of this specific area.

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analysis of rainwater in Chuncheon city (Yu and Park, 2004), located within the present study area. Given that no salt-bearing rocks or evaporites have been reported from the Han River basin (Chough et al., 2000), we have assumed that the Cl in sample NP-3, which was the lowest Cl concentration we found, came entirely from rainwater (Table 1). We then calculated the proportion of atmospherederived elements in the river waters using the element/ Cl ratios in the rain. On this basis, 10–32% of total dissolved cations in the NHR, but only 4–10% of total dissolved cations in the SHR, originated from rain (Table 2). Among the anions, sulfate appears to be derived mainly from the atmosphere in the NHR, whereas only 12–51% of sulfate was explained by atmospheric input in the SHR. This points to the existence of additional sulfate sources in the SHR basin, with the oxidation of sulfides as a result of weathering probably contributing a considerable portion of the sulfate ions in the SHR.

Fig. 6. Histograms comparing (a) TDS, (b) Si, and (c) 87Sr/86Sr of the Han River with the respective values of other world rivers (Gaillardet et al., 1999).

contributions before considering input from the weathering of different rock types within the river basin. An estimate of the proportion of atmosphere-derived constituents in the Han River is provided by a chemical

4.1.3. Sulfide oxidation Three main sources supply dissolved SO4 in natural waters: (1) gypsum/anhydrite, (2) pyrite, and (3) pollution from coal combustion (Thode, 1991; Grasby et al., 1997). In estimating CO2 consumption rates, distinguishing among these sources is very important because pyrite oxidation generates sulfuric acid that can attack surrounding carbonate and silicate minerals during weathering (Moon et al., 2007; Xu and Liu, 2007). As a consequence, the amount of atmospheric CO2 consumed during chemical weathering is reduced when sulfuric acid is involved, and thus the drawdown of CO2 by silicate weathering can be overestimated when the role of sulfide oxidation is ignored (Spence and Telmer, 2005; Wu et al., 2005; Lerman and Wu, 2006; Moon et al., 2007). The only sources of dissolved sulfates in the river waters of the present study area that needed to be considered were pyrite oxidation and atmospheric input, given the absence of salt-bearing rocks and evaporites in the Han River basin (Chough et al., 2000) and the minimal pollution affecting the area (Yu and Park, 2004). Consequently, the sulfates in the NHR basin probably originated mainly from the atmosphere, with minor contributions from sulfide oxidation and pollution. This is easily verified by the stoichiometry that exists between cations and anions. Total Ca + Mg was well balanced by HCO3 with only a minor input required from sulfate in the NHR (Fig. 7a). This result concurs with those of Yu and Park (2004), who found that the metamorphic rocks of the NHR basin contained few sulfides capable of

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Table 2 Results of fluxes and yields of TDS and SPM, and silicate weathering rates from the Han River basin Discharge a

yyyy-mm

km2

km3/yr

mm/yr

Summer samples Main channel of the Han River M-1 M-1

2000-08 2006-08

23,800 23,800

52.9 52.9

North Han River NS-1 NS-3 NS-4 NP-2 NP-2 NP-3 NP-3 N-5 N-5 N-6 N-6

2000-08 2006-08 2000-08 2000-08 2006-08 2000-08 2006-08 2000-08 2006-08 2000-08 2006-08

525.6 1060 2703 3901 3901 4736 4736 7709 7709 9912 9912

South Han River S-2 S-3 S-3 S-3-1 S-5 S-5 S-7 S-7 S-9 S-10

2006-08 2000-08 2006-08 2006-08 2000-08 2006-08 2000-08 2006-08 2000-08 2006-08

– 2283 2283 1524 4690 4690 6648 6648 10,789 12,514

Date

Runoff

SPM

SWR

ΦCO2 by both carbonic and sulfuric acids

ΦCO2 by only carbonic acid

t/km2/yr

106 mol/ km2/yr

106 mol/ km2/yr

9 8

22.2 34.2

0.916 1.46

0.916 1.46

6 21 27 54 22 48 22 39 – 30 17

32 12 18 11 10 12 27 13 – 15 14

14.6 43.3 16.6 9.43 17.9 10.3 5.49 10.8 24.9 13.1 17.3

0.631 1.81 0.689 0.391 0.754 0.423 0.226 0.447 1.05 0.542 0.725

0.631 1.81 0.689 0.391 0.754 0.423 0.226 0.447 1.05 0.542 0.725

80 81 81 81 80 80 82 83 84 84

8 7 6 7 7 6 8 10 9 9

– 19.0 26.0 16.1 21.8 30.5 10.0 4.74 8.42 8.38

– 0.783 1.07 0.672 0.907 1.26 0.418 0.201 0.357 0.350

– 0.545 0.764 0.594 0.724 0.989 0.295 0.166 0.238 0.285

SPM flux

TDS flux

SPM yield

TDS yield

Source of cation

mg/l

106 t/yr

106 t/yr

t/km2/yr

t/km2/yr

2222 2222

– 22.8

– 1.20

5.67 7.04

– 50.5

238 296

29 41

62 51

2.15 4.27 8.02 5.79 5.79 7.43 7.43 11.5 11.5 18.3 18.3

4091 4030 2967 1484 1484 1569 1569 1490 1490 1844 1844

– 4.00 – – 9.50 – 58.0 – 46.0 – 60.5

– 0.0171 – – 0.0550 – 0.431 – 0.528 – 1.11

0.092 0.375 0.478 0.581 0.560 0.652 0.340 0.963 1.04 1.36 1.32

– 16.1 – – 14.1 – 91.0 – 68.5 – 112

176 354 177 149 144 138 71.7 125 135 138 133

63 67 55 36 68 41 50 49 – 56 69

– 9.31 9.31 7.30 23.5 23.5 18.5 18.5 40.4 53.8

4078 4078 4790 5004 5004 2785 2785 3743 4296

6.40 – 4.60 3.80 – 11.0 – 41.0 – 42.5

– – 0.0428 0.0277 – 0.258 – 0.759 – 2.28

– 1.46 2.02 1.23 3.76 4.89 2.78 2.68 6.03 7.95

– – 18.8 18.2 – 55.0 – 114 – 183

– 641 886 810 801 1043 417 403 558 635

12 12 13 12 13 14 10 7 7 7

Sil.

Carb.

Rain

%

J.-S. Ryu et al. / Chemical Geology 247 (2008) 66–80

Surface area a

Sam. #

2000-12 2006-11

23,800 23,800

6.75 6.75

284 284

– 2.00

– 0.0135

0.767 0.852

– 0.567

32.2 35.8

49 58

43 35

8 6

2.83 4.35

0.118 0.180

0.118 0.180

North Han River NS-1 NS-3 NS-3 NS-4 NP-2 NP-2 NP-3 NP-3 N-5 N-5 N-6 N-6

2000-12 2000-12 2006-11 2000-12 2000-12 2006-11 2000-12 2006-11 2000-12 2006-11 2000-12 2006-11

525.6 1060 1060 2703 3901 3901 4736 4736 7709 7709 9912 9912

0.14 0.49 0.49 0.39 0.27 0.27 0.75 0.75 3.34 3.34 3.58 3.58

266 467 467 144 69 69 158 158 434 434 361 361

– – 6.00 – – 9.20 – 18.5 – 11.5 – 17.5

– – 0.00297 – – 0.00249 – 0.0138 – 0.0384 – 0.0627

0.006 0.032 0.040 0.020 0.021 0.022 0.055 0.039 0.198 0.239 0.215 0.234

– – 2.80 – – 0.637 – 2.91 – 4.99 – 6.32

11.6 29.9 37.4 7.51 5.50 5.59 11.5 8.14 25.7 31.0 21.7 23.6

69 64 58 56 38 43 40 53 57 67 59 67

4 21 29 25 49 45 46 26 27 20 26 21

27 14 12 20 13 12 13 22 16 12 15 12

1.09 3.38 3.64 0.678 0.336 0.420 0.782 0.659 2.54 3.92 2.25 3.49

0.047 0.144 0.149 0.0282 0.0139 0.0172 0.0326 0.0264 0.106 0.159 0.0940 0.143

0.047 0.144 0.149 0.0282 0.0139 0.0172 0.0326 0.0264 0.106 0.159 0.0940 0.143

South Han River S-2 S-2 S-3 S-3 S-3-1 S-5 S-5 S-7 S-7 S-9 S-10 S-10

2000-12 2006-11 2000-12 2006-11 2006-11 2000-12 2006-11 2000-12 2006-11 2000-12 2000-12 2006-11

– – 2283 2283 1524 4690 4690 6648 6648 10,789 12,514 12,514

– – 0.45 0.45 0.35 1.39 1.39 0.90 0.90 1.96 2.61 2.61

198 198 232 297 297 135 135 182 208 208

– 3.00 – 0.60 11.8 – 9.60 – 1.00 – – 6.80

– – – 0.000271 0.00418 – 0.0134 – 0.000898 – – 0.0177

– – 0.104 0.096 0.076 0.266 0.298 0.141 0.150 0.289 0.410 0.465

– – – 0.119 2.74 – 2.85 – 0.135 – – 1.42

– – 45.6 42.1 50.1 56.7 63.6 21.2 22.6 26.8 32.7 37.2

13 13 12 15 18 14 16 12 9 2 0 –

81 80 84 80 78 81 80 82 85 90 93 –

6 7 4 5 4 5 5 7 7 7 7 –

– – 1.45 1.38 1.68 1.79 2.01 0.476 0.361 0.236 0.0937 –

– – 0.0601 0.0572 0.0696 0.0745 0.0836 0.0207 0.0155 0.0103 0.00407 –

– – 0.0369 0.0447 0.0640 0.0558 0.0682 0.0161 0.0116 0.0037 0.0000 –

a

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Winter samples Main channel of the Han River M-1 M-1

Data from Water Management Information System, Korea (http://www.wamis.go.kr/).

75

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having an impact on the sulfate concentration of the tributary waters. In contrast, in the SHR waters, total Ca + Mg was not balanced by HCO3 alone; instead, it required significant additional SO4 to achieve ionic balance (Fig. 7b). This requirement suggests that sulfuric acid has played a relatively important role in carbonate weathering of this part of the basin. Sulfur isotope data of our previous study (Ryu et al., 2007) indicate that dissolved sulfates in the NHR originate mainly from atmospheric deposition, but in the SHR both atmospheric input and pyrite oxidation are the principal contributors. To quantify precisely the weathering fraction by sulfuric acid in the SHR, it is important to distinguish gypsum dissolution from both atmospheric deposition and pyrite oxidation. Using three potential end-members for dissolved sulfates in the SHR: i.e., +5.5‰ for δ34Satm (Yu and Park, 2004), +25‰ for δ34Sevap (Spence and Telmer, 2005), and −13.5‰ for δ34Spy (our unpublished data), we obtained that 45% of dissolved sulfates originated from sulfide oxidation, and the rest 55% came from atmospheric deposition. In this calculation, no fraction derived from the dissolution of evaporites was obtained, and this is compatible with the fact that no evaporites have been found in the SHR watersheds (Chough et al., 2000). Furthermore, the absence of a positive correlation between Cl and SO4 in the SHR (not shown) also points to additional sources of sulfates in the watershed, such as those that can arise from the dissolution of sulfides (Edmond et al., 1996). Nonetheless, the charge balance between total Ca + Mg and HCO3 in the SHR (Fig. 7a) makes it likely that carbonic acid-mediated reactions dominated over those related to pyrite oxidation. In contrast, the positive correlation between Cl and SO4 in the NHR waters indicates that they share a common origin in the NHR basin (not shown). 4.1.4. Carbonate weathering In many watersheds of the world, it has been demonstrated that weathering of carbonate minerals is more important than the weathering of aluminosilicate minerals in controlling river water chemistry (e.g., Roy et al., 1999; Karim and Veizer, 2000). Carbonate weathering has also exercised the main control on ion composition in the Han River watershed. Most of the water samples from the SHR were Ca– HCO3 type, except for two samples taken from a tributary draining silicate rocks (Fig. 2a). However, the water samples from the NHR belonged to the Ca–Na– HCO3 type. Calcite saturation index calculated with the PHREEQC model (Parkhurst and Appelo, 1999) showed

that 63% of the total SHR samples were supersaturated with respect to calcite (Table 1). By contrast, most of the NHR waters were undersaturated with respect to calcite, regardless of the season. Therefore, the underlying lithology exerts an important control on the carbonate saturation status of the river. 4.1.5. Silicate weathering Si concentration provides a clear discriminator for distinguishing silicate weathering (Wu et al., 2005). In the NHR, the Si concentration varied from 37.3 to 179 μM (average, 112.1 μM), whereas it ranged from 10.7 to 135 μM (average, 64.1 μM) in the SHR (Table 1). Thus, the average Si concentration in the NHR was about twice that in the SHR, which is consistent with silicate weathering being predominant in the NHR basin (Fig. 2b). The Si/(Na⁎ + K) ratio is also a proxy commonly related to the intensity of silicate weathering (Edmond et al., 1995). Provided that K is not an anthropogenic input and that (Na⁎ + K) is silicate-derived, (Na⁎ +K) accounted for 8– 33% (average, 16%) of TZ+⁎ (TZ+ − Cl) in the NHR samples but only 0–9% (average, 4%) of TZ+⁎ in all except two of the SHR samples. These results are consistent with the Si concentrations and similarly indicate higher silicate weathering in the NHR than in the SHR. 4.2. Model calculation 4.2.1. The model A forward model based on the principles of mass balance modeling initiated by Garrels (1967) and recently summarized by Bricker et al. (2003) can be used to quantify the relative weathering contributions of dissolved ions in river waters from the four possible sources: rain, evaporites, carbonates, and silicates (Galy and France-Lanord, 1999; Krishnaswami et al., 1999; Mortatti and Probst, 2003). Assuming negligible anthropogenic inputs, the forward model is based on mass budget equations for the cations (Na, Ca, Mg, and K) derived from the four possible sources. Given the absence of evaporites from the study area, only rain, carbonate dissolution, and silicate weathering were considered as sources. First, we calculated the input of the four cations derived from rain using the cation/Cl ratios of local rain data given by Yu and Park (2004) and assuming that the low Cl content of sample NP-3, collected from a pristine headwater of the NHR basin, came entirely from rainwater. After accounting for this rain contribution, we assumed that the remaining Na and K ions were derived from the weathering of silicates. The inputs of Ca and Mg from the silicates were then estimated using ratios

J.-S. Ryu et al. / Chemical Geology 247 (2008) 66–80

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The calculations also showed that 15.8± 5.8% of total dissolved cations in the NHR, but only 6.7± 1.5% in the SHR, originated from the atmosphere. No seasonal differences were observed in these contributions. The highest total cation input from rain, 32%, occurred in sample NS-1 from the headwaters of the NHR, whereas the highest carbonate and lowest silicate contributions occurred in sample S-10 from the lower reaches of the SHR. 4.3. Flux calculations

Fig. 7. Plots of (a) HCO3 vs. (Ca + Mg) and (b) (HCO3 + SO4) vs. (Ca + Mg) for the Han River waters. The NHR waters balance well with HCO3, but the SHR waters require SO4 to attain a similar balance.

obtained from waters of monolithologic streams. In sample N-5, (Ca/Na)sil =0.55 and (Mg/Na)sil =0.21. These values are similar to the silicate bedrock of the study area (unpublished data). The [(Ca +Mg)/Na]sil value of 0.71 compares reasonably well with the silicate end-member composition of 0.59 ±0.19 from other world rivers (Gaillardet et al., 1999) and 0.879 for waters of rivers that drain exclusively silicate catchments (Boeglin et al., 1997). The final estimate of input from carbonate weathering was made by subtracting the rain and silicate contributions from the total dissolved Ca and Mg. 4.2.2. Model results Based on the forward model, 55 ± 11% (n = 23) of the dissolved loads in the NHR came from silicate weathering, and 30 ± 14% (n = 23) was from carbonates. In the SHR, 82 ± 3% (n = 24) of the dissolved loads arise from carbonate weathering, and only 11 ± 4% (n = 24) was from silicates (Table 2). These results are compatible with the lithologies of the respective drainage basins, i.e., silicates in the NHR and carbonates in the SHR.

The TDS yields and atmospheric CO2 drawdown arising from bedrock weathering were calculated by combining the major ion concentrations with river discharge data. The TDS yields varied both seasonally and spatially. In the NHR, TDS yield was 165 (71.7–354) t/km2/yr in summer and 18.8 (4.37–37.4) t/km2/yr in winter. In the SHR, TDS yield was 913 (403–1849) t/km2/yr in summer and 43.5 (21.2–86.8) t/km2/yr in winter (Table 2). These TDS yields in the Han River basin are much higher than the world average (24 t/km2/yr) reported by Gaillardet et al. (1999). In addition, the suspended particulate material (SPM) was measured for river water samples collected in 2006 (Table 2). The SPM is mainly delivered by the high discharge in summer, and seems to be controlled by flood control dams because dams greatly facilitate sedimentation of the SPM. Several samples collected from dam discharge during summer (NP-3, N-5, N-6, S-7, and S-10) have much higher SPM yields than other samples (Table 2). This is likely related to SPM remobilization due to increased discharge during the rainy season. A good correlation was observed between the TDS and SPM yields in the Han River (Fig. 8), which may indicate the important role of river water discharge and SPM remobilization on chemical weathering. The silicate weathering rate (SWR) can be calculated as the sum of cations arising from silicate weathering, whether obtained by carbonic acid or sulfuric acid (Millot et al., 2003), as follows: SWR ¼ ΦCationsil ¼ ΦðNasil þ Ksil þ Mgsil þ Casil Þ; where Φ indicates yield (t/km2/yr). Our best silicate weathering rate (SWR) estimates for the NHR and SHR basins were 17.2 (5.49–43.3) and 21.9 (4.74–54.1) t/km2/yr in summer and 1.99 (0.27– 3.92) and 1.19 (0.09–2.91) t/km2/yr in winter, respectively. Because the SWR was a little higher in the SHR compared to the NHR in summer, in contrast to what we expected, a possible cause is that high concentrations of

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Like many rivers in which multiple, codependent geologic and climatic factors influence chemical weathering processes (White and Blum, 1999; Oliva et al., 2003; West et al., 2005), chemical weathering in the Han River basin is also likely to be heavily impacted by geologic and climatic factors (e.g., lithology, relief, runoff, temperature) (Fig. 9). The uptake of atmospheric CO2 by silicate weathering depends on whether the dissolved sulfate in rivers arises from gypsum dissolution or pyrite oxidation (Xu and Liu, 2007). The rate of CO2 consumption during silicate weathering can be determined from the equation of Moon et al. (2007), as follows: Fig. 8. Plot of TDS yield versus SPM yield, showing the relationship between the chemical and physical weathering rates, which indicate that the amount of precipitation and mobilization of SPM can have important effects on chemical weathering. Data of world rivers are from Meybeck and Ragu (1997) and Gaillardet et al. (1999).

Ca, Mg, and HCO3 associated with carbonate weathering along with high pH can facilitate SWR in the SHR. River water chemistry affected by carbonate dissolution is generally characterized by high concentrations of dominant dissolved ions such as Ca, Mg, and HCO3. Although the concentrations of these ions were poorly correlated with their measured pH in the SHR (not shown), the greatest SWR was calculated from samples having the highest pH, indicating an influence of pH on the SWR, a result compatible with Oliva et al.'s (2003) report. However, based on the available data in the Han River basin, it remains difficult to evaluate the effect of carbonate weathering on the SWR. On the other hand, the SHR draining karst terrain is likely influenced by deep groundwater input because of the close hydrological connection between river water and groundwater in the SHR. This has also been documented by uranium isotope data in the SHR (Ryu et al., submitted for publication). Thus, water–rock interactions are likely more intensive in the SHR than in the NHR. Furthermore, the much higher SWR in the NHR relative to the lower silicate section of the SHR could be attributable to the presence of six major dams, which greatly prolong the contact time between the solution and minerals in stagnant waters (Oliva et al., 2003). Moreover, dams for flood control could promote physical erosion rates both by high discharge and by delivering much higher SPM. Therefore, in this study, the higher SWR in the NHR and that in the upper carbonate section of the SHR could be due to dams and water–rock interactions in deeper environments, respectively.

ΦCO2 ¼ ΦCationsil −γ  2ΦSO4 ¼ ΦðNasil þ Ksil þ Mgsil þ Casil Þ−γ  2ΦSO4 where Φ represents yield (mol/km2/yr) with an adjustment factor (γ) for sulfuric acid that can vary from 0 to

Fig. 9. Scatter plots of silicate weathering rates versus multiple codependent factors, (a) temperature and (b) runoff, which indicate that the SWR in the Han River basin is closely related to global weathering rate laws.

J.-S. Ryu et al. / Chemical Geology 247 (2008) 66–80

1. Assuming that the sulfate in the NHR was mainly from rainwater and that sulfuric acid played only a minor role, the consumption rate of CO2 is approximately equal to the SWR, which was 0.72 (0.23– 1.81) × 106 mol/km2/yr in summer and 0.08 (0.01– 0.16) × 106 mol/km2/yr in winter. On the other hand, as discussed in Section 4.1.3, silicate weathering by sulfuric acid plays a significant role in the SHR. With γ = 0.15, calculated by cationsil/(cationsil + cationcar), the consumption rate of CO2 in the SHR was estimated to be 0.71 (0.17–1.75) × 106 mol/km2/yr in summer and 0.04 (0.00–0.11) × 106 mol/km2/yr in winter. Consequently, the CO2 consumption rate was somewhat higher in the NHR than in the SHR regardless of season, which is to be expected considering the lithologies of the respective basins as well as the presence of sulfuric acid in the SHR basin. 5. Conclusions A detailed investigation of the fluvial geochemistry of the Han River system, the largest river in South Korea, allows to estimate the rates of chemical weathering and the consumption of CO2. The dissolved loads of the river waters showed significant differences between the two major tributaries, the NHR and SHR, related to the different lithologies of the respective catchments. The NHR drains silicate rocks and exhibited low concentrations of TDS, major ions, and Sr, but a high Si concentration and 87Sr/86Sr ratio. The SHR drains karstic terrains and displayed trends opposite to those of the NHR. A forward model was used to quantify the relative ion fluxes arising from rain and the weathering of carbonates and silicates. The result showed that the dissolved loads in the NHR originated mostly from silicates (34–69%), with a lesser portion from carbonate weathering (4–54%), whereas the dissolved loads in the SHR came mainly from carbonates (78–93%), with a minor portion from silicate weathering (0–18%). The annual average silicate weathering rate (SWR) was 9.59 t/km2/yr in the NHR basin and 11.5 t/km2/yr in the SHR basin. It is likely that dams and deep-environment water–rock interactions have great effects on the SWR in the NHR and the SHR, respectively. However, the associated CO2 consumption rate was slightly higher in the NHR basin than in the SHR basin, as might be expected on the basis of their surface lithologies and the presence of sulfuric acid in the SHR. The good correlations between the SWR and multiple codependent factors (temperature and runoff) suggest that global weathering rate laws could be also applied to the Han River basin.

79

Acknowledgements We would like to thank Y. Park, W.J. Shin, M.S. Yi and D.C. Koh for their help in field works and sample analysis. We also thank the BK 21 program of the Seoul National University. This work was supported by a grant (code 3-2-3) from the Sustainable Water Resources Research Center of 21st Century Frontier Research and a grant from the KBSI (N27052) to K.S. Lee. The authors would also like to thank Bernard Bourdon, Editor-in-Chief of Chemical Geology for his editorial assistance, and Sigurdur R. Gislason and an anonymous reviewer for their valuable comments. References Boeglin, J.-L., Mortatti, J., Tardy, Y., 1997. Chemical and mechanical erosion in the upper Niger basin (Guinea, Mali). Geochemical weathering budget in tropical environment. C. R. Geosci. 325, 185–191. Bricker, O.P., Jones, B.F., Bowser, C.J., 2003. Mass-balance approach to interpreting weathering reactions in watershed systems. In: Drever, J.P. (Ed.), Surface and Ground Water, Weathering, and Soils. Treatise on Geochemistry, vol. 5. Elsevier-Pergamon, Oxford, pp. 119–132. Cheong, C.H., 1969. Stratigraphy and paleontology of the Samchang coalfield, Gangweondo, Korea. J. Geol. Soc. Korea 5, 13–56 (in Korean with English abatract). Chough, S.K., Kwon, S.-T., Ree, J.-H., Choi, D.K., 2000. Tectonic and sedimentary evolution of the Korean peninsula: a review and new review. Earth-Sci. Rev. 52, 175–235. Dalai, T.K., Krishnaswami, S., Sarin, M.M., 2002. Major ion chemistry in the headwaters of the Yamuna river system: chemical weathering, its temperature dependence and CO2 consumption in the Himalaya. Geochim. Cosmochim. Acta 66, 3397–3416. Dosseto, A., Bourdon, B., Gaillardet, J., Allègre, C.J., Filizola, N., 2006. Time scale and conditions of weathering under tropical climate: Study of the Amazon basin with U-series. Geochim. Cosmochim. Acta 70, 71–89. Edmond, J.M., 1992. Himalayan tectonics, weathering processes, and the strontium record in marine limestones. Science 258, 1594–1597. Edmond, J.M., Palmer, M.R., Measures, C.I., Grant, B., Stallard, R.F., 1995. The fluvial geochemistry and denudation rate of the Guayana Shield in Venezuela, Colombia, and Brazil. Geochim. Cosmochim. Acta 59, 3301–3325. Edmond, J.M., Palmer, M.R., Measures, C.I., Brown, E.T., Huh, Y., 1996. Fluvial geochemistry of the eastern slope of the northeastern Andes and its foredeep in the drainage of the Orinoco in Colombia and Venezuela. Geochim. Cosmochim. Acta 60, 2949–2974. Faure, G., 1986. Principles of Isotope Geology, 2nd ed. John Wiley & Sons. 589 pp. Gaillardet, J., Dupré, B., Louvat, P., Allègre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol. 159, 3–30. Galy, A., France-Lanord, C., 1999. Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget. Chem. Geol. 159, 31–60. Garrels, R.M., 1967. Genesis of some ground waters from igneous rocks. In: Abelson, P.H. (Ed.), Researches in Geochemistry. Wiley, pp. 405–420.

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