silicate weathering, co consumption and...
TRANSCRIPT
CHAPTER 3
SILICATE WEATHERING, CO2
CONSUMPTION AND ITS RATE
CONTROL PARAMETERS IN A
HUMID TROPICAL RIVER BASIN,
SOUTHWESTERN INDIA
Chapter 3
35
Abstract: The Silicate Weathering Rate (SWR) and associated Carbon dioxide
Consumption Rate (CCR) in tropical silicate terrain are assessed through a study of the
major ion chemistry in a small west flowing river of Peninsular India, the Nethravati River.
The specific features of the river basin are high mean annual rainfall and temperature, high
runoff and a Precambrian basement composed of granitic-gneiss, charnockite and minor
meta-sediments. The water samples were collected from three locations along the
Nethravati River and from two of its tributaries over a period of twelve months. Chemical
Weathering Rate (CWR) for the watershed is calculated by applying rainwater correction
using river chloride as a tracer. Chemical Weathering Rate in the Nethravati watershed is
estimated to 44 t km-2
y-1
encompassing a SWR of 42 t km-2
y-1
and a maximum carbonate
contribution of 2 t km-2
y-1
. This SWR is among the highest reported for granito-gneissic
terrains. The assessed CCR is 2.9 x 105 mol km
-2 y
-1. The weathering index (Re),
calculated from molecular ratios of dissolved cations and silica in the river, suggests an
intense silicate weathering leading to kaolinite-gibbsite precipitation in the weathering
covers. The intense SWR and CCR could be due to the combination of high runoff and
temperature along with the thickness and nature of the weathering cover. The comparison
of silicate weathering fluxes with other watersheds reveals that under similar morpho-
climatic settings basalt weathering would be 2.5 times higher than the granite-gneissic
rocks.
3.1. Introduction
The interaction of water, atmospheric CO2 and rocks at the surface of the continents results
in the dissolution of soluble salts of the primary rock forming minerals, which are
transported by rivers into the oceans. The dissolution of CO2 in water provides the
necessary protons for the dissolution of primary rock forming minerals, resulting in the
formation of secondary clays and clay minerals (White and Brantley, 1995, and references
therein). Chemical weathering of silicate rocks is of primary importance in the long-term
global climate change because for every mole of silicate rock weathered an equal amount
of CO2 is withdrawn from the atmosphere and sequestered in sediments (Eq. 1 and 2)
(Garrels and Mackenzie, 1971; Berner et al., 1983; Berner, 1991). On the other hand, the
major ion chemistry of most of the world rivers are dominated by weathering of carbonate
rocks where there is no significant atmospheric CO2 drawdown (Gaillardet et al., 1999);
Chapter 3
36
that is, CO2 utilized for the dissolution of rock mineral will be further released to the
atmosphere during precipitation of carbonates in the ocean (Eq. 3 and 4).
CaSiO3+ 2CO2+ H2O → Ca2+
+ 2HCO3- + SiO2
(1)
Ca2+
+ 2HCO3 → CaCO3↓ + H2O + CO2↑ (2)
CaCO3 + CO2 + H2O ↔ Ca2+
+ 2HCO3- (3)
Ca2+
+ 2HCO3 → CaCO3↓ + H2O + CO2↑ (4)
Understanding the forcing factors and retroaction loops on the long-term Chemical
Weathering Rates (CWR) and associated Carbon dioxide Consumption Rate (CCR) is a
major challenge. CWR is regulated by multiple co-dependent factors such as lithology,
relief (i.e. tectonics), climate (runoff and temperature) and vegetation (see e.g. Garrels and
Mackenzie, 1967; Bluth and Kump, 1994; Gaillardet et al., 1995; White and Blum, 1995,
Drever and Zobroist, 1992, Stallard, 1995, Viers et al., 1997).
Worldwide, the studies are based on the examination of small to medium watersheds (1 to
100 km2) with, as much as possible, homogeneous lithology (e.g. granito-gneiss or basalt)
or on large river basins including variable lithologies. In the Indian subcontinent, studies of
the chemical weathering mostly focus on the large Himalayan River basins (e.g. Sarin et
al., 1989, Pande et al., 1994, Galy and France Lanord, 1999, Singh et al., 2005) and on
smaller rivers draining the Deccan basalts (Dessert et al., 2001, Das et al., 2005). The
rivers and streams draining the Precambrian silicate basement of Peninsular India have
received much less attention, particularly the rivers originating from the Western Ghats
and flowing towards the Arabian Sea. These rivers are characterized by both high runoff
and temperature. Their combined discharge is about 200 km3, which corresponds to an
average surface runoff of 1775 mm, and accounts for 12.5% of the entire Indian rivers
water discharge (www.nih.ernet.in; Krishnaswami and Singh, 2005). The studies on
watershed having high runoff and warmer temperature are scarce in the present day
worldwide database (West et al., 2005 and Oliva et al., 2003). Therefore, the study of the
west flowing rivers of India would give a new insight to the chemical weathering models.
This study reports new data on the chemical weathering rates and associated carbon
dioxide consumption from a west-flowing river draining granito-gneissic bedrocks, the
Chapter 3
37
river Nethravati. The present study focuses on the (1) present day chemical weathering rate
and carbon dioxide consumption in the Nethravati River, (2) the possible controls on the
silicate weathering rate, by comparing the results with west-flowing rivers draining basalts
and with other small streams draining silicates.
3.2. Results
The chemical compositions of Nethravati main stream and its tributaries are presented in
Table 3.1. The pH varies from 5.2 to 7.4, with lower values during the monsoon and higher
during the dry season (base flow). These values are comparable to the values reported for
rivers draining granitic terrains (e.g. Oliva et al., 2003). The electrical conductivity (EC)
varies from 29 to 87µS cm-1
while the river total dissolved salts (TDS) varies from 29 to 66
mg L-1
(average 43 mg L-1
); lower values are recorded during the monsoon and higher
values during base flow. The TDS of Nethravati River is relatively low compared to the
global river average (283 mg L-1
; Gaillardet et al., 1999), but comparable to other rivers
draining orogenic zones such as the Amazon river (44 mg L-1
), Orinoco (82 mg L-1
),
Brahmaputra (71 mg L-1
) (Galy and France-Lanord, 1999), Congo and Niger (35 mg L-1
and 59 mg L-1
; Gaillardet et al., 1999). At comparable morpho-climatic settings, the
Nethravati River exhibits slightly lesser TDS than the west flowing rivers draining Deccan
traps of India (75 - 91 mg L-1
; Das et al., 2005).
Sodium is the dominant dissolved cation, with concentrations ranging from 86 to 260 µmol
L-1
. It is followed by Ca (38 to 157 µmol L-1
), Mg (26 to 121 µmol L-1
) and K (10 to 40
µmol L-1
). The lowest concentrations are observed in the Shishilahole tributary, located in
the footsteps of the Western Ghats. Overall, major cation concentrations in the Nethravati
watershed decrease during monsoon while spatially, it increases further downstream. The
Kumaradhara tributary, which partly drains the hypersthene-rich charnockites, exhibits
slightly higher Ca/Na and Mg/Na molar ratios than the Nethravati mid region of the
watershed (Uppinangadi station, Fig. 2.2).
The bicarbonate concentrations range from 170 to 540 µmol L-1
and account for an average
of 70% of the total anion budget. Chloride concentrations range from 57 to 137 µmol L-1
.
Like cations, the lowest bicarbonate and chloride values were observed during the
monsoon season. In all sampling stations the SO4 concentrations are ranging from 8 to 21
µmol L-1
which are about 10 times lesser than the bicarbonates. The NO3 concentrations
range from 2 to 18 µmol L-1
with maximum values observed during the onset of monsoon
Chapter 3
38
when fertilizers are added to freshly sown agricultural crops. Chloride and SO4
concentrations are not correlated to NO3 variations. Monthly discharges and concentrations
of dissolved species of the Nethravati River at BC Road, Bantwala (Table 3.1) were used
to calculate the weighted mean concentrations of major ions and to deduce the annual flux
of major ions discharged by this river into the Arabian Sea.
The chemical composition of local rainfall is presented in the Table 3.2. The weighted
average chloride concentration is 47 µmol L-1
, Na 45 µmol L-1
, Ca 20 µmol L-1
, K 5 µmol
L-1
, Mg 7 µmol L-1
and SO4 9 µmol L-1
. The molar Na/Cl ratio (~0.95) in the rain water is
corresponding with the marine ratio suggesting the rain water composition is dominated by
sea salts. However, enrichment of Ca and Mg with respect to sea water composition is
noticed. According to Hegde (2007) who studied the rainfall and aerosol composition in
and around Mangalore city, these enrichments would originate from continental dusts
and/or anthropogenic sources.
3.3. Discussion
3.3.1 Sources of major ions
For the estimation of SWR and CCR in the Nethravati river basin, the quantum of
contribution of major ions from evaporites, anthropogenic sources and atmospheric / sea
salts have to be considered. Since evaporites are absent amongst the bedrocks of the
Nethravati watershed, the contribution from them is ruled out. Anthropogenic chloride in
the river water could result in an increase in Cl content and a decrease in the Na/Cl ratio.
This phenomenon is not observed in the Nethravati watershed. Though the weighted
average chloride concentrations in the river are 1.5 times higher than the rainfall, there is
no significant variation in the Na/Cl molar ratio (Fig. 3.1), with the highest values
corresponding to the base flow. This could be explained by simple evapotranspiration
process in the watershed, and negligible influence of anthropogenic chloride. In order to
quantify the chemical fluxes coming only from bedrock weathering, contributions from
atmosphere have to be corrected. This is achieved using chloride as a proxy and relying on
the understanding that Cl in the Nethravati River originates only from seawater through
atmosphere (rainfall). The sea salt corrected concentration of an element (Xr*) is calculated
following the relation (Stallard and Edmond, 1981; Negrel et al., 1993; Millot et al., 2002
Eq.5):
Chapter 3
39
Table 3.1: Stream major ion composition at five stations of the Nethravati River and tributaries.
Date of Discharge* T pH DO TDS EC Na K Ca Mg F Cl NO3- SO4
2- HCO3
- SiO2 TZ
+ TZ
- NICB
sampling m3s
-1
0C mg L
-1 µS cm
-1 µmol L
-1 µeq L
-1 %
Nethravati @ Bantwala, BC Road (Lat: 12°52'47.63" Long: 75°02'27.53")
October 356 30 6.8 7.2 34 54 151 15 65 56 1 85 4 10 260 204 407 370 10
November 133 29 6.7 5.7 36 52 136 17 68 55 1 88 6 11 290 206 399 407 -2
December 35 25 6.9 7.0 39 51 125 15 63 53 1 91 2 11 330 210 371 446 -18
January 9 28 6.7 6.5 - 54 - - - - - - - - - - - - -
February 3 29 7.0 7.2 47 64 141 18 105 64 2 104 - 12 420 190 497 550 -10
March 2 32 7.1 6.7 53 74 197 26 123 92 2 127 - 15 480 173 653 640 2
April 1 33 7.1 5.9 60 82 260 37 124 121 2 136 - 15 540 191 788 708 11
May 11 32 6.6 6.3 52 80 190 38 94 91 2 128 - 21 450 178 599 622 -4
June 386 26 6.0 7.5 29 32 104 19 46 40 1 87 14 21 180 140 294 323 -9
July 1129 27 6.8 7.5 29 35 107 13 49 41 1 84 9 11 220 186 302 335 -11
August 1139 27 7.0 5.5 27 35 96 13 50 38 2 72 8 10 210 170 286 310 -8
September 510 27 6.6 7.1 28 36 103 12 46 41 1 74 6 9 220 182 289 318 -10
Weighted mean 108 14 51 42 1 79 8 11 221 178 309 331
Kumaradhara @ Uppinangadi ( Lat: 12°50' 10.16" Long: 75°14'34.33")
October 235 27 6.8 7.3 31 34 102 16 54 45 1 73 8 11 240 168 317 344 -8
November 66 29 6.7 6.0 34 41 124 17 62 56 1 83 6 10 270 191 377 380 -1
December 13 25 7.1 6.8 37 47 118 13 77 55 3 84 3 10 310 207 396 420 -6
January 0 27 6.7 6.1 38 50 - - 80 - 1 87 2 9 350 205 160 459 -
February 0 29 7.0 6.6 47 61 148 19 93 76 2 98 - 10 440 208 505 560 -10
March 1 32 7.1 6.4 54 73 192 27 107 98 2 109 - 12 500 207 628 636 -1
April 0 33 7.4 5.6 58 84 185 34 152 103 4 113 - 12 530 225 728 669 8
May 9 33 7.4 6.2 47 75 176 35 79 95 2 101 - 18 410 202 559 549 2
June 189 25 5.7 7.6 27 31 87 17 43 38 1 74 18 12 170 135 266 287 -7
July 488 26 6.7 7.8 27 33 96 12 44 41 1 79 9 10 200 177 277 308 -11
August 677 27 7.0 5.6 26 36 90 12 45 40 1 79 9 9 200 162 273 307 -12
September 187 27 7.1 8.5 26 34 86 11 44 39 1 70 9 8 200 165 264 296 -12
Weighted mean 94 13 47 42 1 77 10 10 206 166 284 314
Nethravati @Uppinangadi (Lat: 12°50'28.69"Long: 75°14' 34.33")
October 121 29 6.7 7.2 34 41 165 16 67 52 1 87 4 9 240 210 419 352 18
November 68 28 6.8 6.3 35 41 126 21 80 45 1 83 7 12 260 198 398 375 6
December 22 25 6.9 7.0 39 54 137 15 72 49 1 95 3 12 330 234 393 452 -14
January 9 27 6.8 6.7 40 63 - - 98 26 1 98 7 13 350 216 247 482
February 3 29 6.9 6.5 44 58 157 19 93 59 2 108 4 14 380 202 481 521 -8
Chapter 3
40
(*) Monthly discharge from Central water Commission; T °C= water temperature measured in the field
TDS- Total dissolved solids (Na+K+Ca+Mg+Cl+SO4+NO3+HCO3 +SiO2)
TZ+ = total dissolved cations; TZ
- = total dissolved anions NICB= TZ
+ -TZ
-/mean x 100 %
Date of Discharge* T pH DO TDS EC Na K Ca Mg F Cl NO3- SO4
2- HCO3
- SiO2 TZ
+ TZ
- NICB
sampling m3s
-1
0C mg L
-1 µS cm
-1 µmol L
-1 µeq L
-1 %
March 1 32 7.1 6.4 44 67 198 26 91 77 2 123 - 15 440 196 560 596 -6
April 1 33 7.3 6.7 58 75 231 40 146 81 2 137 1 20 500 212 726 684 6
May 2 33 6.7 5.7 50 87 204 35 93 74 2 126 - 19 430 192 572 596 -4
June 197 25 6.3 8.1 28 31 97 16 40 34 2 73 8 11 180 140 262 285 -8
July 640 26 6.6 7.5 30 35 122 13 51 40 2 81 7 9 220 196 318 328 -3
August 462 27 7.0 5.6 26 46 93 12 48 33 1 69 7 10 200 169 266 296 -11
September 323 27 6.6 7.6 28 36 106 12 49 37 1 76 8 9 220 186 290 322 -11
Weighted mean 112 14 52 38 1 77 7 10 216 183 305 321
Nethravati @ Dharmasthala ( Lat: 12°57' 51.07" Long: 75°21'53.55")
October 30 6.9 7.3 59 45 156 13 76 48 1 90 2 8 350 283 418 460 -9
November 29 6.6 7.4 45 54 159 14 93 51 1 91 2 9 350 260 461 462 0
December 26 7.0 7.7 46 59 137 12 85 47 1 94 1 10 400 263 413 516 -22
January 28 6.7 5.7 43 59 - 10 99 29 2 106 1 13 390 235 265 525 -
February 31 6.9 6.3 49 64 178 19 112 58 2 115 1 12 430 235 537 571 -6
March 34 7.1 6.2 55 74 251 35 157 85 2 116 3 12 470 223 769 614 22
April 34 6.4 5.2 52 72 226 30 108 68 1 118 - 13 460 229 607 606 0
May 25 5.8 7.6 26 32 89 16 54 33 3 74 10 12 180 135 280 291 -4
June 25 6.2 8.1 30 37 117 12 47 41 1 76 7 8 240 203 306 341 -11
July 25 6.5 7.6 29 37 105 12 52 37 1 76 10 9 220 178 295 324 -9
August 27 6.7 8.3 33 44 123 11 61 42 1 74 - 9 260 219 339 353 -4
Shishilahole @ Parpikal (Lat: 12°52'59.93" Long: 75°25'03.44")
December 24 7.0 7.8 35 39 - - 56 - 1 111 - 10 280 222 113 412 -
January 26 6.6 7.4 29 37 95 12 58 34 1 81 3 11 240 169 291 347 -18
February 30 6.8 6.9 29 37 103 12 58 36 1 83 - 12 230 151 305 338 -10
March 31 7.2 7.3 30 38 125 20 77 42 0 96 2 15 220 163 381 348 9
April 31 6.6 7.5 34 41 144 18 57 45 1 89 - 12 270 197 365 384 -5
May 25 5.2 9.0 24 29 89 12 38 31 2 63 6 10 190 158 238 281 -17
June 25 6.4 8.8 27 29 98 10 44 34 1 67 4 8 210 195 264 298 -12
July 27 6.8 8.9 25 31 91 10 45 33 1 57 4 9 200 180 256 280 -9
August 28 5.9 9.4 26 30 101 11 45 37 1 60 - 12 210 187 276 295 -7
Chapter 3
41
Table 3.2: Chemical composition of the rainwater at Mangalore.
Location Sampling Rain fall F Cl SO42-
NO3- Na K Ca Mg
date (mm) µmol L-1
Manipal Jul 08, 09 40* <dl 103 26 1 85 17 66 16
Manipal Jul 02, 09 23* 0.4 50 7 2 44 3 15 7
Parpikal, Mangalore Jul 29, 09 24* 0.5 26 6 4 39 3 12 6
Mangalore Jul 19,09 61* 0.7 48 11 0 44 2 22 9
Mangalore Jun 25, 09 35* 0.5 72 9 0 64 4 29 13
Mangalore Jul 3, 10 34 <dl 56 8 4 57 7 19 7
Mangalore Jul 18,10 83 <dl 21 4 2 22 3 6 3
Mangalore Aug 11, 10 21 <dl 55 6 3 59 7 8 6
Mangalore Sep 09,10 41 <dl 22 5 1 24 4 8 3
Mangalore Sep 21, 10 2.1 <dl 22 5 3 22 5 6 3
Weighted mean 47 9 2 45 5 20 7
(*) TRMM data (http://disc.sci.gsfc.nasa.gov/precipitation/)
Xr*= (Xriver-Clriver) (X/Cl)rainfall (5)
where, Xriver and Clriver are the molar concentrations in the river water and (X/Cl)rainfall is
the X/Cl molar ratio in the local rainfall. Based on Eq. 5, the proportion of atmospheric
inputs accounts for 70% for Na, 65% for Ca and 65% for K and 20% for Mg. The
correction applied to SO4 led to slightly negative values, which indicates that the entire
dissolved sulfate observed in the river could be of atmospheric origin. There is no
significant difference between local rainfall and seawater corrections on Na fluxes.
However, the impact of the local rainfall correction on the SO4, Ca, K and Mg fluxes is
much higher than the seawater; this is because the local rainfall is enriched with these
elements compared to seawater. For instance, the use of seawater composition as a
reference for atmospheric inputs would lead to a correction of only 1% for Ca and 7% for
Mg. This means that the choice of the reference for atmospheric inputs correction impacts,
at least in this watershed, the weathering fluxes of SO4, Ca, Mg and K. The weighted
concentrations (and related specific fluxes within parentheses) attributed to weathering at
the outlet (B C Road, Bantwala) of the watershed are 32 µmol L-1
for Na* (1.1x105 mol
km-2
y-1
), 17 µmol L-1
for Ca* (0.5 x 105
mol km-2
y-1
), 31 µmol L-1
for Mg* (1.0 x 105 mol
km-2
y-1
), and 5 µmol L-1
for K* (0.2 x 105 mol km
-2 y
-1).
In addition to atmospheric inputs, part of Ca and Mg fluxes could be derived from
dissolution of disseminated calcite from the bedrocks that may lead to an overestimation of
silicate weathering rate and associated carbon dioxide consumption in the watershed
(White et al., 1999, 2005; Jacobson et al., 2000). The occurrence of calcite and Mg-rich
Chapter 3
42
calcite crystals was reported in mafic and ultramafic rocks of the Dharwar craton (Braun et
al., 2009). The dissolution of Ca-Mg calcite is much faster than the Ca-Mg-Na silicates and
should force enrichment of Ca and Mg relative to Na+ in the stream (higher Ca*/Na* and
Mg*/Na*) compared to the bedrock. This contribution is usually estimated by comparing
the Ca*/Na* and Mg*/Na* molar ratios of the stream with those of the local bedrock and
the relative “excess” of Ca and Mg is then attributed to carbonate dissolution
(Krishnaswami et al., 1999). The part of Ca and Mg associated to Ca-Mg silicates may be
estimated according to the following equations (6) and (7):
Casil = Na*. (Ca/Na)rock (6)
Mgsil = Na*. (Mg/Na)rock (7)
where Na* (Nasil) is the weighted mean sodium concentration of the river water corrected
from atmospheric inputs, subscripts “sil” refers to silicate origin and “rock” refers to the
bedrock composition (see Table 3.3). As the watershed encompasses a wide range of
lithologies, the calculations were performed using two different parent rock compositions
(averages for both gneiss and charnockite): the concentrations of Ca and Mg that is derived
from silicate weathering range from 11 µmol L-1
(gneiss) to 15 µmol L-1
(charnockite) for
Ca and from 8 µmol L-1
(charnockite) to 13 µmol L-1
(gneiss) for Mg. These values are
compared with the rainfall corrected values of the respective elements; the difference is
small for Ca with 2-6 µmol L-1
but significant for Mg, with 18-24 µmol L-1
. These values
indicate that (1) Ca-carbonate dissolution in the watershed would be minor or Ca would be
leached out at the same rate as Na during silicate weathering and (2) more than 2/3 of Mg
would originate from Mg carbonate dissolution. The latter interpretation is in contradiction
with the absence of Mg-carbonates in the bedrock (Braun et al., 2009). Moreover, the plot
of Ca*/Na* and Mg*/Na* molar ratios of the river water and bedrocks (Fig. 3.2) shows
that all the river water samples fall close to the silicate end-member defined by Gaillardet
et al., (1999) with a slight orientation towards the Mg-silicate mineral composition. The
composition of the tributary draining partly the charnockites and amphibolite
(Kumaradhara at Uppinangadi; Fig 2.6) tends towards the Mg-silicate mineral composition
whereas the upstream samples and Nethravati at Uppinangadi (Fig 2.6) samples fall close
to the Biotite-Gneiss composition. This questions the pertinence of a carbonate correction
for Mg, especially when the watershed bedrock contains weatherable ferromagnesian
silicate minerals such as biotite, hypersthene or amphiboles. Therefore, it is relevant to
Chapter 3
43
consider the atmospheric inputs corrected concentration of Mg as an upper limit of Mg-
silicates weathering in the Nethravati watershed.
Figure 3.1: Scatter plot of Na/Cl vs Cl (not corrected for sea salts) in the sampling stations.
The horizontal line stands for the “marine Na/Cl” molar ratio.
Figure 3.2: Mixing diagrams of atmospheric input corrected Mg/Na vs Ca/Na molar ratios
of Nethravati River. The silicate and carbonate end-member domains are taken from
Gaillardet et al., (1999). The bedrock compositions are taken from Sharma and Rajamani
(2000), Soman (2002), Braun et al., (2009). Stations marked in blue and green shows
detailed data of Nethravati waters, and stations marked in red shows the discharge
weighted average for each sampling station.
Chapter 3
44
Table 3.3: Major element abundance in the main rock types of the Nethravati watershed.
Sample SiO2 CaO MgO K2O Na2O Ca/Na Mg/Na n Reference
wt% (molar)
Charnockite, Tamil Nadu 69.8 3.8 0.8 1.7 4.0 0.52 0.15 1 Sharma and Rajmani, 2000
Charnockite (mafic granulite) 72.6 4.3 1.0 0.7 4.3 0.55 0.18 1 Rajamani et al., 2009
Charnockite, Kerala 68.8 2.5 1.0 5.0 3.2 0.44 0.24 6 Soman, 2002
Charnockite, Kerala 65.1 2.1 1.2 5.4 2.5 0.47 0.37 1 Soman, 2002
Mean* Charnockites 68.9 2.8 1.0 4.2 3.3 0.47 0.24
SD Charnockites 3.11 1.05 0.16 2.37 0.84 0.05 0.10
Gneiss, Halagur Karnataka 59.5 4.7 2.1 3.4 3.7 0.70 0.44 1 Sharma and Rajamani, 2000
Biotite Gneiss Satnur Karnataka 51.2 4.3 3.2 3.5 3.7 0.64 0.66 1 Sharma and Rajamani, 2000
Gneiss, Mulehole (avg.) 68.2 2.0 2.6 1.7 4.5 0.25 0.45 25 Braun et al., 2009
Gneiss, Kerala 67.6 1.3 1.6 3.9 2.6 0.28 0.47 13 Soman, 2002
Gneiss, Kerala 68.7 2.4 1.0 4.5 2.4 0.54 0.32 11 Soman, 2002
Gneiss, Kerala 67.8 3.6 1.6 1.1 4.4 0.45 0.28 7 Soman, 2002
Mean* Gneiss 67.7 2.20 2.0 2.7 3.6 0.35 0.41
SD Charnockites 7.09 1.35 0.79 1.33 0.87 0.19 0.13
Granites, Satnur Karnataka 68.7 1.1 0.2 7.3 3.4 0.18 0.03 1 Sharma and Rajamani, 2000
Amphibolites, Mulehole (avg.) 46.3 2.0 3.2 0.1 0.8 1.38 3.08 35 Braun et al., 2009
(*) The mean takes into account of the number of samples analysed (n).
Chapter 3
45
Table 3.4: Dissolved chemical fluxes, silicate weathering rate and CO2 consumption in the Nethravati River, its tributaries and in the other
west-flowing rivers of Peninsular India. Silicate weathering rates of other west-flowing rivers of Peninsular India are recalculated from data in
Das et al., (2005), Gupta et al., (2011) and Thrivikramaji and Joseph (2001).
Location T Runoff SWR CCR Area Rainfall Reference Correction applied
0C mm t km
-2 y
-1 10
5 mol km
-2 y
-1 km
2 mm y
-1
Nethravati river 29 3300* 42 2.8-2.9 3657 3600 This study Local rain water and
Ca/Na (0.47) &
Mg/Na (0.41) molar
ratio
Kumaradhara river 28 2594* 30 1.6-1.7 1825 - This study
Nethravati @Uppinangadi 28 2734* 38 2.8-2.9 1750 - This study
Gurupur river 28 3425* 38 5.2 824 - This study
Muvattupuzha River 30 2200* 27 5.5 2004 3385 Thrivikramaji &
Joseph, 2001
Gad river 25 1690# 40 5.7 981 2600
Das et al., 2005
Local rain water and
Ca/Na (1.30) &
Mg/Na (1.0) molar
ratio
Kajli river 26 1657# 48 5.8 762 2550
Shashtri river 25 2117# 58 6.3 2174 3260
Vashishti river 26 2198# 63 7.1 2238 3391
Krishna river 24 477 19 4.2 36268 -
Bhima river 25 215 12 3.3 33916 -
(*) Discharge data from Central Water Commission, Government of India; (#) Discharge data from www.nih.ernet.in
Chapter 3
46
Silicate weathering rate in the Nethravati River
The silicate weathering rate (SWR) (t km-2
y-1
) in the Nethravati watershed is calculated
based on the following relationship (Eq. 8):
SWR = Q/A.(∑(Na+ + K
+ + Mg
2+ + Ca
2+)sil + SiO2) (8)
where Q is the discharge in m3
s-1
, A is the surface area of the watershed in km2 and
subscript ‘sil’ refers to silicate-derived concentrations (Krishnaswami and Singh, 2005).
The SWR in the Nethravati watershed is estimated using the atmospheric input and
carbonate corrected concentrations of riverine cations in mg L-1
. Silicate derived cations
are calculated based on both atmospheric and carbonate rock corrections. The atmosphere
corrected values are considered as upper limit of SWR, while the latter is considered as the
lower limit of SWR, based on the discussions made in section 5.1. In these conditions, the
silicate weathering rate at the outlet of the Nethravati watershed would range from 42
(lower limit) to 44 t km-2
y-1
(upper limit) (Table 3.4), which means that the maximum
carbonate contribution would be 2 t km-2
y-1
.
The silicate weathering fluxes per unit area are comparable to the extensively studied
watershed Rio Icacos (40 t km-2
y-1
; West et al., 2005), which reported one of the highest
silicate weathering fluxes, while it was higher than another extensively studied watershed
Nsimi, Cameroon (7 t km-2
y-1
; Braun et al., 2005). Further, the silicate weathering flux in
Nethravati watershed is higher than the orogenic belt rivers such as Yamuna (28 t km-2
y-1
;
Dalai et al., 2002), Bhagirathi and Alaknanda (14 t km-2
y-1
; Krishnaswami et al., 1999)
and the watershed draining shields such as Amazon (13 t km-2
y-1
), Mackenzie (1.8 t km-2
y-
1), Parana (5 t km
-2 y
-1), Mekong (14.3 t km
-2 y
-1), Congo-Zaire (4.2 t km
-2 y
-1) and Orinoco
(9.5 t km-2
y-1
) (Gaillardet et al., 1999). When compared with the watershed having similar
morpho-climatic settings draining basaltic rocks of peninsular India (53 t km-2
y-1
; Das et
al., 2005), the Nethravati has slightly lesser silicate weathering fluxes.
Degree of silicate-rock weathering in the Nethravati watershed
The degree of silicate-rock weathering occurring in the Nethravati River basin is
determined using Re index proposed by Tardy (1971) and modified by Boeglin and Probst
(1998). This index is based on the silicate-derived dissolved cations and silica
concentrations in the river. The coefficients used in the Eq. 9 correspond to average granite
Chapter 3
47
containing feldspar, mica and Mg-silicate minerals such as amphiboles (Boeglin and
Probst, 1998). Such composition is similar to the mean bedrock composition of the
Nethravati watershed, which makes the above equation suitable for estimating the intensity
of rock weathering within the watershed. Re is expressed as:
(9)
where Na, K, Mg, Ca and SiO2 are the concentrations of each chemical species attributed
to silicate weathering. It is assumed to be equivalent to the molar ratio SiO2/Al2O3
remaining in the weathering profile: if Re = 0, the dominant weathering process is the
genesis of gibbsite (called ‘allitization’), if Re = 2, kaolinite is essentially formed
(‘monosiallitization’), if Re = 4, the weathering products are mainly smectites
(‘bisiallitization’). The well drained terrain results in the formation of gibbsite. It
indicates that the silicate weathering is intense and proceeding to gibbsite phase. Whereas,
as Re of ≥2 indicates relatively lesser intensity of weathering leading to the formation of
kaolinite-smectites in the weathering profile.
The Re calculated for the Nethravati River at the outlet ranges from -0.6 (gibbsite
formation) during the peak flow season to 2.4 (kaolinite formation) during the dry season.
Using the weighted mean concentrations, the annual average Re value is 0.14, suggesting
the formation of gibbsite in the watershed regolith which is usual for humid tropical
climate. The calculated Re values, using data from Gaillardet et al., (1999) for Amazon
(2.1), Congo-Zaire (2.1), Orinoco (1.6), Parana (1.4), Ganges (2.2), Brahmaputra (2),
Mekong (1.7) suggests kaolinite formation in their weathering cover. In contrast to Boeglin
and Probst (1998), who observed a slight increase of Re along the Niger main stream, the
Re values are quite uniform in the Nethravati watershed, suggesting no difference in the
weathering regime between the upper and lower parts of the watershed.
Carbon dioxide consumption during rock weathering
Since the dissolved SO4 in the Nethravati River originates from atmospheric inputs and not
from sulfide oxidation, CO2 may be considered as the only weathering agent in the
watershed. Then, the carbon dioxide consumption rate (CCR, expressed in mol km-2
y-1
)
corresponds to the flux of cations originating from silicate weathering:
Chapter 3
48
CCR = Q /A.∑(Na++K
++Mg
2++Ca
2+) (10)
where, Q is the discharge in m3
s-1
and A the surface area of the watershed in km2.
The CCR in Nethravati watershed is 3 x 105 mol km
-2 y
-1 at station BC road (Bantwala;
Table 3.4). This value is higher than other tropical rivers draining shields such as Amazon
(0.5 x 105
mol km-2
y-1
), Parana (0.9 x 105
mol km-2
y-1
), Orinoco (0.6 x 105
mol km-2
y-1
),
Congo-Zaire (0.5 x 105
mol km-2
y-1
), (Gaillardet et al., 1999), but lesser than rivers
draining mountainous ranges such as Yamuna (5 x 105
mol km-2
y-1
; Dalai et al., 2002) or
Bhagirathi-Alaknanda (4 x 105
mol km-2
y-1
; Krishnaswami et al., 1999), though their SWR
is slightly lower than that of Nethravati River. This apparent contradiction may be
explained by the difference in the degree of silicate-rock weathering in the Himalayan and
Nethravati watersheds. The limited degree of silicate rock weathering in Himalayas (Re ~
3, i.e. between monosiallitization and bisiallitization, for Yamuna Head waters, data from
Dalai et al., 2002) is explained by a high dissolved cation and relatively less silica fluxes
whereas the high degree of weathering in the Nethravati is explained by a relatively lesser
cation flux and high silica fluxes.
3.3.2. Factors controlling the intensity of silicate weathering in the Nethravati
watershed
The Nethravati watershed is characterized by several important meteorological, geological
and geomorphological features such as high runoff (3300 mm y-1
), high mean temperature
(30°C), varying lithologies and steep slopes. The importance of these factors in controlling
the SWR in the Nethravati watershed is investigated. The combination of high runoff and
high temperature is a common feature of basins exhibiting intense silicate weathering (Fig.
3.3), like for instance Rio Icacos (40 t km-2
y-1
, West et al., 2005) and other west flowing
rivers draining the Deccan basalts (53 t km-2
y-1
, Das et al., 2005). In contrast, the
watersheds having high temperature and low runoff (Nsimi, Cameroon; Braun et al., 2005)
or low temperature and high runoff (British Columbia; reviewed in West et al., 2005),
exhibit limited silicate weathering rate, i.e. below 15 t km-2
y-1
. This can be explained in
terms of higher kinetic reaction at high temperature combined with fast renewal of regolith
solutions that prevents oversaturation of soil solutions.
Chapter 3
49
0
10
20
30
40
50
60
70
-20
-10
0
10
20
3040
01000
20003000
4000
SW
R (
t.km
-2y
-1)
Tem
per
ature
0C
Runoff (mm)
Nethravati & Muvattupuzha rivers
Deccan basalt rivers
Small silicate watersheds
Major world rivers
Figure 3.3: Comparison of SWR with temperature and runoff for the Nethravati River,
Deccan basalt rivers (Das et al., 2005; Gupta et al., 2011, Table 3.4), Small worldwide
silicate watersheds (West et al., 2005), Muvattupuzha river (Thrivikramaji and Joseph
2001, Table 3.4) and major world rivers (Gaillardet et al., 1999; Dalai et al., 2002;
Krishnaswami et al., 1999). This plot confirms that the highest silicate fluxes result from
the combination of high runoff and warm temperature as observed by West et al., (2005)
and White and Blum (1995).
The possible influence of two additional factors, namely the basin slope and nature of
bedrock on the weathering of silicate rocks, is investigated. This can be better understood
by comparing river basins that exhibit similar morpho-climatic conditions like temperature,
elevation and soil thickness and more or less similar runoff which is the case for west
flowing rivers of Peninsular India. This is achieved by plotting the sum of cations and
silica concentrations attributed to silicate weathering against the basin slope (Fig 3.4). For
a given lithology, the sum of cations and silica concentrations remain stable irrespective of
the basin slope. This means that, for this range of concentrations, the slope would not exert
a significant control on the silicate weathering fluxes. In the same figure (Fig. 3.4), the
sum of cations and silica concentrations define two clear domains related to nature of the
watershed lithology: the west flowing rivers draining basalts exhibit concentrations ~2.5
times higher than those draining gneissic rocks. On the other hand in the low temperature
Chapter 3
50
watersheds (≤10°C) the difference in concentrations of ions drained from basalts and
gneissic rocks is much larger (by 3.9 times) as calculated from the compilations of West et
al., (2005) for granite-gneissic lithologies and Dessert et al., (2009) and Louvat, (1997) for
basaltic lithology. A possible implication is that the difference of “weatherability” between
granitic-gneisses and basaltic rocks seems to be narrowed at high runoff and temperature,
as against the pattern at low temperature watersheds. Such conclusion deserves further
studies on silicate weathering in varying temperature and runoff environments.
Slope (m.m-1
)
0.00 0.01 0.02 0.03 0.04 0.05
Ca
t sil+
SiO
2 (
g.L
-1)
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050West flowing basaltic streams
Nethravati basin
Muvattupuzha River
Figure 3.4: Scatter plot of basin slope versus silicate cation and silica concentrations in the
West Coast Rivers. Data source: West flowing basaltic streams (Das et al., 2005; Gupta et
al., 2011); Muvattupuzha River (Thrivikramaji and Joseph 2001).
Chapter 3
51
3.4. Summary
A systematic monitoring of the geochemistry of a small, tropical west flowing river of
Peninsular India (Nethravati river), and its tributaries flowing across a hot and humid
environment was carried out for twelve months to determine the rate of silicate weathering
and its associated carbon dioxide drawdown from the atmosphere. The chemical
weathering rate in the Nethravati River is estimated to 44 t km-2
y-1
and the silicate
weathering rate of the watershed estimated to 42 t km-2
y-1
, accounts for one of the highest
ever reported. Due to the absence of dissolved sulphate originating from sulphide
oxidation, the carbonic acid may be considered as the principal weathering agent in the
Nethravati watershed. The carbon dioxide consumption rate associated with silicate
weathering is 2.9 x 105 mol km
-2 y
-1. The degree of weathering intensity within the
watershed, calculated from the molecular ratio of dissolved cations and silica
concentrations in the river (Re), ranges seasonally from -0.6 to 2.4, with an annual mean
value of 0.14. Such value suggests that allitisation, i.e. gibbsite formation, is currently
taking place in the weathering profiles of the watershed as a result of an intense chemical
weathering. The intensity of silicate weathering in the Nethravati watershed is primarily
controlled, like the few other basins in the world that exhibit extreme values, by the
combination of intense runoff (3300 mm y-1
) and warm temperature. Though the basin
slope does not seem to influence the weathering intensity of West flowing rivers of
peninsular India, the nature of lithology still plays an important role as the chemical
weathering rate of basaltic rocks is 2.5 times higher than that of gneissic silicate rocks
under similar morpho-climatic settings. However, this lithology-dependence of silicate
weathering rate is less marked compared to the cooler and less humid environment,
suggesting that intense runoff and high temperature would reduce the difference in
weatherability of silicate lithologies.
Chapter 3
52
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