fit with the smallest propagated errors is given by the com-
bination Na, K, Ca, Mg, SO₄, Sr and Sr·Δ⁸⁷Sr. The quality
of the fits is described by the mean square of the weighted
deviate (MSWD, see Albarède, 1995) which is a measure
of the scatter in excess of the estimated errors. A fit in which
scatter matches the estimated errors gives a fit with MSWD
= 1. Inclusion of Si or Cl substantially degrades the quality
of the fits with MSWD increasing from <2 to 8 or 12 and
the uncertainties on the fits ~100%. Using only four com-
ponents out of Na, Ca, Mg, Sr and Sr·Δ⁸⁷Sr gives results
within error of regressions with the 6 or 7 components.
The results consistently give total discharge (∑Pᵢ) ~ 11 ±
5% greater than the discharge at Farakka which likely
reflects evapo-transpiration losses in the flood plain as dis-
cussed above. The calculated discharges from the Hima-
layas (192 × 10⁹ m³/yr) and southern rivers (66 × 10⁹ m³/
yr), relative to the discharge at Farakka given by Hossain
et al. (1987) are within the uncertainties of the values com-
piled in Tables 2 and 3. The calculated discharge from the
flood plain (160 × 10⁹ m³/yr) is within the range expected
from the rainfall and the rather uncertain magnitude of
evapo-transpiration as discussed above.

Calculation of the relative discharges for each month
throughout the year using the monthly averages at Farakka
for the outputs from the flood plain is not possible because
the compositions of the monthly Himalayan and southern
river inputs are not known.

8. DISCUSSION: MAGNITUDE OF FLOOD PLAIN
CHEMICAL WEATHERING FLUXES

We consider that the best estimate of the discharge and
chemical fluxes from the flood plain is that calculated by
deconvolution of the chemical and Sr-isotopic data because
this does not rely on the very uncertain estimates of the
river discharges and allows for water loss by evapo-
transpiration on the flood plain. The calculated uncertain-
ties should be regarded as minimum estimates because the
averages of nearly all the Himalayan and southern tribu-
taries and flood plain waters do not include adequate
time-series sets to capture the full annual variability.

The flood plain weathering fluxes calculated here range
from 41% for Sr to 63% for Na (corrected for saline inputs)
of the calculated total elemental fluxes from the mountains,
floodplain and the southern rivers (Fig. 16A). Even if the
lower estimates from the differences in discharge are
adopted (Table 4) the flood plain weathering fluxes range
from 28% for Sr to 41% for Na. Given that nearly all the
flood plain comprises sediment from the Himalayas
(Lupker et al., 2012b), these values imply that ~59% Sr,
53% Ca, 60% Mg and 82% of the Na derived from Himala-
yan detritus is supplied by weathering in the flood plain,
confirming its importance to the overall chemical weather-
ing budget.

K, Si and Cl exhibit the largest deficit between the
observed chemical flux at Farakka and that inferred from
compositional deconvolution of the water compositions.
The implication is that significant K and Cl (10 and 23%
of the Ganga flux) is derived from the flood plain in excess
of that carried by the analysed flood plain, Himalayan and

southern river waters. The 23% deficit of Si in the Ganga
waters suggests that Si is precipitated in the flood plain con-
sistent with the interpretation of the Si-isotopic systematics
(Fontorbe et al., 2013). There have been relatively few pre-
vious attempts to calculate the chemical weathering yields
from the Ganga flood plain. Galy and France-Lanord
(1999) estimated from the difference between the input
and output fluxes, that the flood plain provided 14% of
the Na, 41% of the K, 18% of the Ca in the downstream
Ganga but that 14% of Mg was taken up by the flood plain.
West et al. (2002) estimated that weathering of High Hima-
layan silicate material in the flood plain occurs at two and a
half to six-times the rate in the mountains, again from dif-
ferences in input and output river chemistries. Rai et al.'s
(2010) estimates of weathering fluxes on the Ganga flood
plain indicated that inputs exceeded outputs for Na, Mg,
Sr, Si, Cl and SO₄ and were within error for Ca but these
budgets were based mainly on May and October samples.
Also, as noted here, the relatively small difference between
the discharge in the downstream Ganga and the inputs to
the flood plain as well as the potential for water loss by
evapo-transpiration makes calculation of the flood plain
flux by difference unreliable.

Lupker et al. (2012a) calculated flood plain chemical
weathering inputs by the difference in riverine sediment
loads at the margins of the flood plain and the Ganga in
Bangladesh. Given the careful sampling of the riverine sed-
iment, this method is attractive because it averages sedi-
ment compositions over much longer time scales than the
water sampling. The difficulty is that the changes in sedi-
ment composition are small given the scatter in sediment
compositions and the resulting flux estimates have large
uncertainties for some elements (Table 4).

The flood plain chemical weathering fluxes of Na, K, Ca
and Mg calculated from the water data here after allowance
for inputs from saline sources (Na 66 ± 16, K 13 ± 4, Ca
109 ± 22, Mg 57 ± 13 × 10⁹ mol/yr, 1σ errors) are within
error of those calculated by the change in composition of
sediments transported by the rivers across the flood plain
(Na 53 ± 18, K 42 ± 13, Ca 199 ± 100, Mg 86 ± 32 × 10⁹
mol/yr) by Lupker et al. (2012a) (Table 4). If it is assumed
that the loss of ~50% of Ca to secondary calcite is not
returned to solution then the chemical weathering flux of
Ca in the flood plain should be increased to ~220 × 10⁹
mol/yr. The Mg and Sr fluxes should be increased by about
5%. The comparison indicates that the current short-term
estimates based on water chemistries are compatible with
the longer-term estimates based on changes in sediment
chemistry which sample chemical weathering over thou-
sands or tens of thousands of years (e.g. Lupker et al.,
2012b; Granet et al., 2010) and predate the development
of intensive agriculture, but note the uncertainties. The
silicate-derived flux of Ca is small (ca. 20 × 10⁹ mol/yr
allowing for loss to secondary calcite), and Mg (27 × 10⁹
mol/yr) is modest.

The total chemical weathering fluxes and the fractions
derived from salts (rain and evaporates or springs), carbon-
ate minerals and silicate minerals for the inputs from the
Himalayan mountains, the southern rivers and the flood-
plain, calculated as discussed above, are illustrated in
fit with the smallest propagated errors is given by the com-
bination Na, K, Ca, Mg, SO₄, Sr and Sr·Δ⁸⁷Sr. The quality
of the fits is described by the mean square of the weighted
deviate (MSWD, see Albarède, 1995) which is a measure
of the scatter in excess of the estimated errors. A fit in which
scatter matches the estimated errors gives a fit with MSWD
= 1. Inclusion of Si or Cl substantially degrades the quality
of the fits with MSWD increasing from <2 to 8 or 12 and
the uncertainties on the fits ~100%. Using only four com-
ponents out of Na, Ca, Mg, Sr and Sr·Δ⁸⁷Sr gives results
within error of regressions with the 6 or 7 components.
The results consistently give total discharge (∑Pᵢ) ~ 11 ±
5% greater than the discharge at Farakka which likely
reflects evapo-transpiration losses in the flood plain as dis-
cussed above. The calculated discharges from the Hima-
layas (192 × 10⁹ m³/yr) and southern rivers (66 × 10⁹ m³/
yr), relative to the discharge at Farakka given by Hossain
et al. (1987) are within the uncertainties of the values com-
piled in Tables 2 and 3. The calculated discharge from the
flood plain (160 × 10⁹ m³/yr) is within the range expected
from the rainfall and the rather uncertain magnitude of
evapo-transpiration as discussed above.

Calculation of the relative discharges for each month
throughout the year using the monthly averages at Farakka
for the outputs from the flood plain is not possible because
the compositions of the monthly Himalayan and southern
river inputs are not known.

8. DISCUSSION: MAGNITUDE OF FLOOD PLAIN
CHEMICAL WEATHERING FLUXES

We consider that the best estimate of the discharge and
chemical fluxes from the flood plain is that calculated by
deconvolution of the chemical and Sr-isotopic data because
this does not rely on the very uncertain estimates of the
river discharges and allows for water loss by evapo-
transpiration on the flood plain. The calculated uncertain-
ties should be regarded as minimum estimates because the
averages of nearly all the Himalayan and southern tribu-
taries and flood plain waters do not include adequate
time-series sets to capture the full annual variability.

The flood plain weathering fluxes calculated here range
from 41% for Sr to 63% for Na (corrected for saline inputs)
of the calculated total elemental fluxes from the mountains,
floodplain and the southern rivers (Fig. 16A). Even if the
lower estimates from the differences in discharge are
adopted (Table 4) the flood plain weathering fluxes range
from 28% for Sr to 41% for Na. Given that nearly all the
flood plain comprises sediment from the Himalayas
(Lupker et al., 2012b), these values imply that ~59% Sr,
53% Ca, 60% Mg and 82% of the Na derived from Himala-
yan detritus is supplied by weathering in the flood plain,
confirming its importance to the overall chemical weather-
ing budget.

K, Si and Cl exhibit the largest deficit between the
observed chemical flux at Farakka and that inferred from
compositional deconvolution of the water compositions.
The implication is that significant K and Cl (10 and 23%
of the Ganga flux) is derived from the flood plain in excess
of that carried by the analysed flood plain, Himalayan and

southern river waters. The 23% deficit of Si in the Ganga
waters suggests that Si is precipitated in the flood plain con-
sistent with the interpretation of the Si-isotopic systematics
(Fontorbe et al., 2013). There have been relatively few pre-
vious attempts to calculate the chemical weathering yields
from the Ganga flood plain. Galy and France-Lanord
(1999) estimated from the difference between the input
and output fluxes, that the flood plain provided 14% of
the Na, 41% of the K, 18% of the Ca in the downstream
Ganga but that 14% of Mg was taken up by the flood plain.
West et al. (2002) estimated that weathering of High Hima-
layan silicate material in the flood plain occurs at two and a
half to six-times the rate in the mountains, again from dif-
ferences in input and output river chemistries. Rai et al.'s
(2010) estimates of weathering fluxes on the Ganga flood
plain indicated that inputs exceeded outputs for Na, Mg,
Sr, Si, Cl and SO₄ and were within error for Ca but these
budgets were based mainly on May and October samples.
Also, as noted here, the relatively small difference between
the discharge in the downstream Ganga and the inputs to
the flood plain as well as the potential for water loss by
evapo-transpiration makes calculation of the flood plain
flux by difference unreliable.

Lupker et al. (2012a) calculated flood plain chemical
weathering inputs by the difference in riverine sediment
loads at the margins of the flood plain and the Ganga in
Bangladesh. Given the careful sampling of the riverine sed-
iment, this method is attractive because it averages sedi-
ment compositions over much longer time scales than the
water sampling. The difficulty is that the changes in sedi-
ment composition are small given the scatter in sediment
compositions and the resulting flux estimates have large
uncertainties for some elements (Table 4).

The flood plain chemical weathering fluxes of Na, K, Ca
and Mg calculated from the water data here after allowance
for inputs from saline sources (Na 66 ± 16, K 13 ± 4, Ca
109 ± 22, Mg 57 ± 13 × 10⁹ mol/yr, 1σ errors) are within
error of those calculated by the change in composition of
sediments transported by the rivers across the flood plain
(Na 53 ± 18, K 42 ± 13, Ca 199 ± 100, Mg 86 ± 32 × 10⁹
mol/yr) by Lupker et al. (2012a) (Table 4). If it is assumed
that the loss of ~50% of Ca to secondary calcite is not
returned to solution then the chemical weathering flux of
Ca in the flood plain should be increased to ~220 × 10⁹
mol/yr. The Mg and Sr fluxes should be increased by about
5%. The comparison indicates that the current short-term
estimates based on water chemistries are compatible with
the longer-term estimates based on changes in sediment
chemistry which sample chemical weathering over thou-
sands or tens of thousands of years (e.g. Lupker et al.,
2012b; Granet et al., 2010) and predate the development
of intensive agriculture, but note the uncertainties. The
silicate-derived flux of Ca is small (ca. 20 × 10⁹ mol/yr
allowing for loss to secondary calcite), and Mg (27 × 10⁹
mol/yr) is modest.

The total chemical weathering fluxes and the fractions
derived from salts (rain and evaporates or springs), carbon-
ate minerals and silicate minerals for the inputs from the
Himalayan mountains, the southern rivers and the flood-
plain, calculated as discussed above, are illustrated in
<figure><img src="image_17.png" /><figcaption>Fig. 16. Chemical fluxes from the flood plain, Himalayan rivers and southern rivers calculated from chemistry of inputs compared to chemistry of Ganga at Farakka (Eq. (5)) scaled to discharge of Ganga. (A) gives total flux from each source, (B) flux from rain and spring or evaporate source, (C) carbonate minerals and (D) silicate minerals.</figcaption></figure>

Fig. 16. The significance of the chemical weathering in the floodplain is demonstrated by the estimates for the silicate inputs of Na, K, Ca and Mg all being equal or greater in the floodplain than inputs in the mountains (Fig. 16D). Inputs of Ca, Mg and Sr from weathering of carbonate minerals from the mountains and the floodplain are similar and between a factor of two to three greater than inputs from the southern rivers. The similarity between the estimates of the Na dissolved flux from the floodplain from the water chemistries and from the change in suspended load chemistries suggests that short-term variations in inputs from Na salts on the flood plain are insignificant within the error bounds.

The important conclusion is that chemical weathering in the floodplain more than doubles the impact of the rapid erosion and chemical weathering in the Himalayas. This estimate of weathering in flood plain is larger than that of Galy and France-Lanord (1999) and reflects in part significantly higher estimates of the flux-weighted mean compositions of Na, K, Ca and Mg in the downstream Ganga based on the three years time-series sampling. Chemical weathering in the flood plain must take place in the sediments stored on the flood plain and the comparison between the chemical fluxes calculated from the water chemistries and the suspended load compositions confirms the contention of Lupker et al. (2012a) that the suspended load in the Ganga in Bangladesh is representative of the weathering on the flood plain. It is also consistent with

the calculation by Lupker et al. (2012b) that the 90% of the sediment delivered to the flood plain, that is subsequently exported to the Bay of Bengal, is recycled with a ~1400 year time constant.

The preservation of significant plagioclase (~10 wt% in the suspended and bedloads of the Ganga in Bangladesh (Garzanti et al., 2010, 2011) indicates that weathering on the flood plain is incomplete. This is supported by the fractions of the major cations leached from the solid load. Taking (1) Lupker et al.'s. (2012b) estimate that the Ganga sediment flux in Bangladesh is 610 ± 210 Mt/yr from ¹⁰Be analyses and that this is generated almost entirely in the Himalayan mountains, (2) that the average composition of the source rock is given by the mean of the suspended and bedload samples from the mountain rivers compiled by Lupker et al. (2012a) with a small adjustment to this composition for the chemical weathering flux from the mountains, and combined with (3) the chemical weathering fluxes calculated from the water chemistries, implies that the Himalayan-derived silicate sediment has lost only ~28% of Na, 6% of K, 18% of Ca and 15% of Mg by the time it reaches Bangladesh. The loss of 28% Na is consistent with the 25% decrease in Na/Si ratio of the suspended sediment between the Himalayan mountain front and Bangladesh noted by Lupker et al. (2012a). On a mass basis the total dissolved solids (Na₂O + K₂O + CaO + MgO + SiO₂ + Cl⁻ + SO₄²⁻) carried by the Ganga at Farakka comprises only ~5.7% of the solid load (2.7% added in the
<figure><img src="image_17.png" /><figcaption>Fig. 16. Chemical fluxes from the flood plain, Himalayan rivers and southern rivers calculated from chemistry of inputs compared to chemistry of Ganga at Farakka (Eq. (5)) scaled to discharge of Ganga. (A) gives total flux from each source, (B) flux from rain and spring or evaporate source, (C) carbonate minerals and (D) silicate minerals.</figcaption></figure>

Fig. 16. The significance of the chemical weathering in the floodplain is demonstrated by the estimates for the silicate inputs of Na, K, Ca and Mg all being equal or greater in the floodplain than inputs in the mountains (Fig. 16D). Inputs of Ca, Mg and Sr from weathering of carbonate minerals from the mountains and the floodplain are similar and between a factor of two to three greater than inputs from the southern rivers. The similarity between the estimates of the Na dissolved flux from the floodplain from the water chemistries and from the change in suspended load chemistries suggests that short-term variations in inputs from Na salts on the flood plain are insignificant within the error bounds.

The important conclusion is that chemical weathering in the floodplain more than doubles the impact of the rapid erosion and chemical weathering in the Himalayas. This estimate of weathering in flood plain is larger than that of Galy and France-Lanord (1999) and reflects in part significantly higher estimates of the flux-weighted mean compositions of Na, K, Ca and Mg in the downstream Ganga based on the three years time-series sampling. Chemical weathering in the flood plain must take place in the sediments stored on the flood plain and the comparison between the chemical fluxes calculated from the water chemistries and the suspended load compositions confirms the contention of Lupker et al. (2012a) that the suspended load in the Ganga in Bangladesh is representative of the weathering on the flood plain. It is also consistent with

the calculation by Lupker et al. (2012b) that the 90% of the sediment delivered to the flood plain, that is subsequently exported to the Bay of Bengal, is recycled with a ~1400 year time constant.

The preservation of significant plagioclase (~10 wt% in the suspended and bedloads of the Ganga in Bangladesh (Garzanti et al., 2010, 2011) indicates that weathering on the flood plain is incomplete. This is supported by the fractions of the major cations leached from the solid load. Taking (1) Lupker et al.'s. (2012b) estimate that the Ganga sediment flux in Bangladesh is 610 ± 210 Mt/yr from ¹⁰Be analyses and that this is generated almost entirely in the Himalayan mountains, (2) that the average composition of the source rock is given by the mean of the suspended and bedload samples from the mountain rivers compiled by Lupker et al. (2012a) with a small adjustment to this composition for the chemical weathering flux from the mountains, and combined with (3) the chemical weathering fluxes calculated from the water chemistries, implies that the Himalayan-derived silicate sediment has lost only ~28% of Na, 6% of K, 18% of Ca and 15% of Mg by the time it reaches Bangladesh. The loss of 28% Na is consistent with the 25% decrease in Na/Si ratio of the suspended sediment between the Himalayan mountain front and Bangladesh noted by Lupker et al. (2012a). On a mass basis the total dissolved solids (Na₂O + K₂O + CaO + MgO + SiO₂ + Cl⁻ + SO₄²⁻) carried by the Ganga at Farakka comprises only ~5.7% of the solid load (2.7% added in the
floodplain, 2% in the Himalayan mountains and 1% from the southern rivers). Chemical weathering in the Ganga floodplain is ‘weathering limited’ (sense Stallard and Edmond, 1983), as in the mountains, and the consequence is that chemical weathering fluxes from both regions are predicted to be sensitive to changing climatic conditions (cf. West et al., 2005).

The Ganga river system illustrates the complexity of deciphering controls on chemical weathering fluxes. West et al. (2005) showed that both the High Himalayan and Lesser Himalayan catchments were ‘weathering limited’. Discharge and erosion rate data from the Marsyandi Tibetan Sedimentary Series catchment in Nepal (Bickle et al., 2015; Gabet et al., 2008) exhibits a similar physical and silicate chemical weathering relationship (Fig. 17). The silicate cation denudation rate for the whole Ganga catchment, calculated for the area of the Himalayan mountains which dominate the supply of sediment, also lies in the ‘weathering limited’ field, albeit with a substantially increased silicate weathering flux (Fig. 17). The increased silicate weathering flux reflects the weathering in the flood plain and also additions from the southern tributaries. It is probable that chemical weathering in the catchments of the southern tributaries is transport limited but constraints on the rates of physical denudation there are not available. The output from the whole Ganga catchment therefore reflects sub-catchments which range from ‘weathering limited’ to ‘transport limited’ and processing of partially weathered material in the flood plain. Evaluating the response of silicate chemical weathering fluxes to changing climatic parameters in such a complex catchment is clearly complicated. For example increased temperatures and rain-

<figure><img src="image_18.png" /></figure>

Fig. 17. Silicate cation denudation rates versus total denudation rate for catchments listed by West et al. (2005) with addition of data from the Marsyandi catchment in the Tibetan Sedimentary series using the estimates of silicate chemical fluxes by Bickle et al. (2015) and physical denudation fluxes by Gabet et al. (2008). The estimate for the whole Ganga catchment is based on the silicate chemical fluxes given here and the physical denudation rate of the whole Ganga catchment by Lupker et al. (2012b). The Ganga weathering rates (solid symbol) are normalised to the area of the Himalayas as that area dominates the supply of rock. The open symbol illustrates the Ganga silicate chemical and total denudation rates normalised to the whole catchment above Farakka.

fall will increase the chemical flux from the mountainous catchments but supply of the more weathered material might limit chemical weathering on the flood plain. The response of the floodplain weathering to changes in temperature and especially rainfall is likely to differ from that of the mountainous catchments as groundwater flow paths and flow rates are likely to be very different.

## 9. CONCLUSIONS

Tributaries to the Ganga which are restricted to the flood plain have chemistries and oxygen and hydrogen isotope compositions which are distinct from the major rivers rising in the Himalayas and Tibet. The $\delta^{18}$O - $\delta$D arrays (i.e. $\delta$D$_{excesses}$) of the flood plain tributaries reflect evaporative loss from the flood plain which is reflected in the higher $\delta$D$_{excesses}$ of the mountain rivers.

The flood plain tributaries may be divided into two distinct groups on the basis of their Sr-isotopic compositions and Sr/Ca to Na/Ca arrays. Over most of the flood plain tributaries have $^{87}$Sr/$^{86}$Sr ratios < 0.75 and Na/Ca molar ratios < 1.0 except for a small subset of samples close to the Gomti river with elevated Na concentrations thought to be derived by re-dissolution of Na-salts. However in the east of the flood plain, in a restricted area around the Kosi, waters have $^{87}$Sr/$^{86}$Sr ratios greater than 0.75 and higher Na/Ca ratios which correlate with Sr/Ca ratios. The bed load compositions of these tributaries are consistent with the high $^{87}$Sr/$^{86}$Sr ratios being derived from both high $^{87}$Sr/$^{86}$Sr dolomites and even higher $^{87}$Sr/$^{86}$Sr ratio silicate minerals from Lesser Himalayan Series.

Modelling of the Sr-isotopic compositions of the flood plain tributaries allows calculation of the inputs of carbonate and silicate Sr. It also provides estimates of the fractions of Ca, Sr and Mg lost to precipitation of secondary calcite within the flood plain. The fractions of Ca remaining after precipitation range from ~24% to 91% in individual tributaries with a mean of between 36 and 57% depending on the estimate of carbonate Sr/Ca and silicate $^{87}$Sr/$^{86}$Sr ratios. The modelling takes the Sr-isotopic compositions and Sr/Ca ratio of the carbonate input from leaches of mainstem Ganga bedloads and the silicate $^{87}$Sr/$^{86}$Sr, Na/Ca and Na/Mg ratios from analyses of residues of bedloads after leaching. After correction for rain and saline inputs, ~31% of the Sr, 8% of the Ca and 45% of the Mg are estimated to be derived from silicate. It should be noted that the calculations imply substantial ranges for the Mg/Ca ratios of carbonate inputs and Sr/Ca ratios of silicate inputs across the individual tributaries. The impact of assuming single values of Sr/Ca ratios for the carbonate inputs and $^{87}$Sr/$^{86}$Sr, Na/Ca and Mg/Ca ratios for silicate inputs is uncertain.

Calculation of the chemical weathering fluxes from the flood plain by difference between the chemical fluxes in the downstream Ganga and the inputs from the Himalayan mountains and southern tributaries gives combined uncertainties in chemical compositions and water fluxes are of the same order as the difference. Deconvolution of the fluxes using the average chemistries of the mountain rivers, the southern rivers and the downstream Ganga sampled at
floodplain, 2% in the Himalayan mountains and 1% from the southern rivers). Chemical weathering in the Ganga floodplain is ‘weathering limited’ (sense Stallard and Edmond, 1983), as in the mountains, and the consequence is that chemical weathering fluxes from both regions are predicted to be sensitive to changing climatic conditions (cf. West et al., 2005).

The Ganga river system illustrates the complexity of deciphering controls on chemical weathering fluxes. West et al. (2005) showed that both the High Himalayan and Lesser Himalayan catchments were ‘weathering limited’. Discharge and erosion rate data from the Marsyandi Tibetan Sedimentary Series catchment in Nepal (Bickle et al., 2015; Gabet et al., 2008) exhibits a similar physical and silicate chemical weathering relationship (Fig. 17). The silicate cation denudation rate for the whole Ganga catchment, calculated for the area of the Himalayan mountains which dominate the supply of sediment, also lies in the ‘weathering limited’ field, albeit with a substantially increased silicate weathering flux (Fig. 17). The increased silicate weathering flux reflects the weathering in the flood plain and also additions from the southern tributaries. It is probable that chemical weathering in the catchments of the southern tributaries is transport limited but constraints on the rates of physical denudation there are not available. The output from the whole Ganga catchment therefore reflects sub-catchments which range from ‘weathering limited’ to ‘transport limited’ and processing of partially weathered material in the flood plain. Evaluating the response of silicate chemical weathering fluxes to changing climatic parameters in such a complex catchment is clearly complicated. For example increased temperatures and rain-

<figure><img src="image_18.png" /></figure>

Fig. 17. Silicate cation denudation rates versus total denudation rate for catchments listed by West et al. (2005) with addition of data from the Marsyandi catchment in the Tibetan Sedimentary series using the estimates of silicate chemical fluxes by Bickle et al. (2015) and physical denudation fluxes by Gabet et al. (2008). The estimate for the whole Ganga catchment is based on the silicate chemical fluxes given here and the physical denudation rate of the whole Ganga catchment by Lupker et al. (2012b). The Ganga weathering rates (solid symbol) are normalised to the area of the Himalayas as that area dominates the supply of rock. The open symbol illustrates the Ganga silicate chemical and total denudation rates normalised to the whole catchment above Farakka.

fall will increase the chemical flux from the mountainous catchments but supply of the more weathered material might limit chemical weathering on the flood plain. The response of the floodplain weathering to changes in temperature and especially rainfall is likely to differ from that of the mountainous catchments as groundwater flow paths and flow rates are likely to be very different.

## 9. CONCLUSIONS

Tributaries to the Ganga which are restricted to the flood plain have chemistries and oxygen and hydrogen isotope compositions which are distinct from the major rivers rising in the Himalayas and Tibet. The $\delta^{18}$O - $\delta$D arrays (i.e. $\delta$D$_{excesses}$) of the flood plain tributaries reflect evaporative loss from the flood plain which is reflected in the higher $\delta$D$_{excesses}$ of the mountain rivers.

The flood plain tributaries may be divided into two distinct groups on the basis of their Sr-isotopic compositions and Sr/Ca to Na/Ca arrays. Over most of the flood plain tributaries have $^{87}$Sr/$^{86}$Sr ratios < 0.75 and Na/Ca molar ratios < 1.0 except for a small subset of samples close to the Gomti river with elevated Na concentrations thought to be derived by re-dissolution of Na-salts. However in the east of the flood plain, in a restricted area around the Kosi, waters have $^{87}$Sr/$^{86}$Sr ratios greater than 0.75 and higher Na/Ca ratios which correlate with Sr/Ca ratios. The bed load compositions of these tributaries are consistent with the high $^{87}$Sr/$^{86}$Sr ratios being derived from both high $^{87}$Sr/$^{86}$Sr dolomites and even higher $^{87}$Sr/$^{86}$Sr ratio silicate minerals from Lesser Himalayan Series.

Modelling of the Sr-isotopic compositions of the flood plain tributaries allows calculation of the inputs of carbonate and silicate Sr. It also provides estimates of the fractions of Ca, Sr and Mg lost to precipitation of secondary calcite within the flood plain. The fractions of Ca remaining after precipitation range from ~24% to 91% in individual tributaries with a mean of between 36 and 57% depending on the estimate of carbonate Sr/Ca and silicate $^{87}$Sr/$^{86}$Sr ratios. The modelling takes the Sr-isotopic compositions and Sr/Ca ratio of the carbonate input from leaches of mainstem Ganga bedloads and the silicate $^{87}$Sr/$^{86}$Sr, Na/Ca and Na/Mg ratios from analyses of residues of bedloads after leaching. After correction for rain and saline inputs, ~31% of the Sr, 8% of the Ca and 45% of the Mg are estimated to be derived from silicate. It should be noted that the calculations imply substantial ranges for the Mg/Ca ratios of carbonate inputs and Sr/Ca ratios of silicate inputs across the individual tributaries. The impact of assuming single values of Sr/Ca ratios for the carbonate inputs and $^{87}$Sr/$^{86}$Sr, Na/Ca and Mg/Ca ratios for silicate inputs is uncertain.

Calculation of the chemical weathering fluxes from the flood plain by difference between the chemical fluxes in the downstream Ganga and the inputs from the Himalayan mountains and southern tributaries gives combined uncertainties in chemical compositions and water fluxes are of the same order as the difference. Deconvolution of the fluxes using the average chemistries of the mountain rivers, the southern rivers and the downstream Ganga sampled at
Farakka, with flood plain outputs characterised by the flood plain tributaries, is preferred. This defines Na, K, Ca, Mg, SO₄ and Sr fluxes with 1 sigma uncertainties of about 30%. Calculated input water fluxes are about 10% greater than the discharge in the downstream Ganga attributed to evaporation and transpiration as water crosses the flood plain.

Two factors dominate the uncertainties of fluxes calcu-
lated from analyses of river water compositions. (1) Most
of the major rivers from the Himalayas and all of the south-
ern rivers have only been sampled on relatively few occa-
sions whereas repeated bi-weekly sampling over several
years is required to properly constrain the average chem-
istry in rivers which exhibit large seasonal changes in water
chemistries. (2) Calculation of annual discharges requires
knowledge of discharge throughout the season, information
which is only sparingly available for major Indian rivers.

Weathering in the flood plain supplies between 41 and
63% of the major cation and Sr fluxes carried by the Ganga
compared to between 14% (Na) and ~36% (Sr) supplied by
the major Himalayan rivers (Table 4). The flood plain sup-
plies 58%, the Himalayan rivers 24% and the southern riv-
ers 18% of the silicate weathering-associated alkalinity flux
carried by the Ganga at Farakka. Weathering in the Ganga
flood plain makes a dominant contribution to weathering
of material eroded from the Himalayan mountains.

ACKNOWLEDGEMENTS

The research was supported by the UK Natural Environment Research Council grants (NE/E003192/1 and NE/N007441/1). Dr Jyotisankar Ray (University of Calcutta, India) helped organise the sampling of the Ganga at Farakka. The late Dr Fatima Khan gave extensive assistance with the chemical and isotopic analyses. D. Hodell provided the δD and δ¹⁸O analyses. We acknowledge constructive reviews by the associate editor, T.K. Dalai and an anonymous reviewer.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can be
found, in the online version, at https://doi.org/10.1016/j.
gca.2018.01.003.

REFERENCES

Albarède F. (1995) *Introduction to Geochemical Modeling*. Cambridge University Press, Cambridge.

Andermann C., Longuevergne L., Bonnet S., Crave A., Davy P. and Gloaguen R. (2012) Impact of transient groundwater storage on the discharge of Himalayan rivers. *Nat. Geosci.* **5**, 127–132.

Battacharya S. K., Gupta S. K. and Krishnamurthy R. V. (1985) Oxygen and hydrogen isotopic ratios in groundwaters and river waters from India. *Proc. Indian Acad. Sci. (Earth Planet. Sci.)* **94**, 283–295.

Becker J. A., Bickle M. J., Galy A. and Holland T. J. B. (2008) Himalayan metamorphic CO₂ fluxes: Quantitative constraints from hydrothermal springs. *Earth Planet. Sci. Lett.* **265**, 616–629.

Berner R. A., Lasaga A. C. and Garrels R. M. (1983) The carbonate-silicate geochemical cycle and its effect on atmo-

spheric carbon dioxide over the past 100 million years. Am. J. Sci. **283**, 641–683.

Bhargava G. P., Paul D. K., Kapoor B. S. and Goswami S. C. (1981) Characteristics and genesis of some sodic soils in the Indo-Gangetic alluvial plain of Haryana and Uttar Pradesh. *J. Indian Soc. Soil Sci.* **29**, 60–71.

Bickle M. J. (1996) Metamorphic decarbonation, silicate weathering and the long-term carbon cycle. *Terra Nova* **8**, 270–276.

Bickle M. J., Harris N. B. W., Bunbury J., Chapman H. J., Fairchild I. J. and Ahmad T. (2001) Controls on the ⁸⁷Sr/⁸⁶Sr of carbonates in the Garwal Himalaya, headwaters of the Ganges. *J. Geol.* **109**, 737–753.

Bickle M. J., Bunbury J., Chapman H. J., Harris N. B., Fairchild I. J. and Ahmad T. (2003) Fluxes of Sr into the headwaters of the Ganges. *Geochim. Cosmochim. Acta* **67**, 2567–2584.

Bickle M. J., Chapman H. J., Bunbury J., Harris N. B. W., Fairchild I. J., Ahmad T. and Pomiès C. (2005) The relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges. *Geochem. Cosmochim. Acta* **69**, 2221–2240.

Bickle M. J., Tipper E. D., Galy A., Chapman H. J. and Harris N. W. B. (2015) On discrimination between carbonate and silicate inputs to Himalayan rivers. *Am. J. Sci.* **315**, 120–166.

Bookhagen B. and Burbank D. W. (2010) Toward a complete Himalayan hydrological budget: Spatiotemporal distribution of snowmelt and rainfall and their impact on river discharge. *J. Geophys. Res.* **115**, F03019 03025.

Bouchez J., Gaillardet J., Lupker M., Louvat P., France-Lanord C., Laurence Maurice L., Armijos E. and Moquet J.-S. (2012) Flood plains of large rivers: Weathering reactors or simple silos? *Chem. Geol.* **332–333**, 166–184.

Brady P. V. and Gislason S. R. (1997) Seafloor weathering controls on atmospheric CO₂ and global climate. *Geochim. Cosmochim. Acta* **61**, 965–973.

Caldeira, K., Arthur, M.A., Berner, R.A., & Lasaga, A.C. (1993). Cooling in the Cenozoic: Discussion of 'Tectonic forcing of late Cenozoic climate' by Raymo, M.E. and Ruddiman, W.F. Nature **361**, 123–124.

Calmels D., Galy A., Hovius N., Bickle M., West A. J., Chen M. C. and Chapman H. (2011) Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan. *Earth Planet. Sci. Lett.* **303**, 48–58.

Carson M. A. and Kirby M. J. (1972) *Hillslope, Form and Process*. Cambridge University Press, Cambridge.

Chamberlin T. C. (1899) The influence of great epochs of limestone formation upon the constitution of the atmosphere. *J. Geol.* **6**, 609–621.

Chapman H. J., Bickle M., Thaw S. H. and Thiam H. N. (2015) Chemical fluxes from time series sampling of the Irrawaddy and Salween Rivers, Myanmar. *Chem. Geol.* **401**, 15–27.

Coogan L. A. and Dosso S. E. (2015) Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater. *Earth Planet. Sci. Lett.* **415**, 38–46.

Cox K. G. and Hawkesworth C. J. (1985) Geochemical stratigra-
phy of the Deccan Traps at Mahabaleshwar, Western Ghats,
India, with implications for open system magmatic processes. *J.
Petrol.* **26**, 355–377.

Craig H. (1961) Isotopic variations in meteoric waters. *Science* **133**, 1702–1703.

Craig H. and Gordon L. I. (1965) Deuterium and oxygen 18
variations in the ocean and the marine atmosphere. In *Stable
isotopes in oceanographic studies and paleotemperatures* (ed. E.
Tongiorgi). Consiglio nazionale delle Ricerche Laboratorio di
Geologia Nucleare, Spoleto, pp. 9-130.
Farakka, with flood plain outputs characterised by the flood plain tributaries, is preferred. This defines Na, K, Ca, Mg, SO₄ and Sr fluxes with 1 sigma uncertainties of about 30%. Calculated input water fluxes are about 10% greater than the discharge in the downstream Ganga attributed to evaporation and transpiration as water crosses the flood plain.

Two factors dominate the uncertainties of fluxes calcu-
lated from analyses of river water compositions. (1) Most
of the major rivers from the Himalayas and all of the south-
ern rivers have only been sampled on relatively few occa-
sions whereas repeated bi-weekly sampling over several
years is required to properly constrain the average chem-
istry in rivers which exhibit large seasonal changes in water
chemistries. (2) Calculation of annual discharges requires
knowledge of discharge throughout the season, information
which is only sparingly available for major Indian rivers.

Weathering in the flood plain supplies between 41 and 63% of the major cation and Sr fluxes carried by the Ganga compared to between 14% (Na) and ~36% (Sr) supplied by the major Himalayan rivers (Table 4). The flood plain supplies 58%, the Himalayan rivers 24% and the southern rivers 18% of the silicate weathering-associated alkalinity flux carried by the Ganga at Farakka. Weathering in the Ganga flood plain makes a dominant contribution to weathering of material eroded from the Himalayan mountains.

ACKNOWLEDGEMENTS

The research was supported by the UK Natural Environment Research Council grants (NE/E003192/1 and NE/N007441/1). Dr Jyotisankar Ray (University of Calcutta, India) helped organise the sampling of the Ganga at Farakka. The late Dr Fatima Khan gave extensive assistance with the chemical and isotopic analyses. D. Hodell provided the δD and δ¹⁸O analyses. We acknowledge constructive reviews by the associate editor, T.K. Dalai and an anonymous reviewer.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can be
found, in the online version, at https://doi.org/10.1016/j.
gca.2018.01.003.

REFERENCES

Albarède F. (1995) *Introduction to Geochemical Modeling*. Cambridge University Press, Cambridge.

Andermann C., Longuevergne L., Bonnet S., Crave A., Davy P. and Gloaguen R. (2012) Impact of transient groundwater storage on the discharge of Himalayan rivers. *Nat. Geosci.* **5**, 127–132.

Battacharya S. K., Gupta S. K. and Krishnamurthy R. V. (1985) Oxygen and hydrogen isotopic ratios in groundwaters and river waters from India. *Proc. Indian Acad. Sci. (Earth Planet. Sci.)* **94**, 283–295.

Becker J. A., Bickle M. J., Galy A. and Holland T. J. B. (2008) Himalayan metamorphic CO₂ fluxes: Quantitative constraints from hydrothermal springs. *Earth Planet. Sci. Lett.* **265**, 616–629.

Berner R. A., Lasaga A. C. and Garrels R. M. (1983) The carbonate-silicate geochemical cycle and its effect on atmo-

spheric carbon dioxide over the past 100 million years. *Am. J. Sci.* **283**, 641–683.

Bhargava G. P., Paul D. K., Kapoor B. S. and Goswami S. C. (1981) Characteristics and genesis of some sodic soils in the Indo-Gangetic alluvial plain of Haryana and Uttar Pradesh. *J. Indian Soc. Soil Sci.* **29**, 60–71.

Bickle M. J. (1996) Metamorphic decarbonation, silicate weathering and the long-term carbon cycle. *Terra Nova* **8**, 270–276.

Bickle M. J., Harris N. B. W., Bunbury J., Chapman H. J., Fairchild I. J. and Ahmad T. (2001) Controls on the ⁸⁷Sr/⁸⁶Sr of carbonates in the Garwal Himalaya, headwaters of the Ganges. *J. Geol.* **109**, 737–753.

Bickle M. J., Bunbury J., Chapman H. J., Harris N. B., Fairchild I. J. and Ahmad T. (2003) Fluxes of Sr into the headwaters of the Ganges. *Geochim. Cosmochim. Acta* **67**, 2567–2584.

Bickle M. J., Chapman H. J., Bunbury J., Harris N. B. W., Fairchild I. J., Ahmad T. and Pomiès C. (2005) The relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges. *Geochem. Cosmochim. Acta* **69**, 2221–2240.

Bickle M. J., Tipper E. D., Galy A., Chapman H. J. and Harris N. W. B. (2015) On discrimination between carbonate and silicate inputs to Himalayan rivers. *Am. J. Sci.* **315**, 120–166.

Bookhagen B. and Burbank D. W. (2010) Toward a complete Himalayan hydrological budget: Spatiotemporal distribution of snowmelt and rainfall and their impact on river discharge. *J. Geophys. Res.* **115**, F03019 03025.

Bouchez J., Gaillardet J., Lupker M., Louvat P., France-Lanord C., Laurence Maurice L., Armijos E. and Moquet J.-S. (2012) Flood plains of large rivers: Weathering reactors or simple silos? *Chem. Geol.* **332–333**, 166–184.

Brady P. V. and Gislason S. R. (1997) Seafloor weathering controls on atmospheric CO₂ and global climate. *Geochim. Cosmochim. Acta* **61**, 965–973.

Caldeira, K., Arthur, M.A., Berner, R.A., & Lasaga, A.C. (1993). Cooling in the Cenozoic: Discussion of 'Tectonic forcing of late Cenozoic climate' by Raymo, M.E. and Ruddiman, W.F. Nature 361, 123–124.

Calmels D., Galy A., Hovius N., Bickle M., West A. J., Chen M. C. and Chapman H. (2011) Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan. *Earth Planet. Sci. Lett.* **303**, 48–58.

Carson M. A. and Kirby M. J. (1972) *Hillslope, Form and Process*. Cambridge University Press, Cambridge.

Chamberlin T. C. (1899) The influence of great epochs of limestone formation upon the constitution of the atmosphere. *J. Geol.* **6**, 609–621.

Chapman H. J., Bickle M., Thaw S. H. and Thiam H. N. (2015) Chemical fluxes from time series sampling of the Irrawaddy and Salween Rivers, Myanmar. *Chem. Geol.* **401**, 15–27.

Coogan L. A. and Dosso S. E. (2015) Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater. *Earth Planet. Sci. Lett.* **415**, 38–46.

Cox K. G. and Hawkesworth C. J. (1985) Geochemical stratigra-
phy of the Deccan Traps at Mahabaleshwar, Western Ghats,
India, with implications for open system magmatic processes. *J.
Petrol.* **26**, 355–377.

Craig H. (1961) Isotopic variations in meteoric waters. *Science* **133**, 1702–1703.

Craig H. and Gordon L. I. (1965) Deuterium and oxygen 18
variations in the ocean and the marine atmosphere. In *Stable
isotopes in oceanographic studies and paleotemperatures* (ed. E.
Tongiorgi). Consiglio nazionale delle Ricerche Laboratorio di
Geologia Nucleare, Spoleto, pp. 9–130.
Dalai T. K., Krishnaswami S. and 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. Cosmochem. Acta* **66**, 3397–3416.

Dalai T. K., Krishnaswami S. and Kumar A. (2003) Sr and ⁸⁷Sr/⁸⁶Sr in the Yamuna River System in the Himalaya: Sources, fluxes, and controls on Sr isotope composition. *Geochem. Cosmochim. Acta* **67**, 2931–2948.

Dansgard W. (1964) Stable isotopes in precipitation. *Tellus* **16**, 436–468.

Dessert C., Dupre B., Francois L. M., Schott J., Gaillardet J., Chakrapani G. and Bajpai S. (2001) Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the ⁸⁷Sr/⁸⁶Sr ratio of seawater. *Earth Planet. Sci. Lett.* **188**, 459–474.

Dawson T. E., Mambelli S., Plamboeck A. H., Templer P. H. and Tu K. P. (2002) Stable isotopes in plant ecology. *Annu. Rev. Ecol. Syst.* **33**, 507–559.

Edmond J. M. (1992) Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones. *Science* **258**, 1594–1597.

English N. B., Quade J., DeCelles P. G. and Garzione C. (2000) Geologic control of Sr and major element chemistry in Himalayan rivers, Nepal. *Geochim. Cosmochim. Acta* **64**, 2549–2566.

Fekete, B.M., Vorosmarty, C.J., Grabs, W., 2000. Global, composite runoff fields based on observed river discharge and simulated water balances, Documentation for UNH-GRDC Composite Runoff Fields, v. 1.0. Global Runoff Data Centre, Koblenz, Germany.

Fontorbe G., De La Rocha C. L., Chapman H. J. and Bickle M. J. (2013) The silicon isotopic composition of the Ganges and its tributaries. *Earth Planet. Sci. Lett.* **381**, 21–30.

France-Lanord F., Evans M., Hurtrez J.-E. and Riotte J. (2003) Annual dissolved fluxes from Central Nepal rivers: budget of chemical erosion in the Himalayas. *Compte Rendus Geosci.* **335**, 1131–1140.

Francois L. M. and Walker J. C. G. (1992) Modelling the phanerozoic carbon cycle and climate: constraints from the ⁸⁷Sr/⁸⁶Sr isotopic ratio of seawater. *Am. J. Sci.* **292**, 81–135.

Gabet E. J., Burbank D. W., Pratt-Sitaula B., Putkonen J. and Bookhagen B. (2008) Modern erosion rates in the High Himalayas of Nepal. *Earth Planet. Sci. Lett.* **267**, 482–494.

Gaillardet J., Dupre B., Louvat P. and Allegre C. J. (1999) Global silicate weathering and CO₂ consumption rates deduced from the chemistry of large rivers. *Chem. Geol.* **159**, 3–30.

Gajurel A. P., France-Lanord C., Huyghe P., Guilmette C. and Gurung D. (2006) C and O isotope compositions of modern fresh-water mollusc shells and river waters from the Himalaya and Ganga plain. *Chem. Geol.* **233**, 156–183.

Galy A. and France-Lanord C. (1999) Weathering processes in the Ganges-Brahmaputra basin and the river alkalinity budget. *Chem. Geol.* **159**, 31–60.

Galy A., France-Lanord C. and Derry L. A. (1999) The strontium isotopic budget of Himalayan Rivers in Nepal and Bangladesh. *Geochim. Cosmochim. Acta* **63**, 1905–1925.

Galy V., France-Lanord C., Beyssac O., Faure P., Kudrass H. and Palhol F. (2007) Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. *Nature* **450**, 407–410.

Garçon M., Chauvel C., France-Lanord C., Limonta M. and Garzanti E. (2013) Removing the “heavy mineral effect” to obtain a new Pb isotopic value for the upper crust. *Geochem. Geophys. Geosyst.* **14**, 9. https://doi.org/10.1002/ggge.20219.

Garçon M., Chauvel C., France-Lanord C., Limonta M. and Garzanti E. (2014) Which minerals control the Nd–Hf–Sr–Pb isotopic compositions of river sediments? *Chem. Geol.* **364**, 42–45.

Garzanti E., Andó S., France-Lanord C., Vezzoli G., Censi P., Galy V. and Najman Y. (2010) Mineralogical and chemical variability of fluvial sediments 1. Bedload sand (Ganga-Brahmaputra, Bangladesh). *Earth Planet. Sci. Lett.* **299**, 368–381.

Garzanti E., Andó S., France-Lanord C., Censi P., Vignola P., Galy V. and Lupker M. (2011) Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh). *Earth Planet. Sci. Lett.* **302**, 107–120.

Granet M., Chabaux F., Stille P., Dosseto A., France-Lanord C. and Blaes E. (2010) U-series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains: The case of the Himalayan rivers. *Geochim. Cosmochim. Acta* **74**, 2851–2865.

Gupta L. P. and Subramanian V. (1994) Environmental geochemistry of the River Gomti: A Tributary of the Ganges River. *Environ. Geol.* **24**, 235–243.

Hodell D., Turchyn A., Wiseman C., Escobar J., Curtis J., Brenner M., Gilli A., Mueller A., Anselmetti F., Ariztegui D. and Brown E. (2012) Late Glacial temperature and precipitation changes in the lowland Neotropics by tandem measurement of δ¹⁸O in biogenic carbonate and gypsum hydration water. *Geochim. Cosmochim. Acta* **77**, 352–368.

Hossain M., Aminul Islam A. T. M. and Kumar Saha S. (1987) *Floods in Bangladesh: Recurrent Disaster and People's Survival*. University Research Centre, Dhaka, Bangladesh, pp. 64–66.

Huh Y. (2010) Estimation of atmospheric CO₂ uptake by silicate weathering in the Himalayas and the Tibetan Plateau: a review of existing fluvial geochemical data. In *Geological Society* (eds. P. D. Clift, R. Tada and H. Zheng). London, Special Publications, London, pp. 129–151.

Jacobson A. D. and Blum J. D. (2000) Ca/Sr and ⁸⁷Sr/⁸⁶Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river water chemistry. *Geology* **28**, 463–466.

Jacobson A. D., Blum J. D. and Walter L. M. (2002) Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: Insights from the carbonate chemistry of stream waters. *Geochim. Cosmochim. Acta* **66**, 3417–3429.

Jha P. K., Subramanian V. and Sitasawad R. (1988) Chemical and sediment mass transfer in the Yamuna River - a tributary of the Ganges system. *J. Hydrol.* **104**, 237–246.

Kent J., Watson G. and Onstott T. (1990) Fitting straight lines and planes with an application to radiometric dating. *Earth Planet. Sci. Lett.* **97**, 1–17.

Kerrick D. M. and Caldeira K. (1993) Paleoatmospheric consequences of CO₂ released during early Cenozoic regional metamorphism in the Tethyan orogen. *Chem. Geol.* **108**, 201–230.

Krishnamurthy, R., Bhattacharya, S. (1991). Stable oxygen and hydrogen isotope ratios in shallow ground waters from India and a study of the role of evapo-transpiration in the Indian monsoon. In: Taylor, H.P., O'Neil, J.R., Kaplan, R., (Eds.), Stable Isotope Geochemistry: A Tribute to Samuel Epstein, Geochem. Society, Special Publication, pp. 187–203.

Krishnaswami S., Trivedi J. R., Sarin M. M., Ramesh R. and Sharma K. K. (1992) Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic ⁸⁷Sr/⁸⁶Sr. *Earth Planet. Sci. Lett.* **109**, 243–253.
Dalai T. K., Krishnaswami S. and 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. Cosmochem. Acta* **66**, 3397–3416.

Dalai T. K., Krishnaswami S. and Kumar A. (2003) Sr and ⁸⁷Sr/⁸⁶Sr in the Yamuna River System in the Himalaya: Sources, fluxes, and controls on Sr isotope composition. *Geochem. Cosmochim. Acta* **67**, 2931–2948.

Dansgard W. (1964) Stable isotopes in precipitation. *Tellus* **16**, 436–468.

Dessert C., Dupre B., Francois L. M., Schott J., Gaillardet J., Chakrapani G. and Bajpai S. (2001) Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the ⁸⁷Sr/⁸⁶Sr ratio of seawater. *Earth Planet. Sci. Lett.* **188**, 459–474.

Dawson T. E., Mambelli S., Plamboeck A. H., Templer P. H. and Tu K. P. (2002) Stable isotopes in plant ecology. *Annu. Rev. Ecol. Syst.* **33**, 507–559.

Edmond J. M. (1992) Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones. *Science* **258**, 1594–1597.

English N. B., Quade J., DeCelles P. G. and Garzione C. (2000) Geologic control of Sr and major element chemistry in Himalayan rivers, Nepal. *Geochim. Cosmochim. Acta* **64**, 2549–2566.

Fekete, B.M., Vorosmarty, C.J., Grabs, W., 2000. Global, composite runoff fields based on observed river discharge and simulated water balances, Documentation for UNH-GRDC Composite Runoff Fields, v. 1.0. Global Runoff Data Centre, Koblenz, Germany.

Fontorbe G., De La Rocha C. L., Chapman H. J. and Bickle M. J. (2013) The silicon isotopic composition of the Ganges and its tributaries. *Earth Planet. Sci. Lett.* **381**, 21–30.

France-Lanord F., Evans M., Hurtrez J.-E. and Riotte J. (2003) Annual dissolved fluxes from Central Nepal rivers: budget of chemical erosion in the Himalayas. *Compte Rendus Geosci.* **335**, 1131–1140.

Francois L. M. and Walker J. C. G. (1992) Modelling the phanerozoic carbon cycle and climate: constraints from the ⁸⁷Sr/⁸⁶Sr isotopic ratio of seawater. *Am. J. Sci.* **292**, 81–135.

Gabet E. J., Burbank D. W., Pratt-Sitaula B., Putkonen J. and Bookhagen B. (2008) Modern erosion rates in the High Himalayas of Nepal. *Earth Planet. Sci. Lett.* **267**, 482–494.

Gaillardet J., Dupre B., Louvat P. and Allegre C. J. (1999) Global silicate weathering and CO₂ consumption rates deduced from the chemistry of large rivers. *Chem. Geol.* **159**, 3–30.

Gajurel A. P., France-Lanord C., Huyghe P., Guilmette C. and Gurung D. (2006) C and O isotope compositions of modern fresh-water mollusc shells and river waters from the Himalaya and Ganga plain. *Chem. Geol.* **233**, 156–183.

Galy A. and France-Lanord C. (1999) Weathering processes in the Ganges-Brahmaputra basin and the river alkalinity budget. *Chem. Geol.* **159**, 31–60.

Galy A., France-Lanord C. and Derry L. A. (1999) The strontium isotopic budget of Himalayan Rivers in Nepal and Bangladesh. *Geochim. Cosmochim. Acta* **63**, 1905–1925.

Galy V., France-Lanord C., Beyssac O., Faure P., Kudrass H. and Palhol F. (2007) Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. *Nature* **450**, 407–410.

Garçon M., Chauvel C., France-Lanord C., Limonta M. and Garzanti E. (2013) Removing the “heavy mineral effect” to obtain a new Pb isotopic value for the upper crust. *Geochem. Geophys. Geosyst.* **14**, 9. https://doi.org/10.1002/ggge.20219.

Garçon M., Chauvel C., France-Lanord C., Limonta M. and Garzanti E. (2014) Which minerals control the Nd–Hf–Sr–Pb isotopic compositions of river sediments? *Chem. Geol.* **364**, 42–45.

Garzanti E., Andó S., France-Lanord C., Vezzoli G., Censi P., Galy V. and Najman Y. (2010) Mineralogical and chemical variability of fluvial sediments 1. Bedload sand (Ganga-Brahmaputra, Bangladesh). *Earth Planet. Sci. Lett.* **299**, 368–381.

Garzanti E., Andó S., France-Lanord C., Censi P., Vignola P., Galy V. and Lupker M. (2011) Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh). *Earth Planet. Sci. Lett.* **302**, 107–120.

Granet M., Chabaux F., Stille P., Dosseto A., France-Lanord C. and Blaes E. (2010) U-series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains: The case of the Himalayan rivers. *Geochim. Cosmochim. Acta* **74**, 2851–2865.

Gupta L. P. and Subramanian V. (1994) Environmental geochemistry of the River Gomti: A Tributary of the Ganges River. *Environ. Geol.* **24**, 235–243.

Hodell D., Turchyn A., Wiseman C., Escobar J., Curtis J., Brenner M., Gilli A., Mueller A., Anselmetti F., Ariztegui D. and Brown E. (2012) Late Glacial temperature and precipitation changes in the lowland Neotropics by tandem measurement of δ¹⁸O in biogenic carbonate and gypsum hydration water. *Geochim. Cosmochim. Acta* **77**, 352–368.

Hossain M., Aminul Islam A. T. M. and Kumar Saha S. (1987) *Floods in Bangladesh: Recurrent Disaster and People's Survival*. University Research Centre, Dhaka, Bangladesh, pp. 64–66.

Huh Y. (2010) Estimation of atmospheric CO₂ uptake by silicate weathering in the Himalayas and the Tibetan Plateau: a review of existing fluvial geochemical data. In *Geological Society* (eds. P. D. Clift, R. Tada and H. Zheng). London, Special Publications, London, pp. 129–151.

Jacobson A. D. and Blum J. D. (2000) Ca/Sr and ⁸⁷Sr/⁸⁶Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river water chemistry. *Geology* **28**, 463–466.

Jacobson A. D., Blum J. D. and Walter L. M. (2002) Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: Insights from the carbonate chemistry of stream waters. *Geochim. Cosmochim. Acta* **66**, 3417–3429.

Jha P. K., Subramanian V. and Sitasawad R. (1988) Chemical and sediment mass transfer in the Yamuna River - a tributary of the Ganges system. *J. Hydrol.* **104**, 237–246.

Kent J., Watson G. and Onstott T. (1990) Fitting straight lines and planes with an application to radiometric dating. *Earth Planet. Sci. Lett.* **97**, 1–17.

Kerrick D. M. and Caldeira K. (1993) Paleoatmospheric consequences of CO₂ released during early Cenozoic regional metamorphism in the Tethyan orogen. *Chem. Geol.* **108**, 201–230.

Krishnamurthy, R., Bhattacharya, S. (1991). Stable oxygen and hydrogen isotope ratios in shallow ground waters from India and a study of the role of evapo-transpiration in the Indian monsoon. In: Taylor, H.P., O'Neil, J.R., Kaplan, R., (Eds.), Stable Isotope Geochemistry: A Tribute to Samuel Epstein, Geochem. Society, Special Publication, pp. 187–203.

Krishnaswami S., Trivedi J. R., Sarin M. M., Ramesh R. and Sharma K. K. (1992) Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic ⁸⁷Sr/⁸⁶Sr. *Earth Planet. Sci. Lett.* **109**, 243–253.
Kumar B., Rai S. K., Saravana Kumar U., Verma S. K., Garg P., Vijaya Kumar S. V., Jaiswal R., Purendra B. K., Kumar S. R. and Pande N. G. (2010) Isotopic characteristics of Indian precipitation. *Water Resour. Res.* **46**, W12548.

Li D. D., Jacobson A. D. and McInerney D. J. (2014) A reactive-transport model for examining tectonic and climatic controls on chemical weathering and atmospheric CO₂ consumption in granitic regolith. *Chem. Geol.* **365**, 40–42.

Lupker M., Christian France-Lanord C., Valier Galy V., Lavé J., Gaillardet J., Gajurel A. P., Guilmette C., Rahman M., Singh S. K. and Sinha R. (2012a) Predominant flood plain over mountain weathering of Himalayan sediments (Ganga basin). *Geochim. Cosmochim. Acta* **84**, 410–432.

Lupker M., Blard P.-H., Lavé J., France-Lanord C., Leanni L., Puchol N., Charreau J. and Bourles D. (2012b) <sup>10</sup>Be-derived Himalayan denudation rates and sediment budgets in the Ganga basin. *Earth Planet. Sci. Lett.* **333–334**, 146–156.

Maher K. (2011) The role of fluid residence time and topographic scales in determining chemical fluxes from landscapes. *Earth Planet. Sci. Lett.* **312**, 48–58.

McKenzie N. R., Hughes N. C., Myrow P. M., Xiao S. and Sharma M. (2011) Correlation of Precambrian-Cambrian sedimentary successions across northern India and the utility of isotopic signatures of Himalayan lithotectonic zones. *Earth Planet. Sci. Lett.* **312**, 471–483.

Mondal M. E. A., Goswami J. N., Deomurari M. P. and Sharma K. K. (2002) Ion microprobe <sup>207</sup>Pb/<sup>206</sup>Pb ages of zircons from the Bundelkhand massif, northern India: implications for crustal evolution of the Bundelkhand-Aravalli protocontinent. *Precambr. Res.* **117**, 85–100.

Pal D. K., Srivastava P., Durge S. L. and Bhattacharyya T. (2003) Role of microtopography in the formation of sodic soils in the semi-arid part of the Indo-Gangetic Plains, India. *Catena* **51**, 3–31.

Pal S. K. (1986) *Geomorphology of River Terraces Along the Alaknanda Valley, Garhwal Himalaya*. B.R. Publishing Corporation, Delhi-110052.

Palmer M. R. and Edmond J. M. (1992) Controls over the strontium isotope composition of river water. *Geochim. Cosmochim. Acta* **56**, 2099–2111.

Peng Z. X., Mahoney J. J., Hooper P. R., Macdougall J. D. and Krishnamurthy P. (1998) Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy. *J. Geophys. Res.* **103**, 29,843-829,865.

Pogge von Strandmann P. A. E. and Henderson G. M. (2015) The Li isotope response to mountain uplift. *Geology* **43**, 67–70.

Quade J., English N. and DeCelles (2003) Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds. *Chem. Geol.* **202**, 275–296.

Rai S. K., Singh S. K. and Krishnaswami S. (2010) Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes. *Geochim. Cosmochim. Acta* **74**, 2340–2355.

Rao K. L. (1975) *India's Water Wealth*. Orient Longman Ltd., New Delhi.

Ray J. S., Veizer J. and Davis W. J. (2003) C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Super-group, India: age, diagenesis, correlations and implications for global events. *Precambrian Res.* **121**, 103–140.

Raymo M. E. and Ruddiman W. F. (1992) Tectonic forcing of late Cenozoic climate. *Nature* **359**, 117–122.

Raymo M. E., Ruddiman W. F. and Froelich P. N. (1988) Influence of late Cenozoic mountain building on ocean geochemical cycles. *Geology* **16**, 649–653.

Rengarajan R., Singh S. K., Sarin M. M. and Krishnaswami S. (2009) Strontium isotopes and major ion chemistry in the Chambal River system, India: Implications to silicate erosion rates of the Ganga. *Chem. Geol.* **260**, 87–101.

Richter F. M., Rowley D. B. and DePaolo D. J. (1992) Sr isotope evolution of seawater: the role of tectonics. *Earth Planet. Sci. Lett.* **109**, 11–23.

Sarin M. M., Krishnaswami S., Dilli K., Somayajulu B. L. K. and Moore W. S. (1989) Major ion chemistry of the Ganga-Brahmaputra river system: weathering processes and fluxes to the Bay of Bengal. *Geochim. Cosmochim. Acta* **53**, 997–1009.

Singh S. K., Trivedi J. R., Pande K., Ramesh R. and Krishnaswami S. (1998) Chemical and strontium, oxygen, and carbon isotopic compositions of carbonates from the Lesser Himalaya: Implications to the strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus rivers. *Geochim. Cosmochim. Acta* **62**, 743–755.

Singh K. P., Malika A., Mohana D. and Sinha S. (2004) Multivariate statistical techniques for the evaluation of spatial and temporal variations in water quality of Gomti River (India)—a case study. *Water Res.* **38**, 3980–3992.

Singh K. P., Malik A. and Sinha S. (2005a) Water quality assessment and apportionment of pollution sources of Gomti river (India) using multivariate statistical techniques - a case study. *Anal. Chim. Acta* **538**, 335–374.

Singh M., Sharma M. and Tobschall H. J. (2005b) Weathering of the Ganga alluvial plain, northern India: implications from fluvial geochemistry of the Gomati River. *Appl. Geochem.* **20**, 1–21.

Singh S., Rai S. K. and Krishnaswami S. (2008) Sr and Nd isotopes in river sediments from the Ganga Basin: Sediment provenance and spatial variability in physical erosion. *J. Geophys. Res.* **113**, pp. F03006, 03018.

Singh S., Singh M., Choudhary A. K., Saxena A., Singh I. B. and Jain A. K. (2009) Sr isotopic signature of the Ganga Alluvial Plain and its implication to Sr flux of the Ganga River System. *Indian J. Earth Sci. (Geol Rundsch)*, 0.1007/s00531-00009-00479-00534.

Sleep N. H. and Zahnle K. (2001) Carbon dioxide cycling and implications for climate on ancient Earth. *J. Geophys. Res.* **106**, 1373–1399.

Stallard R. F. and Edmond J. M. (1983) Geochemistry of the Amazon. 2. The influence of geology and weathering environment on the dissolved-load. *J. Geophys. Res.* **88**, 9671–9688.

Subramanian V., Sitasawad R., Abbas N. and Jha P. K. (1987) Environmental geology of the Ganga river basin. *J. Geol. Soc. India* **30**, 335–355.

Tipper E. T., Bickle M. J., Galy A., West A. J., Pomiès C. and Chapman H. J. (2006) The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry. *Geochim. Cosmochim. Acta* **70**, 2737–2754.

Torres M. A., West A. J. and Li G. (2014) Sulphide oxidation and carbonate dissolution as a source of CO₂ over geological timescales. *Nature* **507**, 346–349.

Turchyn A. V., Tipper E., Galy A., Lo J. and Bickle M. J. (2013) Isotope evidence for secondary sulfide precipitation along the Marsyandi River, Nepal, Himalayas. *Earth Planet. Sci. Lett.* **374**, 36–46.

Walker J. C. G., Hays P. B. and Kasting J. F. (1981) A negative feedback mechanism for the long-term stabilisation of Earth's surface temperature. *J. Geophys. Res.* **86**, 9776–9782.

West A. J., Bickle M. J., Collins R. and Brasington J. (2002) A small catchment perspective on Himalayan weathering fluxes. *Geology* **30**, 355–358.
Kumar B., Rai S. K., Saravana Kumar U., Verma S. K., Garg P., Vijaya Kumar S. V., Jaiswal R., Purendra B. K., Kumar S. R. and Pande N. G. (2010) Isotopic characteristics of Indian precipitation. *Water Resour. Res.* **46**, W12548.

Li D. D., Jacobson A. D. and McInerney D. J. (2014) A reactive-transport model for examining tectonic and climatic controls on chemical weathering and atmospheric CO₂ consumption in granitic regolith. *Chem. Geol.* **365**, 40–42.

Lupker M., Christian France-Lanord C., Valier Galy V., Lavé J., Gaillardet J., Gajurel A. P., Guilmette C., Rahman M., Singh S. K. and Sinha R. (2012a) Predominant flood plain over mountain weathering of Himalayan sediments (Ganga basin). *Geochim. Cosmochim. Acta* **84**, 410–432.

Lupker M., Blard P.-H., Lavé J., France-Lanord C., Leanni L., Puchol N., Charreau J. and Bourles D. (2012b) <sup>10</sup>Be-derived Himalayan denudation rates and sediment budgets in the Ganga basin. *Earth Planet. Sci. Lett.* **333–334**, 146–156.

Maher K. (2011) The role of fluid residence time and topographic scales in determining chemical fluxes from landscapes. *Earth Planet. Sci. Lett.* **312**, 48–58.

McKenzie N. R., Hughes N. C., Myrow P. M., Xiao S. and Sharma M. (2011) Correlation of Precambrian-Cambrian sedimentary successions across northern India and the utility of isotopic signatures of Himalayan lithotectonic zones. *Earth Planet. Sci. Lett.* **312**, 471–483.

Mondal M. E. A., Goswami J. N., Deomurari M. P. and Sharma K. K. (2002) Ion microprobe <sup>207</sup>Pb/<sup>206</sup>Pb ages of zircons from the Bundelkhand massif, northern India: implications for crustal evolution of the Bundelkhand-Aravalli protocontinent. *Precambr. Res.* **117**, 85–100.

Pal D. K., Srivastava P., Durge S. L. and Bhattacharyya T. (2003) Role of microtopography in the formation of sodic soils in the semi-arid part of the Indo-Gangetic Plains, India. *Catena* **51**, 3–31.

Pal S. K. (1986) *Geomorphology of River Terraces Along the Alaknanda Valley, Garhwal Himalaya*. B.R. Publishing Corporation, Delhi-110052.

Palmer M. R. and Edmond J. M. (1992) Controls over the strontium isotope composition of river water. *Geochim. Cosmochim. Acta* **56**, 2099–2111.

Peng Z. X., Mahoney J. J., Hooper P. R., Macdougall J. D. and Krishnamurthy P. (1998) Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy. *J. Geophys. Res.* **103**, 29,843-829,865.

Pogge von Strandmann P. A. E. and Henderson G. M. (2015) The Li isotope response to mountain uplift. *Geology* **43**, 67–70.

Quade J., English N. and DeCelles (2003) Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds. *Chem. Geol.* **202**, 275–296.

Rai S. K., Singh S. K. and Krishnaswami S. (2010) Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes. *Geochim. Cosmochem. Acta* **74**, 2340–2355.

Rao K. L. (1975) *India's Water Wealth*. Orient Longman Ltd., New Delhi.

Ray J. S., Veizer J. and Davis W. J. (2003) C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Super-group, India: age, diagenesis, correlations and implications for global events. *Precambrian Res.* **121**, 103–140.

Raymo M. E. and Ruddiman W. F. (1992) Tectonic forcing of late Cenozoic climate. *Nature* **359**, 117–122.

Raymo M. E., Ruddiman W. F. and Froelich P. N. (1988) Influence of late Cenozoic mountain building on ocean geochemical cycles. *Geology* **16**, 649–653.

Rengarajan R., Singh S. K., Sarin M. M. and Krishnaswami S. (2009) Strontium isotopes and major ion chemistry in the Chambal River system, India: Implications to silicate erosion rates of the Ganga. *Chem. Geol.* **260**, 87–101.

Richter F. M., Rowley D. B. and DePaolo D. J. (1992) Sr isotope evolution of seawater: the role of tectonics. *Earth Planet. Sci. Lett.* **109**, 11–23.

Sarin M. M., Krishnaswami S., Dilli K., Somayajulu B. L. K. and Moore W. S. (1989) Major ion chemistry of the Ganga-Brahmaputra river system: weathering processes and fluxes to the Bay of Bengal. *Geochim. Cosmochim. Acta* **53**, 997–1009.

Singh S. K., Trivedi J. R., Pande K., Ramesh R. and Krishnaswami S. (1998) Chemical and strontium, oxygen, and carbon isotopic compositions of carbonates from the Lesser Himalaya: Implications to the strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus rivers. *Geochim. Cosmochim. Acta* **62**, 743–755.

Singh K. P., Malika A., Mohana D. and Sinha S. (2004) Multivariate statistical techniques for the evaluation of spatial and temporal variations in water quality of Gomti River (India)—a case study. *Water Res.* **38**, 3980–3992.

Singh K. P., Malik A. and Sinha S. (2005a) Water quality assessment and apportionment of pollution sources of Gomti river (India) using multivariate statistical techniques - a case study. *Anal. Chim. Acta* **538**, 335–374.

Singh M., Sharma M. and Tobschall H. J. (2005b) Weathering of the Ganga alluvial plain, northern India: implications from fluvial geochemistry of the Gomati River. *Appl. Geochem.* **20**, 1–21.

Singh S., Rai S. K. and Krishnaswami S. (2008) Sr and Nd isotopes in river sediments from the Ganga Basin: Sediment provenance and spatial variability in physical erosion. *J. Geophys. Res.* **113**, pp. F03006, 03018.

Singh S., Singh M., Choudhary A. K., Saxena A., Singh I. B. and Jain A. K. (2009) Sr isotopic signature of the Ganga Alluvial Plain and its implication to Sr flux of the Ganga River System. *Indian J. Earth Sci. (Geol Rundsch)*, 0.1007/s00531-00009-00479-00534.

Sleep N. H. and Zahnle K. (2001) Carbon dioxide cycling and implications for climate on ancient Earth. *J. Geophys. Res.* **106**, 1373–1399.

Stallard R. F. and Edmond J. M. (1983) Geochemistry of the Amazon. 2. The influence of geology and weathering environment on the dissolved-load. *J. Geophys. Res.* **88**, 9671–9688.

Subramanian V., Sitasawad R., Abbas N. and Jha P. K. (1987) Environmental geology of the Ganga river basin. *J. Geol. Soc. India* **30**, 335–355.

Tipper E. T., Bickle M. J., Galy A., West A. J., Pomiès C. and Chapman H. J. (2006) The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry. *Geochim. Cosmochim. Acta* **70**, 2737–2754.

Torres M. A., West A. J. and Li G. (2014) Sulphide oxidation and carbonate dissolution as a source of CO₂ over geological timescales. *Nature* **507**, 346–349.

Turchyn A. V., Tipper E., Galy A., Lo J. and Bickle M. J. (2013) Isotope evidence for secondary sulfide precipitation along the Marsyandi River, Nepal, Himalayas. *Earth Planet. Sci. Lett.* **374**, 36–46.

Walker J. C. G., Hays P. B. and Kasting J. F. (1981) A negative feedback mechanism for the long-term stabilisation of Earth's surface temperature. *J. Geophys. Res.* **86**, 9776–9782.

West A. J., Bickle M. J., Collins R. and Brasington J. (2002) A small catchment perspective on Himalayan weathering fluxes. *Geology* **30**, 355–358.
West A. J., Galy A. and Bickle M. (2005) Tectonic and climatic controls on silicate weathering. *Earth Planet. Sci. Lett.* **235**, 211–228.

White A. F. and Blum A. E. (1995) Effects of climate on chemical weathering in watersheds. *Geochim. Cosmochim. Acta* **59**, 1729–1747.

York D. (1969) Least squares fitting of a straight line with correlated errors. *Earth Planet. Sci. Lett.* **5**, 320–324.

*Associate Editor: Andrew D. Jacobson*
West A. J., Galy A. and Bickle M. (2005) Tectonic and climatic controls on silicate weathering. *Earth Planet. Sci. Lett.* **235**, 211–228.

White A. F. and Blum A. E. (1995) Effects of climate on chemical weathering in watersheds. *Geochim. Cosmochim. Acta* **59**, 1729–1747.

York D. (1969) Least squares fitting of a straight line with correlated errors. *Earth Planet. Sci. Lett.* **5**, 320–324.

*Associate Editor: Andrew D. Jacobson*