Geothermal fluxes of alkalinity in the Narayani river system of central

21 Pages·2004·0.64 MB·English

33
Article
GG
GGeeoocchheemmiissttrryy Volume 5, Number8
GGeeoopphhyyssiiccss 25August2004
Q08011,doi:10.1029/2004GC000719
GGeeoossyysstteemmss
ISSN: 1525-2027
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
Geothermal fluxes of alkalinity in the Narayani river system
of central Nepal
Matthew J. Evans
ChemicalSciencesDivision,OakRidgeNationalLaboratory,P.O.Box2008,MS6375,OakRidge,Tennessee37831,
USA ([email protected])
Louis A. Derry
Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA ([email protected])
Christian France-Lanord
Centre de Recherches Pe´trographiques et Ge´ochimiques/CNRS, BP20 54501 Vandœuvre les Nancy, France
([email protected])
[1] NumeroushotspringsflowwithinthesteeplyincisedgorgesofthecentralNepalHimalayanfront.The
springfluidshavetotaldissolvedsolids(TDS)upto7000mg/LandNa+,andK+typicallycomprise>50%
of the cationic charge, indicating that high-temperature silicate alteration is the dominant source of hot
springalkalinity.HCO(cid:1)isnormallythedominantanion.Srisotoperatiosfromthehydrothermalfluidsare
3
similartotherangeofvaluesfoundinthehostrocksandimplysignificantfluid-rockinteractionwithlocal
lithologies. To determine the impact of the hydrothermal solute load on the local and regional river
chemistry, we use a chemical mass balance approach to quantify the hot spring discharge. The springs are
ubiquitously enriched in germanium (Ge) with high but variable Ge/Si. Himalayan rivers upstream of the
hot spring zones have Ge/Si systematics like other unpolluted rivers, but downstream they are highly
anomalous, with Ge/Si from 2 to 20 mmol/mol. Ge and Si appear to behave conservatively during mixing
of spring and river, and the large disparity between river and spring [Ge] and Ge/Si ratios makes
germanium an effective tracer of hot spring input. We use the Ge/Si mass balance to estimate the spring
fluxtoindividualriversystems.OurresultsshowthatthepremonsoonspringflowovertheentireNarayani
basin is about 2 m3/s (with a factor of 2 uncertainty), or 0.5% of the total Narayani river discharge. We
estimatethatthespringsprovide25(±15)%ofthesilicate-derivedalkalinitytotheNarayanisystemduring
the low-flow season from October to May. Available monsoon season data indicate that the spring flux
increases during the monsoon by a factor of 2–3, but this increased flow is diluted by the up to 10(cid:2)
increase in overall river flow. The annual river discharge-weighted mean spring flux is 3.0 ± 1.2 m3/s for
the Narayani; hydrothermal alteration contributes (cid:3)10% of the annual flux of silicate alkalinity to this
large river system.
Components: 15,037words, 7figures, 8tables.
Keywords: alkalinity;germanium; Himalaya;hot springs;weathering.
Index Terms: 1045 Geochemistry: Low-temperature geochemistry; 1065 Geochemistry: Trace elements (3670); 8135
Tectonophysics:Hydrothermal systems(8424).
Received 20February 2004;Revised 8June2004;Accepted8July 2004;Published25 August2004.
Evans, M. J., L. A. Derry, and C. France-Lanord (2004), Geothermal fluxes of alkalinity in the Narayani river system of
centralNepal, Geochem.Geophys. Geosyst.,5,Q08011,doi:10.1029/2004GC000719.
Copyright 2004bythe American GeophysicalUnion 1of 21
33
GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
1. Introduction linity to the river system of central Nepal from
geothermal sources. The results indicate that hy-
[ ] The sources of alkalinity in Himalayan rivers, drothermal activity is an important source of sili-
2
and the impact of this alkalinity on the world cate derived alkalinity to rivers in this tectonically
oceans have been of interest to geochemists seek- active region.
ing to evaluate the influence of mountain building
on global geochemical cycles [Sarin et al., 1989; 2. Geologic Setting
Krishnaswami et al., 1992; Pande et al., 1994;
Quade et al., 1997; Dalai et al., 2002; Dowling et [ ] The Himalayan front of central Nepal consists
4
al., 2003; Jacobson et al., 2003]. In the Ganges- of two north-dipping fault systems that delineate
Brahmaputra (GB) river system, independent stud- the main lithologic units (Figure 1). Moving from
ies have established that the major source of south to north and upsection, the intermediate-
alkalinity is the dissolution of carbonates, with gradeschistsandphyllitesoftheLesserHimalayan
smaller but variable contribution from silicate sequence (LH) are separated from the over-riding
alteration[GalyandFrance-Lanord,1999;English high-grade gneisses of the High Himalayan Crys-
et al., 2000; Jamieson et al., 2002]. These results talline units (HHC) by the Main Central Thrust
placelimitsontheroleofsilicateweatheringinthe (MCT). Both the LH and HHC are derived from
Himalaya as a long term carbon sink but the clastic and carbonate sediments deposited off the
processes that generate silicate alkalinity require passive continental margin of India in the Middle
further quantification. Both climate and geomor- to Late Proterozoic, respectively [Parrish and
phic position appear to play roles in determining Hodges, 1996]. The Main Central Thrust is a 1–
silicate weathering fluxes. At the scale of large 10 kmwide shear zone[Le Fort,1975]. The South
Himalayan catchments, silicate weathering yields Tibetan Detachment System (STDS) consists of a
appear to scale with runoff [France-Lanord et al., series of low-angle normal faults [Burg et al.,
2003]. In the high-relief Himalayan zone, silicate 1984; Burchfiel and Royden, 1985; Hodges et al.,
weathering in the soil environment is limited by 1992] dividing the HHC from the overlying
rapid landscape turnover and short residence time Tethyan Sedimentary Sequence (TSS), a Paleozoic
ofmineralsinthesoil.Regolithweatheringismore to Tertiary low-grade shelf sequence dominated by
extensive in the more stable foreland basin, and carbonates.TheNarayaniriversystemiscomposed
silicate weathering yields are substantially higher of four major south-flowing tributaries which flow
in the Siwalik zone [Derry and France-Lanord, across all the lithologies of the Himalayan front
1997; West et al., 2002; France-Lanord et al., (Figure 1); these are the Kali Gandaki, the Mar-
2003]. syandi River, the Bhuri Gandaki, and the Bhote
Kosi/Trisuli River. Other significant streams sam-
[3] Hydrothermal activity along the Himalayan pled in this study include the Mayagdi Khola, the
front is another, less well investigated, source of Modi Khola, the Seti Khola, and the Langtang
river solutes and alkalinity. Active hot springs are Khola.
present in all the major river valleys along the
topographic front of the central Himalaya. In a
previous study we estimated that hot springs are 3. Hot Springs
the source of up to 30% of the silicate-derived
cations in the Marsyandi river in central Nepal [ ] AlongthebaseoftheHighHimalaya,thereisa
5
[Evans et al., 2001]. Alteration reactions at hydro- significant topographic break which is generally
thermal temperatures can be an efficient means of paralleltoandoftencoincidentwiththetraceofthe
neutralizingcarbonicacidandgeneratingalkalinity Main Central Thrust (MCT) [Seeber and Gornitz,
in silicate terranes. The direct introduction of hot 1983; Sai-Halasz, 1999; Wobus et al., 2003].
spring fluids and the dissolution of hydrothermal Along this topographic break and at or near the
precipitates couldbe significant sources ofsilicate- MCT, numerous hot springs flow within and along
derived alkalinity in geothermally active regions, the south-flowing streambeds [Bhattarai, 1980].
but these fluxes have rarely been quantified in Typicallythespringsoccurasclustersofindividual
studies of chemical mass balance in river systems. water sources along a stretch of river bank from
Here we develop a chemical mass balance tech- tenstooveronehundredmeterslong.Anumberof
niqueusinggermaniumtoestimatethecontribution spring systems are associated with nick points in
of hot springs to the river discharge, and then stream profiles. Geothermal systems in areas of
compute the contribution of silicate derived alka- relatively high regional heat flow often form along
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GeochemistryGG eevvaannss eett aall..:: ggeeootthheerrmmaall fflluuxxeess ooff aallkkaalliinniittyy 1100..11002299//22000044GGCC000000771199
Geophysics
Geosystems
Figure1. Generalizedgeologicmap[afterColchenetal.,1986]withhotspringlocations(opencircles).Sampling
sites often include several flows from the same spring system. The number within the open circle indicates the
number of distinct flows sampled at each location. See Tables 1 and 2 for sample-specific longitude and latitude.
Many active hot springs are located near the trace of the Main Central Thrust. Most springs flow from the High
HimalayanCrystalline(HHC)orLesserHimalayan(LH)formations.SpringsintheupperKaliGandakisystemflow
from the Tethyan Sedimentary Series (TSS) of rocks.
fault/fracture zones and are particularly prevalent andambienttemperatures.Sampleswererefrigerat-
where these fracture zones intersect with topo- ed upon return to Kathmandu, within 1 to 15 days
graphic lows [Rybach, 1981]. Similarly, in central after collection, and after air transport to the labo-
Nepal, hot springs tend to flow where the MCT ratory until analysis. Major element concentrations
shearzoneintersectswiththehighlyincisedgorges were measured by inductively-coupled-plasma
(Figure 1). The majority of the springs in the study atomicemissionsspectrometry(ICP-AES)(cations)
area flow from HHC bedrock although the springs and ion chromatography (anions) at Cornell Uni-
in the Myagdi Khola and a portion of those in the versity and the Centre de Recherches Pe´trographi-
Kali Gandaki and Trisuli flow from Lesser Hima- ques Ge´ochimiques (CRPG), in Nancy, France.
layan rock formations, demonstrating the strong Uncertainty on both cation and anion analyses was
regional topographic control over spring locations. <5%.Srisotopiccompositionsweremeasuredona
An additional set of springs are found within the VG Sector-54 at Cornell University. Dissolved
N–S trending graben in the upper Kali Gandaki silica was determined by both ICP-AES and ‘‘mo-
system and flow from TSS lithologies. lybdate blue’’ spectrophotometry and the results
are comparable to within 10%. Ge concentrations
were determined by isotope-dilution hydride gen-
4. Methods
eration [Mortlock and Froelich, 1996] on a Fin-
nigan Element II inductively coupled plasma mass
[ ] River and hot spring samples for this study
6 spectrometer (ICP-MS) at Cornell University
span multiple field seasons from 1975 to 2001.
[Kurtz, 2000]. Total reagent blanks for Ge are less
Water samples were filtered through 0.22 or
than 0.5 ppt and reproducibility is (cid:3)3%.
0.45 mm filters and stored in acid-washed polyeth-
ylene bottles. Two samples were taken from each [ ] River discharge was measured in the field for
7
location; one was acidified with ultra-clean HNO the Myagdi Khola, Kali Gandaki, Modi Khola,
3
foranalysisofdissolvedcations.Theotherwasnot Seti Khola, and Langtang Khola rivers using a
acidifiedandtakenwithoutheadspacetominimize submersible flowmeter (General Oceanics#
degassing, and used for analysis of anions. Every 2030R) and tape and staff measurements for river
effort was made to minimize exposure to sunlight width and depth. Discharge measurements were
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
taken at the same location and at the same time as range from 20(cid:2) to as high as 68(cid:2)C with an average
water samples to reduce potential uncertainty in of44(cid:2)C(n=27).TDSvaluesrangefromaslowas
subsequent calculations. Multiple flow readings 450 mg/L to 7000 mg/L. The cation balance is
were taken at bridge crossings where depth and dominated by the alkali elements (Na+ + K+) and
width could be readily measured. The Nepali Ca2+ with minor Mg2+ (Figure 2a) while HCO(cid:1) is
3
Government’s Department of Hydrology and typically the principal anion, with concentrations
Meteorology (DHM) has monitored stream gauges up to 56,000 mmol/kg (Figure 2b). Sulfate levels
for 10 years and report average monthly discharge are generally less than 1000 mmol/kg. Most of the
for the Bhuri Gandaki, Marsyandi, Trisuli, and springs are saturated to supersaturated with respect
Kali Gandaki as well as the Narayani over this to both calcite and quartz. Several springs in the
10-year span [Yogacharya et al., 1998]. Uncer- Marsyandi are Cl(cid:1) rich, with up to 114 mmol/kg,
tainty for these discharge measurements has been while two springs in the Myagdi Khola have very
estimated at ±25% [Lave and Avouac, 2001]. The high boron concentrations (up to 2400 mmol/kg).
discharge measured in March, 2001 for this study Sr2+ concentrations and isotopic compositions are
for the Kali Gandaki at Beni is 58 ± 21 m3/s highly variable with [Sr2+] from 167 mmol/kg to
(Table 3). The ten year average for March dis- <1 mmol/kg and 87Sr/86Sr range from typical
charge reported by the DHM for the station at carbonate values of 0.71 to as enriched as 0.98.
Baglung, just downstream from Beni, is 54 ± The Sr chemistry in the springs appears to be
13 m3/s [Yogacharya et al., 1998]. Both values strongly lithology-controlled (Figure 3).
agree well within their reported uncertainties and
this provides a level of confidence for the dis-
6. Chemical Mass Balance
charge measurement technique used in this study.
[ ] In order to assess the overall effect of the
10
5. Results spring systems across the Narayani drainage the
discharge from all the springs must be quantified.
Direct measurement of the spring discharge is
5.1. River Chemistry
difficult, as many springs are physically inaccessi-
[8] New samples analyzed for this study from ble and/or are relatively small and diffuse flows
central Nepal rivers have temperatures ranging which can discharge directly into the streambed. A
from 8(cid:2) to 19(cid:2)C and pH values range from 7.9 to chemical mass balance approach provides a viable
8.9 (Table 1). As most of the rivers have head- alternative to direct measurement and has the
watersintheTSS,carbonate dissolutiondominates distinct advantage of accounting for springs that
the cation budget with Ca2+ and Mg2+ comprising are both hard to reach and as of yet un-sampled or
75 to 95% of the cationic charge, and Na+ + K+ unmapped. This approach has long been used in
making up equal portions of the remaining cation hydrological studies [Kilpatrick and Cobb, 1985;
budget (Figure 2a). Bicarbonate is the dominant Herschy,1995]andwassuccessfully appliedtothe
anion(Figure2b)andtheriversgenerallyhavelow springs flowing near the MCT in the Marsyandi
[Cl(cid:1)]. Total dissolved solids (TDS) for the rivers system [Evans et al., 2001].
averagesaround150mg/Landishigherinsamples
downstreamofspringlocations.Theriversarenear 6.1. Germaniumas aTracer of Hot Spring
or above saturation with respect to calcite. The
Input
chemistry of the Trisuli differs somewhat from the
other major streams, with a larger contribution to [11] Several characteristics must be assessed when
the cationic charge from Na+ and K+. Galy and choosing an appropriate tracer of hot spring input.
France-Lanord [1999] observed that the Bhote Most importantly, the element(s) involved should
Kosi-Trisuli River system is primarily a silicate show conservative behavior during mixing of
bedrock drainage basin, and the water chemistry is spring and river waters. In addition, in order for
less affected by carbonate dissolution than are the the mass balance to be well resolved there should
other major streams. beastrongcontrastbetweenthetracervaluesinthe
springs and surface waters. Evans et al. [2001]
used Cl(cid:1) as an effective chemical tracer of hot
5.2. Hot Spring Chemistry
spring input for the Marsyandi spring system. In
[ ] The central Nepal hot springs are primarily this system, the Cl(cid:1) concentrations in the springs
9
Na+ (cid:1) HCO(cid:1) (cid:1) Ca2+ (cid:1) Cl(cid:1) waters with circum- are up to 4 orders of magnitude higher than those
3
neutral pH (Table 2). Temperatures of the springs in the tributaries and Cl(cid:1) is a conservative element
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
cleat Ge/Si 5.213.5 4.61.07.02.18.21.94.21.47.41.40.91.51.45.20.76.2 4.50.4 3.20.5 12.219.43.60.9
rti
hisa Si 8887 8797101929992101108103811185459605259 67102 75124 8591100109
t
onof 2+Ca 871840 101312081310149013711214921948107252052110159088461244774 702744 714547 9658701349543
si
r
e +
v 2 06 6756191621851713 77 40 8345
g 74 6046775327009169 83 38 2380
L M 44 5779764661353374 14 22 3352
M
T
H +K 5962 60849374877270625466484969727949 4090 5653 545710857
e
h
t
in +Na 191233 6569028528336004232542694956153305212200937135 8976 10381 21329516846
e
bl
vaila (cid:1)NO3 7.72.3 5.65.85.6 7.42.6 7.92.9 9.58.0
a
s
1i B 2.46.3 4.35.14.6 3.7 7.3 0.6 0.50.0 1.72.40.8
e
ullTabl 2(cid:1)SO4 393374 661879889117190673443335157014197832594413962262 266439 297138 40241525780
f
The (cid:1)Cl 7488 473645734663528361188202394282320813315377093 275 227 1422198615
[
ars (cid:1)F 56 810101212878945996226 34 33 77
e
v
NepalRi (cid:1)HCO3 diRiver20612019 Gandaki2061239125232811262424552004258823961194153715141549161423302095 Khola13361639 14251498 Khola1894169736191424
ntral TDS Myag220215 Kali278343360416361314231264290120140180147157358206 Modi145188 156138 Seti214204225129
e
C
or pH 8.68.9 8.88.88.58.88.5 8.68.88.68.68.78.0 8.6 8.4 8.58.5 8.68.8 8.68.46.0
f
a
at C
ntD T,(cid:2) 1415 141510299131711 1418 910 919 121224 mol.
aceEleme Date 4/3/014/3/01 4/5/014/5/014/7/0112/2/9512/3/9512/5/9512/6/953/12/953/13/957/7/987/8/987/26/987/27/987/27/985/16/9311/13/93 4/10/014/11/01 4/11/014/12/01 4/12/014/13/014/13/017/7/98 e/Siinmol/m9].
RepresentativeSample).MajorandTrw.g-cubed.org.] Latitude,Longitude,NEStream 002822.328329.77MyagdiRiver(cid:2)(cid:2)002820.538333.79MyagdiRiver(cid:2)(cid:2) 008333.99KaliGandaki2821.19(cid:2)(cid:2)002824.828336.02KaliGandaki(cid:2)(cid:2)002829.788339.26KaliGandaki(cid:2)(cid:2)KaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandakiKaliGandaki 008349.71ModiKhola2825.10(cid:2)(cid:2)008349.16Kyumunu2824.47(cid:2)(cid:2)Khola008349.54ModiKhola2823.65(cid:2)(cid:2)008353.78MardiKhola2819.30(cid:2)(cid:2) 008357.75SetiKhola2821.72(cid:2)(cid:2)008357.59SetiKhola2821.50(cid:2)(cid:2)00283.00844.50SetiKhola(cid:2)(cid:2)SetiKhola entrationsinmol/kgexceptTDSinmg/kgandGmviouslyreportedbyGalyandFrance-Lanord[199viouslyreportedbyFrance-Lanordetal.[2003].viouslyreportedbyEvansetal.[2001].
Table1(http://ww MLB67MLB75 MLB76MLB78MLB86bNAG32bNAG39bNAG43bNAG45NH6bNH8bcMO308cMO314cMO500cMO515cMO517LO1bbNAG5 MLB88MLB93 MLB94MLB98 MLB99MLB103cMO200cMO304 aAllconcbDataprecDatapredDatapre
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
form arrays on Ge/Si versus Si (or 1/Si) plots
[Froelich et al., 1985]. This relationship has been
interpretedtoreflectmixingbetweenweatheringof
primary minerals which yield high [Si] and low
Ge/Si, and weathering of secondary minerals,
which yield low [Si] and high Ge/Si [Mortlock
and Froelich, 1987; Murnane and Stallard, 1990;
Kurtzetal.,2002].RiverswithGe/Sivaluesabove
3 mmol/mol are rare and are known only from
distinctsettings.Areasofhighweatheringintensity
or blackwater streams both in tropical environ-
ments can produce streams with Ge/Si values
(cid:3)2.5 mmol/mol and in some cases, glacial weath-
ering processes can produce low [Si] waters with
Ge/Si up to 2 mmol/mol, probably from the break-
down of high Ge/Si biotite [Chillrud et al., 1994;
Anders et al., 2003]. Higher values have only been
reported from a few locations affected by hydro-
thermal input and/or possibly sulfide weathering
[MortlockandFroelich,1987;Andersetal.,2003].
[ ] The major rivers in central Nepal have anom-
13
alously high Ge/Si ratios and do not lie near the
global mixing array (Figure 4). A significant num-
Figure2. (a)Cationand(b)aniontriangulardiagrams ber of rivers in the Narayani drainage show Ge/Si
showing river (filled diamonds) and hot spring (open values from 3 to 20 mmol/mol and have [Ge] up to
circles) chemistry. Rivers are dominated by carbonate 2.6 nmol/kg, (Table 1), the highest reported values
dissolution,withhighCa2+andMg2+,whilehotsprings for streams not affected by coal-fly ash input
show astrong silicate alteration influence with Na+ and (Figure 4). These highGe/Si values are only found
K+makingupmostofthecationbudget.Bothriverand
in streams with active hot springs indicating that
hot spring anion balances are controlled by bicarbonate
both the high Ge concentrations and Ge/Si ratios
with some hot springs showing variable but strong Cl(cid:1)
in Himalayan streams result from hot spring input.
enrichment.
Further, rivers sampled upstream of the zone of
geothermal activity and tributaries without hot
inthissetting.However,thestrongCl(cid:1)enrichment springs have lower Ge/Si ratios from 0.2 to
seenintheMarsyandispringsisnotcommoninthe 1.6 mmol/mol, (average Ge/Si = 0.7). These values
central Nepal spring systems, most of which have are typical of low to moderate intensity weathering
relatively low Cl(cid:1). Similarly, springs along the processes in continental rocks and fall near the
Myagdi Khola are highly enriched in boron, but global Ge/Si mean of 0.5 mmol/mol [Mortlock and
such high B levels are not typical of most other Froelich, 1987].
Nepali springs. Germanium levels are ubiquitously
high in Himalayan hot springs (Table 2) [Evans
and Derry, 2002] and the characteristic enrichment 6.3. Germanium in Hot Springs
of Ge in hydrothermal waters makes it an effective
[ ] HydrothermalwatersareenrichedinGeregard-
tracer of hot spring input to streams. 14
less of geologic setting (e.g., Iceland [Arnorsson,
1984], the Massif Central [Criaud and Fouillac,
6.2. Germanium in Rivers
1986], and the seafloor [Mortlock et al., 1993]).
[ ] Germanium and silicon display like behavior HotspringsincentralNepalhaveveryhighgerma-
12
inmostgeologicalenvironmentsduetosimilarities niumconcentrations(upto684nmol/kg)andsome
in both charge and ionic radius. Ge/Si ratios can of the highest (Ge/Si) ratios reported for thermal
therefore be used to trace silica behavior during waterswithvaluesrangingfrom4to1000mmol/mol
surface processes [Mortlock and Froelich, 1987; [Evans and Derry, 2002]. The large and consistent
Murnane and Stallard, 1990; Kurtz et al., 2002]. contrast between spring and surface waters makes
Globally, germanium-silicon systematics in clean germanium ideal for use in the chemical mass
rivers (unaffected by coal fly ash pollution) tend to balance(Table2).Germaniumhassimilarchemical
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
cleat Ge/Si 6751076696696653 274147334293851170 7943 212016 268265199 s.The
rti yse
ofthisa SrSi 5667636455675775706642 392468569795815365315971262078353623850 633991220 532501524 3110218308272967 2(cid:1)SOanal4
version 8786Sr/ 0.9260.9160.916 0.915 0.796 0.733 0.810 0.7300.716 0.8200.798 0.816 alackof
TML 2+Sr 8.712.410.010.58.411.3 1.31.91.11.014.212.519.29.11.112.89.91.46.4167.239.821.3 4.44.75.2 18.016.711.910.2 dueto
H
s
e
nthe 2+Ca 12189141194122212801325 6376888947022243218133082320309166932146892509208524733216 239225342068 4821443149983346 sampl
i e
e m
ailabl 2+Mg 435306423399465317 133129207152133911081375876131126377470348366234362218 100710721059 1449125311891300 forso
v d
2isa +K 696101674786812301928 116941041041351139716510908271696151217610175811172171 166201127 1876148915472261 culate
e cal
ullTabl +Na 205133743422288242721674626446 19661663946104412353801224466407171811137515268122765511104883562705 17522007125 41096296742823038495 (cid:1)O]not3
f C
[The (cid:1)NO3 78854766 7293568175 9086 45 with[H
a
rCentralNepalHotSprings 2(cid:1)(cid:1)(cid:1)FClSOB4 MyagdiHotSprings2679368562145348151997612383283971167814882971040560615712668717672135121815825784 KaliGandakiHotSprings2918843676321378439445413322701047339229138212408879796396692651969824513734778122781063588963149794227458316265936136332316930891313651371206422855280725989 ModiKholaHotSprings132487512152657232312 SetiKholaHotSprings7248216102657516585901937932487411126573395693 SeriesDG-99dataareincomplete
Datafo (cid:1)HCO3 12330215231349014942980514069 1652152022381881731515949 16846647088142008564536424199715718 77148716 54865416201613415381 ol.Note:
ement TDS 172629131861202114742245 2612452492181446 10941286167022551185871692953 734809 4630353029253403 mol/mm
TraceEl CpH 6.86.96.87.06.56.8 8.17.97.67.36.5 7.27.286.56.77.4 6.87.0 6.56.36.26.5 dGe/Siin
and T,(cid:2) 514951425254 3638435466 30695060 201321 3939 45434341 kgan
Table2(RepresentativeSample).Majorhttp://www.g-cubed.org.] Latitude,Longitude,NEDate 00MLB662822.078330.154/3/01(cid:2)(cid:2)00MLB682822.118330.124/3/01(cid:2)(cid:2)00MLB702822.078330.154/3/01(cid:2)(cid:2)00MLB712822.078330.154/3/01(cid:2)(cid:2)NH303/13/95NH323/14/95 00MLB812827.478337.574/6/01(cid:2)(cid:2)00MLB822827.598337.654/6/01(cid:2)(cid:2)008338.454/6/01MLB832828.68(cid:2)(cid:2)00MLB842828.688338.454/6/01(cid:2)(cid:2)00MLB852829.788339.264/7/01(cid:2)(cid:2)MO5207/27/98DG99363/15/99DG99373/15/99NAG1711/21/95NH93/14/95NH113/15/95NH123/15/95LO465/20/93LO475/20/93LO655/21/93LO1055/27/93 00MLB872824.948349.714/10/01(cid:2)(cid:2)00MLB922824.948349.714/10/01(cid:2)(cid:2)DG99393/15/99 008357.614/13/01MLB1012821.65(cid:2)(cid:2)008357.614/13/01MLB1022821.65(cid:2)(cid:2)NH373/16/95NH383/16/95 aAllconcentrationsinmol/kgexceptTDSinmg/mremainingdataarereportedforcompleteness.
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
fected by hot spring input, and the hot springs. We
make the assumptions that the Ge/Si in the main
stem is the weighted sum of the hot spring and
tributary contributions and that Ge and Si behave
conservatively during mixing:
(cid:1) (cid:2) (cid:1) (cid:2)
Ge Ge
(cid:1)
Q ½Si(cid:7) Si Si
FWHS ¼QHS ¼½Si(cid:7)trib(cid:2)(cid:1)Ge(cid:2)R (cid:1)Ge(cid:2)trib; ð1Þ
trib HS (cid:1)
Si Si
HS R
Figure3. Sr/Ca(mmol/mol)versus87Sr/86Srinthehot
springs. The Sr chemistry of the springs reflects that of FHS is the fraction of the total river discharge
W
the bedrock at the spring outlet. Springs flowing from
contributed by the hot springs. Square brackets
the Tethyan Sedimentary Series (TSS) have typical
denote concentrations. Subscript HS denotes
carbonate 87Sr/86Sr (0.712–0.717) and variable Sr/Ca.
average hot spring values for each basin, trib
LesserHimalayan(LH)springshavelowerSr/Cavalues
and very radiogenic 87Sr/86Sr up to 0.98. Springs that denotes all nonspring affected surface water input
flow from High Himalayan Crystalline (HHC) rocks (including the main stem upstream of the hot
havehigherSr/Caand87Sr/86Srthatrangesfrom0.73to springs, surface runoff, and cold springs), and R
0.77.Eachoftheserangesin87Sr/86Srisconsistentwith denotes downstream value for the main stem
bedrock values found in the host formations [France- taken near the point of measurement for the main
Lanord et al., 1993]. The strong reflection of the local stem discharge.
lithologyinthespringwatersindicatesanareallylimited
flow path for hydrothermal waters. [16] FWHS was determined for the spring systems in
each of the eight major individual drainages
(Myagdi, Kali, Modi, Seti, Marsyandi, Bhuri, Tri-
suli/BhoteKosi, and Langtang) that contribute to
behavior to silicon and would only be removed
the Narayani system. Tributaries and upstream
from streams in significant amounts by opal
values from locations without known springs
precipitation. Himalayan streams are strongly
and showing no chemical effect of spring input
undersaturated with respect to opal and Ge should
(low Ge/Si, low [Cl(cid:1)]) were averaged to produce
therefore be conservative. In some environments
a consistent surface water input for all the
sorption of Ge to iron oxides may affect Ge/Si
drainages. The resulting values are (Ge/Si) =
systematics [Anders et al., 2003]. Some of the hot trib
0.7±0.46mmol/moland[Si] =155±38mmol/kg
springs, particularly those known locally as ‘‘rato- trib
(n = 21, uncertainties given as 1 standard devi-
pani’’ (‘‘colored water’’) precipitate iron oxides
ation). (Ge/Si) and [Si] are average values
around the spring vent. Ge/Si ratios in two oxide HS HS
for the sampled springs in each drainage. Calcu-
precipitates were 0.14 and 3.8 (Table 2). On the
lated FHS ranges from as little as 0.1% of the
basis of this limited data set, we make the W
total river flow for the Langtang Khola to as high
assumption that Ge is not significantly fraction-
as 4.2% for the Modi Khola (Table 3). Actual
ated from silica in this environment. While
discharge from the hot springs was calculated
strong enrichment in Ge is ubiquitous in the
using main stem discharge values from two
hot springs in our study area, the degree of
sources. When possible, river discharge was mea-
enrichment is quite variable between individual
sured in the field to reduce potential temporal and
spring systems. Consequently, we calculate the
geographic uncertainties. Where field-discharge
influence of hydrothermal discharge on river
measurements were not available, the DHM
chemistry using measured values from individual
10-year average for the sampling-month was used
river and spring systems.
[Yogacharya et al., 1998]. It is important to note
that the spring discharge values calculated here
6.4. Hot Spring Discharge
are based on dry-season sampling, the Myagdi,
[ ] The fraction of the total river discharge con- Kali, Modi, Seti, Marsyandi, Trisuli, and Lang-
15
tributedbythehotsprings(FHS)iscalculatedusing tang in March/April of 2001 and the Bhuri in
W
an end-member mixing model for Ge/Si ratios, October 1999. The FHS is therefore the fraction of
W
weighted by the silica concentration of the end- total flow derived from the springs during the
members (equation (1)). End-members are the low-flow times of the year. For the sum of the
tributaries or upstream reaches of the rivers unaf- 8 watersheds, FHS = 0.52 ± 0.3%, equivalent to a
W
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
Figure 4. Ge/Si versus [Si] for reported world rivers (open symbols) [Froelich et al., 1985, 1992; Mortlock and
Froelich, 1987; Chillrud et al., 1994]. Himalayan river Ge/Si values (filled diamonds) range from near the world
average of 0.5 mmol/mol (gray dashed line) to as high as 20 mmol/mol, the highest values reported for nonpolluted
streams. Smaller Himalayan streams with no known hydrothermal inputs have Ge/Si near the world average, while
those with Ge/Si between 2 and 20 mmol/mol are sampled near or downstream of hot spring locations.
discharge of (cid:10)2 m3/s across the entire Narayani clusters within the outcrop area of the HHC above
basin. the Main Central Thrust [Evans et al., 2001].
Downstream profiles of chloride (and other ele-
6.5. Uncertainties ments) and Ge/Si ratios clearly show the impact of
hotspringinput(Figure5).Intheheadwaters,both
[ ] OveralluncertaintiesforFHSaredominatedby
17 W the main stem and tributaries have Ge/Si =
the uncertainty that arises in determining the hot
0.3 mmol/mol. A zone of hot springs 30 km from
spring end-member Ge/Si value. While the relative
the headwaters near Chame have Ge/Si from 60–
uncertainties are large ((cid:3)60%) for the tributary
110 mmol/mol, and the main stem value increases
end-member Ge/Si value, this has a relatively
as it passes through this zone. Near Jagat, (55 km
minor effect on the FHS calculation. In systems
W fromtheheadwaters)asecondgroupofhotsprings
withonlyonesampledspringtheend-membersare
with Ge/Si near 180 mmol/mol drive the main stem
easily defined and the uncertainty is relatively
ratio to over 10, while tributary values remain
small (i.e., the Modi Khola). However, in the
<1.8 mmol/mol. Near Bahundada ((cid:3)65 km from
Marsyandi, Kali Gandaki and Trisuli river systems
the headwaters and a few km upstream of the
there are multiple spring locations (Figure 1) with
surface expression of the MCT), springs with
variable Ge systematics and this variability results
Ge/Si (cid:10) 50 and two tributaries with Ge/Si (cid:10) 11
inincreaseduncertaintywhendefiningahotspring
maintain a high ratio in the main stem, despite the
end-member value for the mass balance. The
contribution of other ‘‘normal’’ tributaries with
uncertainty on FHS is compounded following
W low Ge/Si. The two anomalous tributaries near
typical error propagation protocol [Ku, 1969].
Bahundada almost certainly have hot spring input,
although we did not explore them upstream.
6.6. Comparison With Cl(cid:1)(cid:1)(cid:1) Mass Balance
One (Ngadi Khola) has high chloride, consistent
[ ] The Marsyandi river system has been well with geothermal activity. Downstream, dilution
18
sampled, and the hot springs are found in four by tributaries with typical weathering values of
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GeochemistryGG evans et al.: geothermal fluxes of alkalinity 10.1029/2004GC000719
Geophysics
Geosystems
Table3. FractionoftheTotalRiverDischargeContributedbytheHotSprings(FHS)fortheRiversoftheNarayani
W
Basina
River Q , [Si] , (Ge/Si) , [Si] , (Ge/Si) , HotSpring
R R R HS HS
Sample m3/s mmol/kg mmol/mol mmol/kg mmol/mol Sample(s) FHS,% Q ,m3/s
W HS
Myagdi MLB 75 52b 86 13.5 584 795 MLB 66,68, 70,71 0.4 0.23 ±0.09
Kali MLB 76 58b 87 5.4 607 100 MLB 81,82, 83,84, 85 1.3 0.74 ±0.67
Modi MLB 94 7b 74 3.2 517 21 MLB 87,92 4.2 0.29±0.11
Seti MO 200 28c 99 3.6 966 267 MLB 101, 102 0.2 0.05 ±0.02
Marsyandi MO 2 40d 100 6.0 1306 131 MLB 51,55, 58 0.5 0.20 ±0.14
Bhuri GA240 169d 93 1.9 977 129 GA217 0.2 0.25 ±0.14
Trisuli Tri-4-4-94 40d 124 8.6 2053 80 MLB 27,28, 4,5 0.8 0.33 ±0.30
Langtang MLB 32 5b 125 3.4 832 583 MLB 41 0.1 0.004 ±0.001
Sum 399 2.1 ±2.0
aThefractionofthetotalriverdischargecontributedbythehotspringsisFHS.Valuesarecalculatedfromequation(1)forthedryseasonusing
W
[Si] =155mmol/kgand(Ge/Si) =0.7mmol/mol.
trib trib
bDischargemeasuredinfield.
cDischargefromHurtrez[1998].
dDischargefromYogacharyaetal.[1998].
Ge/Si (cid:11) 1 causes the main stem Ge/Si ratio to fall silicate-dominated catchments atmospheric deposi-
to 4.5 mmol/mol just before its confluence with tion and dissolution of trace/minor amounts of
the Trisuli. The overall pattern of Ge/Si in the carbonate supply the remainder of the riverine
Marsyandi is very similar to that for chloride. We alkalinity [e.g., Jacobson and Blum, 2000].
can compare the result of the Ge/Si mass balance
calculation with that based on Cl(cid:1). Average [Cl(cid:1)]
of the Marsyandi tributaries is 34 mM and that of
the hot springs 57,000 mM. With a simple mass
balance, a fractional spring discharge of 0.52 ±
0.4% yields the observed [Cl(cid:1)] of 366 mM at
the Marsyandi-Trisuli confluence. The Ge/Si
mass balance for the Marsyandi yields a FHS of
W
0.50 ± 0.35% and agrees well with the Cl mass
balance of Evans et al. [2001]. The close agree-
ment of these two independent tracers of hydro-
thermal input strongly suggests that the Ge/Si
mass balance is a viable tool for estimating hot
spring fluxes.
7. Alkalinity Flux
[ ] In light of efforts to quantify the silicate and
19
carbonate weathering fluxes and therefore estimate
the effect of weathering in the Himalaya on atmo-
spheric CO budgets, the silicate alkalinity flux
2
fromHimalayanrivershasbeenthefocusofrecent
study [Sarin et al., 1989, 1992; Singh et al., 1998;
Galy and France-Lanord, 1999; English et al., Figure 5. Downstream profile of (a) Ge/Si and
2000]. The solute load of the Ganges-Brahmaputra (b) [Cl(cid:1)] for the tributaries (open diamonds), hot
springs (open circles), and main stem (filled diamonds)
system has been shown to be dominated by car-
of the Marsyandi River basin. Note the different scale
bonate dissolution [Sarin et al., 1989; Galy and
for hot spring samples on the right-hand y axis.
France-Lanord, 1999; Jacobson and Blum, 2000]
Tributary values are too low to have produced the
with an estimated 80–90% of the cations derived
anomalously high main stem values. The largest spikes
from carbonate sources. Nonetheless, for drainages in river water [Cl(cid:1)] and Ge/Si are just downstream
underlain with silicate bedrock the fraction of total from major hot spring sources. The influence of the
alkalinity derived from silicate sources can exceed hydrothermal input on stream chemistry persists 60 km
70% [Galy and France-Lanord, 1999]. In those downstream to the confluence with the Trisuli.
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