Seasonal changes in the sources and fluxes of dissolved inorganic carbon through the St. Lawrence River—isotopic and chemical constraint

Chemical Geology 186 (2002) 117 – 138 www.elsevier.com/locate/chemgeo

Seasonal changes in the sources and f luxes of dissolved inorganic carbon through the St. Lawrence River—isotopic and chemical constraint Jean-Francßois He´lie a,*, Claude Hillaire-Marcel a, Bernard Rondeau b GEOTOP—Universite´ du Que´bec a` Montre´al (UQA`M), C.P. 8888 Succursale Centre-ville, Montre´al, Que´bec, Canada H3C 3P8 b Environnement Canada—Centre Saint-Laurent, 105 rue McGill, Montre´al, Que´bec, Canada H2Y 2E7

a

Received 18 July 2000; accepted 12 December 2001

Abstract The St. Lawrence River ranks 16th in the world rivers for its freshwater discharge into the ocean, but its particulate fluxes are relatively low due the upstream presence of the Great Lakes system. Using notably 13C measurements in total dissolved inorganic carbon (DIC) at several stations along stream, earlier studies provided information on dissolved inorganic carbon (DIC) fluxes through the St. Lawrence, with some constraints on spatial trends with respect to the in situ DIC metabolism. Here, we further document annual DIC-fluxes and seasonal changes in DIC supplies and DIC-metabolism, downstream. Chemical and isotopic measurements were performed in 1998, on a biweekly basis, at four different sites. These sites represent respectively the Great Lakes outflow into the St. Lawrence system, supplies from two tributaries (one with a silicate-rich Precambrian bedrock, the other with a carbonate-rich Paleozoic bedrock), and the St. Lawrence outflow into the maritime estuarine system. The 1998 survey yielded a total annual DIC flux of
6  1012 to 7  1012 g of carbon at the Quebec City outlet, about 40% higher than estimates from earlier studies. This represents about 1.5% of the world river DIC supplies to the ocean. Very strong seasonal variations are observed between the summer low water levels, when water supplies from the Great Lakes into the riverine system may represent up to 80% of the total outflow of the St. Lawrence River, and the spring snowmelt period, when tributaries may provide up to 80% of this outflow. d13C-DIC values respond to this seasonal cycle, with near isotopic-equilibrium values with atmospheric CO2 during summer, and strongly depleted values in other seasons, notably during the spring high outflow period. This seasonal isotopic cycle is more pronounced at the Quebec City outlet, where the influence of 13C-depleted DIC supplies from tributaries is stronger than upstream. The low d13C-DIC values observed from fall to spring, may be due to a combination of the following factors: (i) enhanced supplies of 13C-depeleted DIC from soils and ground waters from watersheds, (ii) higher oxidation rates of dissolved or particulate 13C-depleted organic carbon, and (iii) a reduced incidence of the in situ photosynthetic activity. The outflow and isotopic seasonal cycles result notably in a strong negative correlation between d13C-DIC values and discharge rates. Estimates of DIC supplies from tributaries vs. those from the Great Lakes were calculated using either chemical or isotopic approaches. DIC supplies from tributaries vary between
5% and 60% of total DIC. Examination of pCO2 data indicates a negligible seasonal variability at the Great Lakes outlet, with a mean annual value near equilibrium with atmospheric CO2. In contrast, strong seasonal changes are observed downstream, at Quebec City, with minimum pCO2 values in summer and a mean

*

Corresponding author. Fax: +1-514-987-3635. E-mail address: [email protected] (J.-F. He´lie).

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rigths reserved. PII: S 0 0 0 9 - 2 5 4 1 ( 0 1 ) 0 0 4 1 7 - X

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annual value of
1300 ppmv, suggesting that the St. Lawrence River system in its later course acts as a source for atmospheric CO2 during most of the year. D 2002 Elsevier Science B.V. All rights reserved. Keywords: St. Lawrence River; Carbon fluxes; Dissolved inorganic carbon; Carbon isotopes;

1. Introduction Annual dissolved inorganic carbon (DIC) supplies through the world river systems represent a carbon flux of about 0.38  1015 g into the ocean (Meybeck, 1993). Attempts to determine the sources and mechanisms involved in these DIC fluxes have been made, using 13C measurements (e.g., Amiotte-Suchet et al., 1999; Telmer and Veizer, 1999; Aucour et al., 1999; Barth and Veizer, 1999; Atekwana and Krishnamurthy, 1998; Yang et al., 1996; Cameron et al., 1999). In most cases, a progressive enrichment in 13C of DIC is observed from upstream to downstream, generally due to enhanced isotopic exchanges with atmospheric CO2 and/or in situ photosynthetic activity (Amiotte-Suchet et al., 1999; Telmer and Veizer, 1999; Atekwana and Krishnamurthy, 1998; Cameron et al., 1999). Variable seasonal signals have also been reported, notably in response to seasonal changes in the oxidation rate of the 13C-depleted organic matter from the soils in watersheds (Duplessy and Barbaroux, 1973). Deviations from the above patterns have been observed in the Rhone and St. Lawrence rivers as documented by Aucour et al. (1999) and Yang et al. (1996), respectively. These two rivers are characterised by the presence of large lakes at their head. Due to the long residence time of DIC in the lakes, isotopically heavy DIC signatures are observed upstream, suggesting isotopic compositions near equilibrium with atmospheric CO2. Both studies also report downstream trends of DIC depletion in 13C due to supplies of 13Cdepleted DIC from tributaries. In the case of the St. Lawrence River, Barth and Veizer (1999) further investigated spatial variations in 13C-DIC contents and DIC metabolism, by examining more closely 13C-DIC signatures of given ecosystems of an upstream section of the river (Cornwall area; Fig. 1). Chemical and isotopic

13

C

measurements were made by Barth and Veizer (1999) and Yang et al. (1996). Although restricted to a few sampling periods during the hydrological year the data sets allowed these authors to estimate a mean annual DIC flux of 4.7  1012 g of carbon towards the ocean, and to conclude that the role of the St. Lawrence could switch from that of a source for atmospheric CO2, in winter and spring, to a sink during summer and fall. However, the monthly and seasonal variations in DIC fluxes and metabolism remained insufficiently documented to ascertain both a mean annual DIC-flux (and its determining parameters), and the net annual CO2-budget from the St. Lawrence River towards the atmosphere. In the present study, we will therefore pay a special attention to temporal variations in the water outflow throughout the year, with a sampling resolution of 2 weeks, and to subsequent changes in chemical and physical properties of the water and in the isotopic compositions of DIC and water. For this purpose, a survey of these parameters has been implemented at four different sampling sites. Two of these sites were chosen in order to allow for precise estimates to be made of the balance between respectively the outflow of the St. Lawrence fluvial section into the maritime estuary, and the inflow from Lake Ontario at the head of the river (stations C and D; Fig. 1). Two other sampling stations were used (station A and B; Fig. 1) to document chemical and isotopic properties of tributaries draining, respectively, a silicate-rich sub-basin (mostly in the Precambrian Grenville Province; Fig. 1), and a carbonate-rich sub-basin (in the Paleozoic St. Lawrence Lowlands; Fig 1). The sampling and analytical program started in June 1997, and is still in progress. The stations are visited on a 2-week basis, except when severe weather conditions do not permit it, and on a weekly basis during the snow-melt period.

Fig. 1. The St. Lawrence river and sampling locations. Below, bedrock units of the Great Lakes – St. Lawrence system. On top, enlargement of the St. Lawrence River stricto sensu (i.e., the fluvial section between Lake Ontario, and the maritime estuary starting near Quebec City). Sampling sites: A—Ottawa River at Carillon, B—Mascouche River at Terrebonne, C—Montreal, D—Quebec City.

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J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

The study aims at determining seasonal changes in DIC fluxes, during the initial survey period that took place from January 1998 until April 1999, with special attention to the relative supplies from tributaries and the Great Lakes. Similarly, seasonal changes in the isotopic composition of DIC during this survey period are examined with the objective to determine the relative influence of spatial DIC sources vs. that of the in situ metabolism of carbon . We also examine seasonal and regional changes in the river’s pCO2 in order to further document the role of the St. Lawrence River system either as a source or a sink for atmospheric CO2 (see Yang et al., 1996). Finally, we make an attempt to put further constraints on total DIC exportation from the St. Lawrence system into the Atlantic Ocean, this to better assess global carbon fluxes from the continents to the ocean.

2. Material and methods 2.1. Sampling sites The pumping facility of the city of Montreal (Charles-J.-des-Baillets pumping station), is used as one of the four sampling sites. The water is collected in the central part of the South Channel of the St. Lawrence River (Fig. 1, station C) and represents essentially the outflow from Lake Ontario (Fig. 1; St. Lawrence Centre, 1996). Indeed, the water conductivity measured at this station during the course of our study (in 1998) remained within the range of values reported at the outlet of Lake Ontario, from 1994 to 1997, by either the St. Lawrence Centre (unpublished data) or Barth et al. (1998). Therefore, this site is used to illustrate inputs from the Great Lakes into the St. Lawrence River system. In a similar fashion, the second sampling site, located on the St. Lawrence River, at Quebec City (Fig. 1, station D), is used to constrain the output signal of the river into its maritime estuary section. Actually, the true end of the fluvial section is located slightly upstream, near Portneuf (Fig. 1). However, at Portneuf, the water masses originating from major tributaries are still not completely mixed with those from the Great Lakes (St. Lawrence Centre, 1996; Frenette et al., 1989). Therefore, the site of Quebec City was preferred. It also offers the advantage of easy sampling

and in situ measurements, due to the presence of a permanent pumping facility in the city of Levis, located on the southern bank of the river, right across Quebec City. Due to potential tidal influence at this site, samples were taken 2 h before low tides to insure that no brackish waters from the maritime estuary would be mixed. The third sampling site is located on the Ottawa River at the location of a hydroelectric dam (Fig. 1, station A) operated by Hydro-Quebec. With its 149 000 km2 watershed area (Telmer and Veizer, 1999), the Ottawa River is the most important tributary of the St. Lawrence River. It consists of a series of lakes connected by riverine sections (Telmer, 1996). Its drainage basin is essentially composed of Precambrian gneisses with a few marbles. However, Paleozoic carbonates represent approximately 8% of the watershed (Telmer, 1996) and apparently account for near 40% of the dissolved load of the Ottawa River. The mean annual discharge of the river represents about 1540 m3/s based on unpublished data from HydroQue´bec. On a yearly average, the Ottawa River accounts for about 16% of the total water discharge in the St. Lawrence River, downstream Montreal, but may represent up to 50% of this discharge during strong spring snowmelt events (calculated using unpublished daily discharge rates of the Ottawa River at Carillon and of the St. Lawrence River at the outlet of Lake St. Francois). The fourth sampling site is located in the city of Terrebonne (Fig. 1, station D), on the Mascouche River, a small tributary of the St. Lawrence River. This river was chosen because it drains a carbonate subbasin of the Paleozoic platform of the St. Lawrence Lowlands (Globensky, 1987). 2.2. Sampling period Sampling began in June 1997, however, for consistency in sampling procedures we only use here samples collected between January 1998 and April 1999. Each sampling site was visited every 2 weeks, on Wednesdays at Quebec City, and depending on logistical difficulties, the preceding or following day in other cases. A few samples from the Quebec City site are missing due to a major ice-storm in January 1998. In April 1999, sampling was made every week at all sites, to insure a better recording of chemical and

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

outflow changes during the peak of the spring snow melt. 2.3. Sampling procedures At the Montreal pumping station, samples were recovered directly at the outlet of the untreated water supply pipe. At Quebec City, samples were collected from the well of the pumping facility of the city of Levis, using an all Teflonk pump. At the two other sites, 10-l plastic containers were used to collect water directly from the central part of the river stream. Water samples for 13C-measurements in total dissolved inorganic carbon (DIC), were recovered in 120-ml highdensity polyethylene Nalgenek bottles and stabilised using 10 to 15 drops of mercury chloride (HgCl2—0.1 M) in order to prevent bacterial or algal activity. Bottles were filled completely (without any air left) to avoid exchanges with ambient air CO2, then stored at 4 C until further analysis. 2.4. In situ measurements Temperature (C),pH and redox potential (mV) were measured using a VWR 2000k pH-meter. Replicate measurements of pH yielded a 2r error of F 0.06 pH unit. Alkalinity was measured using a Hach Digital Titratork and a VWR 2000k pH-meter. The 100-ml water sample is poured into an Erlenmeyer vial and the pH is lowered with sulphuric acid (0.16 M). Volumes of acid vs. pH are recorded and plotted using the Gran method (Stumm and Morgan, 1996) to determine alkalinity in meq/l. Replicate measurements of alkalinities yielded a 2r error of F 0.01 meq/1. Conductivity was measured with a Hanna HI 8733k conductimeter and reported in microsiemensper centimeter. Redox potential measurements were made using a Pt electrode with Ag/ AgCl – KCl (4 M) filling solution. The measurements were calibrated against standard hydrogen electrode values, corrected for temperature by using factors provided by the electrode manufacturer. At the Montreal pumping station, pH and temperature measurements were those provided by a built-in monitoring system. At Quebec City, temperature, pH and conductivity were measured using an Hydrolabk probe, permanently placed in the well of the facility, and calibrated at each visit.

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2.5. Laboratory measurements Isotopic analysis of the water samples were made in the stable isotopes laboratory of the GEOTOP research centre at the Universite´ du Que´bec a` Montre´al. Deuterium and oxygen 18 measurements will be reported elsewhere. Herein, we make use of d13C-measurements on DIC, which were determined as follows. About 30 ml of sample water is transferred using a syringe, through a septum, into an Erlenmeyer vial containing 20 ml of 100% orthophosphoric acid under a 15-mTorr vacuum. DIC is then converted into gaseous CO2. Water vapour is eliminated by double trapping (first a liquid N2-trap, then a dry-ice isopropyl alcohol trap). The dried CO2 is then transferred into a sample-holder placed in a liquid nitrogen trap. d13C measurements are made using a VG-Prismk triple-collector mass spectrometer, and d-values are reported against V-PDB (see Coplen-Tyler, 1995). Replicate 13C-DIC measurements from given water samples yielded an overall analytical uncertainty of F 0.2x(1r) using this method. 2.6. Additional data and calculations Discharge outflow values were supplied by the St. Lawrence Centre of Environment Canada. At Montreal, discharge rates were measured daily at the outlet of Lake St. Francßois (Fig. 1). At Quebec City, discharge rates cannot be obtained directly due to tidal influence; discharge rates were thus calculated by adding on a daily basis, discharge rates of all tributaries from Montreal to Quebec City, to those of the St. Lawrence River at Montreal. Corrections were made to take into account the delay in transport of water between Montreal and Quebec City (see details in Rondeau et al., 2000). At Quebec City, the percentage of water from the Great Lakes is obtained by reporting discharge rates at the outflow of lake St. Francßois (which is considered to be the discharge of the Great Lakes, upstream Montreal) over those at Quebec City. As mentioned above, the discharge rates of the Ottawa River, at Carillon, were provided by Hydro-Quebec and measured directly at the dam. Discharge rates were not available for the Mascouche River. Concentrations of carbonate, bicarbonate, aqueous carbon dioxide and partial pressure of CO2 are calcu-

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122

lated using a classical carbonate chemical system described by the following equation: þ 2 CO2 þ H2 O X H2 CO3 X Hþ þ HCO 3 X H þ CO3

ð1Þ with the following dissociation equations:

KCO2 ¼

K1 ¼

½H2 CO3  ½ pCO2 

½Hþ ½HCO 3  ½H2 CO3 

ous carbon dioxide. Uncertainty on DIC calculations is estimated to be
1%, when combining maximum vs. minimum values of temperature, pH and alkalinity. d13C values for DIC in equilibrium with atmospheric CO2 are calculated for each sample using Eq. (9) from Zhang et al. (1995). eHCO3 CO2 ¼ ð0:141 F 0:003ÞT ½C þ ð10:78 F 0:05Þ

ð2Þ

ð3Þ

We used a d13C value of  7.8xvs. V-PDB for atmospheric CO2 (Levin et al., 1987). Eq. (9) then yields: dHCO3 ¼ ð0:141T Þ þ 10:78 þ ð7:8Þ

K2 ¼

½Hþ ½CO2 3   ½HCO3 

ð4Þ

Concentration for each species is given in moles per liter (mol/l), except for pCO2, which is given in volumetric parts per million (ppmv). Concentration can be used here to approximate activities since samples are collected in a fresh water system with very low ionic strength (Drever, 1988). Dissociation constants are calculated using these equations (e.g., Clark and Fritz, 1997): pKCO2 ¼ 7  105 T 2 þ 0:016T þ 1:11

ð5Þ

pK1 ¼ 1:1  104 T 2  0:012T þ 6:58

ð6Þ

pK2 ¼ 9  105 T 2  0:0137T þ 10:62

ð7Þ

Because the ionic strength is near zero, here, we assume, in Eq. (8), that alkalinity is represented by the sum of bicarbonate ions plus 2 times the carbonate ions (Stumm and Morgan, 1996). 2 Alkalinity ðeq=1Þ ½HCO 3  þ 2½CO3 

ð8Þ

DIC concentrations are obtained by adding the concentrations (in milligrams of carbon by liter) of the carbonate and bicarbonate ions to those of aque-

ð9Þ

ð10Þ

We use the equation linking directly the isotopic compositions of bicarbonate and gaseous CO2, since pH values are near 8 in the study samples (i.e., with bicarbonate representing
98% of DIC; Stumm and Morgan, 1996). DIC-flux values are interpolated or calculated for periods without actual measurements, using two different approaches. Firstly, we used a computer program (Kaleidagraphk) to interpolate daily DIC fluxes between sampling dates. This program fits a curve through the data points and calculates slopes between these points. Secondly, we used the product of the mean weighted concentration with discharge on the sampling interval flux calculation method (F4 equation; Meybeck et al., 1992). This method uses the mean annual daily discharge rate, instantaneous DIC concentration and instantaneous discharge rates to calculate a mean annual DIC flux for systems in which DIC concentrations do not show a strong relationship with discharge rates which is the case here (r 2 = 0.07). F4 equation is as follows: 2

n X

3

Ci Qi 7 6 6 i¼1 7 ¯ 6 7 F ¼ 365Q6 X n 7 4 Qi 5

ð11Þ

i¼1

¯ is the mean annual Where F is the annual flux, Q discharge rate, Ci is the instantaneous concentration (measured in situ) and Qi is the instantaneous dis-

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123

Table 1 Data sets for all sampling stations, where T is the temperature, Alk is alkalinity, Eh is redox potential, Cond. is the conductivity, d13C is the measured d13C-DIC, Equ. is the calculated d13C-DIC in equilibrium with atmospheric CO2, %GL is the proportion of water from the Great Lakes at Quebec City, pCO2 is the calculated partial pressure of CO2 and [DIC] is the concentration of DIC (A) Carillon sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C x vs.

Equ. PDB

Discharge (m3/s)

pCO2 (ppmv)

[DIC] (mg/l)

98-01-05 98-02-12 98-03-06 98-03-19 98-04-03 98-04-16 98-05-01 98-05-08 98-05-22 98-06-05 98-06-19 98-06-30 98-07-07 98-07-21 98-08-04 98-08-20 98-09-01 98-09-17 98-10-01 98-10-15 98-10-27 98-11-11 98-11-24 98-12-08 98-12-23 99-01-21 99-02-18 99-03-04 99-03-18 99-04-01 99-04-15 99-04-23 99-04-29

1.0 0.0 1.9 1.2 3.1 8.8 10.0 15.2 18.3 17.5 19.6 23.2 22.9 24.6 23.1 23.4 22.0 19.7 15.3 13.0 11.3 8.4 5.5 2.8 1.2 0.3 0.3 0.6 0.4 0.6 4.5 7.0 8.1

7.0 7.4 7.2 7.4 6.8 7.1 7.5 7.3 7.4 7.5 7.3 7.4 7.4 7.5 7.4 7.4 7.4 7.5 7.5 7.5 7.5 7.6 7.7 7.6 6.8 7.2 7.2 6.9 7.0 6.9 7.2 7.4 7.2

0.48 0.41 0.61 0.64 0.54 0.34 0.30 0.28 0.43 0.39 0.30 0.36 0.44 0.51 0.48 0.31 0.53 0.36 0.33 0.39 0.44 0.41 0.41 0.38 0.54 0.36 0.51 0.45 0.47 0.73 0.72 0.62 0.59

493 533 477 437 431 377 417 351 399 448 409 353

95 105 155 157 123 80 124 70 82 96

 8.46  10.53  10.12  10.07  8.93  8.43  8.95  9.11  9.83  9.83  8.48  9.42 11.33  9.91  10.24  10.24  9.43  9.47  9.68  9.33  7.31  6.43  6.73  6.26  4.08  8.50  10.68  7.02  8.44  10.66  7.27  10.87  4.31

2.84 2.98 2.71 2.81 2.54 1.74 1.57 0.84 0.40 0.51 0.22  0.29  0.25  0.49  0.28  0.32  0.12 0.20 0.82 1.15 1.39 1.80 2.20 2.59 2.81 2.94 2.94 2.90 2.92 2.90 2.35 1.99 1.84

1413 1376 2086 1856 7192 4034 2349 1882 945 747 1385 1507 1269 927 852 655 802 671 974 903 786 1409 1080 1460 1674 2367 2053 2022 1823 2525 4414 2483 1717

2576 745 2139 1259 4323 1496 541 766 989 701 897 872 1169 971 1259 791 1401 661 582 696 823 573 452 533 3865 1167 1543 2789 2106 4961 2166 1323 1974

8.0 5.6 9.1 8.8 10.1 5.1 3.9 3.8 5.6 5.1 4.0 4.7 5.8 6.5 6.2 4.1 6.9 4.6 4.3 5.1 5.8 5.3 5.3 5.0 9.9 5.4 7.5 7.9 7.6 13.3 10.4 8.4 8.5

82 103 77 81 86 104 95 95 94 103 89 94 97 96 77 104 85 86 131 120 100 99

549 541 548 566 486 535 560 531 567 515 449 396 356 361 429 425 477 390 539 521

(B) Mascouche sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

pCO2 (ppmv)

[DIC] (mg/l)

98-01-05 98-01-19 98-01-29 98-02-12 98-03-06 98-03-19 98-04-03 98-04-16 98-05-01 98-05-08

0.8 0.1 1.7 0.0 1.2 0.8 1.9 11.5 15.0 18.6

7.5 7.6 7.7 7.9 7.5 7.7 7.1 7.6 8.9 8.0

2.96 4.38 4.74 4.74 2.99 2.50 1.33 1.48

470 436 425 459 465

852 912 874 1002 695 600 234 310 532 600

 13.32  13.15  13.03  14.21  13.31  13.04  12.56  11.28  12.35  11.62

2.87 2.97 2.74 2.98 2.81 2.87 2.71 1.36 0.87 0.36

4620 5644 5159 3279 5253 2248 5780 1952

39.7 57.8 61.4 60.0 40.6 32.1 21.0 19.0

1730

35.6

2.88

430 403 379 345

(continued on next page)

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

124 Table 1 (continued ) (B) Mascouche sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

pCO2 (ppmv)

[DIC] (mg/l)

98-05-22 98-06-05 98-06-19 98-06-30 98-07-07 98-07-21 98-08-04 98-08-20 98-09-01 98-09-17 98-10-01 98-10-15 98-10-27 98-11-11 98-11-24 98-12-08 98-12-23 99-01-07 99-01-21 98-02-04 99-03-18 99-04-01 99-04-15 99-04-23 99-04-29

16.9 16.6 20.3 21.9 21.7 24.2 21.7 19.1 19.7 17.6 13.6 11.3 7.6 6.4 5.3 2.2 0.7 0.2 0.2 0.6 0.7 0.6 5.6 9.9 11.7

8.1 8.2 7.8 7.9 7.9 8.3 8.0 8.2 8.1 8.0 7.9 7.7 8.4 7.9 8.2 7.9 7.7 7.5 7.5 7.5 7.6 7.3 7.8 7.8 8.5

2.87 2.24 2.52 3.64 2.56 2.95 2.85 2.31 2.33 2.82 3.07 2.72 2.95 2.92 2.49 1.87 3.19 3.98 2.22 2.43 2.85 1.67 2.09 2.31 2.74

394 407 320 341

798 755 532 687 635 796 835 709 715 808 823 721 850 756 842 480 1002 830 1090 1995 1605 319 380 468 565

 11.63  11.63  13.47  12.32  12.66  11.04  11.25  11.27  12.15  10.86  10.75  11.49  9.41  9.19  11.19  12.44  12.14

0.60 0.64 0.12  0.11  0.08  0.43  0.08 0.29 0.20 0.50 1.06 1.39 1.91 2.08 2.23 2.67 2.88 2.95 2.95 2.90 2.88 2.90 2.19 1.58 1.33

1308 845 2687 2938 2011 1048 1625 966 1125 1524 2446 2851 635 1829 910 1290 3440 6035 3290 3963 3370 4625 1674 2124 479

35.3 27.4 31.6 45.2 31.7 36.1 35.1 28.4 28.6 34.8 38.3 34.5 36.1 36.5 30.7 23.6 41.4 53.4 29.7 32.8 37.3 24.3 26.4 29.2 33.5

482 545 516 536 467 512 527 493 528 444 451 394 362 297 224 432 517 435 490 367

 12.65  13.52  12.98  13.10  12.72  12.48  11.15

(C) Montreal sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

Discharge (m3/s)

98-01-05 98-01-19 98-01-29 98-02-12 98-03-06 98-03-19 98-04-03 98-04-16 98-05-01 98-05-08 98-05-22 98-06-05 98-06-19 98-06-30 98-07-07 98-07-21 98-08-04 98-08-20 98-09-01 98-09-17 98-10-01 98-10-15

1.5 0.7 0.9 0.4 1.5 1.2 3.0 6.1 7.5 9.0 11.1 11.8 14.5 17.7 18.1 20.7 19.0 18.9 18.2 17.1 15.5 15.6

8.2 8.1 8.1 8.3 8.2 8.4 8.1 8.7 8.2 8.3 8.8 8.2 8.5 8.4 8.1 9.0 9.0 8.2 8.3 8.0 8.2 7.8

1.68 1.66

490 4170 417 426 456 419 389 413 437 363 394 442 402 294

286 261 276 280 274 280 211 273 335 273 308 294 313 288 291 284 309 292 318 292 291 286

 0.54  0.71  1.92  1.38  1.33  3.84  2.91  1.29  2.97  3.46  1.46  1.46  2.59  1.11  1.19  0.41  0.29  0.21  0.21 0.04  0.40  0.45

2.77 2.88 2.85 2.92 2.77 2.81 2.56 2.12 1.92 1.71 1.41 1.32 0.94 0.48 0.43 0.06 0.30 0.32 0.41 0.57 0.79 0.78

8614 6920 6989 8490 10900 10600 11000 11900 11200 10900 10200 9730 9790 9900 9930 9290 9030 8670 8650 8340 8540 8070

1.63 1.69 1.55 1.30 1.49 1.80 1.70 1.64 1.66 1.77 1.56 1.63 1.62 1.78 1.66 1.86 1.55 1.54 1.57

499 556 527 575 511 533 542

pCO2 (ppmv)

[DIC] (mg/l)

560 709

20.6 20.5

398 538 301 554 165 636 411 140 561 350 352 782 86 107 670 588 995 529 1304

19.9 20.7 18.7 16.0 17.7 22.0 20.6 19.4 20.2 21.2 18.7 19.8 18.7 20.7 20.2 22.5 19.1 18.7 19.5

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125

Table 1 (continued ) (C) Montreal sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

Discharge (m3/s)

pCO2 (ppmv)

[DIC] (mg/l)

98-10-27 98-11-11 98-11-24 98-12-08 98-12-23 99-01-07 99-01-21 99-02-04 99-02-18 99-03-04 99-03-18 99-04-01 99-04-15 99-04-23 99-04-29

10.9 8.0 7.9 5.6 1.2 0.7 0.8 0.5 0.9 0.7 0.3 3.0 4.8 6.9 7.0

8.5 8.2 8.2 8.1

1.61 1.59 1.50 1.49 1.48 1.50 1.59 1.58 1.61 1.63 1.56 1.69 1.58 1.60 1.77

551 531 514 460 409 370 347 299 343 435 406 441 473 535 506

290 275 285 282 291 285 271 260 257 271 267 275 258 269 265

0.11 0.08  0.43  0.47  0.87  0.77  1.01  1.50  1.70  0.45  0.65  1.96  1.85  1.14  0.39

1.44 1.85 1.87 2.19 2.81 2.88 2.87 2.91 2.85 2.88 2.94 2.56 2.30 2.01 1.99

7540 7390 7240 6810 7010 6385 5877 6846 6918 6821 7226 7422 7050 6980 6819

285 502 474 621

19.3 19.3 18.2 18.3

432 480 476 360 503 664 657 531 415 400

18.3 19.4 19.3 19.5 19.9 19.3 20.8 19.3 19.4 21.4

8.2 8.2 8.2 8.3 8.2 8.1 8.1 8.2 8.3 8.4

(D) Quebec City sampling station data set Date

T (C)

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

Discharge (m3/s)

%GL

pCO2 (ppmv)

[DIC] (mg/l)

98-01-21 98-02-04 98-02-18 98-03-04 98-03-18 98-04-01 98-04-08 98-04-15 98-04-29 98-05-06 98-05-20 98-06-03 98-06-17 98-07-02 98-07-08 98-07-22 98-08-05 98-08-19 98-09-02 98-09-16 98-09-30 98-10-14 98-10-28 98-11-12 98-11-25 98-12-09 98-12-22 99-01-06 99-01-20 99-02-03 99-02-17

 0.1  0.2  0.2  0.1  0.1 1.6 3.4 6.6 8.9 11.3 16 16 16.9 20.8 21.5 23.7 22.8 22.8 20.7 18.8 17.1 12.7 10.6 6.6 4.4 3.5 0.8  0.2  0.2  0.2  0.2

7.4 7.56 7.56 7.93 7.63 8.7 7.56 7.75 7.74 7.68 7.75 7.83 7.76 7.58 7.76 7.79 8.24 7.93 8.11 7.96 8 7.79 7.87 7.77 7.77 7.5 7.7 7.83 7.6 7.54 7.39

1.32 1.46 1.42 1.45

439 439 455 481

2.99 3.01 3.01 2.99 2.99 2.75 2.49 2.05 1.73 1.39 0.72 0.72 0.6 0.05  0.05  0.36  0.23  0.23 0.06 0.33 0.57 1.19 1.49 2.06 2.36 2.49 2.87 3 3 3 3

11518 11040 12708 13790 14184 26037 19078 18443 15513 14354 12239 11310 13401 14296 12441 11545 10717 10822 10613 10207 10411 10133 9574 9648 8706 10156 9644 11072 10505 10849 10752

61 65.5 70.1 65.6 69.5 23.1 37.6 49.5 62.1 67 76.4 79.2 68.2 65.6 72.7 75.5 79.3 80.8 80.7 79.3 78.3 75.1 76.1 72 76.1 60.8 65.6 57.3 52.4 62.6 65.3

18.2 19.4 18.8 18.1

420

 2.13  2.12  2.02  2.31  3.96  6.83  4.56  4.81  2.94  2.61  3.95  2.2  2.26  2.4  2.89  2.45  1.7  1.61  1.42  1.11  1.01  1.63  1.01  1.05  1.71  1.71  1.92  1.61  2.23  3.2  1.52

2568 1968 1907 828

1.42 1.13 1.13 1.35 1.39 1.36 1.46 1.44 1.59 1.43 1.56 1.44 1.51 1.40

217 238 258 248 229 172 168 171 204 224 242 266 235 246 246 250 256 250 289 260 263 250 261 249 249 229 241 256 229 235 244

137 1566 1043 1309 1586 1398 1246 1468 2570 1538 1618 516 1118 663

16.9 14.8 14.3 17.0 17.6 17.0 18.1 18.1 20.2 17.8 19.4 17.3 18.6 17.0

766 1163 1058 1210 1131

16.1 16.1 18.0 17.3 16.6

1218 948 1761 1741 2626

16.0 16.6 18.9 16.5 18.3

1.31 1.29 1.44 1.37 1.31 1.24 1.32 1.44 1.24 1.32

372 350 426 405 432 246 478 551 569 558 502 533 527 552 518 553 489 486 457 357 335 322 353

(continued on next page)

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126 Table 1 (continued )

(D) Quebec City sampling station data set Date 99-03-03 99-03-17 99-03-31 99-04-14 99-04-21 99-04-28

T (C)  0.1 1.9 4.3 6.8 7.8

pH

Alk (meq/l)

Eh (mV)

Cond. (mS/cm)

d13C xvs.

Equ. PDB

Discharge (m3/s)

%GL

7.61 7.67 7.6 7.69

1.34 1.24 1.23 1.10 1.09 1.19

431 334 466 495 559 521

238 241 217 193 195 191

 1.37  1.52  3.68  4.72  4.29  2.63

3.11 3 2.71 2.38 2.02 1.88

10796 10682 14036 15382 13804 12041

64.5 66.2 55.3 45.1 52.1 56.6

7.61

charge rate (measured daily). This method takes into account the discharge distribution rather than the fact that concentration is a function of discharge (Meybeck et al., 1992). DIC fluxes were calculated from January 1st to December 31st 1998 using both methods.

3. Results The physical, chemical and isotopic data obtained in situ or in the laboratory are listed in Table 1. This table also includes calculated values for: (i) d13C-DIC under equilibrium with atmospheric CO2; (ii) the outflow from the Great Lakes relative to the total outflow measured at Quebec City; (iii) equilibrium pCO2 value; and (iv) DIC concentrations. The discharge rates of the St. Lawrence River at Montreal varied from 5900 to 11 900 m3/s during the study period, with a mean value of 8781 F 100 m3/s. This is consistent with a mean discharge rate of 8200

pCO2 (ppmv)

[DIC] (mg/l)

1292 1537 1138

16.0 16.1 14.0

1541

15.3

m3/s estimated for the interval 1975 – 1987 (CSSA Consultants and Environnement Illimite´, 1988). The hydrogram at Montreal does not define natural seasonal trends, with droughts in summers and winters, and maximum values during the spring snowmelt event (Fig. 2). This is due to outflow regulation at the outlet of the Great Lakes and upstream dams located between Montreal and Cornwall (Fig. 1). At Quebec City, discharge rates are more variable and range between 8700 and 26 000 m3/s, due to a pronounced influence of tributaries. The average value of 12 311 F 100 m3/s is also consistent with long-term recordings spanning the last decades as reported in the literature (St. Lawrence Centre, 1996). The survey year on which we are reporting here may thus be considered representative of conditions during the last few decades. The St. Lawrence tributaries show large seasonal outflow variations (e.g., St. Lawrence Centre, 1996), as illustrated by the record from the Ottawa River, at Carillon (Fig. 2). Compared with the mean annual

Fig. 2. Hydrograms during the survey period (Jan. 1, 1998 to Apr. 29, 1999). Diamonds correspond to sampling days at the corresponding station. Data for Mascouche River are missing.

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

discharge rate of 1537 F 25 m3/s, the spring snowmelt event peaks show approximately a fourfold outflow increase (reaching about 7000 m3/s in 1998 and about 5700 m3/s in 1999). Water temperatures in the St. Lawrence River ranged from 0.4 to 20.8 C (Montreal) and from
0 to 23.7 C (Quebec City) throughout the sampling period. The two tributaries showed temperature ranges near those of Quebec City (0 to 24.3 C). Alkalinity, pH, conductivity and concentration of DIC yielded maximum values in the Mascouche River, draining the small Paleozoic carbonate basin. They show minimum values in the Ottawa River, which primarily drains Precambrian gneiss terrains. The St. Lawrence River, which represents mixtures between these two major lithological sources, yielded intermediate values. These are slightly higher at the Montreal pumping station (representing essentially the outflow from the Great Lakes) than at Quebec City, where the influence of tributaries from silicate-rich catchments is more important. At all sampling sites, alkalinity tends to show higher values in winter, and minimum values during the spring snow melt period. This is particularly well illustrated by the data set from the Mascouche River. Redox potential values vary moderately at all sites, between about 300 and about 570 mV. One exceptionally low value (225 mV) was recorded in the winter of 1999 at the Mascouche River. There is no clear seasonal pattern in redox potential values. The calculated pCO2 values at the Montreal pumping station indicate near equilibrium conditions with atmospheric CO2 during the summer (using the reference value of 360 ppmv measured in 1997 at the Mauna Loa meteorological station, Hawaii; NOAA, 1999, Fig. 4A). These values rise slightly above the equilibrium threshold in winter (Fig. 4A). At Quebec City, pCO2 values show larger variations. They lie well above the equilibrium threshold with atmospheric CO2 (Fig. 4A), notably in winter, and may even show a peak with exceptionally high values such as in July 1998 (Fig. 4A). The tributaries systematically show much higher pCO2 values all year long (Fig. 4B) with winter maximums. This is particularly the case at the Mascouche River station. The carbon isotope compositions of DIC depict a strong annual cyclic trend in the St. Lawrence River. Very negative values were observed during the spring

127

high outflow period, and maximum values, near isotopic equilibrium with atmospheric CO2, during late summer (Fig. 5A and B). The amplitude of this seasonal oscillation is higher at Quebec City than at Montreal, with d13C-DIC values ranging from  6.83xto 0.93xand3.84xto 0.4x , respectively. The Ottawa and Mascouche Rivers show much lower over all d13C-DIC values, ranging respectively from  11.33 xto4.08xand from  14.21xto  7.31x, and without any clear seasonal trend (Fig. 5C). d13C-DIC signatures in the Ottawa River are consistently higher than those of the Mascouche River throughout the year.

4. Discussion 4.1. Dissolved inorganic carbon chemistry in the St. Lawrence system The St. Lawrence River waters yielded mean DIC concentrations of about 20 mg/l at Montreal and of about 17.5 mg/l at Quebec City. DIC concentrations at Montreal closely follow those measured in the Great Lakes outflow water (St. Lawrence Centre, 1996). The lower values observed at Quebec City result from the dilution of the carbonate-rich waters from the Great Lakes and the St. Lawrence Lowlands basin, by lowDIC, low-salt content waters from tributaries draining catchments where carbonate rocks are less abundant. These include, tributaries from the Appalachian Mountains, to the south-east, and tributaries from the Precambrian Grenville Province, to the north – north-west. The largest tributary of the St. Lawrence River, the Ottawa River, represents primarily inputs from Precambrian Rocks. It accounts for about 15% of the total St. Lawrence discharge at Quebec City, but for only 5% of the DIC load at this station. It also shows the lowest DIC content of all major tributaries of the St. Lawrence River (Cossa et al., 1998). Seasonal changes in DIC concentrations are not very pronounced in the St. Lawrence River (Fig. 3). They correspond essentially to a small decrease in spring, due to the dilution by snowmelt waters (e.g., during the 1998 spring discharge peak in early April; Fig. 3). The Mascouche River, which drains a relatively small catchment, represents a lesser ‘‘buffered’’ sys-

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Fig. 3. Dissolved inorganic carbon contents at the study sites.

tem than the Ottawa and St. Lawrence rivers. As a consequence, it shows much larger fluctuations in its DIC concentrations than these larger rivers do. Minimum values are observed during the spring snowmelt event and also during shorter occasional events (e.g., December 1998). One storm occurred July 2nd 1998. It corresponds to increased discharge rates of tributaries between Montreal and Quebec City, as shown by a comparison made of the corresponding hydrograms (Fig. 2). At the Mascouche River station this event is accompanied with a peak in DIC concentrations (Fig. 3). The Quebec City station also recorded this event and shows a maximum in DIC concentrations at the same time. During winter, both the Ottawa and Mascouche rivers show a rise in their DIC concentration (Fig. 3) and pCO2 values (Fig. 4). Since soils are frozen during winter, the increase in DIC and pCO2 values may be due to increased groundwater drainage, relative to surface runoff. DIC contents in groundwaters, notably in the St. Lawrence Lowland area (e.g., Simard, 1978), are much higher than those of runoff or melt waters, and pCO2 values as high as 31600 and 1400 ppmv have been reported in groundwaters from, respectively, carbonate-rich and silicate-rich bedrock areas (Telmer, 1996). As a consequence, mixed lithologies could be expected to yield intermediate pCO2 values (e.g., Telmer, 1996). The enhanced influence of groundwaters during winter is thus likely to result in significantly higher pCO2 values in the draining rivers. In

comparison to the Ottawa and Mascouche rivers, the pCO2-record at the Montreal pumping station shows much lesser variations (Fig. 4A). This is essentially due to the chemical properties of waters outflowing from the ‘‘buffered’’ system of the Great Lakes (Weiler and Nriagu, 1973; Yang et al., 1996). Accordingly, the Quebec City record shows pCO2 values intermediate between those measured at Montreal and in the tributaries (Fig. 4A). During the spring period, the addition of low pH waters from snowmelt moves the carbonate system towards the H2CO3 end-member, thus inducing a rise in pCO2 (Telmer, 1996). This is illustrated in Fig. 4B by the pCO2 peaks of early April of 1998 and 1999 in the studied tributaries matching the discharge peaks of the corresponding snowmelt events (Fig. 2). Low pCO2 values are usually seen during the summer or the late summer at all sites. A few processes may account for this drop in pCO2 values. For example, a high rate of photosynthetic activity, notably in shallow sites and bays where periphyton and algae are abundant, could result in maximum consumption of CO2, as illustrated by Barth and Veizer (1999) in the Cornwall area. Low water levels could also result in more efficient relative exchange rates between DIC and atmospheric CO2. Such processes are likely to act simultaneously, but with rates strongly depending upon the season, meteorological and hydrological conditions. Nevertheless, the high pCO2 values of tributaries as well as those calculated for the Quebec

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

129

Fig. 4. pCO2 calculations at the study sites (see text). A: Montreal and Quebec city stations on the St. Lawrence fluvial segment; B: Studied tributaries. Arrows show the spring snowmelt events.

City station suggest that the St. Lawrence system acts globally, and throughout the year, as a source for atmospheric CO2. This seems to contradict a conclusion of Yang et al. (1996) about a more contrasted regime, with the St. Lawrence River acting as a source of CO2, to the atmosphere, during the spring, and as a sink, during the fall. Since we use here a much higher resolution time series than Yang et al. (1996), we are quite confident that our conclusion is valid, at least for the year of our survey (1998). However, results from subsequent years (unpublished) seem in agreement with those of year 1998. 4.2. Isotopic constraints on carbon sources The isotopic composition of dissolved inorganic carbon in fresh water systems depends upon several

major processes: (1) oxidation of organic matter; (2) dissolution of carbonate minerals (when present) in soils, aquifers and surface waters; (3) exchanges with atmospheric carbon dioxide and kinetic effects at water/atmosphere interface (e.g., due to CO2-degassing, for example); and (4) photosynthetic activity (of algae and macrophytes). All these processes leave isotopic imprints on DIC. Photosynthetic activity in the aquatic system itself results in preferential use of light carbon, thus in a relative 13C enrichment of DIC. Photosynthetic activity also plays a role in determining the isotopic composition of the organic carbon stored in soils, thus in soil CO2. The two most important photosynthetic cycles are the Calvin cycle, which produces a strongly 13C-depleted organic matter (with a mean value of  27x; Deines, 1980), and the Hatch and Slack cycle, which results in a lesser 13C-depleted

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organic matter (with a mean value of  13x; Deines, 1980). Most plants of the study region follow the Calvin cycle. However, in agricultural sectors of the St. Lawrence Lowlands, corn is abundant. It follows the Hatch and Slack cycle (Bender, 1971) and could thus occasionally enrich soils in organic matter with higher d13C values. In a similar fashion, sparse occurrences of aquatic plants following the Hatch and Slack cycle, have been reported in the St. Lawrence system (e.g., Spartina sp.; Rousseau, 1974). However, their distribution seems restricted to marsh zones. They should not represent a major component of the carbon cycle here. Soil CO2 from areas with plants following predominantly the Calvin cycle shows isotopic compositions near  24x(Cerling et al., 1991; Pearson and Friedman, 1970). Bicarbonate ions in equilibrium with such a CO2 should thus present an isotopic composition near (using the regional mean annual temperature of
6 C; see Eq. (9)). Streams, draining silicate rocks in the Ottawa River catchment area, yielded slightly lower values (
 17; see Telmer and Veizer, 1999) possibly due to their relatively low pH values resulting in a relatively high content in dissolved CO2 vs. bicarbonate ions. In the St. Lawrence Lowlands, however, where soils are generally derived from alteration of Paleozoic carbonate rocks, dissolution of these carbonates results in a shift of d13C-DIC values towards less 13C-depleted values, since the Paleozoic limestones carry a ‘‘marine’’ isotopic signature (
0x , i.e., that of the VPDB standard). Hillaire-Marcel (1979) reports d13C values ranging from + 0.2xto + 1.1xin the major Paleozoic units of the St. Lawrence Lowlands, and a slightly higher value ( + 1.91x) for Precambrian marbles in the Grenville Province. Isotopic exchanges with atmospheric CO2 along river pathways, as well as kinetic effects due to CO2-degassing, should tend to shift d13C-DIC towards more positive values. From Eq. (10) above and using mean annual temperatures, equilibrium with atmospheric CO2 should result in d13C-DIC values averaging +1.6x . The isotopic compositions measured respond to a combination of the above processes. In the Mascouche River, which drains carbonate terrains, mean d13C-DIC values near  12xsuggest almost stoichiometric dissolution of carbonates by soil CO2 (e.g., Dever, 1985) and, likely, reduced exchanges with

atmospheric CO2. Due to the small dimension of the drainage basin and the shorter course of the river, one may indeed expect reduced isotopic re-equilibration with atmospheric CO2. Nevertheless, seasonal variations between  14.21xand  7.31xFig. 5C) are depicted by d13C-DIC values in the Mascouche River. Maximum values are observed during the summer, in response to processes already evoked above, notably to maximum photosynthetic activity. The Ottawa River shows slightly higher d13C-DIC values than the Mascouche River, even though carbonate rocks are not much abundant in its watershed area, where they apparently represent less than 8% of the outcropping rocks (Telmer and Veizer, 1999). However, the same authors calculate that these small carbonate-rich areas may contribute up to 39% of the total DIC of the Ottawa River near its outlet. Nevertheless, the Ottawa River shows relatively low DIC concentrations reflecting primarily drainage conditions in a silicate-rich basin, but its DIC isotopic signatures are actually higher than those of a small river draining carbonate-rich soils, such as the Mascouche River. This suggests more efficient isotopic exchanges with atmospheric CO2. The Ottawa River drains many lakes and reservoirs along its course. Over such water bodies, enhanced isotopic exchanges with atmospheric CO2 are likely to shift d13C-DIC towards heavier isotopic values (see Telmer, 1996). A + 3x shift in d13C-DIC values was measured in late autumn 1998 (Fig. 5C) at Carillon, with reference to summer values. This enrichment in 13C coincides with one of the lowest pCO2 measured throughout the year, a pCO2 actually near equilibrium with atmospheric CO2. Since photosynthetic activity should be lower during late fall, as compared with the summer period, isotopic exchanges with atmospheric CO2 are likely to be the principal acting mechanism here, and should account for 13C-enrichment in DIC. In the St. Lawrence River, the role of the Great Lakes seems important with respect to d13C-DIC signatures. Weiler and Nriagu (1973) reported d13CDIC values for Lake Ontario of  0.43xin January and of  0.59xin April of their survey period. They pointed out that the buffering capacity of the Great Lakes should reduce seasonal or long term variations of DIC isotopic compositions. Given the temperature of the lake, the d13C-DIC values reported by Weiler and Nriagu (1973) are about 2xlighter than those of

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

131

Fig. 5. d13C-DIC data at the study sites. A: Montreal city station; B: Quebec City station; C: Studied tributaries. Note that the left hand scales for discharge rates at Montreal and Quebec City stations are reversed.

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a DIC in isotopic equilibrium with atmospheric CO2. The authors interpreted this DIC-depletion in 13C as the result of organic matter decay in the lake. Leggett (1998) reported for 1994 and 1995 mean d13C-DIC values near  1xat the outlet of lake Ontario into the St. Lawrence River, with seasonal oscillations between
0x(late summer) and approximately  3x(winter). The South Channel of the St. Lawrence River at Montreal is considered to represent primarily the outflow of Lake Ontario (St. Lawrence Centre, 1996). This assumption seems to be supported by the fact that the range and seasonal trends of d13C-DIC values observed at the Montreal station (i.e., from
+ 0.5 to
 3.5x; Fig. 5A) are almost identical to those reported for 1994– 95 in Lake Ontario (Leggett, 1998) near its outlet (
0xto
 3.5x). This observation also supports the conclusion of Weiler and Nriagu (1973) about an isotopically ‘‘buffered’’ Great Lakes carbonate system. At several years interval, the annual range of d13C-DIC values seems practically unchanged. During the winter and spring of 1998, mean d13CDIC values in the St. Lawrence River at the Montreal sampling station (Fig. 5A) were practically identical to those reported for Lake Ontario (Leggett, 1998) near its outlet in 1994– 95. The few relatively large amplitude oscillations observed during the winter – spring seasons of 1998, at Montreal (Fig. 5A), are likely due to snow melt events and more generally to hydrographic variability. Nevertheless, the seasonal cycle depicted by d13C-DIC values is probably due to a combination of factors. Firstly, isotopic exchanges with atmospheric CO2 are likely to be less efficient in winter due to the ice cover. Secondly, with respect to DIC isotopic budgets, the light CO2 resulting from the decay of organic matter is likely to contribute more to the d13C-DIC budget in winter than in summer, due to both the reduced photosynthetic activity and lower outflow from Lake Ontario during the winter season. At Quebec City, d13C-DIC values fall systematically below those measured at Montreal, and also below isotopic equilibrium values with atmospheric CO2 (Fig. 5B). DIC at Quebec City shows isotopic compositions intermediate between those of the Montreal station and those in the tributaries. Winter d13C-DIC values are slightly lower than summer values for the Quebec City station, due to a combination of processes already

evoked above. During the spring snowmelt event, high discharge rates from tributaries (which can account for up to 80% of total water discharge at Quebec City) result in significant drops in d13C-DIC values (with values as low as  6.83x). The most striking features of the DIC-isotopic record at Quebec City is its very strong negative correlation with discharge rates (Eq. (12); Fig. 6A), and positive correlation with conductivity (Eq. (13); Fig. 6B), described by the following equations: y ¼ ð0:000347F0:000032Þx þ 1:82F0:42 R2 ¼ 0:76

ð12Þ

y ¼ ð0:0369F0:0047Þx  11:1F1:1 R2 ¼ 0:64

ð13Þ

Since these parameters are inversely correlated (e.g., Cossa et al., 1998), the second relationship is a corollary of the first one. A less robust relationship, at least partly interdependent of the former ones, exists between d13C-DIC values at Quebec City and the percentage of water originating from the Great lakes (Eq. (14); Fig. 6C), described by the following equation: y ¼ ð0:078F0:011Þx  7:5976F0:75 R2 ¼ 0:57

ð14Þ

More complex mixing equations should be considered, due to variable DIC contents in tributaries. However, they seem to result in a sub-linear trend (Fig. 6C), suggesting an equivalent influence from DIC-rich and DIC-poor tributaries. Therefore, with some reservation, Eq. (14) could be used to calculate isotopic compositions for two end-members, one averaging DIC supplies from all tributaries and the other one representing DIC supplies from the Great Lakes. The first one corresponds to a d13C-value of  7.60 F 0.75x, the second one, of + 0.22 F1.80x. Isotopic compositions actually measured for DIC in tributaries vary between  8.8 F 1.8xand  12.1F 1.1x(Ottawa River and Mascouche River, respectively). The Great Lakes mean annual end member is not as well constrained. However, values in the  0.5x to  1xrange seem reasonable, based on data from Weiler and Nriagu (1973), from Leggett (1998) or from the Montreal pumping station. Within standard

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

133

Fig. 6. d13C-DIC vs. A—discharge rate, B—conductivity and C—contribution of the Great Lakes to total discharge at the Quebec City outlet of the St. Lawrence River.

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deviations, the calculated and measured values for the ‘‘Great Lakes’’ end-member agree. Scatter in the data (d13C-DIC values at Quebec City and the percentage of water originating from the Great Lakes) can account for part of the disagreement between calculated and measured DIC isotopic value for the tributaries endmember. However, one can note a slight 13C enrichment in the calculated end-member for tributaries. It probably account for partial re-equilibration with atmospheric CO2 and/or photosynthetic activity, along the St. Lawrence River course, upstream Quebec City. 4.3. Fluxes of dissolved inorganic carbon Two estimates for DIC fluxes through the St. Lawrence River have been obtained from DIC concentrations and outflow rates, using either interpolated values or one of the equations proposed by Meybeck et al. (1992; Eq. F4). These methods yield total DIC fluxes of 6.85  1012 and 5.94  1012 g of carbon per year, respectively. About a third of the annual amount of DIC carried by the St. Lawrence River at Quebec City is recorded in approximately one month (in 1998: from February 18 to March 20), i.e., during the spring snowmelt peak. Yang et al. (1996) report a DIC flux of 4.68  1012 g of carbon per year, which is about a third less than the present estimates. Here again, differences in the time series used and in their temporal resolution may account for the discrepancy, although interannual variability cannot be totally dis-

carded. Nevertheless, the brackets above (i.e., approximately 6  1012 to 7  1012 g of carbon/year) should represent adequately DIC fluxes during year 1998. A more precise assessment of the total DIC carried to the ocean would require shorter sampling intervals than those retained here. At Quebec City, daily DIC fluxes are higher than at Montreal (Fig. 7), due to inputs from tributaries downstream from Montreal. However, from May to December 1998, the difference between the two sampling stations is not important. In contrast, it is significant in winter and during the spring snowmelt event. Therefore, the tributaries entering downstream of Montreal seem to contribute more to DIC fluxes during this period, than on a yearly basis. Nevertheless, maximum values in daily DIC fluxes are recorded during the spring snowmelt event both at Montreal and Quebec City. These maximum values are not due to increased DIC concentrations, but to increased water discharge as discussed above (Figs. 2 and 3). DIC-supplies from tributaries relative to those from the Great Lakes can be estimated using two approaches (Fig. 8). The simplest one consists of the difference in daily DIC fluxes between Quebec City and Montreal. In the second one, isotopic data may provide constraints on the two major DIC-sources, the 13C-enriched one from the Great Lakes vs. the 13C-depleted one from the tributaries. We used a mean d13 C-DIC value of  10xfor the ‘‘tributary’’ end-member in Eq. (15), which seems acceptable on a yearly basis since d13C-

Fig. 7. Daily DIC fluxes on sampling dates in the St. Lawrence River at Quebec City and Montreal.

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

135

Fig. 8. Relative DIC contributions from tributaries (in percent (%) vs. total DIC fluxes at Quebec City) calculated from isotopic data (open diamonds) or chemical data (filled diamonds), vs. their relative discharge rate (see text). The dashed lines represent mixing curves between the ‘‘Great Lakes’’ end-member and a DIC-rich end member (the Mascouche River) or a DIC-poor end-member (the Ottawa River), respectively. The solid line represents the best fit curve with both sets of data points (see text and Eq. (16)). It suggests a stronger influence of tributaries draining the Precambrian basement vs. those draining other lithologies. The open circle corresponds to the mean annual relative discharge from tributaries.

DIC of the Ottawa and Mascouche rivers yield mean annual values of  9xand  12xrespectively. Furthermore, Yang et al. (1996) measured d13C of DIC in several tributaries of the St. Lawrence River, between Montreal and Quebec City. They found a mean value of about  9.5xin the spring and slightly lighter values in the fall of their survey year (1991). The actual d13C-DIC values measured at Montreal is used for the ‘‘Great Lakes’’ end-member.  R¼

d13 C DIC at Montreal  d13 C DIC at Quebec City d13 C DIC at Montreal  d13 C DIC of Tributaries



ð15Þ With both the chemical and isotopic methods, a nonlinear rational function should account for the mixing of the two end-members with different DICconcentrations, thus allowing to estimate the relative DIC-supplies from tributaries and their contribution to DIC-fluxes at Quebec City (Fig. 8). The equation of

the rational function is obtained firstly by setting one end-member value at 19.6 mg/l, i.e. at the level of the mean annual DIC concentration at Montreal (considered to represent the outflow from the Great Lakes). In a second step, the other end-member value, i.e. the mean annual DIC concentration in tributaries, is set at the value resulting in the best fit of the mixing curve (Fig. 8). The best fit is obtained when the highest correlation coefficient (r2 value) is reached between the data points derived from the chemical or isotopic approaches and the rational function curve. The corresponding equation (Eq. (16)) suggests mixing of a combination of DIC-rich and DIC-poor tributaries with Great Lakes supplies, and a mean DIC concentration of approximately 8 mg/l in the tributaries. This value is close to that measured in the Ottawa River (
7 mg/l). Based on the preceding constraints, the relative contribution of each component of the hydrographic system of the St. Lawrence to the mean annual DIC fluxes at Quebec City can be estimated. We

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

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assume that the DIC concentration of the Ottawa River (
7 mg/l) is representative of supplies from rivers draining primarily silicate rocks from the Precambrian basement, and those from the Mascouche River (
36 mg/l), of supplies from the carbonate rich St. Lawrence Lowlands. Then, the
8 mg/l endmember indicated by Eq. (16) suggests a
5% contribution of the supplies from the St. Lawrence Lowlands, to DIC fluxes from tributaries (Table 2). Since DIC from tributaries account for 18% of total DIC at Quebec City, those from the St. Lawrence Lowlands should not account for more than 1% of total DIC export to the St. Lawrence estuary. y¼

ð8xÞ ð8xÞ þ ðð1  xÞ19:6Þ

ð16Þ

In the preceding calculation, one does not take into account the fact that about 39% of DIC from the Ottawa River actually originates from carbonate rich areas of its drainage basin (Telmer and Veizer, 1999) that could include both Precambrian marbles of the Grenville province and Paleozoic carbonates of the St. Lawrence Lowland, at the very end of the river’s course. Therefore, the actual contribution of DIC from the St. Lawrence Lowland tributaries sensu lato, to DIC export in the estuary could be somewhat slightly higher than that estimated above. Nevertheless, as illustrated in Fig. 8 and in Table 3, on a mean annual basis, the DIC supplies from the Great Lakes slightly exceed 80% of total DIC fluxes at Quebec City. In the above calculations, tributary supplies from the Appalachians (see Fig. 1) were ignored. These tributaries carry a heavy suspended particulate matter load (Rondeau et al., 2000). Their dissolved substance Table 2 Relative supplies of tributaries from the St. Lawrence Lowlands and the Precambrian basement to total DIC discharge at Quebec City and their mean DIC concentration

Tributaries from the St. Lawrence Lowlands Tributaries from the Precambrian basement

Relative DIC supply to total DIC discharge at Quebec City (%)

DIC concentration (mg/l)


1

36

17

7

Table 3 Relative proportions of the contribution to total water discharge and to total DIC discharge at Quebec City for the Great Lakes and tributaries

Great Lakes Tributaries

Relative contribution to total water discharge at Quebec City (%)a

Relative DIC supply to total DIC discharge at Quebec City (%)b

66 34

82 18

a

St. Lawrence Centre of Environment Canada, unpublished data from January 1994 to January 1999. b Present study.

content and alkalinity are somewhat intermediate between those of the St. Lawrence River itself, and of the tributaries draining the Precambrian basement (Yang et al., 1996). Based on outflow measurements from 1994 to 1999 (St. Lawrence Centre, unpublished), the tributaries from the southern sector of the St. Lawrence Lowlands (including all rivers from the Appalachian foothills plus a few others from other minor watersheds) account for less than 10% of the mean annual discharge at Quebec City. Taking into account the fact that they have alkalinities representing approximately 50% of DIC concentrations in waters from the Great Lakes and that their discharge to the St. Lawrence River are unimportant, they should have small or even practically no influence at all on the total DIC budget at Quebec City.

5. Conclusion One conclusion resulting from this study has conceptual and methodological implications. It concerns both the duration and the sampling frequency needed to monitor inorganic carbon fluxes through the St. Lawrence system and to determine the corresponding carbon sources in the drainage basin. The seasonal variability is high. DIC fluxes at Quebec City vary from 12 Gg of carbon/day during low-water periods, to approximately 24 Gg of C/day in spring time, with peaks as high as 38 Gg of C/day during major snowmelt events. Therefore, a survey spanning the whole year is needed to calculate annual fluxes. At the beginning of the study, we set a sampling and monitoring interval of 2 weeks. Clearly, this resolution could result in biases, notably due to the variability during the spring snowmelt period. Thus, we increased the resolution to 1-

J.-F. He´lie et al. / Chemical Geology 186 (2002) 117–138

week intervals in the second year of the monitoring program during this critical period (i.e., approximately from late March until early May). This study also shows that chemical data supported by information on discharge rates provide robust estimates for DIC-fluxes and precise indications of the source areas in the hydrological system. Isotopic analysis of DIC may allow for further insight. Herein, isotopic data provided unique information, though not fully conclusive, on the in situ carbon transformation (i.e., organic matter oxidation, exchanges with atmospheric CO2, etc.), and an independent assessment of the relative contributions of the Great Lakes vs. the tributaries to total DIC fluxes. The chemical and isotopic approaches yielded concordant results. However, the isotopes failed to allow for the deciphering of the specific contributions of tributaries draining Precambrian silicate-rich basins vs. those draining Paleozoic catchments. Both categories do not differ much from one another and show overall low d13C-DIC values. The relative importance of tributaries draining primarily Precambrian areas vs. those draining the carbonate-rich St. Lawrence Lowlands, with respect to DIC fluxes, constitutes a somewhat unexpected but important result. On a yearly basis, between 15% and 20% of the total DIC fluxes at Quebec City originate from tributaries of the St. Lawrence River. Most of this contribution is due to inputs from tributaries draining primarily the Precambrian basement and, to a much smaller extent, from rivers draining the Appalachian foothills and the St. Lawrence Lowlands. Nevertheless, the outflow from the Great Lakes represents the most important DIC supply to the ocean (about 80% to 85% of total). Overall, the total annual DIC supply from the St. Lawrence River into the maritime estuary represented approximately 6  1012 to 7  1012 g C in 1998, i.e. about 1.5% of the mean annual DIC exports of world rivers into the ocean, during the survey year (1998). Based on the present data, the St. Lawrence River seems to represent a source for atmospheric CO2 along its course downstream Montreal, with respect to its in situ metabolism of carbon. Indeed, pCO2 calculations indicate near equilibrium with atmospheric CO2 at Montreal in opposition to strong partial pressures downstream, with mean pCO2 reaching values almost threefold that of equilibrium with the atmosphere, at Quebec City. From this view point, the

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annual survey data presented here, compared to the two sets of seasonal measurements by Yang et al. (1996), lead to different conclusions. We do not find evidence for a seasonal pattern as contrasted as they did, nor for a uniform behaviour of the St. Lawrence River, all its way long, with respect to CO2-budgets at water – atmosphere interface. The distinct conclusions of the two studies may be due to either interannual (and interseasonal) variability, but more probably to temporal or spatial differences in the two sampling programs.

Acknowledgements This study has been made possible by funding from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the technical support of the St. Lawrence Centre of Environment Canada. Thanks are due to constructive comments by two anonymous reviewers that helped to clarify several conclusions of the present study. The authors would also like to thank the staff of the GEOTOP research centre who provided support in both the field and laboratory tasks. [JD]

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