Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 74 (2010) 531–539
www.elsevier.com/locate/gca
Noble gas enrichments in porewater of estuarine sediments
and their effect on the estimation of net denitrification rates
Fabien Pitre, Daniele L. Pinti *
Laboratoire GRAM, GEOTOP and Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal,
CP.8888 Succ. Centre Ville, Montréal, Que., Canada H3C 3P8
Received 24 July 2009; accepted in revised form 28 September 2009; available online 7 October 2009
Abstract
The concentration and the isotopic ratios of noble gases He, Ne, Ar, Kr and Xe were measured in porewater trapped in
shallow sediments of the estuary of the St-Lawrence River, Québec, Canada. The gases are atmospheric in origin but most
samples have gas concentrations 1.7–28 times higher than those expected in solution in water at equilibrium with the atmosphere. Elemental fractionation of heavier noble gases Kr and Xe compared to Ar strongly suggests that noble gases were
adsorbed on sediments or organic matter and then desorbed into porewaters due to depressurization, as collected samples
were brought to the surface. Atmospheric Ar in porewater is used as a reference to measure the N2-fluxes at the water–sediment interface. Ignoring the Ar enrichments observed in porewater could lead to a severe underestimation of the denitrification rate in oceans and estuaries.
Ó 2009 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
The concentration of dissolved atmospheric noble gases
(ANG) in groundwater (Air Saturated Water or ASW) and
seawater (Air Saturated Seawater or ASSW) is determined
by the physical conditions – air pressure, water/soil temperature and salinity – prevailing during gas exchange with the
atmosphere (Mazor, 1972). Therefore, ANG can be used
for paleoenvironmental and paleoclimatic reconstructions
(Kipfer et al., 2002 and references therein). Recently, these
applications have been extended to porewater preserved in
sedimentary rocks (Osenbrück et al., 1998; Rübel et al.,
2002) and in unconsolidated lacustrine and oceanic sediments (Brennwald et al., 2003, 2005; Strassmann et al.,
2005). Vertical profiles of dissolved noble gas concentrations in lacustrine porewater are used for the quantitative
reconstruction of past salinity and water levels in lakes
(Brennwald et al., 2003). These profiles may be controlled
*
Corresponding author. Tel.: +1 514 987 3000/2572; fax: +1 514
987 3635.
E-mail address: [email protected] (D.L. Pinti).
0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2009.10.004
by vertical diffusion/advection within the sediment column
and thus serve as proxies to study the transport dynamics of
solutes and fluids in the sediment (Strassmann et al., 2005;
Chaduteau et al., 2009). Finally, noble gases can trace secondary gas exchange processes in the sediment. Brennwald
et al. (2005) showed a distinct depletion of the ANG concentrations in lacustrine porewater of Soppensee, Switzerland. Poorly soluble, lighter noble gases escaped from the
porewater into gas bubbles, due to ebullition of biologically
produced methane. This phenomenon helped to quantify
the release of potential greenhouse gases from lacustrine
sediments during the Holocene (Brennwald et al., 2005).
Being inert, noble gas profiles can be compared against
reactive species to quantify the processes of production/
consumption and the input/output fluxes of those elements
at the water–sediment interface. For example, the N2/Ar ratio is measured in porewater to determine the net rate of
denitrification (N2 production from reduction of nitrates)
in marine and estuarine settings, assuming the Ar concentration to be in solubility equilibrium with the atmosphere
(Nielsen, 1992). Using a rigorous approach to the problem,
the processes controlling the ANG concentration and
distribution in sedimentary porewater need to be fully
532
F. Pitre, D.L. Pinti / Geochimica et Cosmochimica Acta 74 (2010) 531–539
ments of the estuary, these two elements being studied by
research teams from McGill University, Concordia University, Université de Montréal and the GEOTOP research
center at the Université du Québec à Montréal. Sediment
cores, up to 36 cm long, were taken at stations 20, 21, 22,
23 and 23S located in the Laurentian Channel (Fig. 1).
Samples from stations 20 and 21 have been analyzed for
their porewater noble gas content and reported here.
Sediment cores were collected in transparent acrylic
tubes of 62 cm length and external diameter of 10.5 cm
(Fig. 2), which could be mounted on a standard 6- or 8tubes multiple corer. Seven holes were drilled in the plastic
tube, with a spacing of 6 cm, and then closed with a NPT
plug (Fig. 2). Once the multi-corer was onboard, the acrylic
tube was closed at both extremities with two machined PVC
pistons to fit the system with a whole-core squeezer (Bender
et al., 1987). The upper PVC piston has a valve to purge the
air contained in the upper part of the core (Fig. 2). After
installing the core on the squeezer, the lowermost NPT plug
was removed and a standard copper tube (ca. 5 cc volume;
Table 1) with two stainless steel pinch-off clamps (Weiss,
1968) was installed using a male NPT to tube fitting
(Fig. 2). During core squeezing, the air contained in the
copper tube was expelled and then the clamps were closed
elucidated. For example, degassing in lacustrine sediments,
as described by Brennwald et al. (2005), has to be taken into
account to adjust the apparent N2 losses for ebullition (calculated from the Ar losses) and to determine the amount of
excess N2 resulting from denitrification.
Here, we show that noble gases, Ne, Ar, Kr and Xe (and
He in some samples), collected in porewater of the St-Lawrence estuary, Québec, Canada are enriched by a factor of
1.7–28 times the expected ASSW values. Physical adsorption of noble gases on sediments and/or organic matter
and their successive desorption and release into porewater
during the core recovery is likely the cause of these enrichments. The relative Ar enrichment, if not accounted for,
would severely underestimate the net denitrification rate
in estuaries and oceans.
2. SAMPLING AND ANALYTICAL METHODS
Seafloor sediments were collected in the St-Lawrence
estuary, Québec, Canada (Fig. 1) during an oceanographic
cruise in May 2007, onboard the research vessel Coriolis II.
The primary goal was to test whether the noble gases measured close to the water–sediment interface could be used
for calibrating C and N2-fluxes in the organic-rich sedi-
QUEBEC
Sept-Iles
zoomed
area
50°N
Montreal
200 m
20
USA
ce
ren
w
a
23 L
.
23S
St
Tadoussac
ry
ua
Est
21
22
Rimouski
48°N
70°W
68°W
66°W
Fig. 1. Map of the St-Lawrence estuary, including the sampled stations in the Laurentian Channel. Coordinates of the stations are as follows:
St20, 49°25.430 N, 66°19.410 W, St21, 49°05.550 N, 67°16.940 W; St22, 48°52.880 N, 67°59.820 W; St23, 48°42.070 N, 68°38.980 W; St23S,
48°38.840 N, 68°35.800 W.
Noble gas enrichments in sediment porewater
533
Valve
Stopper
Multi-core acrylic tube
Locking ring
Copper tube
Copper tube
Clamps
PVC piston
with O-rings
Hydraulic jack
Fig. 2. Schematic drawing of the acrylic tube used for sampling sediment cores mounted on the squeezing system, together with a zoom of the
NPT to tube fittings used for fixing the copper tube to the acrylic core.
to retrieve uncontaminated bulk sediment. The copper tube
was not removed to safeguard the system against air contamination and the procedure was repeated for each hole,
moving up the core. The lower PVC piston generally
masked the lowermost hole and samples could be not retrieved from it. The uppermost hole was always very close
to the water–sediment interface and thus samples were
not taken, being possibly contaminated by gases diffusing
from the seawater.
In the laboratory, sampled copper tubes were connected
on top of a 2 L pre-evacuated headspace. After the lower
clamp was removed, one end of the copper tube was briefly
heated in order to expel the unconsolidated sediment. An
aliquot of the gas was purified using Ti and St707 getters.
Unspiked isotopic analyses of noble gases (4He, 20,21,22Ne,
36,38,40
Ar, 84,86Kr and 129,132Xe) were carried out using a
quadrupole mass spectrometer PRISMA SEM-200 from
PfeifferÒ, calibrated against pipettes of purified air collected in Montréal, Canada. Precision on standard measurements (1 pipette of 36Ar = 5.76 1011 cm3STP) was
better than 2%. The concentration of each noble gas isotope
was calculated in cm3STP and divided by the volume of
Table 1
Noble gas concentrations and isotopic ratios in porewater from stations 20 and 21.
Sample
ID
Depth
(cm)
±
Volume Porosity 4He
10 8
tube
(%)
[cm3STP/
(cm3)
cm3H2O]
21
Ne
±
10 9
[cm3STP/
cm3H2O]
Station 20: depth = 325 m ; salinity = 24.0 g/L; temperature = 5.07 °C
st20-6
4.9
5.0
84
10.59
1.09
2.52
st20-5
9.0
5.3
82
10.31
0.75
2.43
st20-4
12.2
4.4
80
4.14
0.51
3.31
st20-3
17.2
5.1
80
b.l.
b.l.
1.66
st20-2
21.6
5.2
80
1.81
0.22
2.05
Station 21; depth = 322 m; salinity = 25.2 g/L; temperature = 5.79 °C
st21-5
10.2
5.1
79
b.l.
b.l.
2.28
st21-4
15.6
4.9
77
b.l.
b.l.
1.36
st21-3
21.1
5.1
77
b.l.
b.l.
1.55
st21-2
26.6
4.8
76
b.l.
b.l.
1.35
st21-1
32.0
5.2
78
b.l.
b.l.
1.77
4.31
0.50
ASSWa
36
84
132
Ar
±
10 6
[cm3STP/
cm3H2O]
Kr
±
10 7
[cm3STP/
cm3H2O]
Xe
±
10 8
[cm3STP/
cm3H2O]
0.35
0.19
0.41
0.22
0.42
13.24
5.04
6.68
5.25
7.29
1.25
0.32
0.62
0.54
0.80
9.30
3.47
6.65
6.62
5.78
0.90
0.24
0.66
0.71
0.59
9.81
3.91
6.31
7.42
5.56
0.91
0.29
0.52
0.97
0.98
0.23
0.10
0.12
0.10
0.18
2.17
1.03
0.96
0.33
2.60
1.25
0.18
0.08
0.07
0.03
0.21
2.29
1.19
0.69
0.15
3.81
0.51
0.20
0.09
0.05
0.01
0.34
3.09
1.65
0.60
0.11
5.81
0.35
0.28
0.26
0.10
0.02
0.53
Typical blank values in ccSTP: 4He (1–5 108); 21Ne (0.3–9.9 1010); 36Ar (1.6–8.3 108); 84Kr (0.4–3.3 109); 132Xe
(0.3–7.5 1010). b.l., blank value, assumed for concentrations measured less than two times their respective blanks.
Errors are calculated including analytical ones from the samples and from the standards, and uncertainties on volumes of porewater.
a
Air Saturated Seawater (ASSW) calculated for a mean temperature of 5 °C and 24& of salinity.
F. Pitre, D.L. Pinti / Geochimica et Cosmochimica Acta 74 (2010) 531–539
pic ratios 38Ar/36Ar, 40Ar/36Ar, 84Kr/86Kr and 129Xe/132Xe
were atmospheric within uncertainties and they are not reported in Table 1 for the sake of brevity. The contributions of 40Ar2+ and CO22+ on masses 20 and 22
were underestimated, giving unrealistic 20Ne/22Ne and
21
Ne/22Ne isotopic ratios, not reported in Table 1. But
comparison with neon isotopic ratios obtained for standard air suggests that they are basically atmospheric within uncertainties. Therefore, only the 21Ne isotope, that
does not have isobaric interferences, is discussed here
and assumed to be entirely atmospheric in origin. Brennwald et al. (2003) noticed that radiogenic 4He could be released from the sediment matrix when the copper tube is
heated. We limited heating to one of the extremities for
less than 1 min. The small amount of measured 4He, often
close to the blank (Table 1), suggests that contamination
by an in situ produced radiogenic noble gas component
was minimal.
porewater. The latter cannot be precisely measured because
the amount is small (ca. 5 cc) and it is dispersed in the
extraction system after sediment heating and blowing.
Thus, the amount of porewater was calculated by using
the measured porosity for each sampled interval. The
porosity was measured by weighing wet and dry sediments
at the Department of Chemistry and Biochemistry of Concordia University (Table 1).
3. RESULTS
Table 1 reports the 4He, 21Ne, 36Ar, 84Kr and 132Xe
concentrations measured in the porewater recovered from
stations 20 and 21, together with the volumes of the copper sampling tubes and the porosity of the sediments measured at each sampling interval. The 4He concentrations
were very close to the analytical blank for station 21
and for two samples from station 20 (Table 1). The isoto-
0
0
Station 20
5
20
40
Depth (cm)
60
10
80
100
15
Age (years)
534
120
140
20
160
0
2
36
4
6
8 10 12 0
36
[ Ar]/[ Ar]ASSW
5
10 15 20 25 0 5 10 15 20 25 30 35
84
[ Kr]/[84Kr]ASSW
[132Xe]/[132Xe]ASSW
0
0
Station 21
20
Depth (cm)
60
80
20
100
Age (years)
40
10
120
30
140
0
0.5 1.0 1.5 2.0 2.5
0
[36Ar]/[36Ar]ASSW
[84Kr]/[84Kr]ASSW
2
4
6
8 10
0
5
10 15 20
[132Xe]/[132Xe]ASSW
Fig. 3. The 36Ar, 84Kr and 132Xe measured concentration profiles for station 20 (top) and 21 (bottom). Concentrations have been normalized
to those expected at ASSW conditions for the two stations (see Table 1). Age of sediment is calculated from 210Pb sediment accumulation rates
from Smith and Schafer (1999).
Noble gas enrichments in sediment porewater
The concentration of noble gases 21Ne, 36Ar, 84Kr and
Xe are 1.7–28 times higher than those expected at ASSW
conditions (Table 1), except for sample st21-2 which shows
36
Ar, 84Kr and 132Xe concentrations lower than the ASSW
values (Table 1). The 4He concentration for samples st20-5
and st20-6 is more than twice the ASSW value.
First, we need to evaluate whether the high amounts of noble gases, higher than the expected ASSW values, are produced by (1) excess air (Aeschbach-Hertig et al., 2000) or
(2) by an experimental artifact due to uncertainties in calculating the amount of sampled porewater. Microscopic air
bubbles could have penetrated the sediment core during its
recovery on the ship’s deck. However, if this was the case, elemental ratios among noble gases (Figs. 4 and 5) should plot
on a straight line between the ASSW and the AIR end-member, representing the addition of small air bubbles, which is
clearly not the case (Figs. 4 and 5). Compaction during
squeezing could reduce the porosity inducing an error in evaluating the amount of porewater. However, the compaction
was less than 10% of the total length of the core (and partially
due to the extraction of sediment into copper tubes). Furthermore, if the amount of porewater is smaller than that calculated from the porosity values (Table 1), estimated noble
gas concentrations should be higher (the pore water volume
being at the denominator of the concentrations). Measured
concentrations (Table 1) could be thus considered as minimum values. On the other hand, if our copper tubes contained only porewater, the final concentration should be
lower by 15–25% than the measured values, which corresponds to the volume percentage occupied by sediments in
our samples (Table 1). Thus, we can conclude that the observed enrichments are real and analytical artifacts did not
produce them.
When plotted against depth, the 36Ar, 84Kr and 132Xe
concentrations of the two stations show different profiles
535
(Fig. 3). At station 20, 36Ar, 84Kr and 132Xe do not show
specific trends with depth, except the uppermost sample
(st20-6), whose concentration is 2–3 times higher than that
of the lower samples (Fig. 3). The concentrations of 36Ar,
84
Kr and 132Xe range from 4 to 28 times the ASSW value.
The concentration of 21Ne, not reported in Fig. 3, does not
vary systematically with depth and shows an enrichment of
2.7–6.5 times that expected for the ASSW value in both stations (Table 1). Station 21 (Fig. 3) shows a systematic decrease of 36Ar, 84Kr and 132Xe concentrations with depth,
except for the lowermost sample (st21-2), which shows a
noble gas concentration similar to the value measured for
the uppermost sample (st21-6). The 36Ar concentration decreases from 1.7 times the ASSW value in the uppermost
sample to 0.3 times the ASSW value for sample st21-3.
For sample st21-2, it goes back to two times the ASSW value (Fig. 3). The 84Kr and 132Xe concentrations exhibit a
similar behavior, with concentrations varying between 4.5
and 9 times the ASSW values in the uppermost sample to
0.3 in sample st21-3. In the lowermost sample, they increase
to 7–16 times the ASSW value (Fig. 3).
132
4. DISCUSSION
In the following, we compared the noble gas concentrations measured in porewater with those measured in kerogen and soot from (Frick and Chang, 1977; Frick et al.,
1979). Since Frick and Chang (1977) obtained data for
130
Xe and 22Ne and the isotopic ratios were often fractionated compared to the atmospheric ones, we recalculated the
132
Xe and 21Ne measured in our samples as 130Xe and 22Ne,
in Figs. 4 and 5, for allowing a correct comparison.
The most significant result of this study is the relative
enrichments of noble gas isotopes in the porewater (Table 1;
Fig. 3), which were not previously observed in other studies
100
10
Adsorption
on kerogen
Air
ASSW
0.1
Equilibrium degassing
Continuous degassi
F(22Ne)
1
0.01
0.001
Adsorption
on soot
ng
0.0001
1
10
100
130
F(
22
130
1000
10000
Xe)
Fig. 4. The recalculated F( Ne) vs. F( Xe) at station 20 (black squares) and station 21 (white circles) are shown. The curved dashed line
represents the variation of F(22Ne) and F(130Xe) for an equilibrium degassing; the straight line indicates continuous degassing. The black
triangles are F(22Ne) and F(130Xe) measured in kerogen and soot from Frick and Chang (1977) and Frick et al. (1979).
536
F. Pitre, D.L. Pinti / Geochimica et Cosmochimica Acta 74 (2010) 531–539
Continuous degassing
10-3
1000
0.1
10
10-2
g/w = 10-4
Equilibrium degassing
ASSW
Adsorption
on kerogen
10
100
g
sin
F(84Kr)
as
s
ou
g
de
nu
ti
on
C
10
Adsorption
on soot
Air
ASSW
Equilibrium degassing
1
1
10
100
1000
10000
F(130Xe)
Fig. 5. The recalculated F(84Kr) vs. F(130Xe) at station 20 (black squares) and station 21 (white circles) are shown. The curved dashed line
represents an equilibrium degassing (see zoomed section), while the thick line indicates a continuous degassing model. Symbols are the same as
in Fig. 4.
(e.g., Brennwald et al., 2005; Chaduteau et al., 2009).
Brennwald et al. (2005) suggested degassing (by ebullition)
of methane within the sediment to explain elemental fractionation of noble gases in sediments. If a solubility partition of noble gases at equilibrium is assumed between the
porewater and a separate gas phase, then the less soluble
gases in water (He and Ne) will be preferentially partitioned
in the gas phase and lost during ebullition. The most soluble gases in water (Kr and Xe) will be retained in the residual, degassed water phase. Bosch and Mazor (1988)
modeled the evolution of the ratios (i/36Ar) (where “i” is
22
Ne, 84Kr or 130Xe in our case) in the escaping gas phase.
Rearranging Eqs. 2 and 3 in Bosch and Mazor (1988), gives
the equation governing (i/36Ar) in the residual water phase
after equilibrium degassing:
1
i
i
G
1
G
1
¼
þ
þ
36 Ar
36 Ar
W K Ar
W K iw
w
ASSW
w
i Kw
ð1Þ
K Ar
w
senting the effects of equilibrium degassing on F(22Ne),
F(84Kr) and F(130Xe) are obtained (dotted line of Figs. 3
and 4) by varying the G/W ratio in Eq. (1). The F(22Ne)
vs. F(130Xe) plot (Fig. 4) shows that an equilibrium degassing could only explain the variation observed for F(22Ne),
except for two samples, but not the F(130Xe), which shows
higher values (Fig. 4). This is also true when F(84Kr) and
F(130Xe) are plotted together (Fig. 5). Only three samples
show F(84Kr) and F(130Xe) values compatible with an equilibrium degassing (Fig. 5). The majority of samples show
higher values than those predicted by an equilibrium partitioning of heavy noble gases in the residual water (Fig. 5).
Release of gas bubbles from sediment, where water is
equilibrated with an infinitesimal bubble of gas and the
gas is then subsequently removed, can be modeled by a
Rayleigh distillation. This process is governed by the
equation:
i Kw
1
i
i
K Ar
w
¼
F
ð2Þ
36
Ar
36 Ar
36 Ar
w
ASSW
where “w” denotes the residual porewater phase; “ASSW”
denotes the initial porewater phase at equilibrium with
are solubility constants
the atmosphere. K iw and K Ar
w
(atm1 kg1 mol) of the considered gas isotope, “i”, and
of 36Ar, respectively, as calculated by Smith and Kennedy
(1983). Finally, G/W is the ratio of the volume of gas in
contact with the volume of porewater. The curves repre-
where F 36 Ar is the fraction of 36Ar remaining in the water
after degassing. A continuous degassing model could explain the F(84Kr) and F(130Xe) co-variations (Fig. 5), but
again it cannot explain the relationship between the
F(22Ne) and F(130Xe) (Fig. 4). Furthermore, degassing assumes that the initial total concentration of each isotope
is equal to that expected when in equilibrium with the
Noble gas enrichments in sediment porewater
atmosphere (ASSW), this value decreasing progressively
during degassing. However, measured noble gas isotope
concentrations are clearly higher than those expected for
the ASSW (Table 1).
The only physical process that can explain both an increase in total concentration and elemental ratios is adsorption of gases on fine sediments (clays) and/or organic
matter. Noble gases extracted from ancient and modern
carbon-rich sediments are enriched in Kr and Xe, in comparison to their proportions in air, with observed elemental
130
Xe/36Ar ratios of up to 104 (Frick and Chang, 1977;
Frick et al., 1979; Podosek et al., 1980). The general pattern
of noble gas enrichment (Xe > Kr > Ar; Fig. 3) and the observed fractionation factors F(i) suggest physical adsorption
on sediments or organic matter (Fanale and Cannon, 1971),
preferentially selecting the heavier noble gases. The experimental data of adsorption of lighter (22Ne) and heavier
(84Kr, 130Xe) noble gases on kerogen and soot produced
by electric discharge (Frick and Chang, 1977; Frick et al.,
1979) are plotted in Figs. 4 and 5. The F(84Kr) and
F(130Xe) values could then be explained by the mixing between two sources: atmospheric noble gases dissolved in
water (ASSW) and noble gases adsorbed on kerogen or
any other organic matter preserved in the sediment. The
F(22Ne) vs. F(130Xe) relation and its shift from the degassing line, towards the right side of the diagram, might indicate that both degassing of an ASSW source and mixing
with adsorbed gases might have acted in the sedimentary
column (Fig. 4).
Estuarine sediments are enriched in kerogen or contain
soot produced from combustion of plant tissues or charcoal (Schmidt and Noak, 2000). The Total Carbon Content (TOC) generally tends to decrease with depth
(Emerson and Hedges, 2003). This could explain the decrease in the total 84Kr and 132Xe concentrations with
depth observed at station 21 (Fig. 3). Decreasing availability of adsorbing surface on organic matter would decrease the amount of heavier noble gases in the system.
The net increase of the noble gas concentrations at the
bottom of the core sampled at station 21 (Fig. 3) could
be related to some change in the granulometry and/or organic matter content. Indeed, sediment slumping and
interlayer mixing often disturb the estuarine sediments
of the St-Lawrence.
The adsorption of noble gases on sediments or organic
matter could explain the observed noble gas fractionations
(Figs. 4 and 5), yet noble gases were not measured in sediments but in the porewater, which should be depleted of
heavier noble gases. Interestingly, Torgersen and Kennedy
(1999) observed the same enrichments in Xe and Kr, compared to 36Ar, in petroleum of the Elk Hills Naval Petroleum Reserve (California). Torgersen and Kennedy (1999)
suggested that Xe and Kr enrichments could be explained
by adsorption of noble gases on the organic matter-rich
source rocks. During primary migration of water-pressure
driven oil, the heavier noble gases would be released from
the source rocks and migrate together with oil in the reservoir rocks. Thus the fluid phase would inherit the heavier
noble gas enrichment produced originally by adsorption
on the sedimentary/organic phase.
537
Adsorption of gases on surfaces depends on several
physical parameters. The pressure and temperature are
the most important (Fanale and Cannon, 1971), together
with the intrinsic characteristics of the adsorbent, such as
the effective surface area (Brunauer et al., 1938). The capability of adsorption of noble gases on shales, and possibly
on organic matter, increases with increasing pressure (i.e.,
with burial depth and diagenesis; Fanale and Cannon,
1971) and decreasing temperature (Fanale and Cannon,
1971, 1972). In all cases, the following semi-empirical Freundlich relationship (Freundlich, 1930) is well obeyed:
log V ¼ log K þ
1
log P
n
ð3Þ
where V is the volume of adsorbed gas in ccSTP/g, P is the
equilibrium pressure of that gas, and K and n are constants
dependant on the nature of the adsorbent and adsorbate as
well as the temperature (Fanale and Cannon, 1971).
In the case of the estuarine sediments analyzed here, we
hypothesize that the change in pressure and temperature
during the sediment recovery (32–33 bar corresponding to
322–325 m depths and 5–10 °C between sea bottom and
the vessel’s deck) could have facilitated the release of adsorbed gases from the sediment to the porewater, modifying
the initial ASSW concentrations (Figs. 3 and 4). Fanale and
Cannon (1971) calculated the Freundlich plot for Xe and
Kr, from 0 to 25 °C and from 0.1 to 100 torr, for volcanic
ash shale (Martinez Shale). If we assume a change in partial
pressure of 30 bar between the conditions at the sea bottom
and the surface (1 bar), the capacity of adsorption of Xe
and Kr would be reduced by at least one order of magnitude (the concentration of Xe and Kr adsorbed at 1 bar
would be 45 times less than at 33 bar), at a temperature
of 0 °C. Thus the change in the physical conditions of T
and P during the recovery of the sediment core could produce a partial desorption of gases from the sedimentary matrix to the porewater.
Adsorption of lighter Ne is more difficult to explain. Indeed, experimental adsorption on charcoal at room temperature (Dushman, 1957) showed that Ne should not be
adsorbed. On the other hand, experimental data obtained
on pulverized particles of meteorites showed that at partial
pressures higher than 10–100 torr of the adsorbate, Ne is
effectively adsorbed on the powders (Fanale and Cannon,
1972). Anomalous neon enrichments in terrestrial sedimentary rocks are common (e.g., Podosek et al., 1980; Matsubara and Matsuda, 1995) and they have been explained
by diffusive processes (Matsubara and Matsuda, 1995; Pinti
et al., 1999) or enhanced solubility (e.g., Pinti et al., 2004).
Torgersen et al. (2004) developed a model to explain the
contemporary enrichments of lighter noble gas Ne and heavier Xe in sedimentary rocks. In their model, gases diffuse
through half-spaces to fill the rock. In earlier stages, Ne diffuses faster than heavier Ar and Xe producing a neon
enrichment and high F(Ne) values. During emptying of
the rocks (for example during compaction and diagenesis),
Xe slowly diffuses out the rock, producing a relative enrichment compared to lighter and faster noble gas Ar. Partial or
incomplete filling and emptying of the rock could create the
excesses of Ne and Xe, compared to Ar (Torgersen et al.,
538
F. Pitre, D.L. Pinti / Geochimica et Cosmochimica Acta 74 (2010) 531–539
2004). Although the model of Torgersen et al. (2004) is
appealing, we need porosities of less than 1% and timescale
of the order of hundreds of thousands or millions of years
to produce the Ne and Xe enrichments observed in the StLawrence sediments. But the sampled sediments in the StLawrence estuary have porosities higher than 75% and ages
of less than 160 years (Smith and Schafer, 1999). Thus alternative models need to be developed for explaining the Ne
enrichments observed in the St-Lawrence estuary.
The noble gas enrichments observed in porewater have
dramatic consequences when quantifying the processes of
production/consumption and the fluxes of reactive species
at the ocean–sediment interface. The computation of the
net denitrification rate in oceans, i.e., the amount of produced N2 from the reduction of nitrates by heterotrophic
bacteria (Sigman and Casciotti, 2001) is the perfect example. Denitrification is the major sink of nitrogen in the modern ocean, yet its quantification is not straightforward (Eyre
et al., 2002; Groffman et al., 2006). Among the possible approaches, Nielsen (1992) suggested to use the N2/Ar ratio in
porewater, which is often measured in dynamic mode with
a membrane inlet mass spectrometer and compared to standards of water at known salinities and temperatures (Kana
et al., 1994). The N2 flux is determined from the rate of
change in N concentration with depth. If the N2/Ar ratio
is used, the change in N2 (DN2) is determined from (DN2/
Ar) [Ar], where [Ar] is the Ar concentration at air saturation determined from solubility equations (Weiss, 1970;
Smith and Kennedy, 1983). If the Ar concentration is enriched up to 10 times the expected air saturation value, then
the N2 flux coming out of the sediment is severely underestimated by one order of magnitude. This has remarkable
consequences in the computation of the N cycle balance
in the ocean and estuaries, between the sources (atmospheric N2 fixation and nitrate production) and the sink
(N2 by denitrification).
5. CONCLUSIONS
This study revealed the occurrence, in porewater from
young, unconsolidated estuarine sediments, of noble gas
concentrations in excess of those expected for seawater in
equilibrium with the atmosphere. These excesses might be
produced by physical adsorption on the organic matter
abundant in the sediment and their successive desorption
during the recovery of the core at the surface. Documenting
these excesses is very important for the correct use of inert
noble gases in normalizing fluxes in/out of solute species,
such as the N2 fluxes produced by nitrate reduction. Denitrification is often quantified by using the N2/Ar ratio measured in porewater. This study has shown that Ar can be
enriched up to 10 times the expected ASSW value, which
would produce a severe underestimation of the N2 production in the sedimentary column. To compensate for this
problem, the absolute concentration of Ar must be measured (e.g., Kana et al., 1994), possibly in static mode (this
study), to overcome analytical artifacts and not assumed a
priori as equal to that at air saturation. The measurement of
the Kr/Ar and Xe/Ar ratios, together with the N2/Ar ratio,
could also be helpful for determining whether the system is
close to the ASSW value or fractionated by adsorption/
desorption processes.
ACKNOWLEDGMENTS
Discussions with C. Ballentine, P. Burnard, Y. Gélinas, R.
Maranger B. Marty and J. Kowarzyk-Moreno were appreciated.
Comments and suggestions from two anonymous reviewers greatly
improved the manuscript. We thank R. Lapointe for helping in
building the sampling apparatus and the noble gas line at GEOTOP and M. Lehmann for valuable suggestions. C. Pickler helped
during isotopic analyses and corrected English. Colleagues from
McGill, Concordia, UdM and GEOTOP and the crew of Coriolis
II are thanked for their help and friendship. R. Panetta (Concordia
Un.) measured the porosities in the sediment cores. M. Laithier
helped in improving illustrations for this manuscript. Scholarships
of GEOTOP and Fondation UQAM to F.P., NSERC Grants No.
314496 to D.L.P. and NSERC Ship-Time No. 342808 to Y. Gélinas
funded the project. This is GEOTOP contribution 2009-0018.
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Associate editor: Sidney R. Hemming