N, : Ar, nitrification
and denitrification
California borderland basin sediments1
in southern
R. 0. Barnes2
Scripps Institution
of Oceanography,
P.O. Box 1529, La Jolla, California
92093
K. K. Bertine
Department
of Geology,
San Diego
State University,
San Diego,
California
E. D. Goldberg
Scripps
Institution
of Oceanography
Abstract
The first quantitative
observations
of dissolved molecular
nitrogen
( NP) in deep-sea
sedimentary pore waters show increases of up to 17% above adjacent bottom water values.
The N, : Ar concentration
ratio similarly increases. This increase is greater than expected
from reduction of seawater nitrate in the sediment.
A mechanism is suggested in which
ammonia, derived from nitrogen-containing
organic matter, is converted to Nz through a
nitrite intermediate
under conditions
of low oxygen tension.
Ammonia
is not converted
to N, in anoxic sediments that are undergoing sulfate reduction even though Nz is the thermodynamically
stable species.
Dissolved molecular nitrogen ( N2) is one
of the most abundant nitrogen spccics in
marinc sediments. Its role, if any, in the
chemistry of such sediments has been unknown owing to the difficulty of determining its concentration in interstitial waters;
no such dctcrminations have been reported
for open ocean sedimentary pore waters.
Emery and Hoggan (1958) determined
N2 in the pore waters of several southern
California borderland basin sediments but
used the N2 measurements to normalize
other gas concentrations,
assuming that
pore water N2 concentrations
were the
same as in the overlying seawater. Thus,
they did not determine absolute concentrations of dissolved Nz.
Port water N2 was also observed on leg
15 of the Deep Sea Drilling Project (Hammond et al. 1973), but the observations
were not quantitative and interpretation
of
the results was correspondingly ambiguous.
Reeburgh ( 1969) determined N2, Ar, and
Cl& in Chesapeake Bay sediments in water
’ This research was supported in part by OfIice
of Naval Research contract N00014-69-A-200-6049
and Geological Society of America grant 1749-73.
’ Present acldress: Walla Walla College Marine
Station, Rte. 3 Box 555, Anacortes,
Washington
98221.
LIMNOLOGY
AND
OCEANOGRAPHY
depths of 15 to 30 m. The methane
reached saturation concentrations at scdiment depths of less than 1 m. The N2 and
Ar concentrations
decreased with depth,
apparently due to gas stripping by methane
bubbles rising through the sediment which
obscured any concentration
relationships
between the two gases that may have resultcd from nitrogen chemistry in the sediment.
The great hydrostatic pressures at open
ocean depths (>300 m) would prevent
formation of gas bubbles in surface sediments. Therefore concentrations should reflect only chemical processes, bioturbation,
or diffusive-advective
mixing,
We report the first evidence of the role
of molecular nitrogen in the chemistry of
marine sediments with high or low electron
activity.
Accurate determinations
of N2
were made possible by the development of
a pore water sampler that filters and encapsulates pore water in situ, greatly reducing
gas loss and atmospheric contamination
( Barnes 1973). Analyses of duplicate samplcs show a precision of better than *0.5%
for the Na determinations (Table 1).
Some possible reactions involving molecular nitrogen in marine sediments are given
below. Sulfate reduction is included as a
962
NOVEMBER
1975,
V.
20( 6)
N, : Ar in California
Table 1. N, and AT concentrations
borderland
basins.
and N8 : Ar ratios for the May 1974 cruises to southern
I-_---__--
-_--
-.
------
-.
____-
%)?
Dcp th
Sample
(ml Z&/kg)
.._ ___ -_-Santa
*
California
-____.~
~--
~_-__---c__I-.-
-~--~
Solubility
963
seclinumts
%*
N2:Ar
(ml k/kg)
Barbara
Basin
12.02
36.8
0.327
+2.0
0.323
-1
37.9
Basin
water
550
m
12.24
Basin
water
550
m
12.20
+1.5
0.315
-4
38.7
cm
12.73
+6.0
0.322
-2
39.5
43.2
SupernatanL
+8
Box core
-23
cm
14.03
416.5
0.325
-1
Box core
-39
cm
13.76
+14'.5
0.332
+2
41.4
Box core
-54
cm
13.54
$12.5
0.321
-2
42.2
-69
cm
14.37
-119.5
(0.366)+
+12
39.3
Probe
Box core
-336
cm*
13.97
+L6.0
(0.359)
+10
38.9
Probe
-385
cm
12.95
+7.5
(0.334)
+2
38.8
Probe
-431
cm
14.13
f17.5
(0.354)
+8
39.9
PI-Obc.
-480
cm
13.58
+13.0
(0.351)
+7
38.7
Santa
Solubility
*
Cruz
Basin
12.59
Sllpernatant-pore
water mixture
0
0.344
0.335
-3
37.5
Probe
-111
cm@
12.65
-t0.5
0.337
-2
37.5
Probe
-202
cm
13.14
+4.5
0.333
-3
Li9.5
P1-ObC
-251
cm
13.50
-t7.0
0.341
-1
39.6
I'rOhC
-298
cm
13.25
-t5.0
0.339
-1
39.1
Probe
-393
cm
20.00
-1-59.0
(0.484)
Solubility
12.56
San Diego
,r;
SupernaLant
0.0
36.6
12.80
0.351
cm
12.78
0.0
-1.0
sampler
-21
cm
12.71
Interface
sampler
-21
cm
12.81
Interface
sampler
-38
cm
13.24
14.05
41.3
Trough
f17
InLcrTacc
+41
0.335
36.5
0.351
0.0
0
-3
37.5
+3.5
0.345
-2
38.4
+Jo.o
0.347
-1
40.5
39.7
-2 1 1 Cl11 ”
Probe
-307
cm
13.33
A-4.0
0.336
-4
Probe
-402
cm
13.66
-?-6.5
0.339
-3
----- _--.._
___-
__--
-~___.~-~
Pcrccntagc
deviation
from gas solubilitics
Tar tcmperaturc
wnLers.
Solubili.ties
arc from Weiss (1970).
Tnfcrior
analyses
= (
), about twice
normal
error.
l'robc depths + 25 cm.
Depths + 15 cm.
Probe, -minimum
depths.
rcferencc on the
equilibrium
pE”
taken from Morris
are rcfcrrcd to the
neutral solution.
pE” (W) scale. The
(W) values listed are
and Stumm (1967) and
hydrogen ion activity in
36.2
0.342
Pr0bC2
- --__--------
38.L
-5
l/5
and salinity
-.__
____
oE the
NO:!- + 6/S H+ + e
= l/10 Na(g) + 3/5 Hz0
l/8 SOd2- + 9/8 EI+ + e
= l/8 HS- + l/2 I-120
basin
40.3
_- --__
bottom
PEO (W>
+12.65
(1)
-3.75
(2)
Barnes et al.
964
Table 2. Summary of pertinent information
on sample collection
stations.
The basin abbreviations
are given in the text. “Box core” refers to samplers mounted inside or outside of a 20-cm-square
box
core. “Probe” refers to samplers mounted in-line on a 3 or 4 m long, 25cm diameter rod attached to a
1.2-m-long coring weight.
No sediments were recovered with the probe samples.
--.
~--____--_------.-____----__-Basin
Date
Sf3f)
12 Sep
Position
Collecting
_-----Box core
mechanism
& probe
Water
depth
(ml
_
590
34°16.8'N,
12OoO2.4'W
9 May 74
34O14.6'N,
120°01.7'W
Box core
588
SBB
9 May 74
34O15.6'N,
120°02.61W
Probe
588
SCB
9 May 74
33'42.6'N,
119'34.3'W
Probe
1970
32'32.7'N,
117'33.6'W
- .-..- --- --- - - --
Probe
1260
__----
SISB
SDT
____
l/6
____
73
24 May 74
-.._ _.-. --
N2(g) + 4/3 H+ + e
= l/3 NH4+
3 N03- + 5 NH4’= 4 Na(g) + 9 HsO + 2 H+
-4.68
(3)
-
(4
These generalized equations do not represent the actual reaction pathways through
which the transformations may bc accomplishcd.
To investigate the extent to which the
above reactions or others influence the nitrogen concentrations in marine sediments,
pore water samples were collected from
the top few meters of oxidizing and reducing sediments in three basins of the southcm California continental borderland. The
positions of the stations and sampling information are given in Table 2.
Santa Barbara Basin (SBB) is a shallow
nearshorc basin with a sedimentation rate
of 4 mm yr-1 (Koidc et al. 1973) and a
high concentration of reactive organic material at the sediment surface. The content
of dissolved O2 in the basin water is low,
about 0.1 ml liter-l, except after periods of
flushing when it may briefly rise to 0.4 ml
liter-’ ( Sholkovitz and Gieskcs 1971). Conscquently, the oxygen in the pore waters is
depleted in the first few centimeters of
sediment and sulfate reduction
begins
within a few centimeters of the sedimcntwater interface ( Sholkovitz 1973; Kaplan
,et al. 1963). The basin sill depth is 475 m
( Emery 1960).
The two deeper basins, Santa Cruz (SCB)
and San Diego Trough ( SDT), have higher
02 concentrations in their bottom waters
(0.2 ml liter-l)
and sedimentation
rates
lower by a factor of 3 to 10. Consequently
the zone of oxygenated pore water (low
electron activity) is thicker in these basins
( Emcry 1960).
Analytical
methods
Dissolved gclses-The
pore water collected with the in situ sampler was analyzed
for dissolved Na, Ar, and CH4 by a gas
chromatographic
technique using a tungstcn filament detector. The sample cylinder
( -16 ml) was mounted in-line with a gas
stripper ( Swinnerton et al. 1962)) drying
tube, and collecting trap. The trap was a
15-cm molecular sieve 5,A U-shaped column
immcrscd in liquid N2. After the water
sample was stripped and the gases collected
on the trap, the trap was warmed to room
tcmpcrature and switched to the carrier
gas line of the gas chromatograph.
The
CH, results will be reported elsewhere.
There are several reasons for trapping
and subseqrrently injecting
the stripped
gases into the chromatograph:
the stripping and carrier gas pressures and flow
rates can bc independently
adjusted for
optimum result; injection from the trap
gives sharp, reproducible
peaks independent of variations in the stripping process;
I-1$ and CO2 arc retained on the drying
column and gas trap and do not degrade
the performance of the analytical columns.
The carrier gas flows in one stream
through a first MS 5A column, one side of
N, : Ar in California
the dual detector, a second MS 5A column,
and then through the other side of the detector. Nz is separated from Ar + 02 and
CI-14 in column 1, operated at 40°C. Ar and
O2 arc scparatcd on column 2 immersed in
a Dry Ice-methanol bath (-7O”C), passed
through the other side of the dctcctor, and
vented. Nitrogen and methane remain on
the cold column for about 1 day. If the
column is not allowed to warm overnight,
nitrogen begins to bleed from the column,
producing an unstable baseline. Alternate
sides of the detector act as reference for
the effluent from the two series columns.
The detector response was calibrated by
periodic injections of dry air from a 0.5-cm”
calibration loop. The injcctcd air was collected on the trap and then rcinjectcd into
the chromatograph like the stripped water
samples. The loop volume is calculated
using the nominal inside diameter of the
l/16-inch
(1.6 mm) tubing. Such calculated volumes are usually listed as accurate
to +-2%. The absolute values listed in Table 1 are variable by this amount, but this
dots not affect the relative precision since
the error is systematic.
The precision of the Ar mcasurcments
( +1.5% ) is limited by the analytical precision for the Ar determination;
further improvemcnt of the technique is desirable.
The Ar concentrations for SBB listed in
parcnthescs in Table 1 have lower than normal precision ( *2.5%)
due to baseline
drift at the Ar peak caused by column overloading from the large amounts of CI14 in
the pore waters at these depths. Conscquen tly, Ar concentrations
were determined from the combined Ar + O2 peak
(negligible O2 present) that elutcs from the
first column, This peak is very sharp and
the precision is lower than for the later
eluting Ar peak.
Although the method is sensitive ( with
an oxygen blank of 0.006 ml liter-* ) , the
oxygen concentrations measured are not accurate indicators of the low in situ concentrations of the samples. A small amount of
oxygenated water is automatically
introduced in the sampling procedure (Barnes
1973). This amounts to about 0.045 ml O2
965
sediments
RELATIVE
Santa Barbara
Basin
%
-5 0 I,,,,, 10
-,
550 1
588rn
OCm
A! 1
GAS CONCENTRATION
San Diego
Trough
%
Santa Cruz
Basin
%
70
-5
?
II,
10
Z?
T
rr11
-5
0
‘P
r
N2 o
Ar .-l
1
,
100
I
200
1
300
t
N2: Ar RATIOS
Fig. 1. Vertical distribution
of gas conccntrations relative to solubility
and Nz : Ar ratios. The
horizontal
2” indicates that samples above and
below it are from separate casts at the same station location.
li tcr-I. The collected sample is sealed in
situ and not sterilized, so that biological
oxygen utilization
can continue after collcction. All but three of the samples shown
in Fig. 1 had O2 concentrations less than or
equal to 0.01 ml liter-l. The highest conc&ration
measured was 0.04 ml liter-l,
still lower than the 02 introduced to the
sample during collection.
However, the
O2 concentration is used as a monitor of
possible sample contamination from inadvertcnt addition of air or oxygenated water
in amounts great enough to significantly
affect measurement of the other gases.
Possible sample storagg effects-The
September SBB samples were stored in a
rcfrigcrator for 6 months bcforc analysis.
The May samples from SBB and SCB were
analyzed 1 month after collection and the
Barnes et al.
966
Table 3. Comparison of average measured gas
concentrations
in basin waters below 500 m and
pore waters below GO cm for September 1973 and
May 1974 cruises to Santa Barbara Basin. The
figures in parentheses indicate the number of individual samples in each average.
Basin
_--__11_
N2--
water
Pore
___---N2
Ar
0.319(6)
NO;
.A
----.- - -- ___ _ . _._.__.___
_
12 September
12.24(h)
CONCENTRATION (pm)
water
Ar
on0
l o
NO;
AA
AN,
B
SEP 1973
MAY 1974
-
1973
15.73(9)
0.352(9)
9 Play 1974
12.22(2)
_- _._____--
0.320(2)
______
13.80(5)
---_-__.-__
0.353(5)
._____
SDT samples after 2.5 weeks. All pore waters used for gas analysis were collected
with the in situ sampler, as were the duplicatc 550-m basin water samples listed in
Table 1. Basin water samples were collected in September from Niskin bottle hydrocasts and stored in glass tubes with
stopcocks at each end. Table 3 shows the
averaged N2 and Ar concentrations in basin
and pore waters of SBB for the two cruises.
Only pore waters below 60 cm are included
since no samples above this depth were
collected in September,
The averaged values for the two cruises
agree within 0.3%, except for pore water
NZ concentrations which differ by 13%. It is
not likely that any chemical transformation
increased the Nz concentration during storage. All the pore water samples from the
September 1973 cruise wcrc collected from
the anoxic sediment zone where dissolved
nitrate and nitrite are absent. The only
other nitrogen species present in sufficient
amount to produce the observed increase is
ammonia. However, there is no evidence
for the existence of a reaction mechanism
that converts significant quantities of ammonia to N2 in the absence of O2 or an oxidized nitrate or nitrite intermediate
(see
Therefore, this diffcrdiscussion below).
encc probably represents a real horizontal
variability in SBB sediments. The methane
profiles at the September and May stations
show slope changes at 2- and 3-m depth,
which suggests a horizontal variability
in
the thickness of the sulfate reducing zone.
I
30
ii
35
ow
40
Fig. 2. Vertical
distribution
of nitrate, nitrite,
and relative Nz concentrations
in basin and pore
waters of Santa Barbara Basin. Samples from both
September
1973 and May 1974 cruises arc includcd.
The thickness of the nitrate reducing zone
may also vary, with a consequent change in
the ultimate N:! concentration (see below).
The close agreement of the other values
indicates no significant systematic storage
effects for unreactive species.
Because the important upper 60 cm is
missing in the September 1973 profile,
these samples are not included in the development of the model presented below,
although, as is pointed out, they are consistent with it.
Nitrate and nitrite-Nitrate
was determined by the method of Wood et al. (1967)
and nitrite by that of Bcndschneider and
Robinson ( 1952).
Basin waters on the September and May
cruises were sampled from Niskin or plastic Nansen bottles. Surface sediment samples were removed from a box core on the
IV2 : AT in California
May cruise and immediately squcczcd to
extract the pore water. Supernatant water
was also collected from the top oE the box
core. All samples were frozen until analysis, after 6 days for the September cruise
and after 37 days for the May cruise. The
nitrate and nitrite data for SBB arc plotted
in Fig. 2.
Discussion of data
As expected, our results show that nitrate and nitrite are reduced to molecular
nitrogen when dissolved oxygen is absent
or at very low concentration.
An intercsting feature of the observed increase in
nitrogen concentration is the apparent contribution from ammonia by way of an oxidized intermediate.
This possible mcchanism is discussed below.
In SBB the Nz concentration in the topmost pore water sample is 10% higher than
in the basin water just above the sediment.
This water in turn has a higher Nz conccntration than the basin water about 38 m
above it. The Nz : Ar ratio shows a similar
increase ( Fig. 1).
The increased Nz concentration
just
above the sediment cannot be due to in situ
nitrate reduction via Eq. 1. Figure 2 shows
the nitrate, nitrite, and Nz concentrations
expressed in PM for basin and port waters
of Santa Barbara Basin. The maximum
decrease in nitrate measured from 550 m to
the sediment surface is 6.2 PM. If we assume that this decrease is the result of in
situ nitrate reduction to N2, the corresponding increase in dissolved Nz would be 3.1
PM; the increase actually found is 23 PM.
The nitrate decrease and the NS increase
are apparently due to diffusion into and
out of the sediment. The smooth nitrite profile also appears to be an eddy diffusion
phenomenon. A small amount of nitrate reduction in the basin water cannot bc ruled
out by our data (Sholkovitz and Gicskcs
1971)) but its effects would bc swamped
by the nitrate reduction occurring in the
sedimentary pore waters.
If all the 24.7-28.3 PM of nitrate present
above the scdimcnt were reduced to NZ in
the scdimcnt, the N2 concentration would
sediments
967
increase by 14.2 PM, if we assume no diffusive effects. The actual increase is 59
PM at -23 cm, or 47 PM for the average of
the upper three pore water samples above
60 cm. This is about four times the increase
expected from in situ nitrate reduction in
the sediment.
A sharp gradient of nitrate concentration
is evident in the first few centimeters of
sediment (Fig. 2) and extrapolates to zero
nitrate at about lo-cm depth. Previous
analyses in Santa Barbara Basin sediments
have shown no nitrate below l-cm depth
(Rittcnbcrg
et al. 1955; Sholkovitz 1973);
thcsc analyses were performed on gravity
cores and the soft surface strata may have
been disturbed or lost. Alternatively,
the
measurements may reflect horizontal variability in SBB sediment. Our samples were
taken from a 20-cm-square box core that
collects the surface sediment undisturbed.
Such a sharp nitrate gradient is Gonsistent
with significant diffusion into the sediment,
also indicated by the apparent, small negative concentration
gradient in the water
overlying the sediment ( Fig. 2).
Under steady state conditions, the flux
of nitrate into the sediment would bc balanced by a flux of Ng out of the sediment.
If all the nitrate is reduced to Na and the
diffusion coefficients are assumed to be
equal, the resulting Na concentration gradient will be half the magnitude and of opposite sign to the nitrate gradient. Refcrring to Fig. 2 again and assuming zero
nitrate concentration at lo-cm depth, we
find the average gradient for the upper 10
cm of sediment is -2.8 PM cm-l. The average gradient for di.ssolved Na to 23-cm
depth is +2.6 ,uM cm-l; this is a minimum
estimate. As noted below, the Na concentration probably increases over the same
interval as the nitrate concentration
decreases. This would lead to a Na gradient
of 5.9 ,uM cm-l at the sediment surface.
The corresponding flux would be 4.2 times
larger than that required to balance the
NOa- flux into the sediment.
Apparently then there is a source of Nz
in the sediment in addition to diffusion of
nitrate into the scdimcnt and its subsequent
968
Barnes et al.
reduction. This source must bc the organic
nitrogen compounds that release ammonium ion on degradation in the sediment.
The concentration of NH4+ reaches 1,000
PM in the upper 50 cm of sediment in
SBB (Sholkovitz 1973). However, there is
no indication that a reaction such as Eq. 3
produces Nz in the anoxic sulfate reducing
zone below the first few centimeters. Although the NH4+ concentration continues to
increase, reaching 11,000 PM at 200 cm
(Rittcnbcrg et al. 1955), our data show Na
to remain approximately constant below the
upper few centimeters where the NIL+
concentration is still relatively low. A biochemical pathway for this reaction apparently does not exist, even though N2 is the
thermodynamically
stable species in the
sulfate reducing zone (see Eq. I, 2, and 3).
This observation accords with the findings
of others (Richards and Benson 1961; Rittenbcrg *et al. 1955). It also agrees with
the measurements of ammonia accumulation in sulfate reducing anoxic seawaters
(Richards et al, 1965). The reaction of Eq.
4, whose importance in natural systems is
doubtful ( Cline and Richards 1972 ) , would
also not product a nitrogen excess of the
observed magnitude.
In light of our observations that concentrations of dissolved Na remain approximately constant throughout the sulfate reducing zone and below into the region of
high methane concentrations, the conclusion of Hammond et al. ( 1973) and Hammond (1974) that N2 is reduced to ammonia at depth in the anoxic Cariaco
Trench sediments should be rc-examined.
The basis for .their conclusion is the apparent dccreasc of Na concentrations below
depths of 50 m in the sediment to less than
25% of that in the overlying basin water.
IIowever, their data are only semiquantitativc because of core dcgassing before gas
sample collection and uncertaintics in reducing the analytical data to in situ concentrations. IIowever, our examination of
their results suggests a diffcrcnt interprctation in accord with the cvidcncc that N2
and ammonia are not effectively coupled
as a rcdox pair in the absence of oxygen.
The dccreasc in pore water Na in Cariaco
Trench sediments occurs relatively
suddenly over an interval of several meters at
about 50-m depth in the sediment (see
Hammond 1974: figure 5, table 2). In this
interval apparent concentrations decrease
from about 200% to about 25% of overlying
basin water values. A reasonable correction
for gas loss due to core degassing shows
that methane partial prcssurcs may have
reached in situ hydrostatic pressures at 5O60-m depth in the sediment where the Na
concentration
decreased abruptly
(see
IIammond 1974: figure 6).
WC suggest that below about 50 m the
Na is stripped from the pore water by rising methane bubbles and is then redissolved above this depth because oE the
lower in situ partial pressure of methane.
Such a model explains the large apparent
Nz supersaturation above 50 m as due to
the upward transport of NZ. This supersaturation could not be explained, in Fact
is contraindicated,
by the chemical rcduction model.
Carlucci
and McNally
( 1969) have
demonstrated the bacterial oxidation of
ammonia to nitrite under conditions of low
oxygen tension ( 02 < 0.1 ml liter-l).
Such
conditions prevail in the bottom waters
and surface pore waters of SBB ( Sholkovitz
1973). A continuing supply of dissolved
oxygen could bc provided by diffusion
across the sediment-water
interface into
the upper few centimctcrs of pore water.
The nitrite in turn would be reduced to NB
as a step in Eq. 1.
Comparison between the September and
May data is suggestive of such a mcchanism. At the September station the supcrnatant nitrate concentration is lower, nitrite higher, and probable pore water Nz
higher (Table 3) than at the May station.
In addition, the depth at which sulfate is
depleted is probably shallower at the Septcmbcr station (see above), which suggests
a faster reduction sequence at this station.
Thus the nitrate reducing zone would be
thinner with correspondingly
greater diffusive nitrate loss from seawater to the
sediment. Diffusion of 02 into the scdi-
N, : Ar in California
mcnt would also increase, as would the
pore water ammonia concentration at the
scdimcnt surface, resulting in increased nitrite concentrations reflected in the greater
supernatant nitrite concentration (Fig. 2).
IIighcr nitrite concentration from ammonia
oxidation would in turn produce more N2
leading to the 13% higher concentrations of
N2.
Below 50 cm in SBB, the Ar concentration increases by an average of &% over that
in the upper part of the core. The high
Na : Ar ratio in the upper 50 cm disappears
below this depth, where the ratio assumes
bottom water values ( Fig. 1). This increase in Ar complicates interpretation
of
the Na profile below 60 cm. Argon is a
conservative inert gas that is not produced
or consumed in significant amoul~ts in the
scdimcnt column. Increases of inert gases
of even larger magnitude than those reported here have been measured in sedimentary pore waters ( Barnes in prep. ) ; the
cause is not known, The important point
for the prcscnt discussion is that the process affcc ting argon concentrations may
also affect nitrogen concentrations and thus
bc indcpcndent of redox reactions involving
nitrogen spccics. However, this uncertainty
dots not affect the previous discussion
since the N2 increase in the upper part of
the sediment is not accompanied by a significant Ar increase ( Fig. 1).
In the deeper basins, SCB and SDT, the
upper sedimentary port waters have N2
concentrations
identical to those in the
overlying basin water. The N2 concentration increases between 1 and 2 m in SCB
and between 20 and 40 cm in SDT.
Emery and Rittcnberg ( 1952) found sulfate reduction beginning at about 2 m in a
SCB core. This is consistent with our Na
measurements, since reduction of nitrate
to Na should occur before sulfate is rcduccd.
The increase of NB below l-2 m in SCB
(4 to 10%) is less than that found in SBB
but still greater than the increase of 3.7%
in Na that would be produced by reduction
of the 42 PM of nitrate in the deep basin
water. The few published data for nitrate
sediments
969
in oxidizing sediments show concentrations
higher than in seawater and rising to more
than 100 PM (Rittenbcrg ct al. 1955; Grecnslate et al. 1974; Hartmann et al. 1974).
This nitrate is apparently produced by in
situ oxidation of ammonia as discussed
above and would then bc reduced to NZ at
depth in the nitrate reducing zone. The
reduction of 100 PM of nitrate would lead
to increases of NZ within the range found
for the two deeper basins.
There is no reason to suspect an error
in the N2 and Ar analyses rcportcd for the
393-cm sample from SCB. Increases in Ar
more than twice that of the 393-cm sample
have been measured in other borderland
basins (Barnes in prep. ), but this is the
first sample that includes NZ determinations
as well. The Na concentration is similarly
affected. Deeper samples would bc neccssary to profile this concentration anomaly.
Conclusions
In SBB, the zone of nitrate reduction
straddling
the sedimcn t-water
intcrf act
seems to support relatively large fluxes of
nitrate, nitrite, and ammonia into and out
of the sediment. The steep concentration
gradients allow diffusive fluxes to play an
important role in the nitrogen chemistry of
the surface sediments, such as maintaining
a large excess Na concentration at the interface because of oxidation of ammonia by
diffusing oxygen. Further verification and
clarification of the proposed model will rcquirc analysis of all relevant spccics ( in
closely spaced samples) both above and
below the sediment-water interface so that
detailed concentration profiles can bc obtaincd and fluxes calculated.
In the deeper basins with low electron
activity in the surface sediment, the nitrate
reducing zone is deeper in the sediment
and diffusive fluxes assume less importance
in the nitrogen chemistry of the scdimcnt.
The argon concentration anomalies illustrate the value of measuring a chemically
inert substance to help interpret the conccntration profile of a chemically active
spccics with similar physical properties.
The mechanism of the inert gas concentra-
970
Barne S et al.
tion anomalies is an important problem in
its own right, in addition to its bearing on
the interpretation of chemically active species.
References
R. 0.
1973. An in-situ interstitial
water sampler for use in unconsolidated
sediments.
Deep-Sea Res. 20: 1125-1128.
BENUSCI-INEIDER,
K., AND R. J. ROBINSON.
1952.
A new spectrophotometric
method for the dctermination
of nitrite in sea water.
J. Mar.
Res. 11: 87-96.
CARLUCCI,
A. F., AND P. M. MCNALLY.
1969.
Nitrification
by marine bacteria in low concentrations of substrate and oxygen.
Limnol.
Oceanogr. 14: 736-739.
CLINE,
J. D., AND F. A. RICIIARDS.
1972. Oxygen deficient conditions and nitrate reduction
in the eastern tropical North Pacific Ocean.
Limnol. Oceanogr. 17 : 885-990.
EMERY, K. 0.
1960. The sea off southern California.
Wiley.
AND D. IIoGGAN.
1958. Gases in marink sediments.
Bull.
Am. Assoc. Petrol.
Geol. 42 : 2174-2188.
AND S. C. RITTENBERG.
1952. Early
diagenesis of California
basin sediments in
relation to origin of oil.
Bull. Am. Assoc.
Petrol. Geol. 36: 735-806.
GREENSLATE,
J., J. Z. FRASER, AND G. ARRIIENIUS.
of selected
1974. Origin
and deposition
transition
elements in the seabed, p. 45-69.
In M. Morgenstein
[ea.], The origin and distribution of manganese nodules in the Pacific,
and prospects for exploration.
Hawaii
Inst.
Geophys.
D. E. 1974. Dissolved
gases in
HAMMOND,
Cariaco Trench sediments : Anaerobic diagenIn I. R. Kaplan [ed.], Natuesis, p. 71-89.
ral gases in marine sediments.
Plenum.
R. M. HOROWITZ,
AND W. S. BROECKER.
l&3.
Interstitial
water studies, leg 15, dissolved gases at site 147, p. 765-772.
In B. C.
Heezcn et al. [eds.], Initial
reports of the
Deep Sea Drilling
Project, v. 20. U.S. Gov.
Printing Office.
HARI‘MANN,
M., F. C. K~GLER, P. MUELLER, AND
E.
suEss.
1974. Preliminary
results
of
geochemical
and soil mechanical
investigations on Pacific Ocean sediments, p. 71-76.
In M. Morgenstein
[ed.], The origin and distribution of manganese nodules in the Pacific,
BARNES,
prospects for exploration.
Hawaii Inst. Geodw .
KAPLAN,
I. R., K. 0. EMERY, AND S. C. RITTENBERG.
1963. The distribution
and isotopic
abundance of sulphur in recent marine sedimcnts
off
southern
California.
Geochim.
Cosmochim. Acta 27: 297-331.
KOIIIE, M., K. W. BRULAND,
AND E. D. GOLDBERG.
1973. Th-228/Th-232
and Pb-210 gcochronologies in marine and lake sediments.
Gcochim. Cosmochim. Acta 37: 1171-1187.
MORRIS,
J. C., AND W. STUMM.
1967. Redox
equilibria
and measurements
of potentials in
the aquatic
environment,
p. 270-285.
In
Equilibrium
concepts in natural water systems. Am. Chem. Sot.
REEBTJRGII,
W. S. 1969. Observations
of gases
in
Chesapeake
Bay
sediments.
Limnol.
Oceanogr. 14 : 368-375.
RICHARDS, F. A., AND B. B. BENSON.
1961. Nitrogen/argon
and nitrogen
isotope ratios in
two
anaerobic
environments,
the Cariaco
Trench in the Caribbean Sea and Dramsfjord,
Norway.
Deep-Sea Res. 7: 254-264.
J. D. CLINE, W. W. BROENKOW, AND L. P.
A&NSON.
1965. Some consequences of the
decomposition
of organic matter in Lake Nitinat, an anoxic fjord.
Limnol. Oceanogr. 10
( suppl. ) : R185-R201.
RITTENBERG,
S. C., K. 0. EMERY, AND W. L. Ona.
1955. Regeneration
of nutrients in sediments
of marine basins.
Deep-Sea Res. 3: 2345.
SIIOLKOVI’IZ,
E. 1973. Interstitial
water chemistry of the Santa Barbara Basin sediments.
Geochim. Cosmochim.
Acta 37: 2043-2073.
AND J. M. GIESKES.
1971. A physicalchkmical study of the flushing of the Santa
Barbara Basin,
Limnol. Oceanogr. 16: 479489.
SWINNERTON,
J, W., V. J. LINNENBOM,
AND C. II.
CREISK.
1962. Determination
of dissolved
gases in aqueous solutions by gas chromatography.
Anal. Chcm. 34: 483485.
1970. The solubility
of nitrogen,
WEISS, R. F.
oxygen and argon in water and scawatcr.
Deep-Sea Rcs. 17: 721-735.
Woon, E. D., F. A. J. ARMSTRONG,
AND F. A.
1967. Determination
of nitrate
RICIIARDS.
in sea water by cadmium-copper
rccluction
to nitrite.
J. Mar. Biol. Assoc. U.K. 47: 2331.
Submitted:
Accepted:
27 February 1975
12 June 1975