SEDIMENT COMMUNITY OXYGEN CONSUMPTION
IN THE DEEP GULF OF MEXICO
by
Gilbert T. Rowe1, 2, John Morse2, Clifton Nunnally2 and Gregory Boland3
1Department
of Marine Biology, Texas A&M University, Galveston, TX.
of Oceanography, Texas A&M University, College Station, TX
3Minerals Management Service, U.S. Dept. of the Interior, New Orleans, LA
2Department
1
ABSTRACT
Sediment community oxygen consumption (SCOC) has been measured from the continental shelf
out to the Sigsbee Abyssal Plain in the NE Gulf of Mexico (GoM). SCOC rates on the
continental shelf were an order of magnitude higher than those on the adjacent continental slope
(450 to 2750 m depth) and two orders of magnitude higher than those on the abyssal plain at
depths of 3.4 to 3.65 km. Oxygen penetration depth into the sediment was inversely correlated
with SCOC measured within incubation chambers, but rates of SCOC calculated from either the
gradient of the [O2] profiles or the total oxygen penetration depth were generally lower than
those derived from chamber incubations. SCOC rates seaward of the continental shelf were lower
than at equivalent depths on most continental margins where similar studies have been
conducted, and this is presumed to be related to the relatively low rates of pelagic production in
the GoM. The SCOC, however, was considerably higher than the input of organic detritus from
the surface-water plankton estimated from surface-water pigment concentrations, suggesting that
a significant fraction of the organic matter nourishing the deep GoM biota is imported laterally
downslope from the continental margin.
Introduction
The measurement of sediment community oxygen consumption (SCOC) has
become a standard approach for estimating the carbon and energy requirements of
sediment-dwelling organisms within aquatic sediments. It has been estimated on the
continental margins and abyssal plains of the western North Atlantic (Smith 1978; Smith
and Hinge 1983), the eastern Pacific(Archer and Devol 1992; Smith 1992; and Jahnke
and Jackson 1991); the sub-Arctic Atlantic (Glud et al. 1998); the Arabian Sea (Witte and
Pfannkuche 2000); the eastern Mediterranean (Buhring et al. 2006); and the deep south
Atlantic (Wenzhofer and Glud 2002). It is generally presumed that SCOC at shallow
depths is a direct estimate of the coupling between benthic and pelagic processes (Rowe
et al. 1975; Grebmeier and Barry 1991; Graf 1992). Oxygen utilization by deep-ocean
sediments has been equated stoichiometrically to the net input of organic matter
delivered to the sea floor (Jahnke 1996, Smith et al. 2001). In the Gulf of Mexico,
numerous SCOC measurements have been made in the area of seasonal hypoxia
associated with the Mississippi River effluent on the continental shelf (Miller-Way et al.
1994; Morse and Rowe 1999, Rowe et al. 2002, Rowe and Chapman 2002). Calcium
carbonate shell deposit dissolution rates were shown to be directly proportional to SCOC
on the upper continental slope of the northern GoM(Powell et al. 2002). To date, SCOC
has been reported at two locations on the Sigsbee Abyssal Plain, one just north of the
Yucatan Strait (Hinga et al. 1979) and the other in the west central gulf (Rowe et al.
2003).
The present investigation was designed to complement comprehensive sampling
of the biota associated with the benthic boundary layer. Our goal was to determine if
community structure, in terms of biomass, diversity and species composition, was
related to community function, or rates of biogeochemical processes. Most investigations
of biological stocks and processes in surficial sediments focus either on community
structure (e. g., Sibuet et al. 1984) or function (Devol and Christensen 1993, Sayles et al.
2
1994, Wenzhofer and Glud 2002), not both. Previous investigations of this nature have
been limited to single sites (Smith 1992, Rowe et al. 2003, Smith et al. 2006) or at best a
single transect (Smith 1978). The complete set of investigations in this special issue
(“Deep Gulf of Mexico Benthos” or DGoMB) has attempted to address both for the
northern GoM.
This five-year study (DGoMB) was conducted in stages, with the initial phase in
2000 being a survey of benthic community structure of the continental slope of the
northern GoM from the Texas/Mexico border to northern Florida. The second phase, in
2001, initated “process” measurements at five contrasting locations selected on the basis
of the community structure information available from the survey. The selection was
based on extremes in standing stocks or densities of the sediment-dwelling bacteria,
meiofauna and macrofauna. Fortunately, the stocks all displayed extremes at the same
sites. The original five sites chosen (Fig 1) were MT1, MT3 and MT6, for maximum and
minimum standing stocks at upper slope and lower slope depths associated with the
Mississippi Canyon in the central GoM; and S42 and S36 for minimum and maximum
densities at comparable and intermediate depths in carbonate-rich sediments associated
with the Florida continental margin. The following year (2002), two additional sites (S1
and S4) were added in order to extend the sampling out onto the Sigsbee Abyssal Plain
(Fig 1), a logical extension to encompass the gulf’s greatest depths.
Methods
SCOC in this study was measured using both in situ and ship-board laboratory
incubations of surficial sediments and bottom water within chambers, as described
previously(Tengberg et al. 1995, 2005), with modifications indicated below. A freevehicle bottom lander was utilized to the carry incubation chambers to the sea floor in
order to measure SCOC without decompression or alteration of temperature. The
“GOMEX” lander, as we call it (Rowe et al. 1994), is composed of an aluminum frame,
glass floatation spheres, disposable anchor weights released by an electronic timer, a
B&W film camera and strobe, a strobe and radio direction finder to assist recovery, and
two incubation chambers. The two chambers themselves are constructed of plexiglass
cylinders covering an area of 900 cm2 each, with the top sealed by a flat piece of lexan.
They are held above the bottom of the lander on deployment, but then are dropped to the
sediments by a timer approximately 30 minutes after the lander has settled to the sea
floor. The chambers slide down steel runners with hydraulic dampers to slow their
sinking and minimize disturbance. One-way flapper valves in the lexan top release water
as the chambers descend into the sediments. Once on the bottom, each chamber contains
7 liters of sea water when fully engaged with the surface sediment. A two-cm rim around
the outside of each chamber prevents deeper penetration of the cylinders into the
sediments. The thickness of the diffusive boundary layer within incubation chambers is
controlled by circulation rates. Circulation in the chambers appears to be adequate to
make reasonably accurate oxygen flux measurements with the stirring motors and
pumping system utilized (Rowe et al. 1994; Tengberg 2005). Oxygen concentration is
monitored with polarographic electrodes, with the output recorded on a data logger
(produced by Sea Bird for use in CTDs). The camera is positioned to take photographs of
the chambers’ contact with the sea floor. Spring-loaded 50 cc syringe samples are taken
3
at the beginning, middle and end of each incubation in order to estimate nutrient fluxes
between the water and the sediments. All in situ sea-floor mechanical operations are
controlled by an electronic timer and burn-wire release system.
Small plexiglass chambers measuring 25 cm in diameter were utilized aboard ship
to make parallel incubations in the laboratory (Miller-Way et al. 1994). These samples
were procured by inserting the chambers into the sediments and water contained in 0.2 m2
box cores (Boland and Rowe 1991). The area of sediment covered is 125 cm2 and sea
water volume above the interface varied from 0.8 to 1.3 liters. A polarographic oxygen
electrode (YSI) is screwed into the sealed top of each chamber and a small stirring
magnet is suspended below the electrode membrane. The mixing appears to be adequate
for reasonably accurate flux measurements to be made (Tengberg et al. 2005). These
chambers were maintained in the ship’s laboratory in the dark at in situ temperature,
during which oxygen concentration was measured continuously over periods of 6 to 36
hours. In general two to three replicate, ship-board incubations were made at each site.
Following each incubation, the sediments were sieved using a 300 micrometer mesh sieve
to separate out the macrofauna for comparison with standard faunal sampling that had
already been conducted at each site.
The flux of oxygen into the sediments in the incubations is calculated using the
following formula:
Flux = [Change in concentration] x [Volume of incubation chamber]
[Area covered by chamber] x [Time]
The oxygen flux values (SCOC) are reported (Table 1) as mg carbon in carbon
dioxide remineralized by respiration, wherein the flux of oxygen is multiplied by a
Respiratory Quotient of 0.85. An RQ of 0.85 assumes that the organic matter consumed
is proportioned equally between lipid, protein and carbohydrate.
Oxygen concentration profiles within the sediments were produced at six process
stations (MT3, MT6, S36, S42, S1, and S4) using a microelectrode, according to the
method described by Brendel and Luther (1995). Measurements were made at depth
intervals of 2 mm. Concentrations in the porewaters of the top 10 to 15 cm of sediment,
and 1 cm of overlying bottom water, were made by cathodic-stripping voltammetry, using
solid state amperometric microelectrodes and an Analytical Instrument Systems model
DLK-100 voltammetric analyzer (Brendel, 1995; Brendel and Luther, 1995; Luther et al.,
1998). Measurements were made with an Au/Hg amalgam glass microelectrode.
Instrumental parameters for the linear sweep and cyclic voltammetry modes were
typically 200 mV s-1 scan rate over the potential range -0.1 to -1.8 V with a 10 s
deposition at –0.1 V. Minimum detection limits for O2 were
Calibration of each electrode was based on the pilot ion method where Mn2+ was the
standardized ion (Brendel, 1995).
Sediment oxygen consumption within the sediment was estimated from the
oxygen gradient within the sediments and from the oxygen penetration depth at each site,
4
utilizing Fick’s first law, assuming molecular diffusion-limited rates in the sediments
(Berner 1980):
Js = -
sdc/dz
where Js is the flux in moles per unit area over a layer of surface sediment per unit time,
s is the whole sediment diffusion coefficient and dc/dz is the
concentration gradient. The simplified approach, however, of Cai and Sayles (1996) was
also used to calculate consumption of oxygen based oxygen penetration depth (OPD),
rather than the gradient. In doing so, we used the temperature-dependent O2 diffusion
coefficient in water (Jahnke et al. 1987) corrected for tortuosity, according to Berner
(1980), yielding a value of Ds = 3.08E-6 cm2 s-1 at 4oC. An average porosity of 0.8 was
assumed for each site. The flux was calculated from:
F=2
s
[O2]BW/L
where F is the oxygen flux, L is the OPD and [O2]BW is the bottom water concentration.
Results
SCOC was measured at seven sites (Fig 1, Table 1), from depths of 460 m in the
Mississippi Canyon (MT1) down to 3,650 m on the Sigsbee Abyssal Plain (Fig 1). The
three deep sites (LD97, S1, S4) located on the abyssal plain have been averaged as a
single value (Table 1). The variability in the measurements at any given site was high,
with the Standard Deviation often half the mean, reflecting meager precision in the
methods or small-scale variation on the sea floor. The mean rates at each site decreased
from a range of 32 to 37 mg C m-2-d-1 on the upper continental slope down an order of
magnitude to the low mean value of 3.9 mg C m-2d-1 on the Sigsbee Abyssal Plain.
Oxygen penetration depth (OPD)and concentration profiles for stations MT3,
MT6, S36 and S42, are illustrated in Figure 2. At station MT3, the OPD was close to 0
mm. The S36 and S42 stations displayed rather similar OPD of 45 and 40 mm,
respectively. The profiles were parallel at depth, but the shallower site (S42) had a lower
initial value, reflecting the oxygen concentration in the water column. The deep station on
the central Sigsbee Abyssal Plain, S1, had the deepest OPD (100 mm), as might be
expected, but at S4, just north of the Yucatan Strait, it was only 30 mm (Table 2).
Dissolved iron and sulfide were not detectable at any of the sites within the
sediment depths sampled. Dissolved Mn2+ was observed only at MT3 (Fig 3), where it
exhibited a classic (e.g., Berner 1980) broad subsurface maximum from about 30 to 80
Above this zone diffusive transport of manganese is the primary process and below this
zone manganese is precipitated primarily as the carbonate mineral pseudokuntnahorite
(MnCa(CO3)2).
Sulfate reduction rates were below detection limits. The deep oxygen penetration
depths at these sites, with the exception of MT1 and MT3, indicate no sulfate reduction is
likely to occur, at least to the sediment depths sampled. It could however be occurring at
5
deeper depths than those sampled (~20 cm). This was clearly evident in the earlier work
of Lin and Morse (1991), where reduced sulfur species were not observed in sediments
for water depths greater than about 200 m, until several meters of sediment were
penetrated.
Discussion
Comparison of Methods:
The SCOC values measured with incubation chambers (Table 1)were
considerably higher than the estimates from the gradient of oxygen into the sediments and
the OPD (Table 2), with the exception of MT3 and S4. The estimate at MT3, however,
was not based on a profile or penetration, since there was none. As we cannot divide by
zero, a penetration of 1 mm was assumed. At S4, the two estimates were very close. This
set of relationships is similar to other efforts to compare the two approaches (Wenzhofer
and Glud 2002): the chamber fluxes, referred to in previous work as “total oxygen
utilization” or TOU, are generally larger than the “diffusive oxygen utilization” or DOU.
In the GoM, with the exception of MT3, the DOU rates were lower that the TOU rates.
We interpret this to indicate that considerable activity occurs at the sediment – water
interface involving motile invertebrates that consume reactive particulate organics as
soon as it reaches the sea floor. The subsurface diffusive flux of oxygen (DOU) down
into the sediments on the other hand is driven by bacterial activity that is limited by the
particulate organic matter that is mixed downward by bioturbation, the diffusion of
dissolved organics downward following its remobilization from particulates, and
chemolithotrophic bacterial oxidation of reduced metabolic end-products diffusing up
toward the sediment – water interface. The SCOC, which is referred to as the TOU, is the
sum of these two processes, one at the surface dominated by the metazoans and the other
at depth dominated by heterotrophic bacteria and possibly protists. The low rates of
bacterial organic carbon utilization measured in these same sediments (Deming and
Carpenter, this volume) may reflect this dichotomy. That is, the oxygen profiles and OPD
may be more closely related to measured bacterial activity than is the SCOC (TOU)
measured by incubation chambers, in which the rates are dominated by the biological
processes very near or on the interface.
Patterns within the Gulf of Mexico:
All the SCOC rates presently available for the northern GoM continental margin
have been plotted as a log-normal function of depth in order to infer what controls SCOC
in the GoM (Figure 4). The rates on the continental slope and abyssal plain were
substantially lower than those on the adjacent continental shelf just inshore of MT1
(Figure 4). Mean values dropped by about one order of magnitude for each km increase in
depth. The wide range in values on the continental shelf reflects oxygen limitation during
hypoxic periods and seasonal variation in temperature (ca. 20o C in winter and just under
30o C in summer, Rowe et al. 2002). The seasonal controls, as far as we know, are
limited to the continental shelf. This comparison of the shelf and slope illustrates that the
upper slope rates, although high compared to abyssal plain rates, are low compared to the
continental shelf, a relationship which is typical of all continental margins. The pattern
parallels that of benthic macrofaunal biomass (Figure 5).
6
Comparison with Other Ocean Margins:
The pattern of SCOC relative to depth in the GoM (Fig. 4) is similar to that
described along other ocean basin margins, and reflects, we believe, the variation in
offshore deposition patterns of organic matter. The ‘flat’ part of the regresson on the
upper slope (0.5 to 2.0 km) is believed to correspond to a depocenter in which fine
particulates exported from the continental shelf are accumulating (Rowe et al. 1994). The
rates then decline from that plateau down at least one order of magnitude out on the
abyssal plain. The values encountered on the GoM slope SCOC plateau were10 to 50%
lower than SCOC rates at equivalent depths at suspected depocenters on the western
margin of the Atlantic (Anderson et al. 1994; Rowe et al. 1994), as well as rates measured
on the eastern boundary of the Atlantic (Wenzhofer and Glud 2002) and Pacific (Archer
and Devol 1992) just below the “oxygen minimum zone” (OMZ) characteristic of coastal
upwelling ecosystems. Likewise, the rates on the GoM Sigsbee Abyssal Plain at 3.4 to
3.7 km depth appear to be slightly lower than rates at similar depths on other margins
(Rowe et al. 1994, Witte and Pfannkuche 2001), although the data are too sparse to
confirm this statistically. The GoM SCOC appears to be similar to the eastern
Mediterranean (Buhring et al. 2006).
Organic carbon deposition and cycling in the Northern GoM:
Yeager et al. (2004) demonstrated that inventories of excess Pb-210 accumulation,
a function of both mixing and deposition, correlated with macrofaunal abundances at
these same sites. Morse and Beazley (this volume) noted too that the standing stocks of
the biota were directly related to carbonate-free organic carbon concentrations, when all
the sites studied by DGoMB are included. We thus conclude that the log-normal decline
in SCOC reflects a similar decline in the delivery of the POC required to nourish the biota
responsible for community respiration and production. The mid-depth data however are
by no means linear. The high SCOC mean on the continental shelf, which controls the Y
intercept, dips sharply just off the shelf to the ‘plateau’ on the upper slope, and the values
of this plateau are below the regression mean. Then, the single value below that, at MT6,
is above the regression line. The deep values control the extreme minimum. It might be
suggested that the non-linearity of the pattern may reflect down-slope export, a geologic
process that has long been inferred for the northern GoM, and in particular for the
Mississippi Canyon and deeper sediment fan. It might be inferred that material exported
from the shelf to the upper slope, including both canyon and non-canyon environments, is
in residence at upper slope depths for relatively short periods of time, in a geologic sense,
but it is then exported intermittently to deeper sediments by mass wasting processes, such
as slumps and turbidity flows.
Caution needs to be exercised with this interpretation for several reasons. The
SCOC at MT6 was closer to the upper slope plateau of values, but it was characterized by
standing stocks that were far lower than on the upper slope and even lower than those at
greater depths on the abyssal plain. Also, the relatively smooth [O2] profile gave rise to an
OPD that was twice as deep as the penetration at S36 and S42, which suggests that the
SCOC, or at least the diffusion-controlled fraction of it, would be on the order of half the
plateau rate. Santschi and Rowe (this volume) suggest that relatively negative C-13
values and greater age, based on C-14 values, of MT6 sediments are evidence that it has
7
a more terrestrial source and is less reactive biologically than most deep GoM sediments.
If this is valid, then it would explain the low standing stocks and deeper penetration of
oxygen at the MT6 site. This would also imply that our SCOC measurement at that site
was too high.
Lateral export from continental margins into deep ocean basins is not a new idea.
Jahnke and Jackson (1991) suggested that on the order of 50% of deep-ocean POC
deposition arrives horizontally from an adjacent margin, rather than only a slow vertical
rain of POC. The vertical POC delivery to any depth has been calculated using
exponential regressions of empirical data over large depth intervals, given estimates of
primary production and standing stock of POC in surface waters (see equations from
Betzer, as modified from Berger et al.). Biggs et al. (this volume) documented the
seasonal distribution of phytoplankton pigments across the entire DGoMB area as a tool
for estimating where the greatest biological activity would be located in the deep GoM.
These seasonal maps provide a basis for calculating the potential vertical pelagic delivery
of POC, termed “export flux”, to each of the sites in this study (Table 3). This “export
flux” from surface water should equal the SCOC, if there were no input downslope along
the sea floor. If the “export flux” is subtracted from the measured SCOC, the difference
can be considered the input from the margin, whatever the mechanism. Thus, these few
data points support the contention that a substantial input (ca. 25 to 70% of the total) of
organic matter to the deep GoM originates from the continental margin, as might be
expected in a relatively small ocean basin.
The site at the head of the giant Mississippi Canyon was the only location where
predicted input from the overlying water was greater than the measured SCOC (Table 3),
but this is the depth and type of geologic setting where export would originate, not its
final resting place. The northern Gulf of Mexico thus typifies continental margin
conditions where down-slope movements dominate organic carbon supplies to the deep
benthos, whether as mass wastings or as a gradual, rather benign translocation of particles
along the bottom.
In conclusion, we suggest that there are two dominant mechanisms that transfer
organics to the deep benthos: the vertical rain of detrital plankton and a lateral input down
the slope, along the sea floor, either as mass wasting or as a gradual process of
shelf/upper slope export. We suspect that it has been large-scale, mass movements that
have given rise to extensive burial of reactive organics that serve as the source material
for the extensive fossil fuel deposits that are characteristic of the deep Gulf of Mexico.
These deposits support the rather patchy chemosynthetic communities of the GoM,
whereas the slow and meager rain of POC from the plankton and the downslope accretion
of material from up-slope together support the typical deep benthos that has been the
subject of this study.
ACKNOWLEDGEMENTS
Thanks are due to the men and women of the RV GYRE, without whom this research
could not have been accomplished. Contract 30991 with the Minerals Management
Services of the U.S. Department of the Interior supported the work. This was a
8
cooperative project between the Geochemical and Environmental Research Group and the
Department of Oceanography at Texas A&M University. The work in Exclusive
Economic Zone of Mexico was accomplished as a cooperative study with the Universidad
Autonoma de Mexico, coordinated by Dra. Elva G. Escobar Briones, to whom
considerable gratitude must be expressed. Special thanks are due Matthew Ziegler for his
assiduous attention to the electronics of the benthic lander.
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LIST OF FIGURES
2
Figure 1. Sites where SCOC, oxygen profiles and associated biological sampling were
conducted during the DGoMB program. LD97 is from Rowe et al. (2003) and S4 was
originally occupied by Hinga et al. (1979) and was re-sampled in this study.
Figure 2.Dissolved oxygen concentration in the sediment pore water with depth into the
sedimens (in mm) at stations MT3 - depth of 0.9 km in Mississippi Canyon (solid purple
diamond; note - all values below detection limit), MT6 - depth of 2.75 km on Mississippi
Cone (red square), S36 – depth of 1.8 km in DeSoto Canyon (yellow solid triangle), and
S42 – depth of 0.74 km (blue x).
Figure 3. Dissolved Mn++ in the pore water at MT3.
Figure 4. SCOC measurements in the GoM, in terms of carbon remineralization (see
text), on the continental shelf out to the Sigsbee Abyssal Plain. The shelf values were
determined at depths of 10 to 30 meters, predominantly from the seasonally hypoxic area
just inshore of MT1, with comparative data several hundred kms to the west of the
seasonal hypoxic zone (Rowe et al. 2002).
Figure 5. SCOC as a function of macrofauna biomass.
LIST OF TABLES
Table 1. Sediment community oxygen consumption (SCOC), in terms of organic carbon
remineralization rate (see text for conversions).
Table 2. Oxygen fluxes estimated from Oxygen penetration depth (Fig 2).
Table 3. Comparison of SCOC rates versus estimated POC delivery to the sea floor based
on surface water pigment concentrations. The difference is assumed to be lateral
transport from the continental margin.
3
4
200
180
Pore Water Oxygen (microMoles)
160
140
120
100
80
60
40
20
0
0
20
40
60
80
100
120
140
160
Sediment Depth (mm)
2+
Mn (M)
0
50
100
150
200
250
0
Sediment Depth (mm)
)
m
m
(
50
h
t
p
e
D
100
150
5
3
Log SCOC = 2.31 - 0.48x R = 0.93
Log SCOC (mg C/m^2-day)
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Depth (km)
6
45
SCOC (mg C/m^2-day)
40
35
30
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
450
Macrofauna Biomass (mg C/m^2)
7
500
Site
MT1
S42
MT3
S36
MT6
S1, S4, WSAP
Depth (km)
0.46-0.5
0.75
0.9-1.0
1.85
2.75
3.4-3.65
SCOC(mgC/m^2day)
36.5
32.4
36.3
29.1
21.3
3.9
Std. Dev.
15.1
7.1
13
2.1
n
4
3
3
1
1
5
Table 1.
Site
DO
ml/L
DO
L
nmol/cm^3 cm
MT3
S42
S36
MT6
S1
S4
4.09
3.63
5.03
5.13
5.10
5.10
183
162
225
229
228
228
0.1
3.3
4.0
7.5
10.0
3.0
F
mmol/cm^2
s
1.13E-8
3.03E-10
3.46E-10
1.88E-10
1.4E-10
1.4E-10
F
mmol/m^2
d
9.53
0.29
0.29
0.16
0.12
0.40
SCOC
mgC/m^2day
97.2
2.96
2.96
1.63
1.22
4.1
Table 2.
Site
Depth (km)
SCOC(mgC/m^2day) Pelagic
Downslope
Export Flux1
Flux2
MT1
0.46-0.5
36.5
113
-76.5
S42
0.75
32.4
14.8
17.6
MT3
0.9-1.0
36.3
23
13.3
S36
1.85
29.1
14.9
14.2
MT6
2.75
21.3
4.9
16.4
Abyssal Plain 3.4-3.65
3.9
1.1
2.8
1Calculated from surface water pigment concentrations in Biggs et al., this volume.
2Difference between SCOC and Pelagic Export Flux.
8
Table 3.
9