www.elsevier.com/locate/margeo
Marine Geology 222–22
Shallow gas and flood deposition on the Po Delta
D. Orange a,b,*, A. Garcıa-Garcıa a,b, T. Lorenson c, C. Nittrouer d, T. Milligan e,
S. Miserocchi f, L. Langone f, A. Correggiari f, F. Trincardi f
a Department of Earth Sciences, University of California, Santa Cruz, CA 95064, USAbAOA Geophysics Inc., Moss Landing, CA 95039, USAc U.S. Geologic Survey, Menlo Park, CA 94025, USA
d School of Oceanography, University of Washington, Seattle, WA 98195, USAe Habitat Ecology Section, Bedford Institute of Oceanography, PO Box 1006, B2Y 4 A2, Dartmouth NS, Canada
f Istituto di Geologia Marina-ISMAR (CNR), Bologna 40129, Italy
Accepted 15 June 2005
Abstract
Sediment cores acquired on the Po delta, northwestern Adriatic Sea, in water depths of 10 to 25 m yielded anomalous
concentrations of methane up to 41,300 ppm. Of the 19 unique sites analyzed 5 sites (9 cores) had more than 90 ppm of CH4
and of those, 3 sites (4 cores) had more than 13,800 ppm methane. Compositional and isotopic analyses of the gas support a
bacterial origin. Anomalous methane concentrations were found in kasten, gravity, and box cores; where more than one core
type was acquired at a single location, all cores contained anomalous methane. Despite the regional high productivity in the
Adriatic, all of the highest gas concentrations were found in the region associated with the thickest accumulation of recent flood
deposits off the most active channel of the Po delta (Po di Pila). Cores acquired in this region contain primary sedimentary
structures that indicate rapid burial of thick (N10 cm) flood deposits and a relative lack of bioturbation down to the base of the
flood layer. We propose that in the Po delta, flood deposits deliver significant amounts of terrigenous organic matter that can be
rapidly buried on the prodelta, effectively removing this organic matter from aerobic oxidation and biological uptake, and
leading to the potential for methanogenesis with burial. In areas unaffected by this high flux of organic matter and rapid/thick
flood deposition, or in between flood events, our data indicate that the conditions for methanogenesis and gas accumulation
have not been met. We suggest that in these areas, the physical and biological reworking of the surficial sediment between flood
events may effectively oxidize and mineralize organic matter (derived from both marine and terrestrial sources) and limit
bacterial methanogenesis in the subsurface.
D 2005 Elsevier B.V. All rights reserved.
Keywords: shallow gas; biogenic methane; flood deposits; flocculated fraction; Po delta; Adriatic Sea
0025-3227/$ - s
doi:10.1016/j.m
* Correspondi
831 633 2684.
E-mail addre
3 (2005) 159–177
ee front matter D 2005 Elsevier B.V. All rights reserved.
argeo.2005.06.040
ng author. AOA Geophysics Inc., Marine Geoscience Division, Moss Landing, CA 95039, USA. Tel.: +1 831 633 7400; fax: +1
ss: [email protected] (D. Orange).
D. Orange et al. / Marine Geology 222–223 (2005) 159–177160
1. Introduction
Deltaic environments are characterized by very
high rates of sediment accumulation and are the gate-
way by which much of the world’s terrigenous
organic matter is delivered to the marine environment.
Anaerobic bacterial methanogenesis of this buried
organic matter can lead to the generation of shallow
gas in sediments where sulfate reduction and methane
production dominate decomposition (Berner, 1980;
Martens and Klump, 1984). If present in sufficient
concentrations to exceed solubility, shallow gas can
impact impedance contrasts in the sub-surface, the
bathymetry and backscatter of the seafloor, and the
backscatter within the water column (Hovland and
Judd, 1988). As part of the EuroSTRATAFORM pro-
gram, we have been evaluating the causes of anoma-
lous sub-surface and seafloor features, and specifically,
quantifying shallow gas and identifying its impact on
geophysical properties.
The primary productivity of the Northern Adriatic is
one of the highest in the Mediterranean Sea (Giordani
et al., 2002) and provides a widespread source of
organic matter to the seabed. Both primary production
estimates and carbon respired in the sediment show a
water depth dependence with decreasing production
and respiration with increasing water depth (e.g.: pri-
mary production of 588 g C m�2 yr�1 in the Northern
shelf off the Po river delta and 62 g C m�2 yr�1 in the
Ionian Sea, and carbon respiration between 130 and 2 g
C m�2 yr�1 at the same sites; Giordani et al., 2002).
The main sediment sources of the northern and
central Adriatic are localized along its western and
northwestern shores, originating in the Alps and the
rapidly uplifting Apennines (Cattaneo et al., 2003).
The two main sediment source regions for the Adriatic
basin are the Po river (15�106 t yr�1 from a single
entry point) and the eastern Apennine rivers (a com-
bined total of 32.2�106 t yr�1; Cattaneo et al., 2003).
The Po Delta system represents the northern por-
tion of the late-Holocene highstand systems tract
along the Western Adriatic Sea (Correggiari et al.,
2001). Human manipulation and redirection of the
river flow since 1600 AD have resulted in the forma-
tion of the present day delta (Gandolfi et al., 1982).
Five distributary channels discharge from the delta
with roughly three quarters of the flow passing
through the Pila mouth (Fig. 1).
Human impact on the Po river regime has had three
main impacts: an increase in river sediment yield due
to deforestation phases since ca 3–4000 cal. yr BP
(Oldfield, 1996; Oldfield et al., 2003); controlled
avulsion with enhanced sediment delivery to the sea
through artificial banks since Roman times (Ciabatti,
1967); and river bed excavation and/or damming
leading to a marked decrease in sediment supply,
particularly during the last ca 50 yrs, (IDROSER,
1983).
Extremely rapid sedimentation occurs seaward of
the Pila mouth of the Po river (Nelson, 1970, Boldrin
et al., 1988) with delivery of sediment and organic
matter to the marine realm as flocculated aggregates.
Fox et al. (2004) confirmed previous hypotheses that
this rapid deposition was the result of flocculated
material (dflocsT) settling by providing direct observa-
tions of flocs in the water column. Maximum
observed floc size was in excess of 1000 Am and
settling velocities were in excess of 1 mm s�1. Floc
settling resulted in the deposition of fine-grained sus-
pended material in depths as shallow as 4-m during
normal flow conditions. Deposition of fine-grained
sediment in high-energy environments has been
observed in several different environments where
rapid flocculation leads to the delivery of sediment
to the seabed in sufficient quantities to overwhelm its
removal by waves and currents (e.g. Kineke and
Sternberg, 1995; Hill et al., 2000). Under certain
conditions where sediment flux is very high this can
lead to the development of mobile fluid mud layers
(Traykovski et al., 2000; Scully et al., 2003). Under
normal flow conditions, rapid clearance and deposi-
tion at or near the Pila mouth occurs in part due to the
packaging of fine sediment into flocs in the river
itself, well upstream of the freshwater/seawater inter-
face (Fox et al., 2004). During floods, sediment con-
centrations in the Po River can reach close to 1g l�1
leading to the rapid deposition of flocculated sediment
out to the 25-m isobath over an extended area of the
Po prodelta (Vignati et al., 2003; Milligan et al.,
2003b).
1.1. Biogenic gas
One of the goals of our sampling program was to
understand the relationship between flood deposition,
primary productivity, and gas (methane) formation.
Fig. 1. (a) Satellite image of the Italy and the northern Mediterranean Sea, showing the location of the Po delta EuroSTRATAFORM study area
(http://visibleearth.nasa.gov). (b) High-resolution image of the Po River mouth, showing the four main distributaries of the Po River, and their
relative contributions of water discharge and sediment load (http://eurodelta.bo.ismar.cnr.it; data from Nelson, 1970). The Po di Pila is the
primary active distributary of the Po delta.
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 161
The two main mechanisms for methane formation are:
1) low-temperature bacterial methanogenesis, and 2)
high-temperature kerogen thermal cracking (Friedman
et al., 1992). In geological literature, the gas generated
by the decomposition of organic matter by anaerobic
microbes at low temperatures is commonly referred to
as dbiogenicT gas, which is distinguished from
dthermogenicT gas by its composition and isotopic
signature (Kotelnikova, 2002). Biogenic gas has a
high concentration of methane relative to other hydro-
carbons (more than 99%; Clayppol and Kaplan, 1974;
Vogel et al., 1982) and a very depleted (light) stable
carbon isotope (13C) content (d13C PDBb�25x;
Schoell, 1983; Whiticar et al., 1986).
In marine sediments, the degradation of organic
matter by bacteria is coupled to the consumption of
free oxygen, nitrate, manganese, iron oxyhydroxid
and sulfate (Berner, 1980; Bottcher et al., 2000).
Organic matter oxidation is driven mainly by micro-
bial activity with dissolved sulfate as the final electron
acceptor (Jørgensen, 1982), and this leads to the for-
mation of hydrogen sulfide. Although methane can
form anywhere in the sediment column, it does not
accumulate in appreciable quantities until the dis-
D. Orange et al. / Marine Geology 222–223 (2005) 159–177162
solved sulfate is consumed because sulfate-reducing
bacteria also consume methane. As a result, the max-
imum rate of anaerobic methane oxidation typically
occurs at the base of the sulfate-reducing zone
(Iversen et al., 1987; Reeburg and Alperin, 1988;
Blair and Aller, 1995). Anaerobic methane oxidation
provides a source of carbon for the formation of
carbonate concretions and cements (Raiswell, 1988)
that may be geophysically imaged.
The factors that favor significant generation of
biogenic gas are rapid sediment deposition, sufficient
pore space for methanogenic bacteria (which require
~1 Am; Boone et al., 1993), and abundant organic
matter (Rice, 1993). Hydrogen-rich and bioavailable
organic substrates are associated with young, shallow
sediments (Kotelnikova, 2002) which typically have
the pore space to support methanogenic bacteria.
Biogenic gas formation requires an anoxic environ-
ment with available CO2 and low sulfate concentra-
tions (Oremland, 1988) with temperatures in the
range of 98 (Kotelnikova et al., 1998) to 110 8C(Stetter, 1992; Huber et al., 1994). The upper tem-
perature limit is associated with the maximum con-
ditions at which subsurface methanogens are active.
Biogenic methane can be accumulated in large and
commercially significant quantities; Rice (1992) esti-
mates that 20% of the worldwide reserves of natural
gas are generated by decomposition of organic mat-
ter via anaerobic bacteria.
Although present day shelf seas make up 8% of the
total ocean surface area, about one-fifth to one-third of
the global marine primary production takes place in
these coastal areas (Wollast, 1991). Shelves therefore
are potentially important sinks for large amounts of
organic carbon and the formation of biogenic gas.
There is some disagreement, however, about the fate
of this carbon.
Submarine deltas that supply large amounts of fine
grained sediments, and the shelf area proximal to
deltas, are sites of increased sediment accumulation
and organic matter burial. Ingall and Van Cappellen
(1990) suggest that organic matter buried at high
accumulation rates can avoid even the most efficient
decomposition mechanisms, and would therefore be
less degraded than organic matter buried at lower
accumulation rates. Deltas, therefore, could provide
regions with efficient delivery of organic matter to
methanogenic bacteria.
In contrast to the direct correlation of sediment
accumulation and carbon preservation proposed for
most delta environments, Aller (1998) suggests that
deltas can alternatively be characterized by efficient
decomposition of organic matter, with decomposi-
tion percentages of z70% (terrestrial) and z90%
(marine). Despite high primary productivity and
organic input associated with most deltas, Aller
(1998) demonstrated that on the deltas studied the
reactivity of organic material is low, and a larger
proportion of organic carbon is often degraded
compared to other marine deposits of similar net
accumulation rate. The primary cause of efficient
remineralization is intense physical and biological
reworking of sediment associated with oceano-
graphic fronts, upwelling, tides, bioturbation and
waves, as well as the cometabolism of refractory
carbon upon burial with relatively reactive carbon
(Aller, 1998). Although Aller (1998) documents
efficient decomposition of organic matter in deltaic
environments, Aller and Blair (2004) note that ter-
restrial material dominates the slower overall net
loss of organic carbon from particles in the top-
set–upper foreset zone of submarine deltas. de Haas
et al. (2002), in a review of shelf environments,
concluded that during repeated cycles of erosion
and redeposition of organic matter and associated
sediments, biological and chemical processes lead to
mineralization of more than 95% of the organic
carbon supplied by primary production and fluvial
input. Furthermore, a significant percentage of the
fine-grained sediments containing organic carbon on
the shelf can be removed by currents and waves. To
evaluate the presence and distribution of shallow
gas on the Po delta, we analyzed a suite of cores
acquired with the dR/V Seaward Johnson IIT in 2003
to evaluate the gas concentration and composition
and its association with anomalous seafloor and
sub-bottom acoustic data (Fig. 2).
1.2. Shallow gas and seismic/acoustic signature
Methanogenic fluids can alter the geophysical sig-
nature of the seafloor, water column, and sub-surface.
Fluid expulsion can change the local seafloor physio-
graphy by either creating local bathymetric highs
(pinnacles, mounds, mud volcanoes), or lows (pock-
marks; Hovland and Judd, 1988). Gas bubbling into
Po river(major distributaries)
Po di Goro
Po di Gnocca
Po di Pila
Po delleTolle
Line Po-1
Line Po-2
12.2 12.3 12.4 12.5
44.55
44.65
44.75
44.85
44.95
45.05
N22
N14
O19
J19
HI20
GH19
F16
E11 E20, E21
O10P10
Q21 Q25
S25
U25
PQQ15
M18G
12.6
10 km
Fig. 2. Simplified drawing of the Po delta area, showing the location of the four main distributaries, the location of coring sites sampled for this
study as part of the EuroSTRATAFORM program, and the location of high-resolution Chirp lines that indicated anomalous sub-bottom
reflectivity suggestive of shallow gas (Correggiari et al., 2001). Samples were acquired on transects labeled by letter, from north to south. Each
transect was oriented approximately perpendicular to isobaths. The number in the sample name corresponds to the approximate water depth of
that sample. The location of core E20, analyzed by Correggiari et al. (in press), but not evaluated for headspace gas in this study, is shown.
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 163
the water column can also create a distinctive signa-
ture on single-beam or side scan systems (Hovland
and Judd, 1988; Hornafius et al., 1999; MacDonald et
al., 2002).
On high-resolution seismic records, regions of
anomalous gas concentrations in the sub-surface may
be associated with anomalous seismic character (ori-
ginally proposed by Schuler, 1952; Heggland, 1998;
Hovland et al., 1999; Løseth et al., 2003; note that
anomalous seismic character may also be caused by
variations in attenuation and resonance). Such anoma-
lies are caused by saturated fluids and/or free gas
migrating through porous rocks, which may affect the
density and/or compressional velocity of the sediment
(and therefore the impedance), or the scattering of
seismic wave energy. Normal impedance contrasts pre-
sent in marine sediments may be disrupted or comple-
tely occluded by the presence of gas-rich pore fluids,
leading to bacoustic blankingQ or bacoustic wipeoutQ(Hovland and Judd, 1988). Acoustic plumes or gas
chimneys are vertical disturbances showing the upward
pathway of seeping gas, sometimes reaching the sea-
floor and continuing into the water column. The pre-
sence of gas in the shallow sub-surface may also affect
D. Orange et al. / Marine Geology 222–223 (2005) 159–177164
the backscatter energy as imaged by multibeam and/or
side scan systems (Fonseca et al., 2002).
Gas seeping from the Adriatic seabed was first
reported as distinct and continuous bubbling at the
sea surface (Morgante, 1940). More recently Conti et
al. (2002), Correggiari et al. (2001); Cattaneo et al.
(2003) report numerous of gas seeps and gas within
the shallow sediments. High-resolution sub-bottom
profile lines acquired on the Po Delta (Correggiari et
al., 2001, in press; Cattaneo et al., 2002) and else-
where in the western Adriatic (Trincardi et al., 1996)
Fig. 3. CHIRP sub-bottom profiler records from the Po delta region (see F
prodelta clinoform are masked by acoustic blanking inferred to be due to ga
cores, and concentration of gas from these cores. Depth of vertical line su
imaged acoustic anomalies in the sub-surface inter-
preted to be due to shallow gas (Fig. 3).
2. Methodology
EuroSTRATAFORM scientists examining the Po
Delta took advantage of a significant amount of pre-
vious work that had been conducted in the area,
including the acquisition of high resolution sub-bot-
tom profiler data. Sub-bottom data in the area had
ig. 2 for location). Sub-surface reflectors in the topset/foreset of the
s-prone deltaic deposits (Correggiari et al., in press). Note location of
b-surface shows actual depth of penetration of core.
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 165
identified regions of anomalous to occluded sub-sur-
face reflectivity which had been interpreted to be due
to the presence of shallow gas (Trincardi et al., 1996;
Correggiari et al., 2001, in press). Correggiari et al.
(2001) and this study both use the overall nature of
anomalous sub-bottom character, its extent and the
nature of the margins (both vertically and horizon-
tally) to infer an association with shallow gas. In other
parts of the world (e.g.: Chesapeake Bay, Hagen and
Vogt, 1999), the distribution of gas as inferred from
geophysical data can vary in both space in time, where
the change in bottom water temperature (or change in
the rate of biologic activity) was interpreted to affect
the methane solubility in the upper several meters of
sediment to lead to gas coming out of solution (Hagen
and Vogt, 1999). Gas sampling in the Po Delta does
not span the seasons, and so cannot constrain the
temporal variation in gas concentrations.
South of the Po di Pila distributary, sub-bottom
profiler data east-southeast of the Po di Tolle distri-
butary image buried channels, but do not show the
anomalous wipeout zones characteristic of shallow
gas (Correggiari et al., 2003). As part of a EuroSTRA-
TAFORM program to understand the origin of stratal
formation on continental margins, we participated in a
multiple principal investigator cruise to sample and
analyze the Po and the western Adriatic (Fig. 2). Our
goals were to groundtruth the interpretation of shallow
gas origin for the anomalous sub-bottom character,
and to understand the origin of this spatially restricted
distribution of shallow gas. On the Po Delta we also
acquired cores in areas that lacked anomalous sub-
bottom signatures suggestive of gas (note that the
geophysical signatures of gassy sediments can vary
with time even in the same location; Hagen and Vogt,
1999).
As part of a multi-PI program, we evaluated cores
from the Po Delta and northwestern Adriatic coast by
sub-samping the basal portion of gravity cores, kasten
cores and box cores. Station locations are designated
by a letter indicating the coring transect (with letters
ascending from north to south; each transect is
oriented approximately perpendicular to isobaths),
and a number indicating approximate water depth
(e.g.: core F16 was acquired on the bFQ transect in
approximately 16 m of water). Cores sub-sampled for
shallow gas were acquired in the Northern Adriatic
during a EuroSTRATAFORM cruise using the
research vessel dSeaward Johnson IIT in the summer
of 2003. On this cruise cores were acquired for multi-
ple purposes, including shallow gas studies (this
paper), flood deposit studies, sediment accumulation
rates, detailed sedimentological analysis, flocculation
studies, etc. All cores acquired on Leg 4 of the 2003
bSeaward Johnson IIQ cruise were sub-sampled for
headspace gas, regardless of whether that core’s pri-
mary purpose was gas studies. In this way, cores were
acquired from regions suspected to be characterized
by shallow gas as well as from regions where no gas
was expected.
Cores were acquired in water depths of 10 to 25 m;
no salinity measurements were taken at these sites
during coring operations, or on the cores themselves.
Although salinity affects gas accumulation, with
lower salinity favoring gas higher up in the sediment
column, we have no reason to believe that the samples
in 10 to 25 m of water come from varying salinity
conditions. Instead, we believe samples in these water
depths were thoroughly mixed with seawater prior to
their deposition.
Cores were sampled for headspace gas in the fol-
lowing manner:
A 5-cm-long section near the base of the core was
removed and placed into a metal can equipped with
a septum and weighed. Next, the sample can was
filled with water to the rim and 100 ml of water was
removed. About 2 to 3 g of sodium chloride salt was
added as a bacteriacide. The sample and water were
sealed in the can leaving a 100 ml headspace. The
sealed can was then inverted and frozen. Frozen
samples were transported to the organic geochemis-
try laboratory at the U.S.G.S. in Menlo Park, Cali-
fornia where they were placed in a freezer. Samples
were processed and analyzed until they were thawed,
processed and analyzed. Headspace analyses can
provide an assessment of the constituents in a gas
(C1, C2+, etc.), as well as a relative determination of
gas concentrations. In this study, headspace analyses
were conducted on a single sample from a core
(typically the deepest undisturbed region) so that
the rest of the core could be used by colleagues
studying sediment accumulation rates, preservation
of primary structures, etc. Although this sample
may not be representative of the entire core, our
experience has shown that the deepest sample,
which has the highest probability of being below
D. Orange et al. / Marine Geology 222–223 (2005) 159–177166
the methane oxidation–sulfate reduction horizon,
typically has the highest gas concentrations.
Because headspace analyses measure gas concen-
trations on cores that are not pressurized (and thus
can de-gas on their way to the surface), the measure-
ments provide a minimum estimate of the amount of
gas in a sample; absolute gas concentrations require
pressurized sampling equipment to keep the gas from
coming out of solution during ascent through the
water column. For the Adriatic samples, which ori-
ginate in relatively shallow water, the headspace gas
analyses would more closely relate to the actual
concentration of gas than cores recovered from dee-
per water. Recovered samples with low concentra-
tions of gas may be below saturation both at their
original depth and at the sea surface. Headspace
analyses of these samples may better reflect in situ
gas concentrations. Note that the isotopic composi-
tion of the carbon in the methane would change very
little even if there were degassing.
In the laboratory, the frozen samples in cans were
allowed to thaw until they reached about 20 8C, thenplaced into a high-speed shaker and shaken for 5 min.
The partitioned hydrocarbon gases in the headspace
were analyzed for methane through hexane (C1–C6),
CO2, and H2S by gas chromatography. Samples with
more than 90 ppm methane are considered to be
anomalous; background methane values in this and
similar coastal environments are usually less than
about 30 ppm.
The coring program included 28 cores from 17
distinct locations offshore the Po in the northern
Adriatic Sea (Fig. 2). Headspace gas samples were
obtained from Kasten, gravity, and box cores. Kasten
and gravity cores typically penetrate deeper into the
sediment, whereas box cores typically penetrate only
50 cm. Because the peak of methanogenesis typically
occurs below the base of the sulfate reduction zone,
shallow cores as a rule contain a lower gas concentra-
tion than deeper cores. Core recovery on all samples
was excellent, with up to 240 cm in the Kasten cores
and up to 450 cm in the gravity cores. The average
recovery of the box cores was 30 cm.
In many of the Po samples, the sediment immedi-
ately below the sediment-water interface was dark
colored, with a moderate to strong H2S smell. The
presence of strong H2S suggests that at that location
the depth to the base of the sulfate reduction zone is
shallow, possibly indicating upward advection of
reduced fluids (and gas?).
In several of the Po samples detailed visual inspec-
tion of the core suggested the presence of gas due to
the presence of small bubbles and micro-parting.
Subsamples of the box cores were collected for
disaggregated inorganic grain size (DIGS) analysis
between December 2000 and May 2003. Sample ana-
lysis was carried out on a Coulter Multisizer IIe
following the procedure of Kranck and Milligan
(1991). Floc fraction, an estimate of the amount of
material in the sediment that was deposited as flocs,
was determined by parameterization of the DIGS
spectra using the method described in Milligan and
Loring (1997) as modified by Curran et al. (in press).
3. Results
Of the 17 unique sites analyzed 5 sites (9 cores)
yielded anomalous gas concentrations, with more
than 90 ppm of methane (see Table 1; Fig. 4). Of
these, 3 sites (4 cores) yielded more than 13,800
ppm methane. The highest gas concentrations were
from core F16 (gravity core), which had a methane
concentration of 41,385 ppm; the box core at F16
had an anomalous gas signature that was much lower
of 126 ppm. The next highest gas concentrations
were from site E11. The gravity core at this site
yielded 27,500 ppm, whereas the box core at this
location had a surprisingly higher gas concentration
of 32,300 ppm. The gravity core at site GH19 had
13,800 ppm methane; the box core at this location
again had an anomalous, but much lower, concentra-
tion of 231 ppm methane. All of these cores come
from 10 to 20 m water depths on the delta offshore
the Po di Pila distributary. The remaining core in this
region, E21, had anomalous gas concentrations in
both the gravity (368 ppm) and box (132 ppm)
cores, although the maximum gas values were
lower than the other cores in this region. The only
other site that had anomalous gas was PQQ15, where
a Kasten core yielded 91 ppm methane (close to our
threshold value of 90 ppm).
A previously acquired core in this area also had
anomalous gas in it. Correggiari et al. (2003) report
that core KS02-154, acquired at site E20 (very close
to E21, Fig. 2; see Fig. 3a for the location of E20
Table 1
Headspace gas analyses and isotopic results for cores acquired on the Po delta
Site Core
type
Water
depth
(m)
Sample
depth b
ml* (cm)
C1
(ppm)
C2
(ppm)
C3
(ppm)
CO2
(ppm)
C1
(Al/l)C2
(Al/l)C3
(Al/l)d13C
(CH4)
d13C
(CO2)
E21 BC 21 30 132.01 0.77 0.37 15,180.00 129.422 0.750 0.358
E21G GC 21 436 368.26 0.12 0.07 5369.00 305.189 0.099 0.061 �77.0 �24.3
E11 BC 11 30 32300.00 0.00 0.52 23,020.00 24,771.271 0.000 0.418 �70.0 �23.9
E11G GC 11 280 27500.00 0.00 1.13 5670.00 23,843.931 0.000 0.976 �74.0 �8.0
F16 BC 16 30 125.34 0.70 0.11 1268.00 93.837 0.523 0.081
F16G GC 16 377 41385.00 0.00 0.90 1992.00 46,326.493 0.000 1.002 �86.1 �13.3
GH19 BC 19 30 231.40 0.23 0.18 818.00 216.936 0.218 0.167
GH19G GC 19 449 13800.00 0.11 0.23 1954.00 18,318.584 0.151 0.309 �91.9 �20.2
H120 BC 20 30 10.00 0.25 0.11 203.00 6.380 0.159 0.070
H120G GC 20 450 41.66 0.18 0.16 2281.00 61.265 0.265 0.241
J19 BC 19 30 16.18 0.32 0.13 402.00 11.445 0.224 0.090
J19G GC 19 350 67.15 0.22 0.05 3109.00 58.224 0.193 0.041
M18G GC 18 315 71.50 0.38 0.10 1021.00 65.398 0.346 0.091
N114C BC 14 30 12.93 0.21 0.09 131.00 14.475 0.238 0.103
N14B BC 14 30 17.54 0.34 0.12 116.00 17.536 0.238 0.121
O10 BC 10 30 44.19 0.57 0.10 1180.00 33.312 0.430 0.076
N22D SC 22 40 10.38 0.38 0.14 452.00 8.029 0.297 0.110
N22D BC 22 30 8.00 0.38 0.12 200.00 5.742 0.276 0.086
N22C BC 22 30 10.17 0.82 0.17 701.00 7.226 0.580 0.119
N22A BC 22 30 7.81 0.47 0.11 286.00 6.162 0.370 0.085
P10 BC 10 30 45.08 0.30 0.10 746.00 30.325 0.204 0.069
O19 BC 19 30 19.06 0.32 0.05 274.00 14.223 0.239 0.038
O19G GC 19 373 30.04 0.32 0.08 2292.00 36.637 0.393 0.095
PQQ15 KC 15 230 91.68 0.32 0.25 634.00 91.683 0.322 0.252
Q21 BC 21 30 11.21 0.40 0.07 309.00 6.809 0.343 0.044
Q25 BC 25 30 13.84 0.23 0.08 290.00 10.079 0.167 0.056
S25 BC 25 30 8.35 1.02 0.10 1361.00 5.772 0.706 0.071
U25 BC 25 30 12.82 0.37 0.07 351.00 8.820 0.253 0.048
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 167
relative to E21 on Chirp line Po-1), showed evidence
of the 1994 flood 90 cm below mud line in this 20 m
core. The flood deposit in this core was characterized
by primary sedimentary structures that indicate that
the flood layer had not been re-worked by physical or
biological processes. Correggiari et al. (2003) report
that the evidence of gas was present in this core 160
cm below mud line; previously published sub-bottom
data from this site (Correggiari et al., 2001) showed
this region to be characterized by anomalous sub-
bottom profiler data and acoustic wipeout.
With the exception of PQQ15 (discussed below),
the highest values of gas were associated with the
portion of the delta proximal to the Po di Pila, the
most active distributary of the Po River. Sub-bottom
profiler data acquired in this region imaged anoma-
lous sub-bottom imagery (acoustic blanking), pre-
viously interpreted to be due to shallow gas (Fig. 3).
Cores E11 and E21 were acquired along previously
acquired sub-bottom profiler line Po-1 (Correggiari et
al., 2001, in press); GH19 was acquired coincident
with previously acquired sub-bottom profiler line Po-
2 (Correggiari et al., 2001, in press). Core F16 is not
located along a previously acquired sub-bottom line,
but instead is offset 1.6 km north of line Po-2 (Fig. 4).
Cores acquired proximal to the less active distri-
butaries of the Po, the Po di Tolle and the Po di
Gnocca, all had gas concentrations significantly
below the threshold for anomalous gas (Fig. 4).
Sites with more than one core type (gravity and
box) had low gas in both (e.g.: HI20, J19, O19),
indicating the lack of gas regardless of depth of
penetration in these sites. Core PQQ15, a Kasten
core, occurs southwest of the Po di Goro. This core
penetrated 230 cm. Note that the Po di Goro and Po di
Gnocca are less active than the Po di Pila, in particular
12.2 12.3 12.4 12.5
44.55
44.65
44.75
44.85
44.95
45.05
12.6
10 km
M18G (B= 71)
N22 (B= 8-10)
N14 (B= 13-17)
O10 (B= 44)
P10 (B= 45)
Q21 (B= 11)
Q25 (B= 14)
S25(B= 8)
U25 (B= 13)
PQQ15 (K= 91)
O19B= 19G= 30
HI20B= 10G= 42
J19B= 16G= 67
F16B= 126G= 41385
GH19 B= 231G= 13800
E21B= 132G= 368
E11 B= 32300G= 27500
Po river(major distributaries)
Po di Goro
Po di Gnocca
Po di Pila
Po delleTolle
Fig. 4. Sample distribution of the Po delta area, showing the concentration of gas from each core (in ppm). Methane concentrations are shown
for each core type; B=box core, G=gravity core, K=Kasten core. Gas values higher than 90ppm are anomalous; background methane values
are b30 ppm. Note that in locations with multiple core types and anomalous gas concentrations, all cores in that location showed anomalous gas.
At sites E21, F16 and GH19, gravity cores contained higher gas concentrations than the box core, whereas at E11 the box core contained higher
gas than the gravity core. With the exception of Kasten core PQQ15 (91 ppm), all anomalous gas samples came from cores proximal to the most
active distributary of the Po River, the Po di Pila. These samples are also located in the area of anomalous sub-bottom acoustic character.
D. Orange et al. / Marine Geology 222–223 (2005) 159–177168
since the end of the Little Ice Age, but are not
abandoned. Additional cores acquired south of the
PQQ transect, as far south as the bUQ transect, had
gas concentrations that were at background concen-
trations, up to a maximum of 14 ppm.
The cores that contained anomalous gas concentra-
tions were dominantly (N99%) methane, with trace
amounts of ethane (C2) and higher order hydrocar-
bons (Table 1). Due to their high methane concentra-
tions, the 5 highest gas samples were further analyzed
for their isotopic composition (Table 1). These sam-
ples come from the 4 sites proximal to the Po di Pila
channel (E11, E21, F16, GH19), and include both the
box core and gravity core from E11. The d13C (relative
to PDB) of the methane ranges from �70.0x (E11
box core) to�91.9x (GH19 gravity core). At E11, the
isotopic value for the box core (�70.0) is similar to the
gravity core (�74.0x). At the same sites, the d13C
(PDB) of the CO2 ranges from�8.0 (E11 gravity core)
to �24.3x (E21 gravity core). Methane and CO2
carbon isotopic composition considered in tandem
demonstrate that the methane was likely generated
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 169
by carbonate (CO2) reduction by microbes, a common
process in marine sediments.
4. Discussion
All of the highest gas concentrations occur in the
region of rapid and thick flood deposition proximal to
the most active distributary, the Po di Pila. Anomalous
gas concentrations were found in all core types in this
region. At sites E21, F16 and GH19, gravity cores had
higher gas values than the co-located box core,
whereas at E11, the box core had a higher value
than the gravity core. In all cases, though, both core
types (gravity and box) showed anomalous concentra-
tions at the same location. In the saline environment
of the Po delta in water depths of 10–20 m, the
anomalous gas concentrations even in the shallowly
penetrating box cores argue for a component of ver-
tical gas migration via advection (driven by buo-
yancy?) or diffusion.
The anomalous gas documented at sites E11, E21
and GH19, and by Correggiari et al. (2003) for core
E20, spatially coincides with portions of the Po delta
characterized by anomalous sub-bottom profiler
records. Our headspace gas data confirm the presence
of anomalous gas in the areas where the sub-bottom
data show acoustic wipeout. Gas concentrations in
cores E11, F16, and GH19 are higher than the 1–2%
suggested as the lower limit necessary for acoustic
wipeout (Fannin, 1980; Løseth et al., 2003; note that
in the western Baltic, acoustic turbidity is associated
with less than 0.5% gas; Abegg and Anderson, 1997;
Missiaen et al., 2002), Core GH19 is the only core
where our gas analyses were more than the ~30 ml/l
suggested to be the lower limit of gas necessary to
create acoustic turbidity/wipeout based upon borehole
samples correlated with sub-bottom profiler data in
the Gulf of Mexico (Whelan III et al., 1978); Core
E11 has gas concentrations close to, but lower than,
30 ml/l.
Our core analyses suggest that significant gas con-
centrations can be present 30 to 450 cm below the
sediment-water interface in areas characterized by
acoustic wipeout. The presence of high gas concen-
trations in the sub-surface in these regions could sig-
nificantly impact the frequency-dependent backscatter
properties of the substrate through bubble reverbera-
tion (Vagle and Farmer, 1992; Jackson et al., 1998;
Holland, 2002; Holland et al., 2003).
The lack of higher other hydrocarbons (C2+) in the
headspace gas analyses suggests a microbial origin of
the gas off the Po. Note that there is thermogenic
hydrocarbon production in the Adriatic, as indicated
by numerous active oil/gas platforms. Our data sug-
gest that thermogenic hydrocarbons are an insigni-
ficant component of the gas present on the Po delta.
Isotopic analyses of the methane in the headspace gas
further support a biogenic (microbial) methane origin
(d13CH4 (PDB) of �70.0x to �91.9x). Note that
some of these values are slightly more negative than
expected (�80x is standard). The headspace gas
values of d13CO2 and d13CH4 are as expected for
shallow marine sediments high in organic matter
(N1% TOC). The d13CO2 values analyzed in conjunc-
tion with the d13CH4 values define the kind of bacter-
ial process responsible for the formation of the
methane. In the case of the Po samples (d13CO2=�8
to �24), carbonate reduction (CO2+H2=CH4) is
interpreted to be the microbial process generating
methane.
Compositional and isotopic data from organic
matter in near-surface sediments on the Po delta
indicate a predominantly terrestrial (riverine) organic
carbon source. Misserocchi et al. (2003) defined end
member values for the origin of the organic matter in
the Adriatic sea of d13Corg of �19.3x for marine
organic matter, and d13Corg of �27.8x for riverine
organic matter. The mixture of terrestrial vs. marine
organic matter changes as a function of distance
from the main Po di Pila mouth (increasing the
distance decreases the percentage of terrestrial
organic matter), and as a function of season (with
more terrestrial organic matter during the main run-
off season). For example, at E20, Miserocchi docu-
ment a d13Corg of �23.7x in April 2002, and
d13Corg of �25.5x in October 2002. Using a two
end-member mixing model, this suggests that in
October ~73% of the organic matter is terrestrial,
whereas in April ~52% is. Analyses at E11 and
G15 indicate that in October ~83% (E11) /~70%
(G15) of the organic matter is terrestrial, whereas
in April the terrestrial component is ~68% (E11) /
~62% (G15). These analyses represent the recent
sediments and flood deposits, and therefore may
not be directly applicable to shallow gas produced
D. Orange et al. / Marine Geology 222–223 (2005) 159–177170
in these locations. At E20, d13Corg from the surface
to 1m depth range from �24.6x to �26.9x. This
range indicates differences down core in sediment
input, grain size, etc., but all values indicate a dom-
inance of terrestrial carbon.
In summary, 4 of the 5 sites characterized by
anomalous gas on the Po delta occur proximal to the
most active distributary, the Po di Pila, and analysis of
recent sediments indicates that this is a region domi-
nated by terrigenous organic matter. The remaining
site, PQQ15, yielded gas concentrations close to the
threshold (91 ppm) from a relatively deep Kasten
core. This site occurs southwest of the Po di Goro
distributary. None of the remaining 12 sites (18 cores)
yielded anomalous gas concentrations, despite these
cores being located down-current of the main south-
west-directed Po flood plume, and in a region of high
primary productivity.
Sub-bottom profile lines from the region east-
southeast of the Po di Tolle distributary image the
sub-surface in the area of samples N14, N22, M18G
and J19 (Correggiari et al., 2003). Anomalous sub-
bottom reflectivity on these lines is interpreted to be
due to buried shallow channels related to the paleo Po
di Tolle (Correggiari et al., 2003); the lack of acoustic
blanking below these anomalous zones indicates that
the acoustic incoherence is not due to gas. Analyses of
all cores in this region yielded near-background levels
of gas measured in this area.
We propose that the biogenic gas in the Po delta
reflects the locus of rapid and deep burial of terrestrial
organic matter in flood deposits, such that the organic
matter is efficiently buried beyond the reach of bio-
turbation and physical re-working. To support this
hypothesis, we will examine evidence from recent
flood deposits, recognizing that there is a spatial
(depth) and temporal disconnect inherent to biogenic
gas (related to methanogenic bacteria active on buried
and ancient carbon) and recent sediment accumula-
tion. We believe, however, that the present is the key
to the past, and that the rapid accumulation at the
foreset/topset of the prodelta provides the most effi-
cient means of isolating organic carbon from oxida-
tion and biological uptake as well as from physical
reworking and bioturbation. Moreover, this associa-
tion of recent (year 2000) flood deposition and rapid/
deep burial of organic matter can be extrapolated to
the past, and at least since the Little Ice Age, the locus
of flood deposition in 2000 correlates with the loca-
tion of the Po delta topset/foreset. As such, we believe
that the observations of the most recent flood deposits
can in fact be tied into the distribution of biogenic gas
production, even if the gas itself is being formed by
methanogenesis of buried ancient organic matter.
Martens et al. (1998) present a model of coupled
biochemical processes that control methane in coastal
sediments, and show that methane production is
highly sensitive to two factors that increase the
amount of organic matter delivered to the sediment
column below the base of the sulfate reduction zone.
The first factor is increased flux, which would be
equivalent to the rapid delivery of flood sediment
discussed here. The second factor is the degradation
in the rate constant of organic matter, or the decrease
in the reactivity of organic matter (which allows more
organic matter to dsurviveT its transit through the
upper sediment column). This second factor would
be applicable to the Po if the reactivity of the organic
matter in the flood deposit were lower than the reac-
tivity of the organic matter away from the flood
deposit. Because the flood deposits contain highly
reactive terrigenous organic matter, we suggest that
increased flux is responsible for increased methane
production, rather than a decrease in the reactivity of
organic matter.
In addition to the acoustic blanking observed on
the Po prodelta, sub-bottom profiler records also
show acoustic blanking farther below the seafloor
in deeper parts of the delta (Fig. 3). These acoustic
blanking zones occur below the interpreted maxi-
mum flooding surface in water depths of 45–50 m.
Analysis of the high resolution data off the late-
Holocene Po delta indicates it is a composite deposi-
tional system built during the last 5000 yr (Trincardi
et al., 2003). Within this context, the deeper gas
shown on Fig. 3 originates at or below the maximum
flooding surface. This contrasts with the shallow gas
imaged in the highstand tract (Cattaneo et al., 2003)
and sampled by our cores.
If the anomalous reflectivity deeper on the delta is
caused by shallow gas, than the gas may be formed by
biogenic methanogenesis of organic matter concen-
trated at the maximum flooding surface. The max-
imum flood surface can be prone to high organic
matter sediments (Posamentier et al., 1988; Vail and
Mitchum Jr., 1977; Vail et al., 1991) deposited during
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 171
periods of high sediment accumulation rate. Similar to
today’s flood deposits on the topset/foreset, the high
sediment accumulation rates and high organic content
may have teamed to remove organic-rich deposits
from the reach of physical and biological reworking
such that the organic matter could be metabolized by
anaerobic methanogenic bacteria. The sub-bottom
data suggest that there is little gas migration above
the maximum flood surface in this region. Limited
sub-bottom profiler data suggest that the base of the
highstand deposits/top of the maximum flood surface
may serve as an intraformational seal that may limit or
hinder the vertical migration of shallow gas from
deeper on the delta. Alternatively, biogenic methano-
genesis may not have lead to the formation of a
sufficiently large gas column to exceed the capillary
pressure/critical pore throat surface tension necessary
to lead to expulsion of gas-rich fluids through the finer
grained deposits of the clinoformal wedge, or that
upward diffusing methane is oxidized anaerobically
in the sulfate reduction zone of the clinoformal strata.
The portions of the Po River prodelta near the
major distributaries (especially the Po di Pila distri-
butary) contain evidence that the seabed accretes pri-
marily during periods of rapid sediment deposition
associated with episodic floods of the river. This
includes regions in water depths of ~20 m or less
along the E-G transects. Sediment cores collected in
this vicinity contain sedimentary structures (examined
by X-radiography) that reveal a dominance of physi-
cal stratification (Palinkas et al., 2005—this volume;
Palinkas and Nittrouer, submitted for publication;
Wheatcroft et al., submitted for publication).
Although bioturbation can be intense in surficial
deposits (b10 cm) of these regions (biodiffusion mix-
ing coefficients N10 cm2/yr, Palinkas et al., 2005—
this volume), the sediment showing primary fine scale
structures appears to be emplaced and buried by thick
(N10 cm), rapidly deposited flood layers. This con-
clusion is supported by 210Pb profiles in the northern
portions of the study area, which indicate very rapid
(N1 cm/yr) non-steady-state sediment accumulation
(Palinkas and Nittrouer, submitted for publication).
These profiles occur when large quantities of sediment
are discharged by rivers, such that suspended particles
are not able to scavenge their normal ratio of dis-
solved 210Pb before reaching the seabed. Farther south
(i.e., generally south of N transect), 210Pb profiles are
steady-state and indicate slower accumulation rates
(b1 cm/yr), suggesting that particles are supplied in
a slower and more constant manner.
Independent verification of the potential for flood
deposition comes from direct observations associated
with the autumn 2000 flood of the Po River. Mea-
surements of 7Be in cores collected soon after the
flood indicate new layers at least 15 cm thick,
reflecting input of sediment supplied from the land
surface in the Po drainage basin (Palinkas et al.,
2005—this volume). Sedimentary structures from
these same cores indicate that the earlier stages of
the flood sediment preceded the particles containing7Be, and the total flood deposits can be substantially
thicker (N20 cm in some cases; Wheatcroft et al., in
preparation). The thickest flood layers were located
near the Pila and Tolle distributaries, and generally
in the northern portions of the study area (E–G
transects; see Palinkas et al., 2005—this volume,
their Fig. 5a).
Much of the sediment supplied to the Adriatic Sea
during flood events is buried and preserved relatively
close to deltaic distributaries (especially the Pila).
Burial in these regions occurs in the form of thick
layers (N10 cm), which put sediment below the depth
of bioturbation. Therefore the carbon deposited along
with these sediments is quickly removed from the
oxygenated portion of the seabed, where aerobic bac-
teria can remineralize the carbon and return it to the
atmosphere as CO2 (Hedges and Keil, 1995). Rapid
and deep burial of organic matter in flood deposits
may move the carbon through the sulfate reduction
zone rapidly enough to be impacted by methanogen-
esis. In contrast, particles (with carbon) moving
farther along the dispersal system spend a greater
period of time in the surface mixed layer, where
their carbon can be remineralized by aerobic bacteria
(the bfluidized bed reactorQ of Aller, 1998). The lack
of anomalous gas concentrations in cores acquired in
these distal areas with thinner flood deposits (Palinkas
et al., 2005—this volume) and slower accumulation
rates suggest that they have a lower potential for
methanogenesis. Portions of the Po delta farther
from the active Po di Pila distributary, or in deeper
water depths, may have a flood deposit that is thinner,
such that the processes of biologic or physical re-
working can oxidize or utilize the organic matter, lead-
ing to a lower potential for methanogenesis.
Riverine Input
Corg Primary productivity + non-flood fluvial input
Slow sediment accumulation rate (< 1cm/yr); organic matter with flocsRework (physical,
biological),remineralize
Organic matter: oxidize, remineralize, consumed by fauna
Flood input: LOTS of sediment and silt-sized organic matter
Rapid sediment accumulation rate (>> 1cm/yr)Rapid burial of
sediment + organics below reach ofreworking; preservation of primary structures
Organic matter: rapidly buried below reach of reworking (>10cm)Available for methanogenesis
Non-flood Flood
Riverine Input
Corg
Fig. 5. Schematic diagram showing the relationship of sediment delivery, reworking, organic matter burial, and methanogenesis. During non-
flood times, organic matter may be delivered by both fluvial processes and primary productivity. Organic matter may settle to seafloor as
flocculated material. The relatively low sediment accumulation rate allows physical and biological reworking processes to consume or oxidize
organic matter, leading to relatively little organic matter availability for biogenic methanogenesis. During flood events, in contrast, rapid and
thick accumulation of particulate organic matter and sediment may effectively isolate organic carbon from physical or biological reworking,
leading to enhanced methanogenesis in the actively accreting foreset/topset portion of the delta.
D. Orange et al. / Marine Geology 222–223 (2005) 159–177172
In the marine environment, most organic carbon
is associated with fine-grained sediment particles and
follows the paths of particles due to its dependence
on surface area (Mayer, 1994). This typically leads
to a close correlation in the sediment between
organic carbon concentration and floc fraction, the
amount of material deposited as flocs from suspen-
sion (Milligan and Loring, 1997). Comparison of
sediment accumulation rates, carbon, and floc frac-
tion in the flood deposit off the Po indicate that
contrary to expectation, there is a disconnect
between organic matter delivery to the delta and
floc fraction, and that the organic matter more clo-
sely tracks the silt-sized fraction. As observed in the
near field during normal flow conditions, floc
deposition dominated the emplacement of sediment,
closely following the path of the plume as observed
by satellite (Milligan et al., 2003a,b). At most sta-
tions, X-ray and disaggregated inorganic grain size
analyses showed a complex pattern of interbedded
fine grained and coarser sediment resulting from
variations in the amount of floc deposited sediment.
This pattern was similar to that found on the Eel
shelf after the 1995 and 1997 flood events (Wheat-
croft et al., submitted for publication) and to that
observed in turbidites (Kranck, 1984; Curran et al.,
in press).
Low floc fractions were observed out to the 20 m
isobath near the Pila mouth suggesting that in this
region turbulence was sufficient to prevent floc deposi-
tion. Following the 2000 Po flood event, the concen-
tration of organic carbon in the sediments in this region
did not correlate with floc fraction as expected. Sam-
ples had low floc fractions and high organic carbon
concentrations (Milligan et al., 2003a,b; Misserocchi et
al., 2003; recall that this is the region of anomalous gas
and thick flood deposits). Sampling over the next 2 yrs
showed a gradual return to higher floc fractions but also
a decrease in carbon content.
The disconnect between floc fraction and organic
carbon levels at stations near the Pila mouth suggests
that during the flood the carbon was not associated
with the fine grained sediment but rather with the
coarser silt fraction. The lack of association with the
high surface area found in the finest fraction normally
incorporated into flocs indicates that the carbon was in
large particulate form. 13C analysis shows that the
carbon sequestered in the near field was of terrestrial
origin. Davide et al. (2003) report that during the 1994
flood, of similar magnitude to the 2000 flood, C /N
D. Orange et al. / Marine Geology 222–223 (2005) 159–177 173
ratios show an increase in terrestrial material during
flood conditions.
Over the next 2 years, the expected correlation
between floc fraction and organic carbon failed to
appear. While floc deposition in the near field was
re-established, carbon concentrations decreased
rapidly. At the same time, the marine carbon became
the dominant source as indicated by isotopically hea-
vier d13C (Misserocchi et al., 2003). This continued
lack of correlation between floc fraction (surface area)
and organic carbon supports the hypothesis of Aller
(1998) that regions of rapid fine grained sediment
deposition can respire carbon at extremely high
rates. On the Po delta, however, rapid sediment accu-
mulation during flood events may effectively remove
organic matter from the influence of biologic and
physical re-working, with an increasing potential for
bacterial methanogenesis.
Aller (1998) discuss the dominance of physical and
biological processes in remobilizing and oxidizing
carbon offshore river systems such as the Amazon
and Yangtze. We suggest that the Po represent a
system where the dominance may be on different
processes in the carbon cycle. Specifically, we believe
that burial of terrigenous carbon in thick and rapidly
deposited flood events may lead to locally enhanced
methanogenesis, where the flood deposit acts to mini-
mize the oxidation of carbon. Protected deltas such as
the Po may experience less intense physical re-work-
ing due to the decreased intensity of oceanographic
forcing. On the Po, we find enhanced methanogenesis
in a region offshore to the most active distributary,
which provides terrigenous organic matter to the delta
as silt-sized particles during floods. On the Po, there-
fore, different processes are active at different times,
and in different locations. Between flood events, and
away from the thick flood deposits, bioturbation and
physical re-working may effectively oxidize organic
matter and limit the potential for methanogenesis.
On the Po delta proximal to the active distribu-
taries rapid sediment accumulation related to flood
events may effectively bring carbon below the reach
of physical and biological reworking processes, such
that there is an increasing potential for anaerobic
bacterial methanogenesis (Fig. 5). Between flood
events, when the rate of sediment accumulation is
lower, physical and biological reworking processes
can effectively remobilize, oxidize, and consume
organic matter delivered to the seabed. Anthropo-
genic impact may further impact the delivery of
organic matter to the Po delta. The Po River has
been channelized in recent years by construction of
artificial levees (Marchetti, 2002), which inhibit
overbank flow to floodplains and may facilitate
rapid delivery of sediment to the Adriatic during
flood events (Palinkas et al., 2005—this volume).
This channeling may therefore focus flood deposition
proximal to the most active river distributaries, and
also lead to focused biogenic gas production on the
prodelta.
5. Conclusions
Shallow biogenic gas has been sampled on the
active topset and foreset of the Po delta, and is asso-
ciated with anomalous reflectivity in sub-bottom pro-
filer data. Cores acquired on the Po delta yield gas
data that successfully ground-truth the interpreted
relationship between anomalous reflectivity in high-
resolution sub-bottom data and shallow gas. Cores
also serve to examine the relationship between flood
deposition, high sediment accumulation rates and the
formation of shallow gas.
Shallow biogenic gas in the sub-surface occurs
only in the region characterized by thick and rapid
sediment accumulation associated with modern
floods. We propose that this is due to the spatial
association of gas formation (high flux of organic
matter burial and removal from oxidation/bioturba-
tion) at the region of the prodelta characterized by
the maximum sediment accumulation rate and deposi-
tion of particulate organic matter. Organic matter in
these recent flood deposits shows a dominance of
terrestrial carbon. Organic carbon appears to be deliv-
ered to the delta during flood events as silt-sized
particles, and is not associated with flocculated mate-
rial. We suggest that the flood deposits limit bioturba-
tion and biouptake of organic matter, resulting in
organic matter that can be transported rapidly through
the sulfate reduction zone and into the zone of anae-
robic methanogenesis. Flood deposits also minimize
oxidation potential by removing the organic matter
from exposure to oxygen. In contrast to these flood
deposits, organic matter that arrives at the seabed due
to the continuous rain of primary productivity or
D. Orange et al. / Marine Geology 222–223 (2005) 159–177174
during non-flood events may be reworked by physical
and biological processes and oxidized or consumed,
with a decreased level of biogenic methanogenesis.
One possible mechanism for the sequestering of
large amounts of carbon and its subsequent isolation is
the changing modes of sediment deposition as a flood
event evolves. Prior to a flood event, highly floccu-
lated, organic rich sediment is deposited in the near
field, and reworked in the mobile upper layer of
sediment giving high floc fraction low carbon sedi-
ments. During a flood the plume extends out over the
shelf, increasing bottom stress in the near field so that
deposition is dominated by single grain settling.
Coarse-grained particulate carbon would be deposited
with the silt fraction at this time resulting in low floc
fraction, carbon-rich sediments. As flow decreases,
floc settling would be re-established, capping the
coarse carbon fraction with a relatively impervious
layer of fine-grained sediment.
We suggest that in between flood events organic
carbon is delivered to the seabed with the floc frac-
tion. At low sediment accumulation rates, this material
could be reworked by physical and biological pro-
cesses, such that the floc fraction gets digested by
benthic organisms and/or oxidized, and does not sur-
vive long enough to be broken down biogenically to
form shallow gas.
We anticipated widespread shallow gas in the
northwest Adriatic based upon the very high regional
primary productivity, yet we found anomalous gas
only in direct proximity to the active channels of the
Po delta. Based upon the spatial association of anom-
alous gas with the rapid deposition of recent flood
deposits, we suggest that the shallow gas we sampled
on the Po delta is related to the relatively coarse-
grained organic matter brought in by flood deposits,
and is unrelated to the organic matter brought in with
the flocculated material.
Acknowledgements
We thank all of the scientific staff from the R/V
Seaward Johnson II and our EuroSTRATAFORM col-
leagues for their collaboration and assistance. The
research herein was funded by the U.S. Navy through
the Office of Naval Research EuroSTRATAFORM
project (contract number N00014-03-1-142, Orange).
Ana Garcıa-Garcıa acknowledges post-doc research
grant EX2002-0627 by the MECD. Partial support to
this work came from the EU projects EURODELTA
(Contract EVK3-CT-2001-20001) and EUROSTRA-
TAFORM (EVK3-CT-2002-00079).We thank the Uni-
versity of Victoria (M. Whiticar) for isotopic analyses
of headspace gases. Jennifer Dougherty assisted with
headspace gas analyses at the USGS. This is ISMAR-
Bologna contribution number 1411. Constructive
reviews by Dr. Larry Mayer and an anonymous
reviewer have improved the manuscript.
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