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: Dan_Orange@aoageophysics.com (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.

References

Abegg, F., Anderson, A.L., 1997. The acoustic turbid layer in

muddy sediments of Eckernforde Bay, Western Baltic: methane

concentration, saturation and bubble characteristics. Mar. Geol.

137, 137–147.

Aller, R.C., 1998. Mobile deltaic and continental shelf muds as

suboxic, fluidized bed reactors. Mar. Chem. 61, 143–155.

Aller, R.C., Blair, N.E., 2004. Early diagenetic remineralization of

sedimentary organic C in the Gulf of Papua deltaic complex

(Papua New Guinea): Net loss of terrestrial C and diagenetic

fractionation of C isotopes. Geochim. et Cosmochim. Acta 68

(9), 1815–1825.

Berner, R.A., 1980. Early Diagenesis: A theoretical Approach.

Princeton University Press. 241 pp.

Blair, N.E., Aller, R.C., 1995. Anaeroic methane oxidation on the

Amazon shelf. Geochim. Cosmochim. Acta 59, 3707–3715.

Boldrin, A., Bortoluzzi, G., Frascari, F., Guerizoni, S., Rabitti, S.,

1988. Recent deposits and suspended sediments off the Po Della

Pila (Po River, main mouth, Italy). Mar. Geol. 79, 159–170.

Boone, D.R., Whitman, W.B., Rouviere, P., 1993. Diversity and

taxonomy of methanogens. In: Ferry, J.G. (Ed.), Methanogen-

esis. Chapman & Hall, London, pp. 35–81.

Bottcher, M.E., Hespenheide, B., Llobet-Brossa, E., Beardsley, C.,

Larsen, O., Schramm, A., Wieland, A., Bottcher, G., Berninger,

U.-G., Amann, R., 2000. The biogeochemistry, stable isotope

geochemistry, and microbial community structure of a temperate

intertidal mudflat: an integrated study. Cont. Shelf Res. 20,

1749–1769.

Cattaneo, A., Trincardi, F., Correggiari, A., Marsset, T., Masson,

D.G., 2002. Growth of mud reliefs within the Adriatic late-

Holocene prodelta deposits through sediment load and fluid

escape processes. Mediterranean and Black Sea turbidite systems

and deep-sea fans. CIESM Worshop Series, vol. 17, pp. 33–36.

Bucharest, 5–8 June.

Cattaneo, A., Correggiari, A., Penitenti, D., Trincardi, F., Marsset,

T., 2003. Morphobathymetry of small-scale mud reliefs on the

Adriatic shelf. In: Locat, J., Mienert, J. (Eds.), Submarine Mass

Movements and Their Consequences. Kluwer Academic Pub-

lishers, Amsterdam, pp. 389–396.

Ciabatti, M., 1967. Ricerche sull’evoluzione del Delta Padano. G.

Geol. 34, 381–406.

D. Orange et al. / Marine Geology 222–223 (2005) 159–177 175

Clayppol, G.E., Kaplan, I.R., 1974. The origin and distribution of

methane in marine sediments. Natural Gases in Marine Sedi-

ments. Plenum, New York, pp. 99–139.

Conti, A., Stefanon, A., Zuppi, G.M., 2002. Gas seeps and rock

formation in the northern Adriatic Sea. Cont. Shelf Res. 22,

2333–2344.

Correggiari, A., Trincardi, F., Langone, L., Roveri, M., 2001. Styles

of failure in heavily-sedimented highstand prodelta wedges on

the Adriatic shelf. J. Sediment. Res. 71, 218–236.

Correggiari, A., Wheatcroft, R., Milligan, T., 2003. Episodic sedi-

mentation and recent clinoforms offshore Po river delta. Com-

Delta Open Conference on Comparing Mediterranean and Black

Sea Prodeltas, 26–28 October 2003, Aix-en-Provence, France. ,

pp. 21–22.

Correggiari A., Cattaneo A., Trincardi F., in press. Depositional

Patterns in the Late-Holocene Po delta system. In Bhatta-

charya J. and Giosan L. (Eds.) SEPM Special Pubblication

no. 83, pp. 365–392.

Curran, K.J., Hill, P.S., Schell, T.M., Milligan, T.G., Piper, D.J.W.,

in press. Inferring the mass fraction of floc deposited mud:

application to fine-grained turbidites.

Davide, V., Pardos, M., Diserens, J., Ugazio, G., Thomas, R.,

Dominik, J., 2003. Characterization of bed sediments and sus-

pension of the River Po (Italy) during normal and high flow

conditions. Water Res. 37, 2847–2864.

de Haas, H., van Weering, T.C.E., de Stigter, H., 2002. Organic

carbon in shelf seas: sinks or sources, processes and products.

Cont. Shelf Res. 22, 691–717.

Fannin, N.G.T., 1980. The use of regional geological surveys in the

North Sea and adjacent areas in the recognition of offshore

hazards. In: Ardus, D.A. (Ed.), Offshore Site Investigation.

Graham & Trotman, pp. 5–21.

Fonseca, L., Mayer, L., Orange, D., Driscoll, N., 2002. The high-

frequency backscattering angular response of gassy sediments:

model/data comparisons from the Eel River margin. California.

J. Acoust. Soc. Am. 111, 2621–2631.

Fox, J.M., Hill, P.S., Milligan, T.G., Boldrin, A., 2004. Floccula-

tion and sedimentation on the Po River Delta. Mar. Geol. 203,

95–107.

Friedman, G.M., Sanders, J.E., Kopaska-Merkel, D.C., 1992. Prin-

ciples of sedimentary deposits. Stratigraphy and Sedimentolo-

gyMacMillan Publishing Company, New York. 717 pp.

Gandolfi, G., Mordenti, A., Paganelli, L., 1982. Composition and

longshore dispersal of sands from the Po and ADILE Rivers

since the pre-Etruscan age. J. Sediment. Petrol. 52, 797–806.

Giordani, P., Helder, W., Koning, E., Miserocchi, S., Danovaro, R.,

Malaguti, A., 2002. Gradients of benthic-pelagic coupling and

carbon budgets in the Adriatic and northen Ionian Sea. J. Mar.

Syst. 33-34, 365–387.

Hagen, R.A., Vogt, P.R., 1999. Seasonal variability of shallow

biogenic gas in Chesapeake Bay. Mar. Geol. 158, 75–88.

Hedges, J.I., Keil, R.G., 1995. Sedimentary organic matter preser-

vation: an assessment and speculative synthesis. Mar. Chem. 49,

81–115.

Heggland, R., 1998. Gas seepage as an indicator of deeper pro-

spective reservoirs. A study based on exploration 3D seismic

data. Mar. Pet. Geol. 137, 41–47.

Hill, P.S., Milligan, T.G., Geyer, W.R., 2000. Controls on effective

settling velocity of suspended sediment in the Eel River flood

plume. Cont. Shelf Res. 20, 2095–2111.

Holland, C.W., 2002. Shallow water coupled scattering and reflec-

tion measurements. IEEE J. Oceanic Eng. 27, 454–470.

Holland, C.W., Etiope, G., Milkov, A.V., Michelozzi, E., Favali, P.,

2003. Mud volcanoes discovered offshore Sicily. Mar. Geol.

199, 1–6.

Hornafius, J.S., Quigley, D.C., Luyendyk, B.P., 1999. The world’s

most spectacular marine hydrocarbon seeps (Coal Oil Point,

Santa Barbara Channel, California): quantification of emissions.

J. Geophys. Res. 20 (C9), 20,703–20,711.

Hovland, M., Judd, A.G., 1988. Seabed pockmarks and seepages.

Impact on Geology, Biology and the Marine EnvironmentGra-

ham & Trotman Ltd., London. 293 pp.

Hovland, M., Løseth, H., Bjørkum, P.A., Wensaas, L., Arntsen, B.,

1999. Seismic detection of shallow high pore pressure zones.

Offshore, 94–96 (Dec.).

Huber, H., Huber, R., Lodermann, H.-D., Stetter, O.K., 1994.

Search for hyperthermophilic microorganisms in fluids obtained

from the KTB pump test. Sci. Drill. 4, 127–129.

IDROSER, 1983. Piano progettuale per la difesa della costa adria-

tica Emiliano-Romagnola - Vol. IV Il trasporto fluviale nei

bacini tributari dell’Adriatico. 429 pp.

Ingall, E.D., Van Cappellen, P., 1990. Relation between sedimenta-

tion rate and burial of organic phosphorus and organic carbon in

marine sediments. Geochim. Cosmochim. Acta 54, 373–386.

Iversen, N., Oremland, R.S., Klug, M.J., 1987. Big Soda Lake

(Nevada): 3. Pelagic methanogenesis and anaerobic methane

oxidation. Limnol. Oceanogr. 32, 804–814.

Jackson, D.R., Williams, K.L., Wever, T.F., Friedricks, C.T., Wright,

L.D., 1998. Sonar backscatter. J. Atmos. Ocean. Technol. 9,

630–644.

Jørgensen, B.B., 1982. Mineralization of organic matter in the sea

bed—the role of sulphate reduction. Nature 296, 643–645.

Kineke, G.C., Sternberg, R.W., 1995. Distribution of fluid muds on

the Amazon continental shelf. Mar. Geol. 125, 193–223.

Kotelnikova, S., 2002. Microbial production and oxidation of

methane in deep surface. Earth-Sci. Rev. 58, 367–395.

Kotelnikova, S.V., Macario, A.J.A., Pedersen, K., 1998. Methano-

bacterium subterraneum, a new alcalophilic, euthermic and

halotolerant methanogen isolated from deep granitic ground-

water. Int. J. Syst. Bacteriol. 48, 357–367.

Kranck, K., 1984. Grain-size characteristics of turbidites. In: Stow,

D.A.V., Piper, D.J.W. (Eds.), Fine-grained Sediment: Deep-

water Processes and Facies. Blackwell Scientific Publications,

Oxford, pp. 83–94.

Kranck, K., Milligan, T.G., 1991. Grain size in oceanography. In:

Syvitski, J.P. (Ed.), Theory, Methods and Applications of Par-

ticle Size Analysis. Cambridge University Press, New York, pp.

332–345.

Løseth, H., Wensaas, L., Arntsen, B., 2003. Gas chimneys—indi-

cating a fractured cap rock. Am. Assoc. Pet. Geol. (Abstracts

with Programs).

MacDonald, I.R., Leifer, I., Sassen, R., Stine, P., Mitchell, R., Gui-

nasso Jr., N., 2002. Transfer of hydrocarbons from natural seeps

to the water column and atmosphere. Geofluids 2, 95–107.

D. Orange et al. / Marine Geology 222–223 (2005) 159–177176

Marchetti, M., 2002. Environmental changes in the central Po Plain

(northern Italy) due to fluvial modifications and anthropogenic

activities. Geomorphology 44, 361–373.

Martens, C.S., Klump, J.V., 1984. Biochemical cycling in an

organic-rich coastal marine basin: 4. An organic carbon budget

for sediments dominated by sulfate reduction and methanogen-

esis. Geochim. Cosmochim. Acta 48, 1987–2004.

Martens, C.S., Albert, D.B., Alperin, M.J., 1998. Biogeochemical

processes controlling methane in gassy coastal sediments: Part

1. A model coupling organic matter flux to gas production,

oxidation and transport. Cont. Shelf Res. 18, 1741–1770.

Mayer, L., 1994. Surface area control of organic carbon accumula-

tion in continental shelf sediments. Geochim. Cosmochim. Acta

58 (4), 1271–1284.

Milligan, T.G., Hill, P.S., Miserocchi, S., Langone, L., 2003. The

relationship between floc dynamics, carbon, and metal transport

in the western Adriatic. ComDelta: An Open Conference on

Comparing Mediterranean and Black Sea Prodeltas, Aix-en-

Provence, France, pp. 86–87.

Milligan, T.G., Hill, P.S., Wheatcroft, R.A., Fox, J.M., 2003. Post

flood evolution of grain size in surficial sediments on the

prodelta of the Po River, European Geophysical Society. Geo-

phys. Res. Abstr. 5 (01597).

Milligan, T.G., Loring, D.H., 1997. The effect of flocculation on the

size distributions of bottom sediment in coastal inlets: implica-

tions for contaminant transport. Water Air Soil Pollut. 99, 33–42.

Misserocchi, S., Langone, L., Palinkas, C., Nittrouer, C.A., 2003.

Dispersal of riverine organic carbon along the western Adriatic

continental shelf. ComDelta: An Open Conference on Compar-

ing Mediterranean and Black Sea Prodeltas, Aix-en-Provence,

France, pp. 88–89.

Missiaen, T., Murphy, S., Loncke, L., Henriet, J.-P., 2002. Very

high-resolution seismic mapping of shallow gas in the Belgian

coastal zone. Cont. Shelf Res. 22, 2291–2301.

Morgante, S., 1940. I dbromboliT delle coste istriane. Not. Ist. Biol.Rov. 16 (2), 1–12.

Nelson, B.W., 1970. Hydrography, sediment dispersal, and recent

historical development of the Po River Delta. In: Morgan, J.P.

(Ed.), Deltaic Sedimentation: Modern and Ancient, Soc. Econ.

Paleontol. Mineral. SEPM, New York, pp. 152–184.

Oldfield, F., 1996. The PALICLAS project: synthesis and overview.

Mem. Ist. Ital. Idrobiol. dr. Marco De Marchi 55, 329–357.

Oldfield, F., Asioli, A., Juggins, S., Langone, L., Rolph, T., Trin-

cardi, F., Wolff, G., Gibbs, Z., Vigliotti, L., Frignani, M., Van

der Post, K., 2003. A high resolution Late-Holocene palaeo-

environmental record from the Adriatic Sea: core RF 93-30.

Quat. Sci. Rev. 22, 161–184.

Oremland, R.S., 1988. Biochemistry of methanogenic bacteria. In:

Zehnder, A.J.B. (Ed.), Biology of Anaerobic Microorganisms.

Wiley, New York. 641–707 pp.

Palinkas, C.M., Nittrouer, C.A., submitted for publication. Spatial

variability of sediment accumulation on the Po River shelf,

Adriatic Sea. Cont. Shelf Res.

Palinkas, C.M., Nittrouer, C.A., Wheatcroft, R.A., Langone, L.,

2005–this volume. The use of 7Be to identify event and seasonal

sedimentation near the Po River delta, Adriatic Sea. Mar. Geol.

222–223, 95–112.

Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988. Eustatic controls

on clastic deposition: I. Conceptual framework. In: Wilgus,

C.K., Hastings, B.S., Kendall, C.G.S.C., Posamentier, H.W.,

Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-level Changes: An

Integrated Approach, Soc. Econ. Paleontol. Mineral., Spec.

Publ., vol. 42, pp. 109–124.

Raiswell, R., 1988. Chemical model for the origin of minor lime-

stone shale cycles by anaerobic methane oxidation. Geology 16,

641–644.

Reeburg, W.S., Alperin, M.J., 1988. Studies on anaerobic methane

oxidation. Sci. Am. 66, 367–375.

Rice, D., 1992. Controls, habitat, and resource potential of ancient

bacterial gas. In: Vially, R. (Ed.), Bacterial Gas. Editions Tech-

nip, Paris, pp. 91–118.

Rice, D.D., 1993. dBiogenicT gas: controls, habitats, and resource

potential. In: Howell, D.G. (Ed.), The Future of Energy Gases.

US Government Printing Office, Washington, pp. 583–606.

Schoell, M., 1983. Genetic characterization of natural gases. AAPG

Bull. 67 (12), 2225–2238.

Schuler, F., 1952. Untersuchungen uber die Machtigkeit von

Schlickschichten mit Hilfe des Echographen. Dt. Hydrogr. Zei-

tung 5, 220–231.

Scully, M.E., Friedrichs, C.T., Wright, L.D., 2003. Numerical

modeling of gravity-driven sediment transport and deposition

on an energetic continental shelf: Eel River, northern Califor-

nia. J. Geophys. Res. 108 (C4), 3120–3134.

Stetter, K.O., 1992. Life at upper temperature border. In: Tran

Tranh Van, J.K., Mounlou, J.C., Schneider, C., McKay, C.

(Eds.), Frontiers of Life. Colloque Interdisciplinaire Comite

National de la reserche Scientifique, Gif-sur-Yvette, France.

195–219 pp.

Traykovski, P., Geyer, W.R., Irish, J.D., Lynch, J.F., 2000. The role

of wave-induced density-driven fluid mud flows for cross-shelf

transport on the Eel River continental shelf. Cont. Shelf Res. 20,

2113–2140.

Trincardi, F., Cattaneo, A., Asioli, A., Correggiari, A., Langone,

L., 1996. Stratigraphy of the late-Quaternary deposits in the

central Adriatic basin and the record of short-term climatic

events. Mem. Ist. Ital. Idrobiol. dr. Marco De Marchi 55,

39–70.

Trincardi, F., Cattaneo, A., Correggiari, A., 2003. Growth of the

modern Po delta and prodelta system. ComDelta: An Open

Conference on Comparing Mediterranean and Black Sea Pro-

deltas, Aix-en-Provence, France. , pp. 141–143.

Vagle, S., Farmer, D., 1992. The Measurement of Bubble-Size

Distributions by Acoustical.

Vail, P.R., Mitchum Jr., R.M., 1977. Seismic stratigraphy and global

changes of sea level. Part 1: Overview: Seismic Stratigraphy—

Applications to Hydrocarbon Exploration, AAPG Memoir, vol.

26, pp. 51–52.

Vail, P.R., Audemard, F., Bowman, S.A., Eisner, P.N., Perez-Cruz,

C., 1991. The stratigraphic signatures of tectonics, eustacy and

sedimentology. In: Einsele, G., Ricken, W., Seilacher, A.

(Eds.), Cycles and Events in Stratigraphy. Springer Verlag,

Berlin, pp. 617–659.

Vignati, D., Pardos, M., Diserens, J., Ugazio, G., Thomas, R.,

Dominick, J., 2003. Characterisation of bed sediments and

D. Orange et al. / Marine Geology 222–223 (2005) 159–177 177

suspension of the River Po (Italy) during normal and high flow

conditions. Water Res. 37, 2847–2864.

Vogel, T.M., Oremland, R.S., Kvenvolden, K.A., 1982. Low-tem-

perature formation of hydrocarbon gases in San Francisco Bay

sediment (California, U.S.A.). Chem. Geol. 37, 289–298.

Wheatcroft, R.A., Hunt, L.M., Stevens, A., Lewis, R., submitted for

publication. The distribution and internal geometry of the Octo-

ber 2000 Po River flood deposit: digital X-radiographic evi-

dence. Cont. Shelf Res.

Whelan III, T., Coleman, J.M., Suhayda, J.N., Roberts, H.H., 1978.

Acoustical penetration and sheath strength in gas-charged sedi-

ment. Mar. Geotechnol. 2, 147–159.

Whiticar, M.J., Faber, E., Schoell, M., 1986. dBiogenicT methane

formation in marine and freshwater environments: CO2 reduc-

tion vs. acetate fermentation-isotopic evidence. Geochim. Cos-

mochim. Acta 50, 693–709.

Wollast, R., 1991. The coastal organic carbon cycle: fluxes,

sources and sinks. In: Mantoura, R.F.C., Martin, J.-M., Wol-

last, R. (Eds.), Ocean Margin Processes in Global Change.

Dahlem Workshop Reports. Wiley Interscience, Chichester,

pp. 365–381.