ARTICLE IN PRESS

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doi:10.1016/j.ds

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Deep-Sea Research II 54 (2007) 1292–1311

www.elsevier.com/locate/dsr2

Nature and origin of diagenetic carbonate crusts and concretionsfrom mud volcanoes and pockmarks of the Nile deep-sea fan

(eastern Mediterranean Sea)

S. Gonthareta,�, C. Pierrea, M.-M. Blanc-Valleronb, J.M. Rouchyb, Y. Fouquetc,G. Bayonc, J.P. Foucherc, J. Woodsided, J. Masclee, The Nautinil Scientific Party

aUPMC, LOCEAN, UMR 7159, 4 Place Jussieu, 75252 Paris cedex 05, FrancebMNHN, Histoire de la terre, et CNRS UMR 5143 ‘‘Paleobiodiversite et paleoenvironnements’’, CP 18, 57 rue Cuvier,

75231 Paris cedex 05, FrancecIFREMER, Centre de Brest, Geosciences Marines, Plouzane 29280, France

dVrije Universiteit Amsterdam, De Boelelaan, 1085, 1081 HV Amsterdam, The NetherlandseGeosciences-Azur, UMR 6526, Observatoire de Villefranche-sur-mer, BP 48, 06235 Villefranche/mer, France

Accepted 20 April 2007

Available online 12 July 2007

Abstract

During the NAUTINIL cruise (September–October 2003), mud volcanoes and pockmarks located in four selected areas

of the Nile deep-sea fan (caldera, central, eastern, North Alex) were investigated at water depths ranging from 500 to

3019m. Authigenic carbonate crusts were observed directly by a submersible in each of these fluid-venting areas, in close

association with specific chemosynthetic biological communities. Authigenic carbonates occur typically as pavements,

slabs and mounds on the seafloor, but are also present as millimeter- to centimeter-size concretions dispersed within

sediments. Mineralogical analyses of carbonate crusts and concretions indicate that aragonite and high-Mg calcite

represent the most dominant carbonate phases. Low-Mg calcite, dolomite and ankerite also occur as minor components.

Petrographic observations of carbonate crusts and concretions show that they are composed mainly of microcrystalline

carbonate cement, with minor amounts of detrital minerals, lithoclasts and bioclasts. Aragonite is present as

microcrystalline cement or acicular crystals infilling bioclasts and voids. Pyrite occurs as framboids or cubic crystals,

which are often associated with authigenic carbonates, thereby indicating that sulfate reduction was active during

carbonate precipitation. Numerous millimeter- to centimeter-size euhedral gypsum crystals have been observed within

carbonate crusts and concretions, and as isolated crystals in sediments recovered from the eastern province. In this area,

precipitation of gypsum is related to the presence of rising sulfate-rich fluids, which originate from the dissolution of

underlying Messinian evaporites. Millimeter-size barite concretions have also been discovered in sediment from the central

province and precipitated from ascending fluids, which are enriched in barium due to the dissolution of biogenic and/or

authigenic barite below the depth of sulfate depletion. The oxygen and carbon isotopic compositions of the carbonates

display very large ranges, from �0.67% to 4.15% Vienna PeeDee Belemnite (V-PDB) and from �42.14% to 3.10%V-PDB, respectively. Most carbonates exhibit d18O values around 3%, indicating that they have precipitated in isotopic

equilibrium with bottom seawater. In contrast, two carbonate concretions from the caldera and the eastern areas are

e front matter r 2007 Elsevier Ltd. All rights reserved.

r2.2007.04.007

ng author. Tel.: +33 1 44 27 84 79; fax: +33 1 44 27 71 59.

esses: sglod@locean-ipsl.upmc.fr (S. Gontharet), Catherine.Pierre@locean-ipsl.upmc.fr (C. Pierre), valleron@mnhn.fr

alleron), rouchy@mnhn.fr (J.M. Rouchy), fouquet@ifremer.fr (Y. Fouquet), germain.bayon@ifremer.fr (G. Bayon),

r@ifremer.fr (J.P. Foucher), john.woodside@falw.vu.nl (J. Woodside), mascle@geoazur.obs-vlfr.fr (J. Mascle).

ARTICLE IN PRESSS. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1293

characterized by lower d18O values (�0.67% V-PDB and 0.98% V-PDB), which may reflect a contribution from 18O-poor

(continental?) water or, most likely, a local high heat flow. A few carbonate crusts exhibit slightly positive d13C values,

which indicate that seawater was the main source of carbon for those carbonates. Authigenic carbonates are typically

depleted in 13C, revealing that the major carbon source for those carbonates derives from anaerobic oxidation of methane

driven by microbial consortia of archaea and sulfate reducing bacteria.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Nile deep-sea fan; Cold seeps; Mud volcanoes; Pockmarks; Diagenetic carbonates; Oxygen and carbon stable isotopes

1. Introduction

Mud volcanoes and pockmarks represent com-mon superficial topographic features of cold seepenvironments created by ascent of mud and over-pressured pore fluids from deeper sedimentarylayers to the seafloor (Hovland and Judd, 1988;Brown, 1990; Dimitrov, 2002). They are knownalong convergent plate boundaries on active con-tinental margins as well as on passive continentalshelves where over-pressure in sediments and/or salttectonism control the fluid emissions (e.g., Carsonet al., 1994; Limonov et al., 1996; Milkov, 2000;Huguen, 2001; Dimitrov, 2002; Bohrmann et al.,2002; Loncke and Mascle, 2004; Huguen et al.,2004). Methane brought in large quantities bythese fluids (Charlou et al., 2003; Haese et al.,2003) is produced from microbial methanogenesisand/or thermochemical evolution of organic matter

Fig. 1. Location map showing the different areas of the Nile deep-sea f

carbonate crusts and sediments were collected during the NAUTINIL

(Gornitz and Fung, 1994); methane may escapedirectly as gas bubbles at the seafloor (Limonovet al., 1996) or be stored as gas hydrates in sedi-ments whenever pressure and temperature condi-tions favor their stability (Gornitz and Fung, 1994;Dickens et al., 1995; Ivanov et al., 1996; Vogtet al., 1997; Kvenvolden, 1998; Ginsburg et al., 1999).

At cold seeps, fluid and gas vents supportcommonly specific benthic communities of molluscs(bivalves, gastropods) and tubeworms, which relyon chemosynthetic energy for their metabolism(MacDonald et al., 1989; Fiala-Medioni andFelbeck, 1990; Aharon, 1994; Carney, 1994; Sibuetand Olu, 1998; Van Dover et al., 2003; Olu-Le Roiet al., 2004; Duperron et al., 2005), and microbialassociations of archaea and bacteria (Pancostet al., 2000; Boetius et al., 2000; Michaelis et al.,2002). Authigenic carbonates occur typically ascrust pavements, slabs, mounds and concretions,

an (caldera, central, eastern, North Alex) and the dive sites where

cruise.

ARTICLE IN PRESSS. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–13111294

both on the seafloor and within sediments. They arerelated to the microbial anaerobic oxidation ofmethane (AOM) on the basis of carbon isotopecomposition and organic geochemistry (Ritgeret al., 1987; Hovland et al., 1987; Jørgensen, 1992;Paull et al., 1992; Roberts and Aharon, 1994; VonRad et al., 1996; Bohrmann et al., 1998; Stakeset al., 1999; Peckmann et al., 1999, 2001; Aloisi,2000; Aloisi et al., 2000, 2002; Greinert et al., 2001;Campbell et al., 2002; Peckmann and Thiel, 2004;Stadnitskaia et al., 2005). Previous investigations onthe mineralogy and geochemistry of carbonatecrusts from mud volcanoes of the eastern Mediter-ranean Sea have been made on samples collectedduring MEDINAUT (November 1998) and ME-DINETH (July 1999) cruises in the areas of theMediterranean Ridge (Olimpi area) and the Anaxi-mander Mountains. In these settings, carbonatecrusts are composed of a mixture of aragonite,magnesian calcite and dolomite, exhibiting negatived13C values, which indicate clearly that carbonateprecipitation is closely related to AOM (Aloisi,2000; Aloisi et al., 2000).

In September–October 2003, during the NAUTI-NIL cruise on the R.V. l’Atalante, 19 dives with thesubmersible Nautile were made on selected mudvolcanoes, pockmarks and gas chimneys from theNile deep-sea fan (Loncke and Mascle, 2004)(Fig. 1): (1) the caldera area (2875–3032m waterdepth) located in the Western Nile margin domain,(2) the central area (1683–2132m water depth)located on the lower and middle slope of the centralprovince, (3) the eastern area (991–1122m waterdepth), and (4) the North Alex gas chimney, whichlies in 500m water depth on the upper slope of thecentral province. This work presents and commentson the petrographic textures, the mineralogy andthe stable isotope geochemistry of carbonate crustsand concretions collected during 12 dives. On thebasis of oxygen and carbon isotopic compositions,the possible sources of water and carbon from whichcarbonates have been precipitated are discussed.

2. Geological setting

The Nile deep-sea fan is the most importantPliocene–Quaternary sedimentary clastic accumula-tion in the Mediterranean Sea; it covers a passivemargin segment implicated in the subduction/collision of African and Eurasian plates along theHellenic and Cyprus arcs (Sage and Letouzey, 1990;Chaumillon, 1995; Huguen, 2001). The sedimentary

sequence is mainly composed of terrigenous sedi-ments delivered from the Nile River (Salem, 1976)and deposited in part on thick Messinian evaporitelayers. The total sedimentary thickness of themargin including the Pliocene to Quaternary coverand the underlying Messinian evaporites reaches9–10 km (Mascle et al., 2003; Dolson et al., 2005).Due to over-thickening, which results from down-slope gliding (Ryan et al., 1973; Sage and Letouzey,1990; Loncke, 2002; L. Camera, personal commu-nication), the ductile evaporite layer may locallyreach up to 3 km at the base of the lower slope.

The interactions between progressively glidingMessinian evaporites and various sedimentary dis-tribution mechanisms (turbidite dispersal, giantslumps, debris flows) have strongly controlled themorphology of the Nile deep-sea fan (Gaullier et al.,2000; Loncke, 2002). Using swath bathymetry,acoustic imagery and seismic profiling recordedduring Prismed II (January 1998) and Fanil(October 2000) cruises (Bellaiche et al., 1999, 2001;Mascle et al., 2001; Loncke and Mascle, 2004), fourmain provinces (western, central, eastern andLevantine) were identified on the basis of morpho-logy and their controlling sedimentary and tectonicprocesses (Mascle et al., 2005).

The deformation associated with the ductileMessinian evaporite layers at depth within the Niledeep-sea fan has generated significant overpressurein sediments and triggered salt-related diapirism,leading to fluid migrations towards the seafloor(Loncke and Mascle, 2004). Fluid-releasing struc-tures are quite common on the Nile continentalmargin and characterized by different seafloormorphologies (Loncke and Mascle, 2004): sub-circular and conical mud domes (up to few hundredmeters in diameter and 60m high), caldera-likedepressions (up to 7 km in diameter) in the Westerndomain, broad gas chimneys (up to 5 km indiameter) in the eastern province, pockmarks andmounds chiefly in the central but also in the easterndomains.

3. Formation of authigenic minerals

In marine sediments, authigenic carbonates pre-cipitate when pore fluids are supersaturated withrespect to the carbonate phase and kinetics factorsfavor their precipitation (Burton, 1993). Carbonatealkalinity is mainly affected by degradation oforganic matter (Presley and Kaplan, 1968). Bacte-rial sulfate reduction represents the primary process

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increasing the carbonate alkalinity and triggeringcarbonate precipitation in anoxic environments(Berner et al., 1970):

2CH2Oþ SO2�4 ! S2� þ 2CO2 þ 2H2O

S2� þ 2CO2 þ 2H2O! H2Sþ 2HCO�3

Moreover, in cold seep environments, abundantquantities of methane are brought to the seafloor byascending fluids. The methane is mainly oxidizedanaerobically at the sulfate–methane boundary inthe sediments as evidenced by the vertical distribu-tions of methane and sulfate concentrations (Ree-burgh, 1980; Iversen and Jørgensen, 1985; Blair andAller, 1995; Haese et al., 2003; Joye et al., 2004).The AOM promotes an electron intra-speciestransfer between methane and sulfate (Reeburgh,1980; Ritger et al., 1987; Hoehler et al., 1994;Boetius et al., 2000):

CH4 þ SO2�4 ! HCO�3 þHS� þH2O

This biogeochemical reaction is mediated by diversemicrobial consortia (Hinrichs et al., 1999, 2000;Pancost et al., 2000; Elvert et al., 2000; Valentineand Reeburgh, 2000; Boetius et al., 2000; Orphanet al., 2001; Michaelis et al., 2002; Wakeham et al.,2003) of sulfate reducing bacteria utilizing H2 oracetate and methanotrophic archaea capable ofreversing their metabolism (Hoehler et al., 1994). Asignificant portion of bicarbonate ions produced byAOM increases carbonate alkalinity and precipi-tates as authigenic carbonate within the sediment(Ritger et al., 1987; Hovland et al., 1987; Jørgensen,1992; Paull et al., 1992; Roberts and Aharon, 1994;Von Rad et al., 1996; Bohrmann et al., 1998; Stakeset al., 1999; Peckmann et al., 1999, 2001; Aloisi,2000; Aloisi et al., 2000, 2002; Greinert et al., 2001;Campbell et al., 2002; Peckmann and Thiel, 2004;Stadnitskaia et al., 2005).

Other authigenic minerals are also found in coldseep environments and provide diagnostic informa-tion on the chemistry of the diagenetic fluids. Pyriteis often associated with authigenic carbonatesdemonstrating that these minerals are by-productsof sulfate reduction and methane oxidation. Pyriteformation results from the reaction of hydrogensulfide, produced by bacterial sulfate reduction,with reactive iron minerals (Berner, 1984). Gypsummay precipitate from rising sulfate-rich brinesrelated to the presence of Messinian evaporites atdepth, which can provide either relic formationwaters or be dissolved by diagenetic waters as those

produced by clay mineral dewatering (Corselli andAghib, 1987). Barite precipitation may also occur incold seeps when rising barium-rich fluids react withsulfate-rich, downwards-diffusing seawater or as-cending brines (Torres et al., 2003; Aloisi et al.,2004; Castellini et al., 2006). Barium dissolved incold seep fluids is generally thought to originatefrom dissolution of biogenic and/or authigenicbarite below the depth of sulfate depletion in marinesediments (Torres et al., 1996; McManus et al.,1998).

4. Methods

4.1. Mineralogy

The total carbonate content (in wt%) of bulksediments was determined by reaction of 100mg offine powdered sediment with 0.4 cm3 of HCl 8N.The absolute error is 1%. All samples were analyzedby X-ray diffraction (XRD) to determine thequalitative and semi-quantitative mineralogicalcomposition of the carbonate fraction. Mineralidentification was performed on dried and groundsub-samples using a Siemens D-500 diffractometer(Cu Ka, Ni-filtered, radiation) scanning 21–641 2y ata rate of 0.021 2y per second. The position ofcarbonate peaks was corrected by reference to themain quartz peak present in almost all samples.Semi-quantitative estimations (in wt%) of thedifferent carbonate minerals were obtained usingthe peak areas in combination with total carbonatecontent. The relative error on the weight percentof a given carbonate phase is roughly estimatedat 75%.

The petrography of selected carbonate crusts wasstudied in petrographic thin section. The morpho-logy, microstructure and elemental composition ofauthigenic minerals were examined with a scanningelectron microscope (SEM) coupled to an elementalanalysis unit.

4.2. Oxygen and carbon isotopes

The oxygen and carbon isotope compositionswere measured on powdered samples by reactionwith 100% phosphoric acid at 25 1C under vacuum,following the standard procedure of McCrea (1950).As the time of reaction depends on the carbonatemineral composition, reaction lasted one day forsamples containing calcium carbonate (aragonite,calcite). For samples containing a mixture of

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calcium carbonate and dolomite, the CO2 releasedafter 20min of reaction was considered to comesolely from aragonite and/or calcite (Clayton et al.,1968). The released CO2 was analyzed using a dual-inlet VG-Sira 9 triple-collector mass spectrometer.The isotopic compositions are expressed in theconventional d notation relative to the ViennaPeeDee Belemnite (V-PDB) reference (Craig,1957): d ¼ [(RS/RR)�1]� 1000, where R ¼ 18O/16Oor 13C/12C in the sample (RS) and in the reference(RR). Carrara Marble was used as an in-housestandard (d18O ¼ �1.94% V-PDB and d13C ¼2.00% V-PDB), which was calibrated against theIAEA references (NBS19 and NBS20). The analy-tical precision 2s is 0.01% for both d18O and d13C;the reproducibility for replicate analyses is betterthan 0.1% for d18O and d13C.

5. Results and discussion

5.1. Petrography and mineralogy

5.1.1. Mineral composition of carbonate crusts and

concretions

Carbonate crusts and concretions are mainlycomposed of carbonates (from 75 to 94wt% ofthe bulk sediment) associated with detrital minerals(clays, quartz, feldspars, etc.) and minor authigenicphases (mostly barite, gypsum and pyrite). Insurrounding sediments, the carbonate fraction ismuch lower, varying between 7 and 37wt% of bulksediment (Table 1).

Aragonite and calcite represent the dominantcarbonate phases, associated with minor quantitiesof dolomite (Fig. 2). The XRD patterns ofcarbonate crusts and concretions often show amixture of low-Mg calcite and high-Mg calcite.Fig. 3 illustrates the frequency with which calciteand dolomite occur in carbonate samples versus thed(1 0 4) values of calcite and dolomite. The d(1 0 4)values of calcite vary in the range from 3.307 to2.982 A: a low-Mg calcite group, mostly from calciteof pelagic or detrital sediment, is predominantbetween 3.035 and 3.030 A; a high-Mg calcite group,probably authigenic, predominates in the rangefrom 3.000 to 2.995 A. Two groups of dolomite areidentified: one group characterized by d(1 0 4) valuesbetween 2.900 and 2.882 A is considered to be nearstoichiometric dolomite; the second group withd(1 0 4) values between 2.936 and 2.925 A corre-sponds to dolomites in which some Mg2+ aresubstituted by larger cations as Fe2+ or Ca2+.

The position of d(1 0 4) peaks for calcite anddolomite depends on the incorporation of Mg2+,Ca2+, Mn2+ and Fe2+ in the crystal lattice. Usingthe relationship established by Goldsmith et al.(1961) between the d(1 0 4) value for carbonatephases and their proportion of MgCO3 (in mol%),high-Mg calcite and Fe/Ca-rich dolomite from thecarbonate crusts of the Nile deep-sea fan areestimated to contain 6–18mol% MgCO3, and32–36mol% MgCO3, respectively.

5.1.2. Petrographic characteristics and mineralogy of

carbonate crusts and concretions

5.1.2.1. Caldera Area. Most of the carbonate crustsform either centimeter-thick massive pavementsand mounds or very porous pavements and slabs(Fig. 4A and B), with pore diameters varying from afew millimeters to a few centimeters. Their uppersurface is coated by a dark yellow to dark brownoxide layer and colonized by benthic organisms(tubeworms, bivalves). Numerous sinuous tubularvoids, generally under 1 cm in diameter, crosscarbonate crusts and result from carbonate pre-cipitation around tubeworms which originallycolonized the underlying unconsolidated sediment(Naehr et al., 2000; Aloisi et al., 2000). Millimeter-to centimeter-size sub-rounded lithoclasts are oftenpresent in the matrix of carbonate crusts, whichindicates that these crusts derive from the lithifica-tion of mud breccia (Aloisi et al., 2000). Thin-section and SEM observations reveal that carbonatecrusts are formed by a mixture of detrital andbiogenic components cemented by a microcrystal-line carbonate matrix (Fig. 5A). XRD analysesshow that cements are composed of aragonite and/or high-Mg calcite (Table 1). Acicular aragonitecrystals, generally under 20 mm in length, havegrown radially around bioclasts and infilled veinsand voids (Fig. 6A). Aragonite may form palissadicstructures as in NL20CC2 carbonate crust (Fig. 5B).High-Mg calcite is the predominant carbonatephase in two more friable carbonate crusts(NL5CC1, NL5CC2) where microsparitic cementincludes numerous detrital minerals as quartz andfeldspars (Fig. 5C). Low-Mg calcite and dolomiteare often present as minor components in carbonatecrusts. Pyrite occurs either as grains (less than 10 mmin diameter), cubic crystals or framboids, whichprovides evidence for bacterial sulfate reduction.Biogenic components of pelagic sediments cementedby authigenic carbonates include coccoliths, bolbo-forma, planktonic foraminifers, pteropods, bivalve

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Table 1

Mineralogical and isotopic composition of carbonate crusts, concretions (ccr.) and sediments (sed.) collected in different areas of the Nile deep-sea fan

Sample Facies Province Site MCMa

(wt%)

Aragonite

(wt%)

Low-Mg

calcite

(wt%)

High-Mg

calcite

(wt%)

Dolomite

(wt%)

d18O CaCO3

(% V-PDB)

d13C CaCO3

(% V-PDB)

NL3CC1A Concretion Caldera Cheops mud volcano 84 29 21 31 3 2.40 3.10

NL4BC1 ccr Concretion Caldera Mykerinos mud volcano 79 70 7 1 1 2.77 �38.23

NL4BC1 sed Sediment Caldera Mykerinos mud volcano 37 5 25 5 2 0.60 �12.52

NL4PC4 ccr Concretion Caldera Mykerinos mud volcano 83 13 55 9 6 0.98 �13.45

NL4CC1 Carbonate crust Caldera Mykerinos mud volcano 77 72 4 0 1 2.79 �38.86

NL5CC1 Concretion Caldera Mud dome nearby the

caldera

78 4 0 73 3 3.97 2.93

NL5CC2 Carbonate crust Caldera Mud dome nearby the

caldera

79 18 18 39 3 2.46 �14.24

NL5PC4 ccr Concretion Caldera Mud dome nearby the

caldera

52 4 33 14 1 0.98 �13.45

NL16CC1 Carbonate crust Caldera Chefren mud volcano 83 81 2 0 0 2.68 �41.08

NL16CC2 Carbonate crust Caldera Chefren mud volcano 89 85 3 1 0 2.73 �34.84

NL20CC1 Carbonate crust Caldera Cheops mud volcano 88 82 3 2 0 3.07 �38.37

NL20CC2 Carbonate crust Caldera Cheops mud volcano 90 71 8 11 0 2.88 �29.83

NL6CC1 Carbonate crust Central Lower slope 83 42 15 20 5 2.36 �28.41

NL6CC2 Carbonate crust Central Lower slope 85 55 13 15 2 3.11 �27.05

NL6CC2 ccr Concretion Central Lower slope 82 0 16 65 1 3.48 �37.10

NL6CC3 ccr Concretion Central Lower slope 86 0 0 85 1 3.62 �37.14

NL6CC5 sed

(5 cm)

Sediment Central Lower slope 37 4 15 18 0 2.50 �29.18

NL6CC5 sed

(�20 cm)

Sediment Central Lower slope 30 0 11 18 2 2.46 �34.46

NL7CC1 Carbonate crust Central Lower slope 80 0 0 51 27 4.15 �41.78

NL7CC2 (1)

0–1.2 cm

Carbonate crust Central Middle slope 95 96 3 1 0 2.82 �28.04

NL7CC2 (1)

1.2–2.4 cm

Carbonate crust Central Middle slope 92 95 3 1 0 3.00 �37.69

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STable 1 (continued )

Sample Facies Province Site MCMa

(wt%)

Aragonite

(wt%)

Low-Mg

calcite

(wt%)

High-Mg

calcite

(wt%)

Dolomite

(wt%)

d18O CaCO3

(% V-PDB)

d13C CaCO3

(% V-PDB)

NL7CC2 (1)

2.4–3.6 cm

Carbonate crust Central Middle slope 89 91 3 5 1 3.12 �41.74

NL7CC2 (1)

3.6–4.8 cm

Carbonate crust Central Middle slope 89 85 5 7 2 3.20 �42.66

NL7CC2 (1)

4.8–6 cm

Carbonate crust Central Middle slope 83 80 4 10 5 3.37 �44.17

NL7CC2 (2) Carbonate crust Central Middle slope 90 72 5 13 1 2.64 �33.02

NL14CC1 Carbonate crust Central Lower slope 75 4 26 43 2 2.85 �18.72

NL14CC2 Carbonate crust Central Lower slope 81 51 14 15 1 3.04 �31.85

NL14CC3 Carbonate crust Central Lower slope 83 55 13 13 2 3.05 �31.45

NL14CC4 Carbonate crust Central Lower slope 86 75 8 2 2 3.09 �42.14

NL14CC5 Carbonate crust Central Lower slope 90 70 8 12 0 3.17 �29.22

NL11CC2 Carbonate crust Eastern Amon mud volcano 94 78 2 13 1 3.35 �37.83

NL11BC1

sed (0–2 cm)

Sediment Eastern Amon mud volcano 7 0 5 0 1 �2.67 �10.48

NL11BC1

ccr (0–2 cm)

Concretion Eastern Amon mud volcano 84 0 0 83 1 �0.67 �17.69

NL11BC1

sed (2–4 cm)

Sediment Eastern Amon mud volcano 7 0 5 0 2 �1.99 �9.00

NL15CC1 Concretion North Alex North Alex gas chimney 76 74 0 2 0 3.25 �26.02

aMeasurements directly read on the Melieres manocalcinometer.

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100 %

Calcite

Aragonite

100 %

100 %

Dolomite

Caldera area Central area

Eastern area North Alex area

Fig. 2. Composition of the carbonate fraction in authigenic

carbonate crusts and concretions from the Nile deep-sea fan cold

seeps.

Fig. 3. Distribution of d(1 0 4) values of calcite and dolomite in

carbonate crusts and concretions from the Nile deep-sea fan cold

seeps.

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1299

shells and ostracods. In the upper surface ofNL5CC1 carbonate crust, the occurrence of ovoidand elongated filament structures resembling bac-terial colonies (Fig. 6D) suggests the implication ofmicroorganisms in carbonate precipitation. How-ever, these structures might have been developedafter sampling since samples were not stored understerile conditions.

Unconsolidated sediments from seep areas con-tain pyrite concretions derived from the pyritizationof elongated tubes, and carbonate concretionscomposed of a mixture of microcrystalline carbo-nate and pyrite cementing detrital grains (Fig. 7C).

The XRD analyses of concretions found in thetopmost sediments of NL4BC1 and NL5PC4indicate that they are mainly composed of aragoniteand low-Mg calcite, respectively. The presence ofsulfate, high Mg/Ca ratios and temperature higherthan 6 1C in the solutions are considered to favorthe precipitation of aragonite over calcite (Folk,1974; Walter, 1986; Ritger et al., 1987; Hovlandet al., 1987; Matsumoto, 1990; Burton, 1993; Savardet al., 1996; Morse et al., 1997; Aloisi et al., 2000;Naehr et al., 2000; Greinert et al., 2001). High Mg/Caratios and high sulfate concentrations in ascendingbrines (Charlou et al., 2003) explain the predomi-nance of aragonite in carbonate crusts formedin contact with bottom Mediterranean seawater(T ¼ 13 1C).

5.1.2.2. Central Area. Massive pavements (up to8 cm in thickness) and porous slabs (up to 3 cm inthickness) of carbonate crusts (Fig. 4C) associatedwith tubeworms and small bivalve shells dominatethe central area. Millimeter- to centimeter-sizeindurated burrows filled with fine-grained graysediment (NL7CC1) (Fig. 4D) were found insediments from the summit of an active pockmark.In two massive carbonate crusts (NL14CC1,NL14CC2), numerous voids (up to 5 cm in dia-meter) are probably due to carbonate cementationaround tubeworms. Optical microscopy and SEMobservations show that the general texture of thesesamples is a fossiliferous micrite (Fig. 5D). Asrevealed by XRD analyses, aragonite is the mostabundant authigenic carbonate phase of the micro-crystalline cement (Fig. 6B).

Vertical variations of the carbonate compositionof the NL7CC2 (1) crust show that aragonite is thepredominant carbonate in the upper 6 cm. Arago-nite occurs as large needles (350 mm in length and10 mm wide) filling veins or cavities (up to 800 mm indiameter) (Fig. 6C). High-Mg calcite and dolomitecontents increase from 0 to 7wt% and from 0 to5wt% between 0 and 6 cm depth, respectively(Fig. 8). During the downward growth of NL7CC2crust (Bayon et al., 2006), pore-waters in theunderlying sediments became more and moredepleted in sulfate due both to the decrease ofSO4

2� diffusive flux from seawater and to thebacterial sulfate reduction, which should favor theprecipitation of high-Mg calcite and dolomite(Walter, 1986). Alternatively, the increase of high-Mg calcite content could partly result from recrys-tallization of aragonite. Commonly, pyrite is

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Fig. 4. NAUTINIL dive observations of in situ carbonate crusts and concretions on mud volcanoes and pockmarks (A, C, E) located in

selected parts of the Nile deep-sea fan and their macroscopic structure (B, D, F). (A) Pavement of carbonate crust (NL5) associated with

whitish microbial mats in the caldera area. (B) Small porous slabs of carbonate crusts (NL4CC1) collected in the caldera area. (C) Slabs of

carbonate crust (NL6) associated with tubeworms and bivalve shells in the central area. (D) Indurated burrows (NL7CC1) collected at the

summit of an active pockmark in the central area. (E) Small mound of carbonate crusts (NL11CC2) collected from Amon mud volcano

located in the eastern area. (F) Fossilized burrows (NL15CC1) from North Alex gas chimney.

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–13111300

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Fig. 5. Thin-section photomicrographs of carbonate crusts. (A) Fossiliferous micrite with well-preserved bioclasts and detrital minerals

cemented by a fine-grained micrite (natural light, caldera area, NL4CC1). (B) Microcrystalline carbonates associated with large acicular

aragonite crystals (natural light, caldera area, NL20CC2). (C) Microsparite including numerous detrital minerals (natural light, caldera

area, NL5CC2). (D) Fossiliferous micrite including bioclasts and detrital minerals (natural light, central area, NL6CC2).

(E) Microcrystalline carbonates associated with large areas of acicular aragonite crystals (natural light, eastern area, NL11CC2).

(F) Aragonite needles growing radially around a bioclast (polarized light, eastern area, NL11CC2).

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1301

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Fig. 6. SEM photographs of authigenic carbonate crusts (back-scattered electrons: C; secondary electrons: A, B, D, E, F).

(A) Microcrystalline aragonite crystals (caldera area, NL4CC1). (B) Micrite composed of microcrystalline crystals of aragonite (central

area, NL14CC3). (C) Aragonite needles infilling a void (central area, NL7CC2 (2)). (D) Organic filament structures resembling bacteria

(caldera area, surface of NL5CC2 crust). (E) Pyrite framboid trapped in organic filaments (central area, NL7CC2 (2)). (F) Aggregate of

copper sulfide crystals (middle part of the NL7CC2 (2) crust from the central area).

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–13111302

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Fig. 7. SEM photographs of authigenic carbonate micro-concretions and other authigenic minerals (back-scattered electrons: A, B, C, F;

secondary electrons: D, E). (A) Microcrystalline crystals of ankerite associated with microlithoclasts and euhedral cubic pyrite (eastern

area, NL9BC1 (2–4 cm) ccr). (B) Micro-concretion formed by authigenic microcrystalline crystals of calcite and tabular barite crystals

(central area, NL6PC4ccr). (C) Micrite composed of microcrystalline aragonite needles and pyrite cementing detrital minerals (caldera

area, NL4BC1 ccr). (D) Microcrystalline calcite crystals (central area, NL6PC2 ccr). (E) Microsparite mainly composed of ankerite crystals

sometimes gathered in rosette structure (eastern area, NL9BC1 (0–2 cm) ccr). (F) Individual elongated gypsum crystal (eastern area,

NL9BC1 (0–2 cm) ccr).

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1303

ARTICLE IN PRESS

-40 -35 -30 -25

δδ13C ‰ V-PDB

δ18O ‰ V-PDB

2.8 2.9 3.0 3.1 3.2 3.3 3.4

Dep

th (

cm)

-45

δ13C

δ18O

0

0.6

1.2

1.8

2.4

3.0

4.2

3.6

4.8

0.6

1.2

1.8

2.4

3.0

4.2

3.6

4.8

1009080706050403020100

Dep

th (

cm)

0

Relative weight %

Aragonite Low-Mg calcite

DolomiteHigh-Mg calcite

Fig. 8. Variations of carbonate mineralogy and stable isotope compositions of bulk carbonate through NL7CC2 (1) vertical transect.

S. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–13111304

associated with aragonite crystals as isolated grains(around 0.2 mm in diameter) sometimes gatheredin framboid structures (14–15 mm in diameter)(Fig. 6E). The association between authigeniccarbonates and pyrite has already been observedin fossil seeps (Campbell et al., 2002; Peckmann andThiel, 2004) and modern seeps (Ritger et al., 1987;Hovland et al., 1987; Stakes et al., 1999; Aloisi et al.,2000). The occurrence of pyrite indicates thatcarbonate cementation occurred under anoxic con-ditions as the result of methane oxidation in thesulfate reduction zone. In the middle part ofNL7CC2 (2) crust, aggregates of authigenic coppersulfide crystals are also present (Fig. 6F).

In NL14CC1 carbonate crust and in the indu-rated burrows (NL7CC1), high-Mg calcite repre-sents the dominant carbonate phase associated withlow-Mg calcite in the crust, and with dolomite ininfilled burrows.

The unconsolidated sediments contain numerouswhitish millimetric concretions where carbonate isassociated with authigenic barite forming tabularcrystals (Fig. 7B). The carbonate cement of theseconcretions is mostly composed of low-Mg calciteor high-Mg calcite (Fig. 7D). The precipitation ofbarite generally occurs at the boundary betweenoxic and anoxic conditions.

5.1.2.3. Eastern Area. The massive carbonate crust(NL11CC2) from a few meters high mound(Fig. 4E) exhibits pores, generally under 5mm indiameter, which are homogenously distributed. The

surface of this crust is coated by a black layer ofmanganese oxides and pitted by tubeworms. In athin section, large acicular crystals of aragonite areassociated with a compact micrite (Fig. 5E) includ-ing rare detrital minerals and bioclasts (bolboforma,planktonic foraminifers, ostracods and shell frag-ments) (Fig. 5F). Aragonite is the dominantcarbonate phase in this crust, whereas high-Mgcalcite, low-Mg calcite and dolomite representminor components.

In sediments collected from Isis (NL8BC1,NL9BC1) and Amon mud volcanoes (NL11 BC1),between 0 and 4 cm depth, millimeter-size concre-tions are composed of lithoclasts and euhedral cubicpyrite cemented by a microcrystalline carbonatematrix (Fig. 7A). High-Mg calcite is the majorcarbonate component of the carbonate concretionfound in the uppermost 2 cm, whereas low-Mgcalcite is predominant in surrounding sediments.SEM observations reveal that ankerite is alsopresent as lens-shaped crystals sometimes gatheredin rosette structure (Fig. 7E). Numerous millimeter-to centimeter-size gypsum crystals have been foundin sediments from Isis and Amon mud volcanoes.They occur as elongated twinned crystals withsmoothed faces probably due to etching by bacterialsulfate reduction; some lacunas in gypsum crystalsprobably result from a loss of inclusion or a crystalgrowth default. Their euhedral shape and theoccurrence of included biogenic and clastic compo-nents (Fig. 7F) demonstrate that they are authigenicprecipitates from sulfate-rich fluids.

ARTICLE IN PRESSS. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1305

5.1.2.4. North Alex gas chimney. The carbonateconcretions of a few centimeters length correspondto fossilized burrows or filled conduits (Fig. 4F).They have a homogeneous medium-gray color atthe surface and light gray internal color. Micro-sparite is composed of very thin elongated aragonitecrystals associated with minor quantities of high-Mg calcite as shown by SEM observations com-bined with XRD analyses. Detrital carbonate andsilicate minerals are included in the carbonatecement. Rare occurrence of ilmenite crystals, amineral associated with eruptive rocks, is related tothe detrital input from the Nile River.

5.2. Stable isotope geochemistry

Stable carbon and oxygen isotopic compositionsof bulk carbonate from carbonate crusts, concre-tions and sediments are presented in Table 1 andFig. 9.

5.2.1. Oxygen isotopic compositions of carbonate

crusts and concretions

Oxygen isotopic compositions of carbonate crustsfrom the Nile deep-sea fan vary between 2.36% and3.97%. In the caldera sector, the d18O values rangefrom 2.40% to 3.97% and from 2.36% to 3.37% inthe central sector. The carbonate crust from theAmon mud volcano has a d18O value of 3.35%.Moreover, the d18O values of NL7CC2 (1) crustfrom the central area increase from 2.82% to 3.37%between 0 and 6 cm depth (Fig. 8) as expected by theincrease of dolomite content, a 18O rich-carbonatedue to different oxygen isotopic fractionation

-1

0

1

2

3

4

5

6

-45 -35 -25 -15

δδ13C ‰ V-PDB

δ18O

‰ V

-PD

B

Crust Concretion

Caldera area Pockmark area

Eastern area North Alex area

Fig. 9. Carbon and oxygen isotopic compositions of carbonate crusts

deep-sea fan. The dotted lines represent the d18O value of calcium

precipitating in isotopic equilibrium with present-day Mediterranean b

during precipitation (Friedman and O’Neil, 1977;Land, 1985). In carbonate concretions (ccr), thed18O values vary generally from 2.77% to 4.15%and are close to the d18O values measured in crusts.However, NL11BC1 (0–2 cm) ccr from the easternarea (Amon mud volcano) and NL5PC4 ccr fromthe caldera area show lower d18O values, whichare, respectively, �0.67% and of 0.98%. Theoxygen isotopic compositions of carbonate fromthe surrounding unconsolidated sediments (sed) are0.60% (NL4BC1 sed, caldera area) and �2.67%(NL11BC1 (0–2 cm) sed, eastern area).

The oxygen isotope composition of authigeniccarbonates is controlled by a combination of factorsincluding the temperature of formation, the carbo-nate mineralogy and the oxygen isotopic composi-tion of pore fluids. Aragonite and calcite with 12%MgCO3 content precipitated in isotopic equilibriumwith the modern eastern Mediterranean bottomseawater (T ¼ 13 1C, d18O ¼ 1.5% SMOW; Pierre,1999) would have d18O values of 3.3% (Grossmanand Ku, 1986) and 2.9% (Tarutani et al., 1969;Friedman and O’Neil, 1977), respectively. Thisindicates that most of the studied carbonate crustsand concretions with d18O values around 3% wereprecipitated in isotopic equilibrium with bottomseawater similar to present-day conditions. Acarbonate crust from the caldera area (NL5CC1)and indurated burrows from the central province(NL7CC1) are characterized by higher d18O values,which indicate the presence of 18O-rich watersource. The high d18O values might result from claymineral dehydration (especially smectite–illite trans-formation) as it was shown in the mud volcanoes of

-5 5

δ18O high-Mg calcite

δ18O aragonite

and concretions from mud volcanoes and pockmarks of the Nile

carbonates (aragonite and calcite with 12% MgCO3 content)

ottom seawater.

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the Mediterranean Ridge (Dahlmann and de Lange,2003), or from relic Messinian brines in the calderaarea. Two carbonate concretions from the sedi-ments collected in the caldera (NL5PC4 ccr) and theeastern (NL11BC1 (0–2 cm ccr)) provinces displaylower d18O values (0.98% and �0.67%, respec-tively) indicating either a local contribution from18O-poor water (continental?) or, alternatively, alocal high heat flow. The equilibrium temperatures,calculated using an average mineral composition ofcalcite and high-Mg calcite (12% MgCO3) accord-ing to the equation of Tarutani et al. (1969) andFriedman and O’Neil (1977), are from 19.571.5 1Cfor NL5PC4 ccr and 27.571.5 1C for NL11BC1(0–2 cm) ccr. These temperatures are consistent withthe in situ temperature measurements in pore fluidswhich may reach more than 55 1C above fluid-releasing structures (J.-P. Foucher, unpublisheddata).

5.2.2. Carbon isotopic compositions of carbonate

crusts and concretions

The carbon isotopic compositions of carbonatecrusts from the different areas of the Nile deep-seafan exhibit large variations. The d13C values of thebulk carbonates range from �41.08% to 3.10% inthe caldera province and from �42.14% to�18.72% in the central province. The crust fromthe Amon mud volcano shows a d13C value of�37.83%. Similar d13C values of carbonate crusts,varying from �46.7% to 3%, have been reportedfrom the Mediterranean Ridge (Olimpi area) andthe Anaximander Mountains in eastern Mediterra-nean by Aloisi (2000) and Aloisi et al. (2000).Furthermore, the d13C values of calcium carbonatesdecrease systematically from �28.04% to �44.17%from 0 to 6 cm along the vertical transect of thecrust NL7CC2 (1) from the central area (Fig. 8),showing that the contribution of 13C-deple-ted carbon source increased with depth. Thecarbonate concretions are also 13C-depleted andshow a range of d13C values from �41.78%to �13.45%, which is included in the interval ofd13C values measured in crusts. The d13C valuesof the surrounding sediments are less 13C-depletedthan associated carbonate concretions: �12.52%for NL4BC1 sed and �10.48% for NL11BC1(0–2 cm) sed.

Three major carbon sources with different carbonisotopic compositions may be mixed and contributeto the formation of authigenic carbonates: a marineseawater source (d13CDIC ¼ 1.00–1.30%; Pierre,

1999), a marine organic source (��20%) and amethane source (�110% to �30%; Sackett, 1978;Whiticar et al., 1986; Whiticar, 1999).

Two carbonate crusts (NL3CC1, NL5CC1) fromthe caldera area exhibit positive d13C valuescharacteristic of carbonates precipitating in isotopicequilibrium with modern eastern Mediterraneanbottom seawater; the precipitation mechanism ofthe carbonate cement of these two crusts is thusconsidered as abiotic (Allouc, 1990).

The carbonate crusts and concretions from theNile deep-sea fan showing low d13C values arecomposed of a mixture of 13C-depleted cement andof sediments, which contain 7–37wt% carbonate.Due to extreme oligotrophic conditions in theeastern Mediterranean Sea, the organic mattercontent of sediment is very low (o0.5%) andcannot contribute as a significant carbon sourcefor authigenic carbonates. Thus, the 13C-depletedcement is derived mostly from the microbial AOM.In the NL7CC2 (1) crust from the central area, thed13C values show a decrease of �10% in the upper2.4 cm depth and of �6% between 2.4 and 6 cmdepth (Fig. 8). During the downward growth of thiscrust, the underlying sediments were progressivelyisolated from the Mediterranean bottom seawaterand the contribution of methane-derived carbonsource increased.

Since the carbonate fraction of authigenic carbo-nate crusts and concretions is a mixture of thecement and of surrounding sediments, the d13Cvalues of the authigenic carbonate cements of twocarbonate concretions from the caldera area(NL4BC1) and the eastern area (NL11BC1(0–2 cm)) have been estimated by a mass balanceusing the carbon isotopic compositions measured incarbonate concretions and surrounding sediments,and the carbonate fraction of cement and ofsediments:

d13Cconcretion ¼ f � d13Ccement þ ð1� f Þ � d13Csediment

where f is the fraction of carbonate cement in themixture and (1�f) is the fraction of carbonate fromthe sediment in the mixture.

Using the abovementioned d13C values (NL4BC1ccr:d13C ¼ �38.23%; NL4BC1sed: d13C ¼ �12.52%;NL11BC1 (0–2cm)ccr: d13C ¼ �17.69%; NL11BC1(0–2cm) sed: d13C ¼ �10.48%) and total carbonatecontent (NL4BC1ccr: 79wt%; NL4BC1sed: 37wt%;NL11BC1(0–2cm)ccr: 84wt%; NL11BC1(0–2cm) sed:7wt%), the estimations of d13C values of cements are

ARTICLE IN PRESSS. Gontharet et al. / Deep-Sea Research II 54 (2007) 1292–1311 1307

�6175% for NL4BC1ccr and �18.470.2% forNL11BC1 (0–2 cm) ccr. These values indicate thatmethane was the major source of carbon forNL4BC1 ccr whereas the carbon source forNL11BC1 (0–2 cm) ccr might be organic matter or,most likely, a mixture of seawater dissolvedinorganic carbon (DIC) and oxidized methane.During the anaerobic bacterial oxidation ofmethane, the lighter 12C isotope is preferentiallyincorporated in the produced CO2, with a fractiona-tion factor varying between 1.002 and 1.014(Whiticar and Faber, 1986; Alperin et al., 1988).Thus, the carbonate cement with calculated d13Cvalue of �61% originated from methane with ad13C value ranging from E�57% to �71%, owingto both fractionation factors during carbonateprecipitation (E10%) and methane oxidation. Thisrange includes the d13C value of �65.6% measuredby Charlou et al. (2003) in methane from brinesseeping in mud volcanoes from the Olimpi area ofthe Mediterranean Ridge. Moreover, the highmethane to ethane ratios (41000) considered withthe low d13C values of methane were interpretedas characteristic of biogenic methane (Charlouet al., 2003). However, lower methane to ethaneratios in the gas dissolved in pore waters from theNile deep-sea fan cold seeps indicate that there isa deep seated source of thermogenic methane(de Lange et al., 2006; V. Mastalertz, unpublisheddata).

6. Conclusions

Mud volcanoes and pockmarks located in differ-ent areas of the Nile deep-sea fan are sites of activemethane-rich fluid venting and precipitation ofauthigenic carbonates in the topmost sedimentsmediated by microbial consortium activity. Carbo-nate concretions are also present within sedimentssometimes associated with other authigenic minerals(barite, pyrite, copper sulfide). Isolated euhedralgypsum crystals precipitate from seeping sulfate-rich brines in sediments collected from Isis andAmon mud volcanoes (eastern area). In all stu-died areas, carbonate crusts and concretions aremainly composed of aragonite and/or high-Mgcalcite associated with minor quantities of low-Mgcalcite and dolomite. Pyrite produced by bacterialsulfate reduction is associated with carbonates.Most of the authigenic carbonate crusts andconcretions were precipitated in isotopic equili-brium with the modern eastern Mediterranean

bottom seawater, except two 18O-depleted carbo-nate concretions collected from the caldera and theeastern (Amon mud volcano) areas, which likelyformed under the influence of higher heat flow atthese sites. The low d13C values of authigeniccarbonates indicate that the primary source ofcarbon was derived from the AOM contained inthe rising fluids.

Acknowledgments

This study was supported by the ESF-EURO-MARGINS-MEDIFLUX project and by theINSU-GDR MARGES program. We thankthe captain of the l’Atalante and the pilots of theNautile submersible for their helpful assistance atsea and their expertise in collecting the samples. Weare grateful to Vincent Rommevaux for preparingthe thin sections and Omar Boudouma for perform-ing SEM analysis. The fruitful comments andconstructive suggestions of Roy Krouse and ananonymous reviewer considerably helped to im-prove the article.

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