AUTHIGENIC CARBONATES RELATED TO THERMOGENIC GAS HYDRATES IN THE SEA OF MARMARA (TURKEY).

Antoine Crémière , Catherine Pierre, Giovanni AloisiLaboratoire d'Océanographie et du Climat Expérimentations et Approches Numériques

Université Pierre et Marie Curie4 Place Jussieu, 75252 Paris

FRANCE

Marie-Madeleine Blanc-Valleron Muséum National d'Histoire Naturelle

Paléobiodiversité et Paléoenvironnements57 rue Cuvier, 75231 Paris

FRANCE

Pierre Henry, Tiphaine ZitterCEREGE, Chaire de Géodynamique, Collège de France,

Europôle de l’Arbois, Aix en ProvenceFRANCE

Namik ÇağatayFaculty of Mines, Geology Department

Istanbul Technical UniversityMaslak, 34469 Istanbul

TURKEY

ABSTRACTActive gas venting was observed along the submerged part of the North-Anatolian fault system (Sea of Marmara) both by acoustic methods (EK60 echosounder) and Nautile submersible dives during the

MARNAUT cruise (2007). On the Western-High ridge, thermogenic gas and crude oil seep at the seafloorwith deep-sourced brines. During submersible dives at this site, authigenic carbonate crusts outcropping at the seabed were sampled. Coring in the sediments revealed the occurrence of cemented carbonate levels and gas hydrates. Porous carbonate crusts are composed of aragonite-cemented pelagic deposits. The 13C

depletion (-44 < 13C ‰ V-PDB < -12.8) attests that methane and possibly other hydrocarbons were the main source of carbon and implies that microbial oxidation processes have transformed these reduced forms

of carbon into dissolved inorganic carbon. The 18O values of seafloor carbonate crusts (+2.5 < 18O ‰ V-PDB < +3.1) suggest precipitation in equilibrium with the present-day bottom water. Abundant diagenetic

carbonate concretions found in the younger marine deposits are made of high Mg-calcite and aragonite.

Concretions have a wide range of carbon isotope compositions (-22 < 13C ‰ V-PDB < +14.2) suggesting multiple sources or diagenesis. In particular, extreme 13C-enrichment indicates the migration of 13C-rich CO2 produced by microbial activity in the methanogenic zone. The 13C-depleted, methane-derived,concretions are most enriched in 18O, suggesting that gas hydrates provided a source of 18O-rich water

Corresponding author: Phone +33 (0)1 44 27 84 79 Fax +44 (0)1 44 27 71 59 E-mail: antoine.cremiere@locean-ipsl.upmc.fr

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

during their dissociation. Carbonate precipitation was probably enhanced during the last deglaciation when concurrent global climate warming and the transition of the Sea of Marmara from a lacustrine to a marine environment could have caused widespread thermal destabilization of shallow gas hydrates as well as input of sulfate in the shallow sediment.

Keywords: Sea of Marmara, cold seeps, carbonate diagenesis, carbon and oxygen stable

isotopes, thermogenic gas, gas hydrates.

NOMENCLATUREmbsf meter below the seafloorSEM Scanning Electron Microscopy

V-PDB Vienna Pee Dee Belemnite V-SMOW Vienna Standard Mean Ocean Water

notation of the isotopic composition expressed in ‰

INTRODUCTIONSince their first discovery on the passive margin of

the Gulf of Mexico [1], colds seeps have been reported all over the world’s oceans in various geological settings [2]. Produced by the degradation of organic matter, methane-rich fluids migrate within the sediments and reach the seafloor where they sustain the development of

chemosynthetic communities. Near the seafloor, where ascending methane encounters sulfate-rich seawater, the anaerobic oxidation of methane(AOM) coupled with sulfate reduction is mediated by a microbial consortium of methanotrophic archea and sulfate reducing bacteria [3] :

OHHSHCOSOCH 23

2

44

This reaction occurs at the sulfate methane interface (SMI) where bicarbonate ions production in pore waters increases the alkalinity and favours carbonate precipitation:

OHCOCaCOCaHCO 2232

32

Authigenic carbonates are common features

associated with cold seeps and near seafloor gas hydrates deposits.In 2007, the MARNAUT cruise investigated the Sea of Marmara, and numerous seepage sites were found along the fault network [4]. On the Western High, brines and thermogenic oil and gas derived

from Eocene-Oligocene Thrace basin source rocks [5] migrate through long-lived fluid conduits [6].Seafloor surveys at this site indicate widespreadoutcropping carbonate crusts on the seabed; buried carbonate layers, concretions and type II gas hydrates were also cored at a shallow depth (<7

mbsf) within the sediment. Pure methane (type I) gas hydrates wo uld not be stable at the

temperature and pressure conditions of the site (14.5°C and 67 bars) [7]. We report mineralogical observations and stable isotopes analyses on authigenic carbonates in

order to track fluids sources and diagenetic processes.

G EOLOGICAL SETTINGBetween the Mediterranean and the Black Seas,

the Sea of Marmara is an intra-continental basin located in the north-western part of Turkey. The northern branch of the seismic North Anatolian strike-slip fault is submerged within the Sea of Marmara (fig. 1). Along the main active fault, widespread fluid emissions were discovered

during the MARMARASCARPS and MARNAUT oceanographic cruises [4, 8, 9].

Figure 1 Bathymetric map of the Sea of Marmara

with the location of the Western High site and the North Anatolian fault network (black lines).

In the western part of the Marmara Sea, bet ween the Tekirdağ and the Central basins, the Western High formed as a compressive sedimentary structure [10]. The sampling sites are t wo carbonate mounds located along an anticline (40°49’N - 28°47’E, 660m water depth).

DIVES EXPLORATION AND SAMPLING METHODSDuring the MARNAUT cruise, gas emission sites were identified from acoustic anomalies in the water column [11]. These were notably detected along active faults, driving the idea that fluids

migrate preferentially throughout fractures [9, 11].

Figure 2 Seafloor observations from submersible Nautile during dive 1648. Extended area of black

patches with white microbial mats on the rim

associated with outcropping carbonate crusts.

During dives of manned submersible Nautile, seafloor observations on the Western High north of the main fault found carbonate crust outcrops at

two sites, and black patches of reduced sediment with bacterial mats on the rim (Fig. 2). At one of these two sites, free gas and crude oil bubbles escaping from the sediments, and white deposits of precipitated barite were also observed. A total of 3 dives (1648, 1662 and 1669) have

allowed the collection of 12 carbonate crust samples. Küllenberg cores KC-14 (9m length) and KC-27 (7m length) were retrieved at each site; they contain several carbonate-cemented concretion levels. Moreover, the lower part of core KC-27 was filled by brownish porous gas hydrates

[7].

METHODSThe semi-quantitative estimation of mineralcomponents of carbonate crusts and concretions,

and of unlithified sediments, was determined by X-Ray diffraction (XRD) combined with total carbonate content measurements. Magnesium content in calcite was estimated from d104 values [12]. Microfacies observations were carried out with scanning electron microscopy (SEM). The

carbon and oxygen isotopic compositions of

carbonates were performed with a dual-inlet IRMS. The isotopic compositions are expressed in

the conventional delta () units in ‰ relative to the Vienna Pee Dee Belemnite (V-PDB).

RESULTS AND DISCUSSION

Macrofacies

Figure 3 Macrofacies of carbonate samples. A:

Porous aragonitic crust outcropping at the seabed(dive 1662). B: Cemented rounded buried

concretion of high Mg-calcite (core KC-27).

Carbonate constructions outcropping at the

seafloor (fig. 3-A) are centimeter thick porous crusts that have cemented pelagic deposits; they are covered by a thin brownish/orange layer of oxides. Buried carbonate concretions (fig. 3-B) are well lithified forming white/grey rounded pieces and chimneys.

Mineralogy and microscopic observationsIndurated samples exhibit a total carbonate content range from 61 to 94 wt% contrasting with the low values (0-15 wt %) of the surrounding sediments.

Microscope and binocular observations indicate that the unconsolidated sediment contains biogenic carbonates (bivalve shell fragments, foraminifera, coccoliths, ostracodes) that are incorporated in cemented material. A comparison of sediments and lithified samples by XRD analyses allows the

attribution of the low-Mg (<5 mol % MgCO3) calcite component to the biogenic carbonate phase.Dominant authigenic minerals in carbonate crusts and concretions are aragonite and high Mg-calcite (~15 mol % Mg) sometimes associated with minor amount of dolomite. The mean proportion of the

carbonate phases appear variable (fig. 4) between the seabed crusts and the concretions from cores KC-14 and KC-27.

Figure 4 Distribution of the average carbonate mineralogy of samples from dives on the seafloor

(n=12) and cores KC-14 (n=17) and KC-27 (n=20).

Outcropping carbonate crusts are primarily composed of aragonite. Mean mineralogy of core KC-14 concretions is dominated by aragonite differing from core KC-27 concretions essentially composed of high Mg-calcite. Differences in carbonate mineralogy may be related to

differences in the chemical composition of fluids during diagenetic precipitation. In the Cascadia margin authigenic carbonates, the aragonite phase was described as forming in equilibrium with pore waters whereas high-Mg calcite was interpreted to

be linked to gas hydrates dissociation [13] ; a similar interpretation could be consistent with the occurrence of gas hydrates at the bottom of core KC-27. Pore water chemical analyses indicate that there is a significant difference bet ween the two cores [6]; Mg2+ profiles are similar, but the Ca2+

concentration is twice in core KC-27 compared to that of core KC-14. Ca/Mg ratio is an important factor controlling the carbonate mineralogy [14], since when this ratio is high, calcite is favouredover aragonite.SEM observations of freshly broken carbonate

fragments confirm the diagenetic origin of carbonates with well-crystallized botryoïdal, needle-fibers or prismatic crystals of aragonite and rhombs of high-Mg calcite.

Figure 5 SEM photograph (back-scattered electrons) of framboidal pyrite infilling

foraminifera test in aragonitic cement (core KC-14).

Diagenetic carbonates are associated with minor amount of authigenic barite and pyrite (fig. 5). Reaction between detrital iron and H2S producedby sulfate reduction is thought to produce pyrite

[15]. Barite precipitation occurs when rising Ba-rich fluids encounter advecting sulfate-rich seawater [16]. Occurrence of both minerals confirms that carbonate diagenesis was coupled with sulfate reduction.

Stable isotopesCarbon isotopic composition of carbonates reflects the isotopic signature of the pore fluids in equilibrium with precipitating carbonate. The carbon isotope composition of bulk carbonates presents a wide variability ranging from -44 to

+14.2 ‰ V-PDB (figure 6).

The 13C-depletion (-44< 13C ‰ V-PDB <-12.8) of seafloor crusts indicates unequivocally oxidation of methane and possibly heavier hydrocarbons.With a mean of -26 ‰ V-PDB (figure 7), the source of bicarbonate could be a mixture of

oxidized methane and heavier hydrocarbons. Such moderately depleted 13C values were identified in carbonates from the Gulf of Mexico and attributed to the microbial oxidation of oil and hydrocarbons heavier than methane [17, 18].

Figure 6 Carbon and oxygen isotopic compositions of the diagenetic carbonates. Dashed lines

represent the 18O values of carbonate precipitating with the present-day bottom

seawater.

Buried carbonate concretions exhibit less depleted

13C values but with an important variability (-

22.4< 13C ‰ V-PDB <+14.2), varying from 13C-

depleted to 13C-enriched values. For these carbonates, microbial oxidation of hydrocarbons alone cannot explain values as high as +14 ‰. However, biodegradation of crude oil (including methanogenesis) in a deeper reservoir could have

generated 13C–rich CO2 (13C = +25 to +29‰ V-

PDB) [7]. Migration of fluids with positive 13C values into shallower environment would have

contributed to the formation of 13C-rich carbonates.

Figure 7 Range of 13C values of carbonate samples, hydrocarbons and carbon dioxide in free

gas from the Western High Ridge [7].

Oxygen isotopes provide information about the temperature and the isotopic composition of fluids.

With 18O values ranging from +2.4 to +4.9‰ V-PDB (fig. 6), the carbonates display a significant

variability. Theoretical 18O values of carbonate precipitating with the present-day bottom seawater

(t = 14.5°C; 18O = +1.58‰ V-SMOW) would be +3.0 ‰ for aragonite [19] and +2.3‰ V-PDB for

Mg-calcite [20, 21].

The 18O values of seafloor carbonate crusts

present a weak dispersion (+2.5 < 18O ‰ V-PDB < +3.1) and are very close to equilibrium values indicating that they precipitated recently in an environment near the seafloor. Comparatively, buried concretions present higher

18O values up to +4.9‰ V-PDB. As the oxygen isotopic composition of diagenetic carbonates depends on the temperature of precipitation and on the isotopic composition of the mixture of bottom seawater and rising fluids, we propose the following assumptions to explain this enrichment:

1. According to [22], during the transition (starting at 14.7 ka) from the late glacial maximum (LGM) to the Holocene, the Marmara freshwater lake was gradually converted to marine conditions over a 2ky

period by inflow of Mediterranean waters. Moreover, subsequent climatic warmingled to an increase by about 10°C of the sea surface temperature until the hydrological conditions became stable at 8ka. At the seafloor, benthic foraminifers display

variations in 18O values of about ~2‰,which result from the combined effects of changes of temperature and isotopiccomposition of water [23, 24].

2. The contribution of 18O-rich fluids at this site could arise from three main sources.

Clay minerals dehydration and gas hydrates dissociation are known to release 18O-rich water [25, 26]. Additionally, deep-sourced fluids coming from the oil/gas field could also be a source of 18O-

rich water [27].

Both a lower bottom water temperature during precipitation and the influence of 18O-rich rising

fluid might have led to the increase of 18O values of the buried carbonate concretions.

Depth distribution of buried concretions in the sedimentary sequenceCores KC-14 and KC-27 were retrieved from two distinct mounds structures. Two sedimentary units can be distinguished (fig. 8); the marine deposits present intercalations of grey to dark brown mud

with carbonate concretion-rich levels whereas the underlying lacustrine deposits are characterized by low total carbonate contents (~2 wt%).

Figure 8 Lithostratigraphy, radiocarbon datings and location of carbonate concretions sampled in

cores KC-14 and KC-27.

In both cores numerous carbonate concretions and cemented layers were found close to or above the lacustrine to marine transition at ~13 ka. A

widespread authigenic calcite layer in the Sea of Marmara was previously described [28] and interpreted as a consequence of the marine incursion; thermodynamical calculations showed that carbonate saturation state increased as a consequence of the mixing of Mediterranean water

with the residual lacustrine water. Locally at cold seep sites, the increase of sulfate concentration in the pore waters that followed the marine reconnection, favoured microbial oxidation ofhydrocarbons coupled with sulfate reduction and carbonate precipitation. Additionally, a recent

study identified a biogeochemical signal of

methanotrophy in the water column after the transition (≈11kyr BP) and interpreted it as the result of gas hydrates dissociation [29]. Thus, we assume that increase of the bottom water temperature could have destabilized gas hydrates originally present within the sediments; the

resulting massive release of hydrocarbons could have enhanced microbial activity and promoted carbonate diagenesis within the sediments.

CONCLUSIONAlong the main faults on Western-High ridge in the Marmara Sea, seepage of deep sourced fluids and thermogenic gas mixture fueled microbial activity within the sediments resulting in the precipitation of authigenic carbonate crusts near seafloor. Oxygen and carbon isotopic

compositions of these carbonates confirm that they formed close to the seafloor and derived from anaerobic oxidation of hydrocarbons. Buried carbonate concretions occur in the sedimentary sequence starting within the lacustrine/marine transition zone. Carbon isotopic compositions

indicate, in some of them, a contribution from 13C-rich CO2, likely derived from bacterial degradation of oil at a deeper level. The 18O enrichment of the buried concretions and their proximity to gas hydrates lead us to propose that this enrichment

might at least in part be due to gas hydrates dissociation.

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ACKNOWLEDGEMENTSWe would like to thank the Marnaut cruise party for their contribution. SEM analyses were performed with Omar Boudouma UMR 7193

ISTEP that we acknowledge warmfully. We acknowledge Jean Pascal Dumoulin and Christophe Moreau of the AMS 14C measurements realized at the UMS 2572 LMC14 (CEA-CNRS-

IRD-IRSN-MCC) of Saclay.