On continental margins, upward migration of fluids from various sources and various subsurface accumulations, through the sedimentary column to the seafloor, leads to the development of cold seeps where chemical compounds are discharged into the water column. MarsiteCruise was undertaken in November 2014 to investigate the dynamics of cold seeps characterized by vigorous gas emissions in the Sea of Marmara (SoM). Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. seventeen seeps consist of variable mixtures of different components from two or three sources.

in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the

Abstract:The knowledge of the phase behavior of carbon dioxide (CO 2 )-rich mixtures is a key factor to understand the chemistry and migration of natural volcanic CO 2 seeps in the marine environment, as well as to develop engineering processes for CO 2 sequestration coupled to methane (CH 4 ) production from gas hydrate deposits. In both cases, it is important to gain insights into the interactions of the CO 2 -rich phase-liquid or gas-with the aqueous medium (H 2 O) in the pore space below the seafloor or in the ocean. Thus, the CH 4 -CO 2 binary and CH 4 -CO 2 -H 2 O ternary mixtures were investigated at relevant pressure and temperature conditions. The solubility of CH 4 in liquid CO 2 (vapor-liquid equilibrium) was determined in laboratory experiments and then modelled with the Soave-Redlich-Kwong equation of state (EoS) consisting of an optimized binary interaction parameter k ij(CH 4 -CO 2 ) = 1.32 × 10 −3 × T − 0.251 describing the non-ideality of the mixture. The hydrate-liquid-liquid equilibrium (HLLE) was measured in addition to the composition of the CO 2 -rich fluid phase in the presence of H 2 O. In contrast to the behavior in the presence of vapor, gas hydrates become more stable when increasing the CH 4 content, and the relative proportion of CH 4 to CO 2 decreases in the CO 2 -rich phase after gas hydrate formation.

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This article presents gas hydrate experimental measurements for mixtures containing methane (CH 4 ), carbon dioxide (CO 2 ) and nitrogen (N 2 ) with the aim to better understand the impact of water (H 2 O) on the phase equilibrium. Some of these phase equilibrium experiments were carried out with a very high water-to-gas ratio that shifts the gas hydrate dissociation points to higher pressures. This is due to the significantly different solubilities of the different guest molecules in liquid H 2 O. A second experiment focused on CH 4 -CO 2 exchange between the hydrate and the vapor phases at moderate pressures. The results show a high retention of CO 2 in the gas hydrate phase with small pressure variations within the first hours. However, for our system containing 10.2 g of H 2 O full conversion of the CH 4 hydrate grains to CO 2 hydrate is estimated to require 40 days. This delay is attributed to the shrinking core effect, where initially an outer layer of CO 2 -rich hydrate is formed that effectively slows down the further gas exchange between the vapor phase and the inner core of the CH 4 -rich hydrate grain.

The authors regret a mistake on the value of δ13C of methane for one of the investigated gas samples. This concerns sample MRS-DV3-PE03, taken as end member and for which the correct methane δ13C value is -41.9‰ and not -34‰. The sample was collected at in situ pressure (~100 bars) from a CO2rich seep, in a liquid state. The mistake likely results from a partial extraction (i.e. done by expansion of the sampler volume to allow vaporization) of the pressurized sample for subsampling. Additional analyses have been carried out from fully vaporized samples. This error has resulted in updating the geochemical diagrams used for the discussion, and revising both the gas origin for sample MRS-DV3-PE03 and the section entitled "Delineation of sources, reservoirs or migration pathways, and estimation of level of mixing". The conclusion of the manuscript is unchanged. The authors would like to apologize for any inconvenience caused.

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