In the Western High of the Sea of Marmara, where pervasive gas hydrate and hydrocarbon gas seepage occurs, late Pleistocene-Holocene sediments are composed of a lower lacustrine and an upper marine unit. The sedimentary evolution, gypsum formation and carbonate anomaly, under the complex sedimentary environment at cold seep sites on the Western High, have not been systematically well explained. The transition from the lacustrine to marine unit occurs at around 7.5 m below the sea floor (mbsf), with a sulfidization front present in the lacustrine sediments just below it, and the marine unit includes a sapropel layer between 5.20 and 7.20 mbsf. The sedimentary sequence is characterized by authigenic minerals including pyrite, carbonates, and gypsum in the upper 0.50 mbsf, corresponding to the present-day sulfate-methane transition zone (SMTZ) near the seafloor. Mineralogical, chemical and isotopic compositions (13C, 18O, 34S, 26Mg) of the sediments in the core studied show their close relationship with intensive sulfate-driven anaerobic oxidation of methane (AOM) and other hydrocarbons. The dissolution of carbonate, related to the gas and oil migration, and organic matter enrichment might be the main cause of carbonate deficiency in the sapropel unit. Heavy carbon isotopic composition (δ13CCarb = 6.98‰) of carbonate at 4.30 mbsf implies that the carbon might be the residual by-product of subsurface biodegradation of seeping petroleum. The formation of secondary gypsum and partial dissolution of carbonate in the Sea of Marmara is commonly considered of being associated with the aerobic oxidation of pyrite. However, the euhedral authigenic gypsum studied might be with different origins, possibly including anaerobic oxidation of Fe-sulfides, carbonate deficiency in the sapropel unit and AOM. Gypsums below SMTZ are with the size of hundreds of microns. Linear pits and grooves are developed on these gypsum surfaces, probably due to corrosion by the methane-rich fluid of latest hydrocarbon seepage. Platy hexagonal gypsum in SMTZ, without any corrosion on crystal surface, might be associated with the changing flux of the acidic composition (e.g. CO2) in the seepage. Depleted-26Mg by about 1‰ in the SMTZ might be caused by the upward seepage of hydrocarbons. Magnesium isotope of authigenic carbonate could be a valid proxy for the recognition of SMTZ in a hydrate geo-system. 

Origins of authigenic gypsums and carbonate minerals in sediments at a cold seep site in the Sea of Marmara

Cold seeps, featured by their extremely methane-rich sedimentary environments, play a significant role in the geological history and are common in marine sediments across the seafloor. Primary dolomite, possibly mediated by microorganisms, can be widely discovered in methane-rich environments. Hence, cold seeps may provide new insights into the ‘dolomite problem’, which has confused geologists for decades. Magnesium isotope geochemistry of seep carbonates contributes to the understanding of the dolomite formation mechanism in marine environments. In this paper, magnesium geochemical characteristics of carbonates in modern sediments are summarized, along with rare researches on magnesium isotopes of seep carbonates. Methane vigorously interacts with sulfate by anaerobic oxidation of methane at cold seeps, producing vast amounts of dissolved sulfide which can significantly promote dolomitization of seep carbonates. Compared with temperature, alkalinity, mineralogy, etc., the competition between rapid carbonate precipitation rates and aqueous ligands may be the main factor of the magnesium fractionation at cold seeps, which is controlled by the kinetic effect. The range of magnesium isotopes of seep carbonates is narrow (from -3.46‰ to -2.36‰), and an upper limit of magnesium content seems to exist. This characteristic may be a good indicator for identifying dolomitization related to anaerobic oxidation of methane. Whereas, mechanisms of magnesium isotope fractionation and dolomitization at cold seeps remain unclear, necessitating more natural samples tested, stimulated calculation and laboratory experiment. 

Magnesium geochemistry of authigenic carbonate at marine cold seep

Heavy sulfur isotopes recorded in contrasted redox conditions in Holocene sediments from the Yangtze River Delta

Cold-Seep Carbonate Factory: Formation Mechanisms, Geochemical Proxies, and Implications for Global Carbon Cycling

Abstract Methane (CH 4 ) release and consumption in marine cold seeps are key processes influencing seabed ecology, global climate reconstruction, and hydrate resource exploration. Anaerobic oxidation of methane (AOM), the primary methane consumption mechanism, typically couples with microbial sulfate or metal reduction. Iron‐bearing minerals, including iron sulfides and iron oxides, have recently been recognized as key indicators of methane flux and metabolic pathways in cold seeps. However, their mechanistic roles in reconstructing paleo‐seep evolution under varying methane intensities remain unresolved. Here, we employed an integrated geochemical, mineralogical, and magnetic approach to analyze a seep core recording multistage methane expulsion events, revealing iron‐mineral responses to cold seep evolution. Our results show that iron‐bearing sulfides serve as key tracers of sulfidic environments driven by methane seepage. Their mineralogical attributes depend on both methane flux intensity and ambient iron availability. High methane flux combined with elevated reactive iron levels facilitates the coexistence of metastable iron sulfides (e.g., greigite) with euhedral pyrite. During cold seep evolution, declining methane fluxes and/or sustained sedimentation can trigger a transition from sulfidic to non‐sulfidic conditions in seep‐impacted zones. Microbially produced ferrous iron‐bearing oxides, such as magnetite [formed through Fe(III)‐driven AOM or methanogenesis], may indicate the evolution of cold seeps into anoxic, non‐sulfidic methane environments. These findings demonstrate linkages between iron‐mineral signatures and methane‐cycling processes, highlighting their potential as proxies for reconstructing seep evolution history. 

Iron‐Bearing Minerals as Potential Indicators of Paleo‐Methane Seepage Evolution

Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey

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Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey