Occurrence and Genesis of Cold‐Seep Authigenic Carbonates from the South‐Eastern Mediterranean Sea

Methane‐derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane‐enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south‐eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X‐ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ13CVPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan‐shaped aragonite (δ13CVPDB of −52‰ to −30‰) and high‐magnesium calcite cements, with the latter often growing from low‐magnesium calcite spherules. Botryoidal low‐magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low‐magnesium calcite breccias, high‐magnesium calcite nodules (δ13CVPDB of −35‰ to −20‰) and cements, and partially replaced fan‐shaped aragonite cements. Carbonate pavements consist of low‐magnesium calcite microsparite, micritic dolomite and high‐magnesium calcite. Fan‐shaped aragonite is locally present as pore‐lining cement. Iron oxides are often seen coating the low‐magnesium calcite, high‐magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high‐magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.


| INTRODUCTION
Hydrocarbon discharge from below the sediment-water interface is an important source for deep-sea carbon and plays a role in the modulation of the global carbon cycle (Judd et al., 2002).Large amounts of carbon and hydrocarbons are stored in marine sediments as natural gas hydrates (Kvenvolden, 1988), crude oil or gas (Fleischer et al., 2001).Under certain circumstances hydrocarbons stored in the deeper subsurface may be naturally released in large quantities.As an example, estimates suggest 500-2300 Gt of carbon is bound up as hydrates in the sediments at the continental margins (Lee et al., 2022;Milkov, 2004;Piñero et al., 2013;Wallmann et al., 2012) and a rapid release of these hydrocarbons could have a big impact on the global climate.One extreme example is thought to be represented by a large methane hydrate release at the Palaeocene-Eocene boundary, which has been suggested to have increased the sea surface temperatures by about 6°C (Dickens et al., 1995).Nowadays, the depth of gas hydrate occurrence and migration, as well as sediment and water column sinks, may mitigate the impact of gas hydrate dissociation (Ruppel & Kessler, 2017) and/or hydrocarbon migration on atmospheric greenhouse gas concentrations.However, more observational data are necessary to improve numerical climate models considering naturally released hydrocarbons (especially methane; Ruppel & Kessler, 2017).
The various forms of carbon emitted across the sediment-water interface to the deep sea are partly mineralised as calcian or magnesian carbonates.Such calcian and magnesian seep carbonates are formed mostly in reducing environments facilitated by anaerobic oxidation of hydrocarbons, notably methane (AOM), coupled to sulphate reduction (SR).These processes are mediated by a consortium of methane-consuming archaea and sulphatereducing bacteria (Boetius et al., 2000) and can be summarised as: Following the Brønsted-Lowry acid-base theory, the reaction in Equation (1) generates the conjugate bases of carbonic and sulphidic acids.As such, the AOM reaction results in a proton release which promotes the increase in pH, dissolved inorganic carbon (DIC) and alkalinity, leading to the precipitation of carbonates (Luff & Wallmann, 2003): The formation of authigenic carbonates is an important process in carbon sequestration and it is therefore important to understand the controls acting on these processes.Authigenic carbonates have distinctive characteristics.These carbonates can occur in different morphologies and sizes, including pavements forming hardground-like structures, crusts with millimetre to centimetre-sized nodules, or as chimneys, forming metre-high sea floor structures (Feng et al., 2014a;Smrzka et al., 2019).Furthermore, seep carbonates have a diverse carbonate mineralogy, including highmagnesium calcite (HMC), aragonite, dolomite and in some cases low-magnesium calcite (LMC; Aloisi et al., 2000b;Feng et al., 2014b;Greinert et al., 2001;Huang et al., 2019;Naehr et al., 2007).Other associated minerals, such as pyrite, baryte and celestine, can be identified in seep carbonates, forming due to changes in seep fluid chemistry, recrystallisation and sulphatereduction related processes (Bian et al., 2013;Feng & Roberts, 2011).The mineralogy, internal texture (different generations of cements), morphology and geochemistry of seep carbonates are products of changes in the ascending fluid flux and transport mode, as well as interactions between the seep fluids and the surrounding pore and/or sea waters.Seep carbonates are therefore valuable archives for the identification of past seepage activities and the geochemical composition of the ascending fluids and their sources (Bayon et al., 2009;De Boever et al., 2006).
This study focusses on the mapping of different seep carbonate morphologies, and the study of petrographic and geochemical features of seep carbonates retrieved from the south-eastern Mediterranean Sea during the 2016 EUROFLEETS SEMSEEP and the 2011 E/V Nautilus cruises.The aims are to understand through morphology and paragenesis (1) the controlling factors on the formation of authigenic seep carbonates and (2) their link with past seepage activity, that is, recurrent seepage activity versus single event, diffusive (low fluid) versus advective (high fluid) flow.

| Geological background
The Levant Basin (LB; Figure 1A), located in the south-eastern Mediterranean Sea, started forming between the late Palaeozoic and early Mesozoic with the breakup of the northern edge of Gondwana and its collision with Eurasia (Garfunkel, 1998;Granot, 2016;Robertson, 1998;Steinberg et al., 2018).During the late Cretaceous and Tertiary, an inversion of the early Mesozoic structures took place and sediment transport into the basin increased (Gardosh et al., 2010).During the Oligocene and Miocene, Mediterranean connectivity to the global oceans was reduced, while the LB was filled (Bialik et al., 2019;Torfstein & Steinberg, 2020).At the end of the Miocene, the connection between the Atlantic Ocean and Mediterranean Sea was closed leading to the Messinian Salinity Crisis (5.97-5.33 Ma;Hsü et al., 1973;Meilijson et al., 2018Meilijson et al., , 2019)).Evaporites of up to ca 2 km thickness were deposited, mainly consisting of halite (Meilijson et al., 2019).With an abrupt sea-level rise in the Pliocene high amounts of clastic material were deposited in the LB, forming a ca 0.5-1.5 km thick wedge on top of the Messinian evaporites (Steinberg et al., 2011).Recent studies have shown the ongoing deposition of clastic material in the LB with sedimentation rates up to 20.7 cm/ka in the basin (Almogi-Labin et al., 2009;Castañeda et al., 2010) and ca 1 m/ka on the continental slope (Almagor & Schilman, 1995).Ongoing deposition across both the basin and slope regions of the LB comprises Nilotic clastic sediments, primarily clays (Stanley et al., 1998).
The study area in the south-eastern LB, stretches across two major geomorphological domains, the Palmahim Disturbance (PD) and the Levant Channel being part of the Nile Fan (Figure 1A; Gvirtzman et al., 2015).The PD is a prominent (15 km × 50 km) rotational slide across the continental margin, rooted in Messinian evaporites that are mobilised within the now buried Miocene Ashdod Canyon (Figure 1B; Eruteya et al., 2018;Garfunkel et al., 1979).It is composed of an extensional domain (between water depths of 70 to ca 450 m), a translational domain (between 450 m and 900 m; Figure 1B) and a compressional domain (thrust faulted and folded collisional toe; between water depths of 900 m and 1250 m; Figure 1C).To the west, at a water depth of ca 1250 m, the PD toe domain is bounded by the easternmost deep-water channel of the Nile fan, the ca 500 m wide Levant Channel (Gvirtzman et al., 2015).The sediments on top of the PD are clay-rich and slightly bioturbated.In some horizons (between 33 cm and 48 cm below sea floor [bsf]) patches of coarser sediment can be identified.Abundant pockmarks are found in the collision zone at the toe of the PD.The pockmarks are 50-150 m in diameter and can be found in water depths of 1000 to 1300 m, most of them occurring around 1200 m (Tayber et al., 2019).Seismic data reveal prominent subsurface deformations and faults below the pockmarks and the Levant Channel (Eruteya et al., 2018;Lawal et al., 2023;Tayber et al., 2019).
The LB is a prolific exploration basin, with several identified hydrocarbon systems (Feinstein et al., 2002;Gardosh & Tannenbaum, 2014).Two primary confirmed and producing offshore gas systems are the Oligo-Miocene Tamar Sands sub-salt system and the Pliocene Yafo system, located above the Messinian evaporites.Other gas reservoirs might be former channel-lobe systems, located 25-200 m bsf, where high amounts of organic matter and sediment have been transported and accumulated, as well as the base of the Israel slumping complex (ISC, Figure 1; Lawal et al., 2023;Tayber et al., 2019).Multiple sea floor gas seepage edifices and associated features were identified and investigated, primarily on and around PD, through sea floor surveying in the framework of E/V Nautilus cruises of 2010 and 2011 (Coleman et al., 2012;Rubin-Blum et al., 2014b), EUROFLEETS2 SEMSEEPS cruise of 2016 (Basso et al., 2020;Beccari et al., 2020;Makovsky et al., 2017) and follow-up cruises (Sisma-Ventura et al., 2022).These seepage features include a variety of authigenic seep carbonates (Makovsky et al., 2017;Spiro et al., 2021), bacterial mats and macrofauna (Beccari et al., 2020;Makovsky et al., 2017).Eruteya et al. (2018), Tayber et al. (2019) and Lawal et al. (2023) correlated the sea floor seepage observed at the toe of PD and the proximate Nile deep-sea fan with high-amplitude seismic reflectivity observed in commercial 3D seismic datasets.They suggest multiple migration pathways of gas to the sea floor, with potential connectivity to the top of the Messinian evaporites unit and possibly also to deeper levels.In addition, recent discovery of combined brine and gas seepage at the northern part of the PD toe was associated by Herut et al. (2022) with fluid flow from the Messinian evaporites.Tayber et al. (2019) modelled the hydrate stability in the LB and correlated it with the observed seismic reflectivity.They found that at present the top of hydrate stability is at a water depth of ca 1250 m, and argue for the potential presence of gas hydrates below water depths of 1300 m.

| Sample collection and description
Seep carbonate samples were collected during the R/V Aegaeo EUROFLEETS 2 SEMSEEP cruise in 2016 using the MAXROVER remotely operating vehicle (ROV) and the E/V Nautilus 2011 NA019 cruise using the HERCULES and ARGUS ROVs (Table 1).Twelve samples were retrieved from different sites on the flanks of the PD (Figure 1, working area 5 at the northern flank of the PD and working area 6 at the south-eastern flank of the PD), in a pockmark system at the toe of the PD (Figure 1, working area 3), within the Levant Channel (Figure 1, working area 4a), within a pockmark immediately east of the Levant Channel (Figure 1, working area 4b), and within a mega-pock mark structure in the western part of the LB ca 40 km west of PD (Figure 1, working area 2a).Images of the seep carbonate samples and cut seep carbonate slabs were taken at the University of Fribourg, using a Canon 6D camera equipped with a 100 mm macro lens and UV light (wavelength 365 nm).The samples were distinguished based on morphology, colour, facies and location.

| Mapping
The sea floor in the Eastern Mediterranean Sea was investigated during nine dive transects with the MAXROVER ROV aboard the R/V Aegaeo (EUROFLEETS 2 SEMSEEP cruise, September 2016; Table S1, Figure 1A).Over 35 h of dive videos have been analysed, localised within the five working areas to identify different seep carbonate morphologies on the sea floor.Furthermore, bacterial mats and macrofauna have been mapped.

| Thin section analysis
Thin Sections of the different seep carbonates were prepared and polished with a thickness of ca 30 μm at the University of Fribourg.Thin sections were analysed with optical microscopy (LEICA DM4500P) and cathodoluminescence, using a light microscope, equipped with a Cambridge CLmk3A cathodoluminescence device.The samples were excited with 11 kV and 450 mA to emit luminescence.Fluorescence microscope analysis was carried out on a Nikon Eclipse Ci Pol microscope, equipped with a Semrock GFP-3035D filter cube.The samples were excited by blue UV light (472 nm wavelength) and studied using a green fluorescence emission filter (520 nm wavelength).Tables, summarising the sediment petrography of the different seep carbonates can be found in the supplementary material.

| X-ray diffractometry
The mineralogy of the different seep carbonates was determined using X-ray powder diffraction.Powders were drilled from each seep carbonate, covering different cements.All samples were measured using a powder XRD Rigaku Ultima IV system and a DTex position sensitive detector.Powders were drilled from the seep carbonate samples using a hand-held Dremel and distributed onto a 0.2 mm sample holder.The scans were carried out at 40 kV and 40 mA, using Cu-K α radiation between 10° and 75° 2θ with a step size of 0.2° and 0.5°/min.
Peak identification was done using the Rigaku PDXLR2 software and the ICDD database (PDF-4+ 2020).Quantitative results were calculated using the Bruker Topas 5 software and the revised Rietveld method (Titschack, 2011).Magnesium (Mg) and calcium (Ca) content of the different Mg-carbonates were derived from the lattice parameters and the cell volume.The magnesium content was calculated with the formula: where C Mg is the Mg content in the Mg-calcite and V the unit cell volume, based on the lattice parameters (Titschack, 2011).The magnesium content of the dolomites was calculated with the formula of Lumsden (1979): where d 104 is the lattice spacing in angstroms and N CaCO 3 the mol percent of CaCO 3 in the dolomites.

| Electron microscopy
Thin sections of the different seep carbonates were cleaned with ethanol and coated with carbon of approximately 30 nm thickness, using a Balzers CED 030 Carbon Evaporator.These sections were examined (4) under a FEI scanning electron microscope equipped with an energy dispersive spectroscope (SEM-EDS).
To avoid charging of the sample, conductive graphite cement was applied on the edges and the bottom side of the thin section.The samples were examined semi-quantitatively with 20 kV high voltage to evaluate the Mg/Ca ratio in the different sedimentary and diagenetic phases.The different carbonate mineral phases were distinguished based on the Mg/Ca ratio (Table S2).

| Stable isotope geochemistry
The samples for stable oxygen and carbon isotope analysis were drilled from different parts of the seep carbonates using a hand-held Dremel equipped with a 0.6 mm tungsten carbide drill bit.In total, 78 samples of approximately 100 μg were measured using a GasBench II and a ThermoFinnigan MAT 253 isotope ratio mass spectrometer at the University of Lausanne according to a method modified after Spötl and Vennemann (2003).
All carbonates reacted with ortho-phosphoric acid (density 1.88 g/cm 3 ) at 70°C, except for dolomite that was reacted at 90°C.All values were normalised against the internal Carrara Marble standard and the NBS-19 standard (δ 13 C VPDB of 1.95‰, δ 18 O VSMOW of 28.65‰; Spötl & Vennemann, 2003).Acid fractionation factors used to calculate the δ 18 O values for carbonate are from Swart et al. (1991) for calcite and HMC, from Kim et al. (2007) for aragonite, and from Rosenbaum and Sheppard (1986) for dolomite.The reproducibility of the standard for δ 13 C is better than 0.06‰ (1SD) and 0.08‰ (1SD) for δ 18 O (n = 24).Carbonate-water fractionation factors of the different carbonate phases have been calculated using the factor of Mavromatis et al. (2012), assuming HMC formed under equilibrium conditions with the temperature of the surrounding bottom water (13.5°C,Tayber et al., 2019).The δ 18 O of pore water used for this study is estimated to be 1.5‰ (VSMOW; Gat et al., 1996;Herut et al., 2022).Carbonate-water fractionation factors used for aragonite and dolomite were obtained from Böhm et al. (2000;aragonite) and Fritz and Smith (1970;dolomite).
Both δ 13 C and δ 18 O values are reported in ‰ V-PDB, except for the water δ 18 O values that are relative to VSMOW.Tables summarising the different phases for each seep carbonate morphology and their mean stable isotope values can be found in the supplementary material.

| Facies mapping/video analysis
Three different carbonate morphologies were identified in the studied locations: 3.1.1| Chimneys Chimneys are sub-metre in size, they appear to be solitary, emerging from the sediment above the sea floor or extend on top of carbonate pavements (tens of centimetres).The colour of the chimneys is dark brown to blackish.Big solitary carbonate chimneys are associated with seep fauna, and shells of dead bivalves of the species Lucimoni kazani (Figure 2A through E).

| Crusts
Crusts are composed of small (centimetre-sized) carbonate nodules which have been cemented together to form surfaces.They appear on the sea floor as concretions a few cubic metres in size and are often covered by a thin layer of sediment.The colour of the carbonate crusts is similar to that of the chimneys, dark brown to blackish (Figure 2F through J).

| Pavements
Pavements appear on the sea floor as layered carbonates.They are partly covered by a thin layer of sediment or overgrown by chimneys.The colour of the pavements changes from light brown to orange-yellowish.Carbonate pavements are associated with seep fauna at some locations, mainly dead bivalve shells (Lucimoni kazani) and living tubeworm communities (Siboglinidae; Figure 2K  through O).
Carbonate Chimneys are identified at the toe of the PD within the Levant Channel (WA 4a, Figure 3A) and within the pockmarks in the central part of the LB (WA 2a, Figure 3F).Large solitary chimneys were identified within the Levant Channel (Figure 2A).In contrast, the chimneys found elsewhere in the study area are significantly smaller and either reach out of the sediment or grow on top of carbonate pavements.Carbonate Pavements are abundantly identified in the large pockmark (WA 2a, Figure 3F), forming hardgrounds (Figure 2K).Some of the pavements form small domes in this area.Pavements are also present within the Levant Channel (WA 4a, Figure 3A) and the pockmarks at the toe of the PD (WA 3, Figure 3B,C).Unlike the pavements of the LB, those in the Levant Channel are not domeforming.Some of the pavements are covered by greyish or whitish bacterial mats (Figure 2P).
In contrast to the chimneys and pavements, carbonate crusts are mainly present at the northern flank and at the south-eastern flank of the PD at significantly shallower water depths (WA 5 and 6, Figure 3D,E).On the northern flank, the carbonate crusts consist of bigger tube-like structures (possibly formed by bioturbation), cemented together by micrite, and are partly overgrown by soft body corals and Scleractinia (Figure 2F).Moreover, the crusts are not accompanied by other carbonate morphologies, like chimneys and pavements.

| Carbonate chimneys
Chimneys are characterised by mainly non-luminescent (CL) micrite enriched in fossils (mainly foraminifera, bivalves and gastropods) and minor amounts of detrital quartz (petrographic observation (PO) 1, Figure 4A,B), classified as wackestone.Based on Rietveld refinements, the micrite consists mainly of HMC with Mg contents of 11%-15% (Table S3).Locally, it exhibits luminescent centres identified in SEM-EDS analysis as small rhombs (30 μm) with Mg/Ca ratios of 0.39-0.72 (Figure 4C), indicating dolomite or a precursor phase of dolomite.However, the total dolomite content in the chimneys, in general, is close to or below the detection limit (Table S3).Locally, the micrite is replaced by LMC microsparite forming spherules characterised by a dull luminescence (SEM-EDS Mg/Ca ratio 0.07; PO 2, Figure 4A,B,D; Table S4).Ghost structures of peloids can be identified within the LMC microsparite spherules.Non-luminescent (CL) fanshaped aragonite cements cover the LMC microsparite (PO 3, Figure 4A,B; Table S4).Locally, zoned (CL) botryoidal LMC cements fill open spaces (Mg/Ca ratio up to 0.08; PO 4, Figure 4E,F; Table S3).Next to the aragonite fans, CL luminescent HMC cements form (Mg/Ca ratio up to 0.12) and engulf the aragonite fans (PO 5, Figure 4G,H).Dissolution horizons can be identified at the outer edges of the HMC cements (Figure 4G).In some parts, the HMC cements are overgrown by a second generation of non-luminescent needle-like aragonite (PO 8, Figure 4G; Table S4).This sequence (LMC microsparite replacements → aragonite fans → LMC botryoidal cements → HMC cements) is repetitive and can be identified up to three times within the carbonate chimneys.

| Carbonate crusts
The carbonate crusts consist of CL non-luminescent micrite composed of LMC and HMC (PO 1, Table S3; Figure 5A).They are rich in quartz crystals, fossils and CL luminescent rhombs up to 35 μm in size.The Mg/Ca ratios between 0.47 and 1.53 (Figure 5A) suggest dolomite or a precursor phase of dolomite.The overall dolomite content based on XRD is, however, close or below the detection limit (Table S3).The crusts of the PD pockmark (WA 3) consist of higher amounts of dolomite (PO 10).The sediment in this area can be described as wackestone to packstone.
Non-luminescent (CL) aragonite fans grow as porelining cements in the pores between the breccia fragments (Figure 5B) and in between the nodules (PO 3, 5 and 6, Figure 5F,H).Dissolution horizons, peloidal micrite and ferromanganese crusts are present within the aragonite fans forming disruptive surfaces (Figure 5G).The inner part of the aragonite fans is partially replaced by CL nonluminescent LMC microsparite (Figure 5F).Oxidised framboidal pyrites are present within the breccia fragments (PO 7, Figure 5B), the microsparitic HMC nodules and associated with the aragonite fans.The micrite and aragonite fans are partly replaced by blocky HMC crystals (Mg/Ca ratios up to 0.1).These HMC crystals also form blocky cements (PO 9, Figure 5I).Phyllosilicates and pyroxene crystals, minerals with a terrigenous origin, are associated with the HMC.Bioerosion marks are a common feature in the carbonate crusts with burrows transecting the aragonite cements later filled by sediments, similar to the micrite (PO 11, Figure 5F,J).

| Carbonate pavements
The carbonate pavements consist of CL nonluminescent LMC microsparite (Mg/Ca ratios 0.05 to 0.07; PO 2, Figure 6A,C,D; Table S6).Ghost structures of former peloids are visible.Fluorescence luminescence (FL) reveals the presence of bacterial cells within the microsparite (Figure 6B).Additionally, dull to bright CL luminescent clotted HMC micrite and micritic dolomite can be identified (PO 3 and 4, Figure 6A,C,D; Table S6), the former exhibiting BSE-EDS Mg/Ca ratios of up to 0.21 and Mg contents of approximately 22%, based on Rietveld refinements (Table S3).Small, zoned CL luminescent rhombs (up to 80 μm), identified as dolomite and magnesite with Mg/Ca ratios between 0.61 and 0.71 can be found within the HMC micrite (Figure 6E,F).These dolomites are composed of up to 34% Mg, based on the method of Lumsden (1979).Within a fluorescent bioturbated band, porelining aragonite fans were identified growing from peloidal micrite and fossils (PO 6, Figure 6G,H; Table S6).Iron oxides are associated with the LMC microsparite and micritic dolomite (PO 5).Rounded to elongated bioerosion features are partly filled by semi-lithified, quartz-rich and CL non-luminescent sediment (PO 7, Figure 6I).
Due to their size, the microsparitic replacements, micritic replacement and dolomites could not be sampled separately.Mixed phases have a mean δ 13 C value of −23.8‰ (n = 8, SD = 5.95) and a mean δ 18 O value of +2.5‰ (n = 8, SD = 0.27).Mixed phases with high amounts of micritic dolomite reveal a larger depletion, with δ 13 C values of −34.8‰ and δ 18 O values of 2.6‰.The aragonite fans have low mean δ 13 C values of −37.7‰ (n = 3, SD = 4.87) and a mean δ 18 O value of +2.42‰ (n = 3, SD 0.18; Table S5).The aim of this study is to identify the controlling factors on the formation of authigenic seep carbonates and to link these with past seepage activity.To this end, the different phases of the macromorphologies were put into a paragenetic framework (Figure 8).Differences in the formation of chimneys, crusts and pavements were identified.The chimneys are formed in a sequence progressing through peloidal micrite, LMC microsparite spherulites, non-luminescent aragonite and HMC and LMC cements.Ferromanganese crusts form at the edges of the aragonite fans.Unlike the pavements and crusts, the chimneys are formed in three recurrent cycles of advective methane flow (Figures 8A and 9).While the chimneys are rich in aragonite and HMC cement growing around LMC replacements (Figure 8A), the pavements mainly consist of high amounts of LMC microsparite, which is replaced by clotted micrite and micritic dolomite.Aragonite fans in the pavements are formed in a later stage exclusively in a bioturbated band (Figures 8C and 10).Ghost structures of a former peloidal micrite and traces of bacteria can be identified within the microsparite.The crusts are characterised by HMC nodules and LMC breccia fragments, cemented together by a HMC micrite.Aragonite forms in the pore space between the breccias, while patches of LMC microsparite and small dolomite crystals form within the nodules (Figures 8B and 11).The discussion below focusses on: (1) the formation of the distinct authigenic carbonate phases and the thermodynamic and kinetic factors influencing the mineralogy, and (2) the effect of diffusive versus advective methane flux on carbonate mineralogy, morphology and occurrence.) and advective seepage (Burton, 1993;Burton & Walker, 1987;Greinert et al., 2001;Luff & Wallmann, 2003).Aragonite is formed when high amounts of SO 4 2− and Mg inhibit the formation of calcite (Goetschl et al., 2019).Aragonite formation in regions with active seepage (northern Arabian Sea, Gulf of Mexico and in the South China Sea) are often linked to more advective gas flow (Feng et al., 2014a;Ge et al., 2010;Himmler et al., 2015).Therefore, the aragonite within the samples from the PD and WA 2a locations formed due to high and advective seepage flux.The aragonite can be linked to a sea water-influenced diagenetic system close to or directly at the sea floor, where significant advective fluid flow and efficient methane oxidation tends to create optimal conditions for aragonite precipitation.This is congruent with the stable isotope compositions of aragonite, which is being depleted in 13 C (δ 13 C can reduce to −52‰), and influenced only to a minor extent by the surrounding sea water δ 13 C and organic matter.The threefold appearance of the microsparite and aragonite sequence indicates a cyclic fluid flow pattern (Figure 7), possibly due to overpressure-driven gas migration towards the sea floor, where gas is accumulating in a low-sedimentation, low permeability environment, such as the buried channel-lobe systems (Lawal et al., 2023).
The overlying hemipelagic sediment acts as a seal until the gas pressure is sufficient to initiate fractures (Lawal et al., 2023) and generate a seal breach, which might lead to overburden collapse and the expulsion of gas to the sea floor (Judd & Hovland, 2007;Lawal et al., 2023).Another possibility is sedimentation and gas hydrate recycling in the sediment, where free gas can break through the gas hydrate stability zone (Schmidt et al., 2022).This gas hydrate recycling was studied at the Green Canyon (Gulf of Mexico), with similar features (evaporite deposits in the subsurface, palaeo-channel systems and high sedimentation rates; Schmidt et al., 2022).

LMC cements
All seep carbonates studied show LMC replacements of micrite, aragonite or a former peloidal matrix (probably a combination of LMC, HMC and organic matter).Furthermore, carbonate chimneys host LMC cements.
An explanation for the LMC might be that aragonite and HMC are metastable and even under conditions of low undersaturation, aragonite and HMC tend to dissolve and reprecipitate (Gomberg & Bonatti, 1970).However, the time span for the transition of HMC to LMC is not sufficient as this process probably happens during progressive burial, making this hypothesis rather improbable (Hashim & Kaczmarek, 2021).Furthermore, LMC does not precipitate directly from modern marine waters (Sandberg, 1983) due to the high-Mg/Ca ratio and high SO 4 2− concentrations (Bots et al., 2011).As such, the replacement of sediments by LMC requires a lowering of SO 4 2− and Mg concentrations (Burton, 1993).The sequestration of SO 4 2− can be explained by intense organoclastic sulphate reduction (OSR), due to the high amount of organic matter available in the sediment (Glombitza et al., 2013).As the replacements and cements reveal a dull CL luminescence, either an enrichment in reduced manganese (Mn) or higher amounts of Mn and iron (Fe), sufficient for Fe and Mn quenching can be suggested (Machel, 2000).This enrichment can be explained by Mn mobilisation within suboxic conditions (Calvert & Pedersen, 1993).Fluids with a low-Mg/Ca ratio (<2) are a prerequisite for the precipitation of LMC (Stanley et al., 2002).Possible sources of fluids with low-Mg/Ca ratios are Mg depleted and Ca enriched brine seeps, with such an example recently found in the vicinity of the studied sites (Charlou et al., 2003;Herut et al., 2022;Huguen et al., 2009).The 18 O values of the Palmahim brine pools (+2.0‰VSMOW ) are slightly enriched compared to ambient sea water (+1.5‰VSMOW ; Herut et al., 2022).The source of these brine fluids might be the dissolution of Messinian evaporites by sea water (Figure 1B,C), deep subsurface mineral dehydration and/or diagenetic processes (e.g.formation of K-rich Mg-smectite), which are responsible for the depletion of Mg, K and SO 2− 4 (Herut et al., 2022).The presence of LMC microsparite patches might be linked to higher amounts of organic matter in certain parts of the sediment (Riding & Tomás, 2006) and therefore increased OSR (Huang et al., 2019;Jørgensen et al., 2019).

| Blocky HMC replacement, HMC microsparite and HMC cements
The spatial proximity of the HMC crystals to the aragonite fans in the carbonate chimneys indicate that HMC F I G U R E 7 Stable isotope analysis reveal three different δ 13 C clusters, a highly depleted (mainly aragonite), moderately depleted (LMC and HMC microsparite, micritic dolomite) and non-depleted cluster (mainly micrite).Stable oxygen isotope values are enriched, compared to sea water stable oxygen isotope values.
cements are linked to the aragonite fans.Similar HMC cements have been observed at the Dolgovskoy Mound in the Black Sea (Smrzka et al., 2021).
Cements consisting of HMC form next to the HMC replacements.Low SO 4 2− concentrations are needed to form HMC, as SO 4 2− inhibits calcite growth by blocking calcite growth sites (Goetschl et al., 2019;Nielsen et al., 2016).At higher SO 4 2− concentrations, when calcite formation is inhibited, aragonite growth is promoted.Accordingly, HMC tends to form in more restricted sedimentary systems, characterised by low SO 4 2− concentrations (Bots et al., 2011;Luff & Wallmann, 2003), compared to aragonite.The HMCs tend to form at the sulphate-methane transition zone (SMTZ) with the strong influence of SR-AOM, where the alkalinity F I G U R E 9 Schematic evolution of the carbonate chimneys.Similar to pavements and crusts, LMC microsparite forms in the chimneys as small replacements.During advective seepage, the SMTZ zone is pushed upwards and aragonite is formed at the sedimentwater interface.Due to a change in the pore water composition, botryoidal LMC forms.This could also be related to brine flux, but other mechanisms cannot be excluded.In the last phase, luminescent HMC cement is forming next to the aragonite and engulfing it.The cements are probably formed under suboxic conditions in the sediment, as the cements are luminescent.is increased, the SO 4 2− concentrations are lowered and the pore waters are oversaturated in terms of carbonate (Aloisi et al., 2002;Luff et al., 2005;Wehrmann et al., 2011).This is further supported by the 13 C depleted stable isotope values of the HMC microsparite and blocky HMC replacement.

| Dolomite
Within the pavements and crusts, dolomite forms next to HMC and LMC.Traces of bacterial cells can be identified in the pavements (Figure 6B), possibly driving the micritisation.Combined with the dull to bright luminescence of the dolomite, indicating suboxic to reducing conditions (Boggs Jr. & Krinsley, 2006), an enhanced bacterially driven SR-AOM can be assumed as a potential formation mechanism of the clotted micritic dolomite in the pavements (Petrash et al., 2017).This assumption is promoted by the presence of framboidal pyrite, which is probably formed in the sulphate-reduction zone by upwards diffusing hydrogen sulphide (Berner, 1984).The formation of dolomite can be described as a mixture between OSR in a closed system with low pore water replenishment, coupled to a deep SMTZ.High pore water replenishment would lead to increased SO 4 2− concentrations in the pore water, decreasing the amount of hydrogen sulphide and accordingly inhibiting the formation of dolomite.Within the deep SMTZ, additional hydrogen sulphide is produced by SR-AOM, which diffuses into the overlying sediment layers.High amount of hydrogen sulphide is considered to increase the availability of Mg for dolomite formation, as hydrogen sulphide might play an important role in the dehydration of Mg 2+ ions (Lu et al., 2018;Zhang et al., 2012)-increasing the effective activity of Mg 2+ .The stable carbon isotope composition of the dolomite (−23.8‰)supports mixing between the ascending methane-rich fluids and sea water enriched pore fluids.As the formation of dolomite happens in a closed system, a sediment-buffering component might contribute substantial amounts of carbon.The organic matter in the sampling region has a stable carbon isotope composition between −24 to −28‰ (Sisma-Ventura et al., 2022).

| Carbon source to sink
The stable carbon isotopic data of the chimneys can be, similar to the carbonate crusts, divided into three clusters (highly depleted [−52‰ to 30‰], moderately depleted [−35‰ to −20‰] and non-depleted [−10‰ to +5‰]).Sources of the carbon contributing to the formation of the seep carbonates are (1) ascending methane, (2) sea water and (3) oxidised organic matter.Processes fractionating the primary sources are SR-AOM, OSR and organic matter fermentation in the shallow sediment.
Carbonate crusts and chimneys both show similar micrites.The stable carbon isotope compositions of these phases are relatively homogeneous and not as depleted as the other phases in 13 C.The stable isotope composition of the micrite (−10‰ to +5‰, Figure 7) can be explained by carbonate precipitation in equilibrium with the surrounding sea water with minor amounts of organic matter, as organic matter commonly has stable carbon isotope values ca −24‰ (Hudson, 1977;Sisma-Ventura et al., 2022).As the micrite also indicates values enriched in 13 C (up to 5‰ in δ 13 C), the influence of DIC formed by organic matter fermentation in an oxidised environment can be considered.The δ 13 C of CO 2 generated during fermentation of organic matter can be strongly enriched (Boehme et al., 1996;Irwin et al., 1977), making the influence of fermentation during micrite formation very probable.Earlier studies have already reported methanogenesis influenced authigenic carbonates (Aloisi et al., 2000a) and traces of methanogenic microbes in sediments in this region (Rubin-Blum et al., 2014a).The δ 13 C values of the chimney aragonites cluster around −37‰, most probably indicating a mixing of methane-enriched fluids with sea water and/or marine influenced pore water fluids.The stable carbon isotopic compositions of aragonite occurring in the carbonate crusts reach values as low as −52‰ (Figure 7).This can be explained due to less mixing of the ascending fluids with sea water-like fluids during precipitation.As thermogenic methane is generally less depleted (δ 13 C VPDB − 30‰ to −50‰; Sackett, 1978) than biogenically formed methane (−50‰ to −110‰; Bernard et al., 1978;Whiticar et al., 1986), the ascending methane of chimneys and crusts is most probably of biogenic origin.Aragonite precipitating in carbonate pavements has a similar pattern with δ 13 C values down to −41‰.Therefore, different mixtures between biogenic gas and sea water pore fluids can be assumed for all aragonite phases.
Microsparite is present as LMC in all macromorphologies.The microsparite values are highly variable, depending on the macromorphology.While the LMC microsparite of the pavements reveals only a slight depletion (ca −10‰ to −15‰), the microsparite of the chimneys and crusts show a larger spread (ca −30‰ to −40‰ for chimneys and ca −15‰ to −50‰ for crusts).Furthermore, the LMC microsparite of the crust is intermingled with HMC microsparite.This variability might either be linked to different proportions of methane associated with the ascending brine fluids or greater mixing with surrounding sea waterinfluenced pore waters.As the LMC microsparite tends to form in the OSR zone, high amounts of organic matter are associated with this phase.The high amount of organic matter (δ 13 C VPDB −24 to −28‰) might also contribute significantly to the stable carbon isotopic composition of the microsparite of the crusts and chimneys.
Stable oxygen isotope values of all carbonates are between 1.7‰ and 5.3‰.The measured values are more enriched in 18 O compared to values calculated via the fractionation factor of Mavromatis et al. (2012;LMC and HMC), Böhm et al. (2000;aragonite) and Fritz and Smith (1970;dolomite) at a bottom water temperature of 13.5°C (Tayber et al., 2019) and a given bottom water of 1.5‰ (2.44‰ for aragonite, 0.72‰ for LMC, 1.84‰ for HMC and 2.69‰ for dolomite).While the stable oxygen isotope values of chimneys (ca 2‰-3.5‰) and crusts (ca 1.5‰-5.3‰)are variable, the values of the pavements appear to be less variable (ca 2‰-3‰).As chimneys and pavements show less variability and formation close to the equilibrium with ambient sea water, a smaller influence of 18 O enriched fluids on the formation of chimneys and pavements can be assumed.Crusts reveal the largest variability, indicating the largest influence of 18 O enriched pore fluids on the formation of carbonates.The enrichment in aragonite, LMC and HMC 18 O cannot be explained by temperature effects only (Aloisi et al., 2000a), as the potential temperature needed to precipitate LMC and HMC at 5.3‰ and aragonite at 4.5‰ is too low, being 4°C (LMC/HMC) and 6.5°C (aragonite) respectively.Mean western Mediterranean bottom water temperatures averaged ca 10°C over the last 40 ka (Cacho et al., 2006).Possible explanations for the enrichment in 18 O are either clay mineral dehydration, especially the transformation of smectite to illite in the deeper subsurface (Dählmann & de Lange, 2003), or the decomposition of gas hydrates (Davidson et al., 1983;Hesse, 2003), which were observed in other parts of the Eastern Mediterranean Sea (Aloisi et al., 2000b;Charlou et al., 2003) and were argued by Tayber et al. (2019) to be present in LB.Water, bound to gas hydrates, can be enriched by 3‰ in 18 O, compared to pore waters not affected by gas hydrate dissociation (Hesse & Harrison, 1981;Ussler & Paull, 1995).Brine seepage, which was also observed in the region, might be a potential pathway for the 18 O-enriched fluids (Aloisi et al., 2000a;Herut et al., 2022).As the carbonates of Aloisi et al. (2000a) and Charlou et al. (2003) are similarly enriched in 18 O compared to those of this study (up to 4‰), and for which an influence of brine seepage was also inferred, the situation in those studies may well have been similar to that of this study.

| Fluid dynamics and seep carbonate morphologies in the Eastern Mediterranean
Seep carbonates occur at several places in the Eastern Mediterranean Sea as hardgrounds and crusts (Aloisi et al., 2000b;Bayon et al., 2009;Gontharet et al., 2007;Spiro et al., 2021).Samples collected by Gontharet et al. (2007) are predominately composed of aragonite and only minor amounts of Mg-calcite and dolomite.The stable isotope compositions reported by Gontharet et al. (2007) have similar δ 18 O versus δ 13 C values to this study.The proximity of the two sampling regions and this similarity in stable isotope compositions probably indicate a similar source for the ascending fluids.Aloisi et al. (2000a) reported pavements at the Anaximander mound and Olimpi area that are composed of a mixture of mainly Mg-calcite and aragonite, besides one sample, which contains 18% dolomite.Stable isotope compositions have similarly low δ 13 C values, but more enriched δ 18 O values (Figure 7).The formation of the pavement-like structures in those areas was linked to mud volcano activity and brine pool structures (Aloisi et al., 2000b).Spiro et al. (2021) identified layered structures similar to the pavements in several pockmarks on the continental shelf off Israel.However, the mineralogy of these pavements is different with high amounts of HMC and kutnohorite.Spiro et al. (2021) linked kutnohorite to a deeper methane source.Kutnohorite cannot be identified in any samples of this study suggesting different sources in the Levant region.Stable carbon isotopic values have similar clusters compared to this study with δ 13 C values between −3.4‰ and − 39.9‰.However, the stable oxygen isotope compositions reported by Spiro et al. (2021) are higher in 18 O compared to those of this study (up to +6.2‰).

| Chimneys
Carbonate chimneys in this study appear in a deeper pockmark and in the Levant turbidite channel (Figure 3A,F).Chimneys contain high amounts of aragonite and are interpreted as a product of advective flux regimes (Figure 9).During brine seepage, the HMC micrite is replaced by LMC microsparite, due to the low-Mg/ Ca ratio and intensified OSR.Advective methane seepage promotes the formation of aragonite around the LMC F I G U R E 1 1 Schematic evolution of the carbonate crusts: LMC microsparite within the crusts might form because of brine seepage or the higher stability of LMC.Brecciation of LMC might be caused by a strong upwards flux of CH 4 .Aragonite is forming at a very shallow SMTZ as pore-lining cement.Within the sediment, blocky HMC is replacing micrite and aragonite fans.Nodules of HMC microsparite and LMC microsparite form at a shallow SMTZ and lowered SO 4 2− concentrations in the pore waters.Another stage of aragonite growth can be observed.Due to the deepening of the SMTZ and the H 2 S diffusing upwards from the SMTZ, dolomite can form within the nodules.
microsparite spherules.Throughout the aragonite formation, the SMTZ is near the sediment-water interface.Another phase of brine seepage forms LMC botryoidal cements in small cavities.Following the formation of LMC cements, another stage of aragonite forms during advective methane seepage.After the aragonite formation, HMC cements grow into the open pore space, engulfing the aragonite fans.As the cements are luminescent, they are probably formed in slightly reduced pore waters.
4.6.2| Pavements Carbonate Pavements are found in similar places as the carbonate chimneys, at the edges of pockmarks and within the Levant turbidite channel (Figure 3A,B,C,F).In contrast to the chimneys, the carbonate pavements are more probably formed by a diffusive flow and within the sediment.The LMC replaces a former HMC micrite during brine seepage (Figure 10).The clotted micrite is formed by a higher methane flux, but probably within the sediment in reducing environments.Dolomite forms in a diffusive seepage setting, above the SMTZ.The formed H 2 S diffuses upwards, where Mg 2+ ions are dehydrated and become available for dolomite formation.This is also supported by the δ 13 C values, which suggest a higher amount of mixing between the ascending fluid and the surrounding reduced marine pore waters.As a consequence of this mixture between sea water-enriched fluid and methane-enriched fluid, the stable carbon isotopic composition is less depleted in 13 C compared to the aragonites.This agrees with the seismic data, which indicate a shallow gas reservoir in the working area (Eruteya et al., 2018;Tayber et al., 2019).Aragonite fans form in small bioturbated burrows, where methane can ascend and the pore waters are enriched in Mg and SO 4 2− , inhibiting the formation of calcite.

| Crusts
Carbonate crusts appear on the PD at shallower water depths compared to the chimneys and pavements (Figure 3D,E).The crusts are more probably formed under changing flow conditions-first focussed flux and then mostly diffusive flux conditions, interpreted based on the aragonite to HMC and dolomite transition observed within the crusts (Figure 11).The LMC microsparite replaces the HMC micrite because of brine seepage, possibly associated with gas hydrates (Dickens & Quinby-Hunt, 1997).Aragonitic cements form during advective methane seepage.HMC nodules are later formed in a more restricted environment with lower concentrations of SO 4 2− , compared to the aragonite.Because of a deepening of the SMTZ and the upwards diffusion of hydrogen sulphide, the pore waters become reduced and dolomite can form, as the hydrogen sulphide promotes the dehydration of the Mg 2+ ions, making them available for dolomite formation.Furthermore, δ 18 O values are between 1.7‰ and 5.3‰, indicating changing fluid sources.Overall, within the crusts and chimneys, similar seepage patterns can be identified.Aragonite is present in both macromorphologies and is replaced by LMC microsparite.This indicates that similar mechanisms formed both crusts and chimneys, despite the fact that they are found in different water depths.This implies a depth-independent, potentially time-dependent process for the formation of these seep carbonates.Furthermore, three phases of focussed seepage are identified within the chimneys and crusts.

| CONCLUSION
The combined use of video mapping, sediment petrography, XRD analysis and stable isotope geochemistry reveal different authigenic carbonate morphologies and multiple generations of carbonate phases associated with gas seepage.Distinct cement and replacement phases and morphologies can be ascribed to a combination of different modes of hydrocarbon gas transport and variable mixing and oxidation thereof with the surrounding pore waters.Focussed and advective methane fluxes are linked to the precipitation of high amounts of aragonite and the formation of carbonate chimneys and carbonate crusts.Diffusive hydrocarbon fluxes are associated with the precipitation of high amounts of HMC and dolomite and the formation of carbonate pavements.Interactions with brine seepage, affecting the Mg/Ca ratio of the pore waters, leads to the formation of LMC in all carbonate morphologies.This study provides insights into the precipitation of authigenic carbonate phases and morphologies linked to hydrocarbon seepage in the Eastern Mediterranean Sea.Given the unique nature of the LB (warm bottom waters, highly oligotrophic surface waters), these carbonates are an important analogue to understand the effect of methane migration and carbon sequestration during climate change and global warming events.

ACKNO WLE DGE MENTS
This study is funded through the Swiss National Science Foundation "Unconventional carbonate factories in the Eastern Mediterranean: cold-water coral ecosystems and seeps" project (grant no 200021_175587).We would like to thank the crew and scientists aboard the RV Aegaeo for retrieving samples during the EUROFLEETS 2 SEMSEEP cruise funded the EUROFLEETS grant no 312762, and of the EV Nautilus 2011 NA019 cruise, funded by the Institute for Exploration, University of Rhode Island, and Charney School of Marine Sciences, University of Haifa.We would also like to acknowledge Patrick Dietsche, Alexandre Salzmann and Christoph Neururer for the preparation of the thin sections and the help during the EDS mapping and scanning electron microscopy.We would like to thank Prof. Dr. Jörn Peckmann for his precious input and insights to this study.We would also like to thank two anonymous reviewers for their input to further improve this study.
Map of the Eastern Mediterranean Sea (black rectangle indicates position of the bathymetric map).(B)The different working areas (WA) and domains (CD-compressional domain, TDtranslational domain, ED-extensional domain) are indicated.Seismic profile (indicated on B) with the different stratigraphic units(Elfassi et al., 2019) and possible gas reservoirs (modified afterLawal et al., 2023).Within the Jonathan Depression, the Messinian evaporites are pinched out, forming a possible fluid migration pathway for deeper gas.(B,C) Faults, which act as possible fluid migration pathways for deeper gas and possible brine fluids are indicated in black.(C) Shallow gas might be located 25-100 mbsf, based on high-amplitude seismic reflectors in the stratigraphic unit U4(Lawal et al., 2023;Tayber et al., 2019).

F
I G U R E 2 (A) Solitary carbonate chimneys on the sea floor within the Levant Channel during Dive 1 (WA 4a).(B, C) Chimney sample taken from the location in image A. (D, E) Slabs of chimney sample under normal and UV light with the three different growth sequences indicated (red, yellow and white arrows).(F) Carbonate crust sample, overgrown by soft corals (red arrow) and Desmophyllum corals (yellow arrows; WA 5).(G, H) Different crust samples, with breccias (red arrow) and (I, J) nodules (yellow arrows).The fluorescent part in image J corresponds to aragonite cements.(K) Carbonate Pavements forming hardgrounds on the sea floor (WA 2a) (red arrows).(L, M) Pavement samples, collected during Dive 9 (WA 2a).(N, O) Slab of sample, visible on image L under normal and UV light.A bioturbated band (fluorescent-red arrows) and bioerosion features (yellow arrows) can be identified.(P, Q) Next to the pavements, bivalves and greyish-whitish bacterial mats are visible (yellow arrows; WA 4a).F I G U R E 3 Mapping of different carbonate morphologies.(A) Within the Levant Channel (WA 4a), pavements and chimneys can mainly be identified.(B, C) Pavements and chimneys can also be identified in pockmarks (WA 3 and 4b) (B, C) and in a mega pockmark system in the Levant Basin (WA 2a) (F).In contrast to the pockmarks and channels, crusts can mainly be identified at the flanks of the Palmahim Disturbance (WA 5 and 6) (D, E), in significantly shallower water depths.Macrofauna is often associated with the carbonates.

F
I G U R E 4 (A, B) CL non-luminescent micrite (mic) with luminescent LMC spherules (Lm).(A) Ghost structures of peloids are identified within the spherules indicating replacement.(A) Aragonite (arg) is present around LMC spherules.(C) Within the micrite small bright luminescent dolomite crystals are identified with EDS (dol).(D) Replacement of the micrite by LMC is evidenced by EDS mapping with high-Mg content in the primary micrite compared to the replacive LMC.(D) Small baryte crystals (ba) are identified by EDS analysis.(E, F) LMC botryoidal cements (Lb, dull zonated) are forming in small cavities.(E) Aragonite fans are often overgrown by ferromanganese crusts (fm).(G, H) Luminescent HMC cements (cc) are growing from LMC spherules into open pore space, engulfing aragonite fans.(G, H) A clear dissolution horizon (dh) covering the CL luminescent HMC cement can be identified.(I) The LMC and aragonite are covered by ferromanganese crusts.Baryte crystals are identified within the micrite.(J) Aragonite fans, growing into the micrite are partially replaced by LMC.

4. 1 |
Formation of authigenic carbonate phases 4.1.1| Aragonite Aragonite formation is commonly linked to environments with high concentrations of dissolved sulphate (SO 4 2−

F
I G U R E 5 (A) Micrite (mic), enriched in fossils (mainly planktonic foraminifera) and quartz grains.(B) LMC breccias (Br) in matrix and pore-lining aragonite cements (arg)-framboidal pyrite (fp) is associated with all phases.(C) Within the HMC microsparite (Hm) nodule, traces of LMC microsparite (Lm) are identified.(D) The HMC nodule has no CL luminescence, but reveals small bright crystals (dol).(E) EDS-Mg elemental map of an HMC nodule shows the presence of dolomite (dol).(F, G) Aragonite fans growing in pores within the HMC microsparite nodules (partially replaced by LMC microsparite) in several stages.(G) The aragonite fans are interrupted by sedimentation surfaces (sed), a dissolution horizon (dh) and ferromanganese crusts (fm).(H) Pores within the HMC microsparite are filled by aragonite pore-lining cement.(I) HMC crystals (blc) replacing the micrite and aragonite.The crystals are also growing into cavities as blocky cements.Pyroxene (pyr) and Phyllosilicates can also be identified.(F, J) Bioerosion (bio) cutting through aragonite fans.The parts are refilled by a micrite, similar to the micrite in image A.

F
I G U R E 6 (A) LMC microsparite (Lm) and clotted micrite (cl).(B) FL luminescence reveals the presence of bacterial cells (bac) in the LMC microsparite.(C, D) The LMC microsparite reveals no CL luminescence, while the clotted micrite reveals dull to bright luminescence.(E, F) CL and EDS mapping reveals the presence of dolomite crystals (dol).(G-I) Aragonite fans (arg) are growing as pore-lining cements from peloidal micrite (mic) and fossil fragments (fos) within the bioturbated band.(J) Semi-lithified infill within bioerosion features.The infill is enriched in detritic material (det) and fossils.

F
I G U R E 8 Paragenetic sequence observed in (A) the carbonate chimneys, (B) carbonate crusts and (C) carbonate pavements.(A) Three cycles of seepage are identified in the chimneys.The red boxes indicate CL luminescent phases.
Schematic evolution of the carbonate pavements.LMC microsparite forms by replacement of a peloidal matrix, either by brine seepage or due to the higher stability of LMC.Diffusive methane fluxes are shallowing the SMTZ, resulting in the precipitation of clotted HMC.Lower methane fluxes, deepen again the SMTZ zone, hydrogen sulphide is diffusing upwards from the SMTZ resulting in the precipitation of dolomite.Only within one bioturbation, aragonite cements are present possibly due to limited methane escape.