International Conference on Gas Hydrates (ICGH) (6th : 2008)


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1   RE-EVALUATING THE SIGNIFICANCE OF SEAFLOOR ACCUMULATIONS OF METHANE-DERIVED CARBONATES: SEEPAGE OR EROSION INDICATORS?   Charles K. Paull* and William Ussler III  Monterey Bay Aquarium Research Institute 7700 Sandholdt Road Moss Landing, California 95039-9644 USA   ABSTRACT Occurrences of carbonate-cemented nodules and concretions exposed on the seafloor that contain cements with light carbon isotopes, indicating a contribution of methane-derived carbon, are commonly interpreted to be indicators of seafloor fluid venting. Thus, their presence is commonly used as an indicator of the possible occurrence of methane gas hydrate within the near subsurface. While some of these carbonates exhibit facies that require formation on the seafloor, the dominant fine-grained lithology associated with these carbonates indicates they were formed as sediment- hosted nodules within the subsurface and are similar to nodules that are obtained from the subsurface in Deep Sea Drilling Project, Ocean Drilling Project, and Integrated Ocean Drilling Project boreholes. Here we present the hypothesis that the occurrence of these carbonates on the seafloor may instead indicate areas of persistent seafloor erosion.  Keywords: gas hydrates, methane-derived carbonates, continental margin fluid flow, seafloor erosion  NOMENCLATURE AOM - Anaerobic Oxidation of Methane CBC – Chemosynthetic Biological Communities DIC – Dissolved Inorganic Carbon MDAC – Methane Authigenic Derived Carbon mM – milimolar PDB – PeeDee Belemnite isotope standard SMTZ - Sulfate Methane Transition Zone MOA – methane-oxidizing archaea  INTRODUCTION Numerous occurrences of authigenic carbonate nodules and concretions have been found on the seafloor in continental margin settings [e.g., 1-2]. Carbon isotopic values of these cements commonly are distinctly negative (e.g., δ13C <- 25‰ PDB), which indicates that significant amounts of methane-derived carbon are incorporated within the precipitated carbonate cements and illustrates that they were formed in close association with methane-rich environments. In some areas where authigenic carbonates occur on the seafloor, there is independent evidence for active fluid and gas venting nearby. These associations have given rise to the common interpretation that these carbonates indicate areas of seafloor seepage of methane-bearing porewaters and/or gaseous methane venting. Methane-derived authigenic carbonates (MDAC) also have been sampled on the seafloor in other areas that are not believed to be associated with active venting. Usually these sites are inferred to represent fossil discharge areas where the fluid flow has ceased [1, 3-5]. In this paper we re-evaluate the assertion that these MDAC are indicators of methane-rich fluid flow and propose an alternative hypothesis as to what some of these deposits signify.  *Corresponding author: Phone 831-775-1886; FAX 831-775-1620 E-mail: Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. 2 Methane-derived authigenic carbonates The association between authigenic carbonate and methane is based on the distinct carbon isotopic composition of the carbonate cements, which frequently have δ13C values that range from –20‰ to –65‰ PDB [e.g., 1-6]. Authigenic carbonates form from DIC available in the surrounding porewater without significant isotope fractionation and reflect the δ13C composition of the DIC at the time of cement formation. Because methane, especially microbially-produced methane, has distinctly 13C-depleted isotope values (e.g., <-60‰ PDB; [7]), a substantial proportion of the carbon contained in these cements was derived from the addition of 13C-depleted carbon to the DIC pool by anaerobic oxidation of this methane.  Conditions appropriate for concentrating methane- derived carbon in the porewater from which the cements form need to exist to stimulate the formation of these cements. Ocean waters contain ~2 mM DIC with δ13C values more positive than -4‰. Thus to shift the isotopic composition of seawater from its initial DIC concentration and isotopic composition by the addition of methane- derived carbon will require the majority of this DIC carbon to come from 13C-depleted methane.  Methane is oxidized both aerobically and anaerobically by microbially-mediated processes. Aerobic oxidation of methane will occur where active venting brings methane into the oxygenated water column and is of importance for the oxidation of methane within the water column [e.g., 8]: CH4 + 2O2  HCO3 - + H+  + H2O (1) While aerobic oxidation of methane rates may be higher around seafloor gas vents, methane has a residence time in the open ocean water column of several weeks [8-9]. Thus, aerobic oxidation rates of methane are slow compared with the oceanographic processes that will transport, mix and dilute methane carbon within benthic bottom waters. A restricted sub-environment surrounding a vent site is required to allow methane-derived DIC to accumulate sufficiently to drive authigenic carbonate precipitation, but an adequately restricted sub-environment is inconsistent with the need for oxygen during aerobic oxidation of methane. The aerobic oxidation of methane (AOM) will lower the pH because of the net production of hydrogen ions (reaction 1). Thus, aerobic oxidation of methane is not conducive to authigenic carbonate precipitation, but instead can promote carbonate dissolution.  In more isolated sub-seafloor environments where upward migrating methane meets sulfate diffusing downward from overlying seawater, populations of microorganisms engage in AOM [8, 10-11]: CH4 + SO4-2  HCO3- + HS- + H2O (2) This reaction (2) converts methane carbon into HCO3-, spiking the porewater DIC pool with 13C- depleted carbon and increasing the alkalinity of the water [12-13]. This alkalinity increase will stimulate the formation of authigenic carbonates and the bisulfide that is generated provides chemical energy for sulfide-dependent Chemosynthetic biological communities (CBC) [14-15]. Carbonates formed from porewater DIC enriched with methane-derived carbon supplied by AOM (Reaction 2) have distinctive 13C-depleted isotopic values [e.g., 1-2, 16].   Figure 1. Conceptual diagram showing relationship between sulfate and methane concentration gradients, the location of the SMTZ, and the location of methane derived authigenic carbonate formation associated with the AOM.  In marine sediments methane and sulfate will co- occur in a zone at the base of the sulfate-bearing zone and above the underlying methane-bearing sediments (Figure 1). Numerous observations indicate that AOM at the sulfate-methane transition zone (SMTZ; previously called the sulfate-methane interface or SMI) strongly affects the porewater chemistry in continental margin areas where there is a source of methane at depth. The upward flux of methane sweeps the SMTZ towards shallower depths, generating linear sulfate concentration profiles, 13C-depleted DIC, which is 3 in the range of observed authigenic carbonate nodules [e.g., 13, 16-17].  Sulfate reduction of sedimentary organic matter is another widespread process that occurs in most shallow marine sediments [18]: 2CH2O + SO4-2  2HCO3- + H2S (3) Reaction 3 also increases porewater alkalinity. Because sulfate reduction of sedimentary organic matter occurs throughout the sulfate-bearing sediment column, its effects are distributed over a broader area rather than being focused at a relatively narrow zone as in the case of AOM at the SMTZ (Reaction 2). Sulfate-reduction of organic matter is less likely to stimulate local authigenic carbonate precipitation and the addition of carbon to the porewater DIC pool from organic matter decomposition (e.g., -25‰ to -20‰ δ13C) cannot generate DIC with δ13C value of <-25‰.  Of these reactions, AOM is the most capable of inducing conditions appropriate for the generation of MDAC.  SEDIMENT FABRIC MDAC samples collected from continental margins exhibit distinct morphologic features and habits that are easily distinguished in hand sample and thin-sections. Samples collected from the seafloor come in a variety of forms including pavements, slabs, irregular nodules and what have been described as chimneys ([3, 19]; Figure 2). Samples collected from the seafloor can be divided into two distinct facies based on whether they have coarse- or fine-grained textures. A third group comprises nodules commonly found in boreholes, and are lithologically similar to the fine-grained seafloor carbonates. They are distinguished here only because they are obtained from the subsurface.  (1) MDAC facies formed in association with CBC Coarse-grained authigenic carbonates contain multiple styles of MDAC cements including aragonite needles filling voids, aragonite as acicular cements, sparry calcite cements of both low and high Mg-calcite and occasionally dolomite. Multiple events of cementation can also be seen in thin-sections. These samples commonly contain re-cemented intraformational breccias. Breccia formation is believed to be a consequence of deformation associated with gas hydrate formation in the shallow seafloor [20].  Some easily identified criterion help distinguish these carbonates as having formed on or in close proximity to the seafloor. The inclusion of the bivalve shells and other fossils associated with seafloor CBC within these MDAC-cemented samples implies that the SMTZ and a biogeochemical environment appropriate for carbonate formation is at or near the seafloor (Figure 2). The presence of acicular void filling aragonite cements and aragonite needles also distinguishes coarse-grained authigenic carbonates formed near the seafloor. The existence of intraformational breccias is another characteristic of carbonates formed around sites where vigorous seafloor venting has occurred. Such coarse-grained authigenic carbonates are found in the immediate vicinity of obvious fluid vent sites [e.g., 2, 4, 6, 21-24].  (2) MDAC facies formed below the seafloor The most common occurrence of MDAC consists of micritic cements formed within fine-grained clastic sediments. The micritic cements are predominately composed of low Mg-calcite and occasionally dolomite. Typically these samples have a massive appearance and there is no evidence of significant primary porosity within the host sediments.  The cements in these authigenic carbonates fill an established sediment matrix within the host sediment. Typically the host sediments are like the underlying sediments where they are found, which requires that the sediments already existed at the time the cements formed. The presence of the fine- grained clastic host sediments within these nodules requires that the cements formed in the subsurface. Thus, erosion of the seafloor is necessary to have these exposed on the seafloor.  Samples of these fine-grained MDAC are commonly found dispersed over wide areas of the seafloor, rather than being concentrated just around discrete vent sites. Often the seafloor surrounding areas where MDAC are found are devoid of CBCs or unequivocal indications of venting such as active gas bubble plumes.  The exterior shapes of these carbonate nodules are diverse. Massively-cemented fine-grained nodules 4 have smooth rounded exteriors and are commonly ovoid, but sometimes elongated and up to a meter in length. These larger specimens sometimes have central holes (Figure 2B) and have been referred to as chimney structures and are discussed below.   Figure 2. Photographs illustrating MDAC samples. A is a long, narrow fine-grained sample from the Florida Escarpment [2]. White bar is 10 cm. B shows a doughnut “chimney” from Monterey Bay, CA. Black bar is ~10 cm Both samples shown in A and B were picked up off the seafloor, presumably after being eroded out of the sediments in which they formed. C shows a MDAC nodule from ~0.6 mbsf from under a CBC [56]. D shows a MDAC nodule sampled from ODP Site 994 ~41 mbsf [16].  (3) MDAC nodules found in the subsurface MDAC nodules are commonly reported in cores from Deep Sea Drilling Project, Ocean Drilling Project, and Integrated Ocean Drilling Project boreholes [e.g., 16, 25-30]. In general, these nodules come from methane-rich continental margin sites. While the shape of the MDAC samples recovered in boreholes is clearly limited by the sampling method, these cored samples have an appearance in hand sample and thin-section that is generally similar to fine-grained nodules found on the seafloor. Although many have a firm core, they have a broader range of firmness and only partially-cemented samples are common (Figure 2C). Nodules from the seafloor may have the same origin as the nodules found at depth within boreholes.  VARIATIONS IN CEMENTATION WITH METHANE FLUX The formation of authigenic carbonate from DIC with a significant contribution of methane-derived carbon occurs where methane and sulfate co-occur at the SMTZ (supporting AOM) and also where the products of AOM accumulate sufficiently to stimulate carbonate precipitation (Figure 3).  The depth to the SMTZ varies considerably in continental margin settings [13]. The depth of the SMTZ and the rate of AOM are controlled by the upward flux of methane toward the seafloor (Figure 3), which is in turn sensitive to the methane concentration deeper in the sediment column. The flux of methane to the surface varies  Figure 3. Cartoon showing the depth of the SMTZ as a function of the upward flux of methane. A continuum exists from high flux to low flux environments. At high flux sites methane and hydrogen sulfide produced by AOM occur near enough to the seafloor to support CBCs. Areas of decreased flux have no surface expression, and the amounts of MDAC will diminish as the depth to the SMTZ increases. 5 with the inventory of methane, the amount and distribution of gas hydrate, and whether there is active advection in the subsurface. The impact of AOM in terms of the amount and style of carbonate formation will also change as both the SMTZ shoals and the flux of methane increases.  Formation of fine-grained sediment-hosted MDAC Sedimentary sections that do not contain much methane will have minimal vertical methane fluxes and a correspondingly deep (>50 m) SMTZ [13]. Thus, the effect of AOM, if it is occurring at all, will be minimal on porewater alkalinity and thus biogeochemical conditions will be unfavorable for formation of authigenic carbonate nodules.  In methane-rich continental margin settings, which are also sites where the potential for gas hydrate to occur in the subsurface is increased, the SMTZ occurs at somewhat shallower sub-bottom depths (e.g., <20 mbsf [13, 31-32]). An increased vertical flux of methane results in shallower SMTZ depths and steeper sulfate concentration gradients. Increased porewater alkalinity at the SMTZ caused by AOM will result in more extensive cementation with MDAC cements within the subsurface. The amount of authigenic carbonate cementation depends in part on how long SMTZ remains focused at a particular horizon within the pre- existing sediments [17].  In areas of low to moderate methane flux, the SMTZ does not reach the seafloor and CBCs are not supported. However, AOM will still occur along the SMTZ deeper in the sediment column. The amount of authigenic carbonate will depend on the supply of methane, the rate of AOM, the rate at which the DIC diffuses away, and the time interval over which the effects of AOM are focused on a particular horizon [17]. While the amount of carbonate will vary, the textures of fine- grained carbonate do not provide information about how far into the subsurface the host sediments were at the time the cements formed.  Formation of coarse-grained MDAC Most continental margin sediments are fine- grained and thus the fluxes are controlled by diffusion. However, some vents occur, which are usually associated with structural or stratigraphic conduits through which dissolved methane-bearing water and even gaseous methane flows to the seafloor. In these areas of high methane flux the SMTZ will crop out on the seafloor. This will support CBC [15] and dramatically alter the local environment. However, cement formation is restricted to local sub-environments within these seafloor sediments, which allow the methane- derived DIC and alkalinity to accumulate within the pore space. These methane-derived authigenic carbonates reflect the dynamic seafloor environment by being coarse-grained, containing fossils of the CBC that are supported on methane or locally-produced bisulfide, having authigenic carbonate filling the original porosity with multiple generations of cements, and being brecciated. While authigenic carbonates may be formed in a zone surrounding the venting conduit, it is unlikely that vigorous venting will give rise to laterally widespread authigenic carbonate formation.  EFFECTS OF EROSION The fate of authigenic carbonate nodules depends on whether the overlying seafloor is undergoing sediment accumulation or erosion. Below we consider the implications of net sediment accumulation or erosion, and especially the effects of widespread seafloor erosion and erosion by slope failures.  Widespread seafloor erosion The position of the SMTZ and the chronologic sequence of MDAC formation within sediments will differ depending on whether the seafloor is undergoing sediment accumulation or erosion (Figure 4). When there is net sediment accumulation, carbonate nodules formed at the SMTZ will be progressively buried. Thus, unless the upward flux is high enough to vent onto the seafloor, MDAC nodules will never occur on the seafloor.  Erosion of the seafloor will expose MDAC formed by AOM within the sediment column on the seafloor. Carbonate-cemented nodules are more resistant to erosion than the uncemented host sediments. Thus, exhumed carbonate-cemented nodules will accumulate as lag deposits in areas experiencing long-term erosion (Figure 4).  Seafloor erosion will also alter the porewater chemical gradients in seafloor sediments. The progressive removal of seafloor sediments by erosion will result in a shoaling of the SMTZ and 6 enhance the rate of MDAC formation at the SMTZ. The presence of MDAC at these sites might result in the false impression that there had been fluid venting in the past and an over assessment of the chances for gas hydrate to occur in the near subsurface.  Figure 4. Conceptual diagram illustrating how the fate of MDAC nodules formed in the subsurface at the SMTZ depends on the whether the area is undergoing net sediment accumulation or erosion. The pink represents the youngest nodule formed under present conditions (T-3), light red intermediate aged nodules (formed at T-2) and dark red the oldest nodules (formed at T-1). In depositional areas, the nodules will be arranged with the most recently formed nodules on top. However, in eroding areas, the oldest nodules will be concentrated as an erosional lag on the seafloor, while the newest ones will be forming at depth.  Slope failures Submarine slope failure events can expose sediments on the seafloor that contain dissolved methane in the porewaters (Figure 5). The presence of methane within the seafloor sediments will establish conditions that are appropriate to support CBC and the formation of MDAC in the subsurface as re-equilibration proceeds [33-34]. The occurrence of MDAC and CBC in these areas could lead to over interpretation of the potential for gas hydrate occurrences in the near subsurface.  FEATURES ASSOCIATED WITH MDAC Carbonate chimney structures The term chimney [5, 35-37] has been used to describe a recurring morphology associated with fine-grained MDAC samples (Figure 2B). These chimneys consist of carbonate-cemented sediments, which contain a central open-hole. These chimneys are hosted within fine-grained mudstones and the holes are typically a few centimeters in diameter. Where the samples appear to be in place, the holes have a vertical orientation. The shapes of the chimney samples suggest that the cement has developed concentrically around the central hole. The characteristically 13C- depleted δ13C values of these carbonate cements indicate that the DIC from which they formed contained a significant amount of methane-derived carbon. Because these carbonates grew within a fine sediment matrix, they are clearly formed within the subsurface and their presence on the seafloor requires subsequent erosion.  Figure 5. Schematic drawing showing how porewater sulfate and methane concentration gradients in a typical continental margin sediment sequence are altered by a major slope failure event. Part A indicates established profiles of sulfate (red) and methane (yellow) before the slope failure removes overburden. Sulfate decreases with depth from seawater concentrations at the seafloor because of sulfate reduction and AOM. When sulfate is depleted at the SMTZ, methane concentrations increase with greater depth. The location of a future failure surface (FS) becomes the new seafloor immediately after the slide (B). Initially sulfate-free sediments will be exposed on the seafloor. Over time sulfate will diffuse back into the sediments (C), re-establishing the sulfate gradient.  The term chimney is also used to describe other authigenic structures. For example, sulfide mineral chimneys structures associated with hydrothermal vents are well known where hot waters escape from the mid-ocean ridge crests [38]. The hydrothermal vent chimneys form around venting conduits and grow upwards into the water column because of rapid mineralization associated with abrupt thermal and redox changes that occur as the 7 hydrothermal plume encounters seawater [39]. The MDAC chimney structures found on continental margins have a grossly similar shape to the hydrothermal vent chimneys. Because these two types of features have similar morphologic appearance, they are typically described with the same genetic term and consequently have commonly been presumed to have an analogous origin.  In the Black Sea there are authigenic carbonate structures, also described as chimneys, which stick up to 4 m above the seafloor into persistently anoxic bottom water [40]. Streams of methane gas bubbles emanate from the tops of these structures. Stable isotope measurements from these carbonate chimney structures and the presence of methane- oxidizing archaea (MOA) attest to their formation under present-day anoxic bottom water conditions [41-43]. MOA do not survive in oxygenated environments. The presence of anoxic bottom waters in the Black Sea expands the biogeochemical zone where AOM occurs into the water column and prevents venting methane from being oxidized aerobically in the benthic water column. This unusual seafloor environment allows MDAC structures to grow up off the sea floor into the water column. These features may be a special case because they are sediment-free and found within anoxic bottom water.  Although many active methane vents are found on continental margins throughout other ocean basins, comparable MDAC chimney structures are not known to surround active gaseous methane vents from the seafloor into oxic bottom waters [e.g., 6]. Thus, there are no known modern analogues for the formation of the common methane derived- carbonate chimney structures. Although it is possible, we are unaware of any evidence for the occurrence of well-developed MDAC-cemented sub-seafloor plumbing systems associated with active methane venting that have this shape. Below we will propose that these carbonate are not necessarily formed around gaseous methane vents.  Authigenic carbonate chimney formation The genetic term chimney inherently implies a conduit through which something is moving upwards. As a consequence MDAC chimney structures are commonly assumed to have formed around sites of active methane venting. Examples of MOA and sulfate-reducing bacteria preserved within authigenic carbonate structures indicate the role of AOM in the formation of the carbonate [37]. However, the connection with venting gas is an inference that is not directly supported by the available data for many of the chimney structures.  Because the biogeochemical arguments presented earlier suggest that restricted conditions are necessary for authigenic carbonate formation, the carbonate structures described as chimneys most likely have formed within fine-grained sediments, and have not grow upward into oxygenated seawater. Instead, these MDAC structures have formed in the subsurface and have been subsequently exposed on the seafloor by erosion. While the central hole in chimneys could provide a conduit through which water and/or gas could move, how these structures form is unclear. Sediment that once filled the area now occupied by the holes needs to be removed. Upward fluid flow usually occurs along fractures and is not known to generate long cylindrical tubes within an otherwise uniform substrate. Bubbles developed in near-seafloor sediments [44] are not known to produce long cylindrical tubes. Thus, there are difficulties in explaining these structures as being gas conduits.  An alternative view of these authigenic carbonate structures is that they form around animal burrows that extend downward from the seafloor through the SMTZ into methane-bearing sediments (Figure 6). Animal burrows are known to extend to depths in excess of 2 m [45]. The size, shape, and orientation of the open holes preserved within chimneys are consistent with known shrimp burrow tubes [46]. Depending on the occupant, ventilation of the burrow may bring sulfate- bearing oxygenated seawater downward into the methane-bearing host sediments. If the oxygen can be consumed either through respiration or consumption by sulfur-oxidizing bacterial mats lining the burrow (e.g., Thioploca or Beggiatoa), AOM will be stimulated in the methane-rich sediments surrounding the burrow.  The restricted microenvironment within the walls of the animal burrow provides a suitable site for the precipitation of MDAC. Shrimp burrows are known to stimulate microbial colonization within the surrounding sediments [47].  8  Figure 6. Cartoon showing two models of how sediment-hosted authigenic carbonate chimneys may form in fine-grained sediment surrounding a conduit. Both cases depend on methane and sulfate being brought together in the sediments immediately surrounding a tube and AOM to stimulate carbonate formation. Part A shows gas bubbles passing through a central tube, which provide methane to the sulfate-bearing porewater within the surrounding sediments. Part B shows a burrow that extends through the SMTZ into sediments with porewater containing methane. Sulfate may be transported into the burrow by animal ventilation. Bringing sulfate-bearing seawater into contact with methane-bearing sediments surrounding the burrow would facilitate AOM. Parts C and D are cross sections of the tubes, illustrating methane moving upwards into sulfate-bearing host sediment from a gas carrying conduit (A and C), while the burrow (B and D) carries sulfate downward into methane-bearing host sediments.  Erosion, which is required to expose MDAC structures on the seafloor, would also result in a shoaling of the SMTZ depth. When the SMTZ is within the upper ~2 m of the seafloor, it is shallow enough to be reached by animal burrows. Thus, these chimney structures may form around animal burrows in areas where the SMTZ is shallow.  This alternative explanation for the origin of chimney structures changes the biogeochemical environment from one associated with gaseous methane venting to one simply associated with moderate methane gradients. Seafloor erosion may enhance this process by bringing methane-rich sediments closer to the seafloor. Chemoherm formation The term chemoherm was created to describe a large carbonate structure exposed on the western flank of Hydrate Ridge [48; Figures 7 and 8]. Samples from this feature are composed primarily of MDAC [19, 21, 48-50] that are exposed on the western flanks over a zone more than 40 m high and 400 m in width (Figure 8). Although this feature has also been called a pinnacle, without vertical exaggeration its relief is modest (Figure 8). Methane venting is observed in adjacent areas on Hydrate Ridge and in places the surfaces of the carbonates are still coated with bacterial mats. While some samples are essentially pure carbonate, others contain quartz and feldspar grains [50]. MDAC have a wide range of facies including microbial filaments and skeletal debris from CBC, and occur in a variety of forms that range in size up to blocks several meters across (Figure 7). In places these blocks are separated by distinct open cracks, but expose layering that appears to be continuous with the adjacent blocks.   Figure 7. Video images collected with the ROV Tiburon (Dive T-168) on the carbonate structure on the west flank of Hydrate Ridge. Images are ~3 m wide. A shows undercut slabs of apparently exhumed MDAC exposed on top of the pinnacle (790 m). B shows open fractures and layering that appears to be bedding on the flank of this feature 800 m water depth.  A conceptual model for the formation of a chemoherm has been offered which suggests that 9 these features grow upward from the seafloor by addition of MDAC to the outer perimeter of the structure as a consequence of methane venting [50]. This model depends on carbonate forming within a microenvironment created by the bacterial mats that mantle these structures. As the relief of a mound increases, the distance venting gas must pass increases, and because of the open framework of the structure this gas will mix with seawater. Why a restricted microenvironment necessary for MDAC formation would persist on the periphery of these features, allowing continued upward growth, is unclear.  An alternative view is that these structures are exhumed by erosional down-cutting in areas where MDAC form in the sub-surface. The Hydrate Ridge pinnacle (Figure 7) is on the crest of the tectonically uplifted ridge that is subject to erosion by bottom currents. The removal of less competent, uncemented sediments may have exposed this framework of comparatively competent MDAC. Thus, as the level of the surrounding seafloor recedes, the carbonates are left in relief. This model was offered for a very similar structure on the flanks of the Guaymas Basin [24].   Figure 8. Cartoon showing two models for the formation of chemoherms. The heavy line is the present day topography on the western flank of Hydrate Ridge (with 5X vertical exaggeration) crossing the large carbonate structure. Tan shows areas of MDAC. Part A depicts formation by upward accretion. Part B depicts formation by erosional exhumation with grey shading indicating material removed by erosion.  SONAR MAPPING OF SEA FLOOR SEEPS Seafloor exposures of authigenic carbonates are easily detected using sonar because they form rough, hard substrates, which are strong acoustic backscatter surfaces. Extensive areas of high backscatter have been mapped where MDAC nodules have been sampled. The presence of MDAC has been interpreted to be associated with widespread fluid advection [51-54]. Because these carbonates are associated with rough seafloor, they are easily identified using sonar. Thus, sonar surveys have been used to identify and map potential fluid venting areas over large areas [52- 54].  While some MDAC are clearly associated with fluid seepage, we suggest that many samples of MDAC were formed within the subsurface and have subsequently been exposed on the seafloor by erosion. Interpreting all MDAC as being seepage indicators may substantially overestimate the amount of seafloor that has or is experiencing active fluid seepage. Thus, the use of MDAC as an indicator of fluid seepage needs to be re-evaluated.  CONCLUSIONS The occurrence of methane-derived authigenic carbonates (MDAC) on the seafloor is commonly used as an environmental indicator for seafloor methane seepage and gas venting. However, fine- grained MDAC, including authigenic carbonate chimney structures, are not necessarily associated with advective venting of methane. While some MDAC clearly form on the seafloor in areas immediately adjacent to active seafloor vents, others are formed in restricted subsurface environments in which the diagenetic impact of AOM is magnified. Accumulations of MDAC occur on the seafloor when exhumed by erosion. Areas of high seafloor reflectivity associated with MDAC may represent erosional lags and be unrelated to active fluid advection. The distinct positive topography mantled with MDAC, sometimes called chemoherms, and often associated with active methane venting, may also be attributed to differential erosion rather than upward accretion of MDAC on the seafloor. 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