6th International Conference on Gas Hydrates

RE-EVALUATING THE SIGNIFICANCE OF SEAFLOOR ACCUMULATIONS OF METHANE-DERIVED CARBONATES: SEEPAGE OR EROSION.. Paull, Charles K.; Ussler III, William 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.,  ?13C  <-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: Paull@mbari.org 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 ?13C 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 ?13C 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 ?13C 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?  ?13C) cannot generate DIC with ?13C 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  ?13C  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. Because sites of active methane venting are likely to be associated with near-surface gas hydrate, the use  of  seafloor  accumulation  of  MDAC  and authigenic  carbonate  structures  as  a  proxy indicator  for  methane  leakage  needs  to  be  re-evaluated.  REFERENCES [1] Ritger S, Carson B, Suess E. Methane-derived authigenic  carbonates  formed  by  subduction-10 induced  pore-water  expulsion  along  the Oregon/Washington margin. Geological Society of America Bulletin. 1987; 98: 147-156. [2]  Paull  CK,  Chanton  J,  Neumann  AC,  Coston JA,  Martens  CS,  Showers  W.  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