6th International Conference on Gas Hydrates

DETECTION OF METHANE SOURCES ALONG THE CALIFORNIA CONTINENTAL MARGIN USING WATER COLUMN ANOMALIES Ussler III, William; Paull, Charles K. 2008-07-31

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* Corresponding author: Phone: +1 831 775 1879 Fax +1 831 775 1620; Email: methane@mbari.orgDETECTION OF METHANE SOURCES ALONG THE CALIFORNIA CONTINENTAL MARGIN USING WATER COLUMN ANOMALIESWilliam Ussler III?  and Charles K. PaullMonterey Bay Aquarium Research Institute7700 Sandholdt RoadMoss Landing, CA 95039-9644USAABSTRACTWater column methane measurements have been used to understand both the global distribution of  methane  in  the  oceans  and  the  local  flux  of  methane  from  geologic  sources  on  the  continental margins,  including  methane  vents  and  gas-hydrate-bearing  sites.  We  have  measured  methane concentrations in 1607 water samples collected along the central California continental margin. Methane supersaturation of the surface mixed layer (0-50 msbsl) is widespread and above a well-defined  subsurface  particle  maximum  (~50  mbsl)  that  generally  corresponds  with  the  pycnocline. Local production of methane appears to be occurring in the surface mixed layer above the particle maximum and may not be particle-associated. Methane concentrations in water column CTD cast profiles  and  ROV-collected  bottom  waters  obtained  in  Partington,  Hueneme,  Santa  Monica,  and Redondo submarine canyons increase towards the seafloor and are distinctly higher (up to 186 nM) compared to open-slope and shelf waters at similar depths. These values are in excess of measured surface water methane concentrations and could not be generated by mixing with surface water. Elevated methane concentrations in these submarine canyons and persistent mid-water methane anomalies  in Ascension  and Ano  Nuevo  Canyons  could  result  from  restricted  circulation  and/or  proximity  to  gas  vents,  seafloor  exposure  of  methane  gas  hydrates,  recently-eroded  methane-rich sediment, submarine discharge of methane-rich groundwater, or particle-associated methane production. On the Santa Barbara shelf water column methane profiles near known gas vents also increase in concentration with increasing depth. Thus, elevated bottom water methane concentrations observed in submarine canyons may not be diagnostic of proximity to methane vents and may be caused by other processes.Keywords: methane, water column, submarine canyons, gas vents, turbidity, organic detritusProceedings of the 6th International Conference on Gas Hydrates (ICGH 2008),Vancouver, British Columbia, CANADA, July 6-10, 2008. NOMENCLATUREAUV Autonomous Underwater Vehiclecc cubic centimetersCTD Conductivity Temperature DepthGC Gas ChromatographHOV Human Occupied Vehiclembsl meters below sea levelmL millilitersOD Outside DiameternM nanomolar (nanomoles/liter)ppm parts per millionPTFE Polytetrafluoroethylene (Teflon )ROV Remotely Operated VehicleSGD Submarine Groundwater DischargeINTRODUCTIONIn the past few decades the existence of widespread methane venting from the seafloor along continental margins  around  the  world  has  become  well  established.  Methane  venting  is  now  considered to  be  a  process  capable  of  rapidly  transferring large  amounts  of  methane  carbon  from  geologic to  oceanographic  and  atmospheric  reservoirs [1].  Methods  for  detection  of  methane  venting include  acoustic  imaging  of  bubble  plumes  [2], acoustic  identification  of  seafloor  vent  sites  [3], direct  observation  by  HOV/ROV  [4],  and  water column  sampling  [5].  Except  for  a  few  efforts to  measure  bubble  flux  at  small  clusters  of  gas vents [6], measurement of water column methane concentration  is  the  only  practical  means  for estimating the amount of methane emanating from Figure 1: Water column study areas (shown in more detail in Figures 2A-2F) are outlined in red boxes.122?0'0"W118?0'0"W38?0'0"N36?0'0"N34?0'0"NSan FranciscoLos AngelesSanta BarbaraMontereySanta CruzA. Ascension-Ano Nuevo    CanyonsE. Santa Barbara Shelf,Gaviota & Goleta SlidesD. Sur Pockmark FieldC. Partington Canyon &     Point Lopez AreaB. Monterey-Carmel CanyonsF. Santa Monica Bay    Canyons500 m1000 m500 m contoursN        124W                                                            122W    32N                                                                34N                                                              36N   38N                                                               36N                                                                 34N                                                          118W2seafloor  sources  [7]  and  determining  its  vertical distribution in the water column.Methane concentration profiles in the oceanMethane  concentration  profiles  in  the  oligotrophic open ocean [9, 18, 19] provide an end-member for the  spectrum  of  profiles  obtained  throughout  the worlds?s  oceans  where  the  influence  of  riverine methane  input  [12,  20],  and  benthic  sources (such  as  methane-supported  chemosynthetic communities [23], methane bubble plumes [7, 22], or hydrothermal vents [17]) are of little importance. The  few  published  deep-water  open  ocean  water column  methane  profiles  [9,  18,  19]  combined with  the  more  numerous  surface  ocean  methane measurements  [11,  24,  25,  40]  indicate  that  the surface  oceans  are  nearly  always  supersaturated with  respect  to  equilibrium  with  the  overlying atmosphere and this is most likely caused by  in situ biological production (e.g., mixed layer production at  the  base  of  the  pycnocline [14,  16]).  Open  ocean methane  concentrations  decrease  with  depth  from supersaturation at the surface to values close to the detection limits of the analytical methods employed (typically <0.5 nM). Aerobic methane oxidation is the  primary  biogeochemical  process  that  removes methane from the deep ocean water column [14, 21, 48].In  contrast  with  the  oligotrophic  ocean,  methane concentration  profiles  from  the  water  column  over continental margins show enrichment from a variety of  sources,  including  rivers  entering  the  coastal ocean [12, 29], production in the mixed layer (see [43]  for  a  review),  apparent  intrusion  of  methane-enriched  bottom  waters  along  isopycnal  surfaces [7],  and  benthic  fluxes  from  microbial  (Jamaica Ridge [15]), gas hydrate (Hydrate Ridge [22]; South China Sea [44]), and thermogenic sources [5].Sources of water column methane anomaliesSupersaturated  conditions  in  the  upper  water column are not easily explained by physical mixing of  undetected  lateral  sources  containing  elevated methane  concentrations  or  deep  entrainment  of methane-enriched  bottom  waters.  Biological production  in  either  anaerobic  particle-associated microcosms  [28],  digestive  tracts  of  plankton  and nekton  [30,  31],  by  methylotrophic  methanogens [33], or possibly aerobic production of methane via cleavage of the C-P methylphosphonate bond [27, 32, Dave Karl, personal comm.] are likely explanations for  elevated  methane  in  the  upper  water  column. Unless  local  production  is  rapid  and  widespread, intense  mixing  and  aerobic  oxidation  [48]  will attenuate the methane concentration signal.Benthic  sources  of  methane  have  generally  been discovered during acoustic surveys of the continental margins  [34-36],  by  HOV/ROV  diving  [23],  by observing  oil  slicks  and  gas  bubbles  breaking  the ocean  surface  (Santa  Barbara  Channel  [7];  Bush Hill [39]), and by chance detection of water column acoustic  anomalies  in  the  mid-water  (Guaymas Basin [2]; Blake Ridge Diapir [37]). These  sources 3of  methane  gas  bubbles  create  vertical  plumes  of bubbles and methane-enriched water that rise many 100s meters into the water column and dissipate by dissolution, diffusion, and aerobic oxidation.Water  column  methane  concentration  anomalies may  occur:  (1)  in  proximity  to  water  column methane bubble plumes; (2) in areas where steep, rugged  topography  could  be  releasing  methane by diffusion from erosionally exhumed, methane-bearing  anoxic  sediments;  or  (3)  in  submarine canyons  where  restricted  circulation  may  trap water allowing methane released by diffusion from sediments, release from bubble vents, or biological production to accumulate in the water column. The rate of methane addition to the water column must exceed the total methane loss rate in order to produce net  methane  accumulation.  The  most  important mechanisms  that  attenuate  methane  anomalies include loss by oxidation by aerobic bacteria and dilution by eddy diffusive mixing.Detection of water column methane anomaliesRelative  to  the  enormous  area  of  the  continental margins,  methane  bubble  plumes  can  be  viewed as tightly-constrained point sources that would be difficult to detect, much like the proverbial needle in a  haystack.  In  some  cases,  methane  bubble  plumes disperse  along  isopycnal  surfaces  [7]  creating thin,  sub-horizontal  layers  that  may  be  difficult  to detect, if vertical sampling is insufficient. Obtaining sufficient  numbers  of  discrete,  vertically-spaced samples  during  a  CTD/Niskin  bottle  profile, and  then  analyzing  methane  concentrations  at nanomolar  levels  is  analytically  challenging  and time  consuming.  Thus,  the  continental  margins, where substantial amounts of methane production and release from a variety of sources are occurring, are vastly undersampled.Systematic  3-dimensional  surveys  of  methane distribution (niffer surveys have been  conducted for  the  oil  and  gas  industry  decades  ago.  These surveys  were  primarily  conducted  far  shallower  (<200  mbsl)  than  the  occurrence  of  gas  hydrate (<520 mbsl), and the data has generally remained proprietary.  To  our  knowledge  there  are  no  known or published systematic analyses of this data from the  point-of-view  of  locating  discrete  methane vents  on  the  seafloor.  More  recently, AUV  ?sniffer surveys? conducted using a METS  methane sensor [49]  have  targeted  areas  of  the  seafloor  purported to contain vent sites. Results have been ambiguous, primarily because there was no context as to what would happen up and down the margin, where there isn?t  any  significant  venting  of  methane  from  the seafloor.Motivation for this studyPrevious investigations of methane distribution in the water column along the continental margin of California  [5,9]  and  Baja  California  Sur  [26]  have shown  that  water  column  methane  concentrations increase with proximity to known or putative benthic sources,  which  may  include  gas  vents,  diffusion out  of  methane-rich  sediments  and  proximity  to methane  gas  hydrates.  In  the  case  of  the  Santa Barbara Basin [5, 50], the association of gas venting and elevated water column methane concentrations is  clear.  However,  few,  if  any,  systematic  surveys of methane concentration in the water column over the continental margins have been conducted where methane gas vents are not known to occur.In order to further explore the connection between water column methane anomalies and gas venting and to better determine how these methane anomalies are generated, we have accumulated more than 1600 methane concentration measurements from portions of  the  central  and  southern  California  continental margin over the past eight years (Figures 1 & 2). Some  of  these  samples  were  taken  in  areas  known to have methane vents, but others were purely for exploratory purposes. Here we address the question as to whether we can detect seafloor methane venting sites  using  the  water  column  profiling  technique, and whether we are able to associate water column methane  anomalies  with  specific  morphological  or geologic factors.METHODSWater  samples  were  obtained  from  12  10-L Niskin  bottles  mounted  on  a  CTD  rosette  (Seabird Electronics  911plus  profiling  CTD)  or  a  pair  of 5-L  Niskin  bottles  mounted  on  MBARI?s  ROV Tiburon.  ROV  samples  were  collected  ~2  m  above the  seafloor.  Great  care  was  exercised  so  as  not to  disturb  the  seafloor  and  cause  sample  bias  by releasing  potentially  methane-enriched  porewater trapped  in  surface  sediments.  Video  observations made  during  sample  collection  confirmed  the  lack of obvious contamination of these water samples.121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N36?10'0"N36?5'0"N36?5'0"N36?0'0"N36?0'0"N122?20'0"W122?20'0"W122?40'0"W122?40'0"W37?0'0"N 37?0'0"N36?55'0"N 36?55'0"NAscension Canyon100 m500 mAscension Shelf CastsA100 m contoursNAo Nuevo Canyon        1220 W                                                            1220 W     365 N         37 N   122?0'0"W122?0'0"W122?20'0"W122?20'0"W36?40'0"N 36?40'0"N36?20'0"N 36?20'0"N121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N36?10'0"N36?5'0"N36?5'0"N36?0'0"N36?0'0"N122?20'0"W122?20'0"W122?40'0"W122?40'0"W37?0'0"N37?0'0"N36?55'0"N36?55'0"NCarmel Canyon100 m contoursBMonterey    Canyon100 m1000 mN1220 W                                                122 W                 360 N                                                   360 N                  121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N36?10'0"N36?5'0"N36?5'0"N36?0'0"N36?0'0"N121?40'0"W121?40'0"W121?50'0"W121?50'0"W35?50'0"N 35?50'0"N35?40'0"N 35?40'0"N1000 m900 m1100 mD100 m contoursNPockmarks                                 1210 W                      1210 W                                                  350 N                          360 N 1/2 km121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N 36?10'0"N36?5'0"N 36?5'0"N36?0'0"N 36?0'0"NCPartington CanyonLopez Point Area100 m900 m100 m contoursN               1215 W                  1210 W                                                    36 N                      360 N 4Figure  2A-2D:  Locations  for  CTD  casts  (filled colored  circles)  and  ROV  (colored  Xs)  water column  methane  samples  are  indicated  for  each study  area  as  identified  in  Figure  1.  Symbol  colors are keyed to methane and light transmission data shown in Figure 3. CTDs measure conductivity (converted to salinity), temperature, pressure (converted to depth) and light transmission simultaneously. In  this study percent light transmission will be used as a proxy for gross particle  concentration;  lower  light  transmission corresponds  with  higher  particle  concentration. CTD data was collected continuously on the down- and  up-casts  at  1-second  intervals,  and  the  Niskin bottles  were  fired  on  the  up-cast. The  deepest  CTD sample  from  a  cast  was  obtained  ~1  m  above  the seafloor.  The  CTD  was  equipped  with  a  bottom sensor  (lead  weight  hanging  on  a  1-m  cable connected  to  a  trip-switch)  that  indicated  contact with the bottom, to avoid bottom impact by the CTD rosette  during  ship  heave.  Once  the  CTD  was  on 121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N36?10'0"N36?5'0"N36?5'0"N36?0'0"N36?0'0"N122?20'0"W122?20'0"W122?40'0"W122?40'0"W37?0'0"N37?0'0"N36?55'0"N36?55'0"N119?40'0"W119?40'0"W120?0'0"W120?0'0"W120?20'0"W120?20'0"W120?40'0"W120?40'0"W34?40'0"N 34?40'0"N34?20'0"N 34?20'0"N100 mTransect BTransect HTransect KTransect MGaviota Slide AreaGoleta Slide AreaSeep TentE100 m contoursN          1210 W                            1200 W                           120 W                            1190 W                    340 N                            340 N Figure KeyOil PlatformSeep Tent5deck, water was transferred immediately from each Niskin bottle via clear plastic tubing to a previously seawater  rinsed  240-mL  amber  glass  bottle.  The end of the tubing was placed at the bottom of the bottle to prevent formation of bubbles. The bottle was  filled  to  overflowing  and  sealed  with  a  PTFE-silicon  septum  and  screw  cap  without  trapping  a gas bubble under the septum. A 10-mL high-purity nitrogen  gas  headspace  was  added  to  the  240-mL bottle  and  equilibrated  for  15  minutes  using  a commercial  paint  shaker.  Methane  concentration in this headspace was obtained shipboard using a Shimadzu mini-2 gas chromatograph equipped with a  flame  ionization  detector.  Methane  was  separated isothermally  from  other  gases  in  a  high-purity nitrogen carrier gas stream (Whatman nitrogen gas generator) using either a 5? x 1/8? OD stainless steel Figure 2 continued: Panels E and F indicate CTD casts  and  ROV  water  column  methane  sample locations.121?40'0"W121?40'0"W121?45'0"W121?45'0"W36?10'0"N36?10'0"N36?5'0"N36?5'0"N36?0'0"N36?0'0"N118?40'0"W118?40'0"W119?0'0"W119?0'0"W34?0'0"N 34?0'0"N33?40'0"N 33?40'0"NMid-Basin Cast100 m1000 mFSanta Monica Canyon      Hueneme Canyon       Redondo Canyon100 m contoursN                                                             119 W                                                       1180 W   330 N                                                         34 Nchromatographic  column  packed  with  60/80  mesh Carbosieve G (Supelco, Bellefonte, PA) or a 6? x 1/8? OD  stainless  steel  chromatograph  column  packed with  80/100  mesh  Poropak-Q  (Alltech  Associates, Deerfield,  IL).  Oven  temperature  was  typically 100  and  detector  temperature  was  125.  Gas samples  were  injected  into  the  gas  chromatograph via a small volume magnesium perchlorate drying-trap  in  series  with  a  2-mL  stainless  steel  sample loop.  A  primary  methane  standard  (9.93  ppm  in nitrogen) was run approximately every 10th sample. A 60-cc aliquot of nitrogen carrier gas was used to flush residual methane from the gas chromatograph sample  loop  between  every  sample.  This  method has a detection limit of ~0.5 nM.RESULTSSince  initiation  of  our  efforts  in  2000  to  survey  the distribution of methane in the water column along the California continental margin (Figure 1), 1607 6individual water column samples have been analyzed for  methane  concentration.  Sample  locations  of  the CTD  profiles  and  ROV  dives  are  plotted  for  each study  area  in  Figure  2.  Selected  concentration profiles  and  ROV  samples  are  plotted  in  Figure  3, organized  by  the  study  areas  illustrated  in  Figure 2.  Data  from  only  4  of  the  13  cross-shelf  transects for  the  Santa  Barbara area are  shown  in  Figures  3E & 3K; the complete methane concentration dataset for  the  Santa  Barbara  shelf  is  reported  in  Lorensen et al. [10]. Light transmission data obtained by the profiling  CTD  during  collection  of  the  methane samples are also plotted in Figure 3.Except  for  three  of  the  four  profiles  from  the  Sur Pockmark  area  (Figure  3D),  near  surface  methane concentrations  (0-50  mbsl)  are  saturated  or supersaturated with respect to equilibrium with the overlying  atmosphere  (2-2.5  nM).  Excluding  Santa Barbara  shelf  surface  waters,  which  are  highly oversaturated  because  of  widespread  seafloor methane venting, 75% of the methane measurements in the surface layer (67 out of 89) exceed atmospheric saturation  (2-2.5  nM).  Profiles  from  the Ascension Shelf  (black  lines;  Figure  3A)  have  some  of  the steepest  gradients,  increasing  towards  the  ocean surface from 4.2 nM to 9 nM over less than 55 m. All methane concentration profiles, except for those from Ascension and Ano Nuevo Canyons (Figure 3A)  and  the  Goleta  Slide  (orange  line;  Figure  3E), can be grouped on the basis of either decreasing or increasing  concentration  with  depth.  Profiles  from Monterey  and  Carmel  Canyons  (Figure  3B),    the Lopez  Point  area  (black  and  green  lines;  Figure 3C),  the  Sur  Pockmarks  (Figure  3D),  the  upper slope 350-mbsl cast at Point Conception (blue line; Figure  3E),  the  Gaviota  Slide  (black  line;  Figure 3E),  and  the  mid-basin  cast  in  the  Santa  Monica Basin  (black  line;  Figure  3F)  decrease  with  depth; whereas,  profiles  from  Partington  Canyon  (Figure 3C),  the  Santa  Barbara  shelf  (Figure  3E),  and  the Santa  Monica  Canyons  (Hueneme,  Santa  Monica, and  Redondo;  Figure  3F)  increase  with  depth.    In general, profiles with methane increasing with depth are more erratic compared to those with methane decreasing with depth.Persistent  mid-water  methane  anomalies  between 400-700  mbsl  were  observed  in Ascension  Canyon (3  casts,  each  collected  24  hours  apart  in  October 2000, red lines; Figure 3A) and upper Ano Neuvo Canyon (6 years after the Ascension Canyon casts, blue line; Figure 3A). Upper slope casts from near the  mouth  of  Ascension  Canyon  (also  from  the October  2000  cruise,  purple  and  pink  lines;  Figure 3A) have mid-water methane anomalies similar to those in Ascension Canyon. Methane  profiles  from  the  Santa  Barbara  shelf display a rapid increase in concentration with depth. The four transects presented in Figure 3E show a general trend in steeper methane gradients occurring closer  to  shore,  and  going  from  west  (Transect  B; Point Conception area) to east (Transect M).The four water column methane profiles for the Sur Pockmark  area  (data  originally  published  in  [38]) are closely coincident with each other (Figure 3D) starting  at  approximately  atmospheric  saturation near the ocean surface and decreasing with depth, approaching  the  detection  limit  (~0.5  nM)  below 800  mbsl.  These  profiles  are  comparable  to  ?open ocean? methane profiles obtained further offshore of the central California margin [9] and the equatorial Atlantic  [8].  The  mid-basin  cast  in  Santa  Monica Basin  (black  line,  Figure  3F)  and  methane  profiles in  Ascension  Canyon  (below  700  mbsl)  and Monterey  Canyon  (below  600  mbsl)  also  display characteristics of  ?open ocean? methane profiles.ROV  water  samples  were  collected  within  2  m of  the  seafloor  and  often  show  higher  methane concentrations than the deepest CTD profile sample collected  nearby  (e.g.,  Santa  Monica  Canyons, Figure  3F).  The  highest  measured  value  in  this dataset is 184 nM, collected by ROV from the head of Partington Canyon.Percent light transmission data obtained during each CTD/Niskin bottle cast are displayed in Figures 3G through  3L.  These  CTD  profiles  were  collected irrespective of the season, thus, a number of factors may influence the  size of an anomaly or its depth. A well-defined subsurface light transmission minimum (as  low  as  ~80%)  occurs  at  the  base  of  the  mixed layer (~50 mbsl) below where methane is commonly supersaturated.  Except  for  four  profiles,  {one  in upper Monterey Canyon (UM; Figure 3H), two from the  Goleta/Gaviota  Slides  (orange  and  black  lines; Figure 3K), and one in upper Hueneme Canyon (UH; Figure 3L)}, the lowest values of light transmission 01002003004005006008085909501002003004005000501001500200400600800100012001400051015200100200300400500600051015202530354005001000150020000510152002004006008001000051015202530354002004006008001000120014000510152001002003004005007580859095020040060080010001200140080859095050010001500200075808590950200400600800100080859095020040060080010001200140080859095UMUADepth (mbsl)Santa Monica Canyons     184 nMSanta BarbaraMethane (nM)Methane (nM)Methane (nM)Methane (nM)Methane (nM)Methane (nM)Sur PockmarksPartington-Lopez PointMonterey-CarmelAscension-Ano NuevoABCDEFDepth (mbsl)         near only known gas ventpersistent mid-water anomalyPercent TransmissionPercent TransmissionPercent TransmissionPercent TransmissionPercent TransmissionPercent TransmissionLKJIHGUH     84 nMGoGo7Figure 3: Methane concentration profiles (lines) and ROV-collected methane concentrations (filled circles) for each of the six study areas (Figure 2) are plotted versus depth in  panels A-F and corresponding percent light transmission data are plotted in panels G-L (note different scales). Line color correlates between paired panels, i.e., panel A and G,  B and H, etc. Green arrows indicate the light transmission minimum in the surface mixed layer. The red arrows in panel G indicate small light transmission anomalies. Data are  available upon request. Line colors explained in the text. UA=upper Ascension Canyon; UM=upper Monterey Canyon; Go=Goleta Slide; and UH=upper Hueneme Canyon.8in  any  particular  profile  are  found  in  the  upper  50 m of the water column. In most submarine canyon profiles  (Ascension,  Monterey,  Partington,  and  the Santa  Monica  canyons;  Figures  3G-3I,  &  3L)  and outer shelf/upper slope profiles from Santa Barbara, light  transmission  remains  constant  with  depth (~90%)  below the light transmission minimum and decreases slowly below the mid-point of most CTD casts towards the seafloor, reaching values between 85% and 90%. Light transmission is nearly constant with depth below the light transmission minimum for  the  Sur  Pockmark  and  mid-basin  Santa  Monica casts  (Figures  3J  &  3L).  CTD  data  also  reveal  that there  are  no  salinity  anomalies  (data  not  shown) indicative of freshwater input to the surface ocean where water column methane measurements were obtained. Thus, there is no obvious methane input to the surface ocean from terrestrial sources at the sites surveyed.DISCUSSIONOpen  ocean  methane  profiles  provide  a  context  in which to evaluate the effects of geologic, terrestrial, and  biological  sources  of  methane  to  the  coastal oceans.  Studies  of  open  ocean  methane  profiles  [9, 18] and surface ocean methane concentrations [8, 12] have shown that the open ocean mixed layer (0-200 mbsl) is slightly supersaturated (3-6 nM) with respect to  equilibrium  with  methane  in  the  atmosphere (2-2.5  nM),  which  results  in  a  net  flux  of  methane to  the  atmosphere.  Supersaturation  of  the  mixed layer in the open ocean most likely has a biological origin because it is isolated from the influence of the continental  margins.  Karl  and  Tilbrook  [28]  have shown that supersaturation of the open ocean mixed layer is easily achieved. Particle-associated methane released  into  the  mixed  layer  by  sinking  particles can  produce  methane  supersaturation  in  less  than a month [28] and methane concentrations increase towards the base of the mixed layer [18, 28]. Below the  mixed  layer,  methane  concentrations  decrease with depth to values that reach the detection limits of the methods employed (<1 nM), which indicates net loss of methane from the water column. Thus, by comparing profiles obtained in this study with what would be expected for open ocean methane profiles, geologic  and  terrestrial  sources,  and  enhanced biological production of methane can be inferred.Open Ocean-like Methane ProfilesA  small  number  of  methane  profiles  and  their corresponding  light  transmission  profiles  obtained along  the  California  margin  resemble  open  ocean methane  profiles  (Big  Sur  Pockmark  field,  Figure 3D;  and  the  mid-basin  cast  in  Santa  Monica  Basin, black  line,  Figure  3F).  An  ROV  survey  of  the  Big Sur  Pockmark  field  [38]  showed  no  evidence  for methane venting or seepage from selected pockmarks in this giant deep-water (900-1200 mbsl) pockmark field. Methane profiles were collected over the same pockmarks within days of the visit by the ROV.Methane in the Surface Mixed LayerLight transmission profiles (Figures 3G-3L) obtained along  the  central  California  continental  margin commonly have a pronounced subsurface minimum at  approximately  50  mbsl  that  in  most  cases corresponds with the pycnocline (data not shown). Methane in the surface mixed layer (0-50 mbsl) above the  pycnocline  is  commonly  supersaturated,  with concentrations  as  high  as  32  nM  (upper  Hueneme Canyon; Figure 3F). In many profiles undersaturated values begin to appear below the particle maximum and  methane  concentrations  continue  to  decrease with depth, reaching values at or below detection limit  (Figures  3A-3D).  This  profile  data  indicate that local production of methane is occurring in the surface  mixed  layer  above  the  particle  maximum and  may  not  be  particle-associated  as  suggested by  Karl  and  Tilbrook  [28],  or  Sansone  et  al.  [42]. Except  for  nearly  every  profile  from  the  Santa Barbara  shelf  (Figure  3E),  the  majority  of  methane profiles  increase  in  concentration  from  the  particle maximum towards the ocean surface, not the base of  the  mixed  layer.  Methane  concentration  profiles that  increase  towards  the  ocean  surface  cannot be  produced  by  vertical  mixing  of  methane  from greater depths as observed in open ocean methane profiles [18].Although  little  is  known  about  the  capacity  of local  rivers  to  transport  methane  into  the  coastal ocean  along  the  California  continental  margin, their ephemeral nature and low total discharge, and the  absence  of  salinity  anomalies  in  the  surface mixed  layer  indicative  of  freshwater  input,  makes it unlikely that rivers entering the coastal ocean are important sources for methane found in the mixed layer, including those on the Ascension Shelf. These observations suggest that methane supersaturation in  the  surface  mixed  layer  along  the  central California margin is not related to fluvial export nor 9is  it  particle-associated,  but  may  be  biologically-mediated through other processes, such as cleavage of  methylphosphonate  [27,  32]  or  production  by methylotrophic methanogens [33]. Methane in Submarine Canyon SystemsWithin  most  of  the  submarine  canyon  systems surveyed,  elevated  methane  concentrations commonly  occur  in  the  benthic  bottom  water (Partington Canyon, red lines, Figure 3C; Hueneme, Santa  Monica,  and  Redondo  Canyons  in  Santa Monica  Bay,  Figure  3F)  or  appear  as  persistent anomalies  in  the  mid-water  (Ascension  and  Ano Nuevo  Canyons,  red  and  blue  lines,  respectively, Figure  3A).  Monterey  Canyon  (red  lines,  Figure 3B) is a notable exception to this observation.The steady rise in methane concentration with depth in  the  three  canyons  from  Santa  Monica  Bay  and the highly elevated methane concentrations near the seafloor in upper Partington Canyon (184 nM at 200 mbsl; 84 nM at 298 mbsl) may result from restricted circulation  in  these  canyon  systems.  Restricted circulation  would  increase  the  residence  time  of methane  and  other  constituents,  such  as  ocean-margin derived organic-rich particles, which might serve  as  a  methane  source.  Restricted  circulation would also allow the accumulation of methane in the bottom waters released by local geologic sources (vents, methane gas hydrates, and/or eroded anoxic seafloor sediments).Ascension  and  Ano  Neuvo  Canyon  profiles display  mid-water  methane  anomalies  between approximately  400-700  mbsf  of  varying  intensity, that  persist  over  a  few  days  to  perhaps  years, superimposed  on  methane  profiles  that  decrease with depth. These anomalies may also result from increased  methane  residence  time  and/or  lateral proximity to geologic sources of methane. Although there  are  no  data  that  indicate  the  occurrence  of methane  gas  hydrates  along  this  portion  of  the California  continental  margin  (gas  hydrates  have only  been  reported  from  the  Eel  River  [45]  and  an authigenic  carbonate  mound  in  the  Santa  Monica Basin  [23]),  the  depth  interval  for  the  mid-water methane  anomalies  corresponds  with  the  upper depth  limit  for  methane  hydrates  (~520  mbsl). Slowly  decomposing  methane  gas  hydrates  could provide a sustained source of methane to the water column that would maintain water column methane anomalies over annual time-scales.Submarine  groundwater  discharge  (SGD)  has  been shown  to  export  methane-rich  (200+  nM  levels) freshwater to the continental shelves [41]. Although salinity anomalies indicative of freshwater input to benthic bottom waters along the California margin have not been identified, SGD may contribute a yet undetected contribution of methane to the coastal ocean.  Submarine  canyons  may  intersect  off-shore aquifers and provide exit points for fluid discharge, much like that seen in box canyons on land and the base of the West Florida Escarpment [46].Many  of  the  light  transmission  profiles  in  the submarine  canyons  surveyed  trend  toward  lower values near the seafloor indicating higher suspended particle concentrations. Distinctly larger increases in suspended particle concentrations occur in the upper reaches  (<500  mbsl)  of  three  canyons  (UA,  UM, and UH, Figure 3). Although particle concentrations are similar to those obtained in the surface mixed layer,  and  the  upper  canyons  are  closer  to  higher productivity zones of the coastal ocean, there is not a corresponding increase in methane concentration in the water. This observation suggests that either particle-associated  methane  production  is  not important or particle-type is different in the mixed layer  compared  to  the  benthic  boundary  layer  in submarine canyons.Proximity to Known Methane Gas VentsMethane  concentrations  in  Ascension  (up  to  16 nM), Partington (up to 184 nM), and Santa Monica canyons  (up  to  24  nM)  are  comparable  to  values obtained  over  or  near  known  methane  gas  vents  on the  Santa  Barbara  shelf  (up  to  143  nM;  Figure  3E), yet,  there  are  no  known  methane  gas  vents  in  these canyon  systems.  In  Monterey  Canyon,  elevated near-bottom methane concentrations occur adjacent to a known fluid vent at Extravert Cliff (Figure 3B). However,  the  Extravert  Cliff  anomalies  are  modest (up to 6 nM) compared to methane concentrations measured in the other canyon systems. CTD casts taken  on  the  Ascension  Shelf  (Figure  3A),  were located  as  close  as  possible  to  reported  water column acoustic anomalies once interpreted to be methane  bubble  plumes  [47].  These  profiles  show no  indication  of  gas  venting  from  the  seafloor; the methane gradients are opposite what would be expected for a seafloor source.10Methane  concentration  profiles  from  the  Santa Barbara shelf increase with increasing depth (Figure 3E), consistent with a benthic source for the methane. The entire water column is methane-enriched, with only  12  of  the  480  methane  measurements  made  in the surface layer (0-100 mbsl, or less) at or below atmospheric  saturation.  Although  gas  vents  are numerous  and  widespread  along  the  Santa  Barbara shelf [5, 7], it would be difficult to find an individual gas  vent  using  water  column  methane  profiles alone.Water  vapor  plumes  from  power  plants  (Figure 4)  provide  a  physical  analogy  for  the  problem  of identifying a methane source. A single vertical profile through the dispersed vapor plume downwind of the stack would provide evidence that a source of water vapor is nearby, but there would be little directional information.  Samples  a  few  meters  away  from  the vapor plume would not indicate its nearby existence! A  dense  3-dimensional  grid  of  directed-sampling would be necessary to pinpoint the source.CONCLUSIONSAs  a  result  of  prospecting  for  new  seafloor  sources for methane using water column methane profiling, several  classes  of  methane  anomalies  have  been identified.  Supersaturation  of  the  surface  mixed layer (0-50 mbsl) is widespread and is above a well-defined  subsurface  particle  maximum  (~50  mbsl). Particle-associated  methane  production  appears not  to  be  an  important  process  along  the  central California  margin.  Persistent  mid-water  methane anomalies  that  occur  in  the  Ascension  and  Ano Nuevo Canyon systems may be the combined result of  restricted  circulation  and  a  proximal  geologic source for methane, potentially methane gas hydrate or methane-rich, recently eroded sediments on the canyon walls. In some of the upper canyon systems, near-seafloor particle concentrations are comparable to those found at the particle maximum below the mixed  layer,  but  do  not  correlate  with  a  methane increase. In the Santa Barbara Basin, we know there are numerous gas vents, but based on the results we have  obtained,  it  would  be  hard  to  find  one  using water column methane profiles alone.REFERENCES[1] Judd, A.G., Hovland, M., Dimitrov, L.I., Garcia Gil,  S.,  Jukes,  V.  The geologic methane budget at continental  margins  and  its  influence  on  climate change. Geofluids 2002;2:109-126.[2]  Merewether,  R,,  Olsson,  M.S.,  Lonsdale,  P. Acoustically detected hydrocarbon plumes rising from 2-km depths in Guaymas Basin, Gulf of California. J. Geophysical Research 1985;90(B4):3075-3085.[3] Paull, C.K., Ussler, W., III, Peltzer, E.T., Brewer, P.G., Keaten, R,, Mitts, P.J., Nealon, J.W., Greinert, J.,  Herguera,  J.-C.,  Perez,  M.E.  Authigenic carbon entombed  in  methane-soaked  sediments  from  the northeastern  transform  margin  of  the  Guaymas Basin,  Gulf  of  California.  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