association among active seafloor deformation, mound formation

doi:10.1016/j.margeo.2008.01.011Association among active seafloor deformation, mound formation, and gas hydrate growth and accumulation within the seafloor of the
Santa Monica Basin, offshore California
Charles K. Paull a,, William R. Normark b, William Ussler III a, David W. Caress a, Rendy Keaten a
a Monterey Bay Aquarium Research Institute, Moss Landing, California 95039-9644, USA b United States Geological Survey, Menlo Park, California 94025, USA
Received 3 September 2007; received in revised form 16 January 2008; accepted 21 January 2008
Abstract
Seafloor blister-like mounds, methane migration and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, California. Two distinct deep-water (≥800 m water depth) topographic mounds were surveyed using an autonomous underwater vehicle (carrying a multibeam sonar and a chirp sub-bottom profiler) and one of these was explored with the remotely operated vehicle Tiburon. The mounds are N10 m high and N100 m wide dome-shaped bathymetric features. These mounds protrude from crests of broad anticlines (~20 m high and 1 to 3 km long) formed within latest Quaternary-aged seafloor sediment associated with compression between lateral offsets in regional faults. No allochthonous sediments were observed on the mounds, except slumped material off the steep slopes of the mounds. Continuous streams of methane gas bubbles emanate from the crest of the northeastern mound, and extensive methane-derived authigenic carbonate pavements and chemosynthetic communities mantle the mound surface. The large local vertical displacements needed to produce these mounds suggests a corresponding net mass accumulation has occurred within the immediate subsurface. Formation and accumulation of pure gas hydrate lenses in the subsurface is proposed as a mechanism to blister the seafloor and form these mounds. © 2008 Elsevier B.V. All rights reserved.
Keywords: gas hydrate; chemosynthetic communities; mounds; gas vent; methane; mud volcano; diaper; pingo
1. Introduction
Discrete dome-shaped bathymetric features that have been described as diapirs or mud volcanoes within the Santa Monica Basin, offshore southern California are known to contain gas hydrate at shallow depths (Normark and Piper, 1998; Normark et al., 2003; Hein et al., 2006). The seafloor surrounding one mound was explored with a remotely operated vehicle (ROV), characterized with geochemical measurements, and imaged with high-resolution seafloor-mapping surveys. The role that gas hydrate formation and regional faulting plays in the formation of these features is considered here.
Venting of methane and other hydrocarbon gases through seafloor sediment stimulates profound changes to the local
Corresponding author. E-mail address: [email protected] (C.K. Paull).
0025-3227/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.01.011
environment. These changes are focused near the seafloor where mixing occurs between the rising gas and the overlying oxygenated and sulfate-bearing seawater. Anaerobic oxidation of methane (AOM) supports communities of chemosynthetic organisms (Sibuet and Olu, 1998), and induces rapid diagenetic changes within near-seafloor sediments (e.g., Paull et al., 1992; Greinert et al., 2000; Peckmann et al., 2001; Roberts, 2001). The most obvious manifestation of this early diagenesis is the formation of authigenic minerals, primarily methane-derived carbonates. Such carbonates are identified on the basis of their 13C-depleted cements and distinctive textures.
Gas hydrate may also form within seafloor sediments beneath deep-sea gas vents (generally N520 m water depths) where there is an adequate concentration of low molecular weight gas, usually methane (Sloan, 1998). The formation of gas hydrate within seafloor sediments where there is continued supply of methane is believed to exert a significant influence on
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marine sediment because of mechanical property changes and volume increase that occur during the open system addition and conversion of methane and water into solid gas hydrate (Paull et al., 2000a; Hovland and Gudmestad, 2001).
1.1. Seafloor mounds
Elevated bathymetric features that are generally circular and range from a few meters to kilometers across can form by expulsion or expansion of material from deeper within the sediment section (e.g., mud volcanoes; Kopf, 2002). Seafloor gas vents and gas hydrate are commonly found on such topographic highs. While some authors have simply called these bathymetric features mounds (e.g., Chapman et al., 2004), in some cases the morphology and structure seen in seismic-reflection profiles suggest that they might be mud volcanoes or diapirs.
Mud volcanoes are elevated morphologic features constructed from the accumulation of ejected and extruded sediments. Mud volcanoes occur within hydrocarbon-rich and/or tectonically- active areas either on land or on the seafloor and are interpreted to indicate the occurrence of over-pressured fluids at depth (e.g., Westbrook and Smith, 1983; Milkov 2000; Aisan et al., 2001; Kopf, 2002; Yassir, 2003; Huguen et al., 2005). Diapirs, where salt and/or mud are extruded from depth by buoyancy or differential pressure (e.g., Seni and Mullican, 1985; Limonov et al., 1996; Warren, 2006), can also generate seafloor relief.
Pingos are another type of elevated topographic feature that occur in permafrost regions (Mackay, 1998 and references within). Development of subaerial pingos is ascribed to the expansion of water as the active permafrost zone refreezes seasonally and lifts the soils. Some pingos are connected to an additional source of water that enters from below, increasing the ice core volume and enhancing their size. Until recently, pingos were thought to be restricted to permafrost environments, but similar features have been observed in marine environments that are believed to be associated with gas hydrate formation rather than ice (Hovland and Svensen, 2006).
1.2. Santa Monica Basin tectonics and mounds
Numerous active faults associated with the boundary between the North American and Pacific plates occur throughout the southern California region (e.g., Dolan et al., 1995). While much more is known about onshore faults than offshore faults, about 20% of the plate-margin movement occurs along northwest-striking right-lateral faults within the offshore basins (Sorlien et al., 2006).
The existence of offshore faults is known from earthquake occurrence, seismic-reflection profiles, and seafloor-mapping surveys (e.g., Fisher et al., 2004; Sorlien et al., 2006). A number of faults identified in low frequency seismic-reflection profiles that are associatedwith large offsets at depth also can be traced in high- resolution reflection profiles up towards the present seafloor where they may offset near-seafloor strata or be associated with distinct bathymetric offsets in multibeam data (Gardner et al., 2003).
Fisher et al. (2003) and Gardner et al. (2003) indicate that seafloor topography associated with the apron on the Santa
Monica Basin may be associated with the San Pedro Basin Fault Zone (informally called the Santa Monica Basin Fault by Gardner et al., 2003). The San Pedro Basin Fault Zone is associated with oblique shortening, right-lateral offsets, and block rotations (Sorlien et al., 2006).
High-resolution seismic-reflection surveys conducted in 1992 revealed the existence of a distinct topographic feature (hereafter called a mound) on the side of the Santa Monica Basin (Normark et al., 2006a). The first mound to be discovered (NE Mound) was described as a “diapir-like piercement structure” (Fig. 11 in Normark and Piper, 1998) and later as a mud volcano on the basis of its shape (Normark et al., 2003; Hein et al., 2006). Another mound (SWMound) was subsequently identified 2.3 km away in newly acquired surface-shipmultibeam swaths since theHein et al. (2006) study.
A piston core taken on the NE Mound contained lenses of white gas hydrate at ~2 m below the seafloor (Normark et al., 2003; Hein et al., 2006). Methane-derived carbonates and shells of the bivalve family Vesicomyidae (Krylova and Sahling, 2006) that are known to live in sulfide-rich environments (Barry et al., 1996) and to be components of sulfide dependant chemosynthetic biological communities (CBC), were also found in this core (Hein et al., 2006). Authigenic carbonates and shell samples generated δ13C values as low as−19‰, PDB (Pee Dee Belemnite). The low δ13C values were interpreted to indicate that the Vesicomya bivalves living at this site might be incorporating significant amounts of methane-derived carbon into their shells (Hein et al., 2006). Bivalve shells typically utilize dissolved inorganic carbon (DIC) from seawater to form the carbonate in their shells (McConnaughey et al., 1997). A significant amount of methane- derived carbon in the shells would suggest that these clams are getting carbon from the underlying pore waters rather than from the overlying ambient seawater. However, these determinations were made on shell material collected from piston and gravity cores and diagenesis is always a lingering concern.
To further explore these topographic features, to investigate the connections with the active faulting, and to determine whether the fauna in this environment utilized unusual carbon sources in the formation of their shells, ROV dives and detailed seafloor surveys were undertaken. Neither visual imaging nor systematic pore fluid and biological sampling had occurred prior to theseMontereyBay Aquarium Research Institute (MBARI) dives. The dives were followed with detailed autonomous underwater vehicle (AUV) surveys to map the seafloor topography, stratigraphy, and structure of the uppermost (0–20 m) of the seafloor.
2. Methods
2.1. ROV observations, sampling and sample processing
Five dives of theROVTiburon (T-785, T-792, T-793, T-796, and T-799) were conducted in February 2005 to explore the seafloor associated with the NE Mound and the adjacent topographic swell on the floor of the Santa Monica Basin (Fig. 1). The term swell is used to denote a low relief, elongate hummock in the sediment fill near the edge of SantaMonica Basin (Fig. 1B). The existence of the SWMoundwas not known at the time of the ROV surveys. During
ig. 2. Video images collected during dives of the ROV Tiburon showing the seafloor on the NE Santa Monica Mound and photographs of samples collected from the EMound. Part A shows a crack near the crest of the mound from which a continuous plume of gas emanates. Bubbles can be seen above the ROV's manipulator. The anipulator is holding an overturned cylinder with an attached funnel to collect the gas. The white material in the cylinder is a mush of gas-hydrate-coated bubbles. ylinder is 9 cm in diameter. Part B shows the contact between a white-fringed patch of orange microbial mat and a bed of living clams on a terrace on the southwestern ank of the mound. Part C shows a densely packed bed of V. elongata. Part D shows a cross section of authigenic carbonate rock. Articulated valves of V. elongata ells are contained within this sample. A 2.5-cm long V. elongata shell found nearby is shown for scale. It also provides a coarse scale for the images in Parts B and C.
Fig. 1. Maps showing the location of seafloor topographic mounds within the Santa Monica Basin, offshore of Los Angeles, California. Part A shows both mounds (indicated with crosses inside the rectangular box), location of ODP Site 1015, and 100 m bathymetric contours of the Santa Monica Basin. Bathymetry after Chase et al. (1981). Part B shows shaded relief map illuminated from the NNWwith 50 m contours based on existing surface-ship multibeam bathymetry that outlines the two mounds and surrounding bathymetric swells on the floor of the San Monica Basin. Multibeam data published by Gardner et al. (2003) plus two additional swaths collected by Scripps Institution of Oceanography reveal the existence of the SW mound. Location of multichannel seismic-reflection profile A-1-98 L030 shown in Fig. 9 is indicated with a dotted red line (Normark et al., 1999a). Dashed line indicates approximate position of Santa Monica Fault zone (after Gardner et al., 2003). SWM–Southwest Mound; NEM–Northeast Mound.
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F N m C fl sh
this effort 20 h of video observations of the seafloorwere conducted (Fig. 2). The ROV's obstacle-avoidance sonar helped to identify areas of increased seafloor reflectivity associated with biological communities, hard bottoms, and local topography and thus helped to direct the ROV to sites of interest along the transects.
Ten samples of the carbonate mantle of the NE Mound that were solid enough to be picked up with Tiburon's mechanical manipulator (thus called ‘rock’ samples) were recovered. Scoops were used to sample clam beds. Samples of live Vesicomya clams were washed in deionized water, patted dry, and frozen.
Sediment samples from 50 push cores (up to 22 cm long) and 8 vibracores (up to 147 cm long) were obtained on the five ROV dives. Push core sediments were extruded and sampled shipboard after each ROV dive. The lower 16 cm of each vibracore was cut off on shipboard to extract a pore water sample. The remainder of each corewas loggedwith amultisensor track, split, photographed, and preserved in the core repository at theUnitedStatesGeological Survey in Menlo Park, California as expedition W-1-05-SC.
Pore waters were extracted from fresh sediment samples using Reeburgh-style squeezers and analyzed for their sulfate, chloride, and methane concentrations on shipboard. Pore-water samples were preserved for measurement of dissolved inorganic carbon (DIC) concentration and the stable isotopic composition
Fig. 3. Map showing locations of AUV survey lines flown to map the Santa Monica m covered in Fig. 1B (larger box) and Fig. 4 (smaller box). Location of multichannel sei
of the water and DIC. Methane gas for stable isotope analysis was obtained by headspace extraction of the pore water sample.
Gas emanating from the NE Mound at 800 m water depth captured within an overturned funnel quickly formed a mushy mass of gas-hydrate-coated bubbles (Fig. 2A). Once the funnel was filled with gas-hydrate-coated bubbles, the ROVascended. By 420 m water depth all the gas hydrate had decomposed but the resulting gas remained trapped in the funnel. The ROV paused its ascent at 400 m and the gas was transferred to a manifold of pre-evacuated cylinders.
Twenty measurements of shallow temperature gradients in sediment on and around the NE Mound were obtained using an Alvin-style thermal probe deployed from the ROV Tiburon. This 0.6 m-long five-thermistor thermal probe was inserted vertically into the sediment by a hydraulic ram mounted on the ROV. After insertion, temperature was measured at 2-s intervals for 10 min. The data were processed using an analytical function fit to the decay of the frictional heat pulse, modified from Villinger and Davis (1987), with an assumed thermal conductivity of 0.80 W/ m°K based on needle probe determinations of thermal conductivity in sediments from nearby basins (Lee and Henyey, 1975). The uncertainty in the measured gradient values is reported as one standard deviation of a least-squares fit to the lag-corrected
ounds and swells and to establish a tie to the ODP 1015 Site. Boxes show areas smic-reflection profile 98L030 shown in Fig. 9 is indicated by white dotted line.
thermistor temperature gradient and is generally 10% or less of the measured gradient.
The ROV placed a small frame carrying a Nobska acoustic current meter (ACM) equipped with conductivity, temperature, and depth (CTD) sensors on the crest of the NE Mound on February 11, 2005. The CTD was recovered on February 16, 2005, thus providing a 5-day record at a 5-s sampling interval of benthic water column conditions.
Selected carbonate-bearing rock samples were analyzed for their δ13C and δ18O values at the Stanford University Stable Isotope Laboratory, Palo Alto, California. DIC concentrations and δ13C values were measured at North Carolina State University, Raleigh, North Carolina. Thirty-nine pore water δ18O measurements and twenty-six pore water methane δ13C measurements were provided by Isotech Laboratories, Cham- paign, Illinois.
The gas sample collected with the overturned cylinder was analyzed for hydrocarbon gases from C1 to C5, methane δ13C, and 14C content by Isotech Laboratories, Champaign, Illinois.
Vesicomya specimens that had been washed in distilled water after recovery were defrosted and dissected at MBARI in Dr. Bob Vrijnehoek's lab. Tissue components were separated into abductor, foot, and gill. Coupled measurements of 14C content were made on the shell and tissues of the dissected Vesicomya at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratories (LLNL), Liver- more, California. Additional measurements of the δ13C and
Fig. 4. Bathymetric map generated by AUV multibeam surveys across the NE and SW profile sections illustrated in Fig. 6A–E are indicated (black lines). Locations of RO inferred strike–slip faults (blue line) are also indicated.
δ15N of selected clam tissues were obtained at the Stanford University Stable Isotope Lab, Palo Alto, California.
To determine the age of the sediments, the tests of ~1000 planktonic foraminifera specimens were handpicked from the N125 μm fraction from selected intervals in 10 samples from the vibracores. Radiocarbon measurements were made at the National Ocean Sciences AMS (NOSAMS) Facility at the Woods Hole Oceanographic Institution and the CAMS at LLNL. Ages were calculated using the accepted half-life of 14C of 5568 years (Stuvier and Polach, 1977). The reservoir age for the planktonic foraminifera samples in this area is 800 years (Southon et al., 1990; Kienast and McKay, 2001). Radiocarbon ages were converted to calibrated ages using the CALIB 5.0.2 program (Stuiver and Reimer 1993; Hughen et al., 2004; http:// calib.qub.ac.uk/calib/calib.html).
2.2. AUV mapping surveys
The bathymetry and shallow subsurface structure of the Santa Monica Basin mounds and their associated swells were surveyed during four AUV missions operated from R/V Zephyr on June 5–9, 2006. The MBARI Mapping AUV is equipped with a 200 kHz Reson 7125 multibeam sonar, an Edgetech 2– 16 kHz chirp sub-bottom profiler (operated with a 2–10 kHz sweep), and an Edgetech 110 kHz chirp sidescan. The AUV collects high-resolution multibeam bathymetry, achieving a vertical precision of 0.3 m and a lateral resolution of 1.5 m at
Mounds and Swells shown at a 5-m contour interval. Location of chirp seismic V dives (thin red lines), the axial trace of the anticline (red dashed line) and the
100 m survey altitude. The chirp sub-bottom profiles image subsurface structure with a vertical resolution of 0.1 m. The vehicle navigation and attitude data derive from an inertial navigation system (INS) initialized by GPS fixes at the sea surface and then aided by velocity-over-ground observations from a Doppler velocity log (DVL). The INS navigation achieves 0.05% of distance-traveled accuracy provided that constant DVL bottom lock is maintained. Bathymetry, sub- bottom, and sidescan processing are done using MB-system, an open source software package for the processing and display of swath mapping sonar data (Caress and Chayes, 1996; http:// www.mbari.org/mbsystem).
The Mapping AUV surveys were conducted at 3 knots at altitudes ranging from 75 to 100 m. In order to insure continuous DVL bottom lock, the AUV was launched in 100 m water depths
Fig. 5. Parts A and B are detailed maps showing the crests of the SW and NE Mound color codes differ. Parts C and D are perspective views at a 5× vertical exaggeration of with an elevation of 31.5° from horizontal. Red lines are 300 m long.
on the shelf and transited down slope to the survey areas. A total of 130 km of seafloor survey data was collected associated with the mound surveys. These included (Figs. 3 and 4) four descents down the slope to the mounds (N800mwater depth), two detailed surveys with ≤100 m line spacing of the mounds, a reconnais- sance run to the NW, and a transit out to ODP borehole site 1015 in 912 m water depths on the floor of the Santa Monica Basin (Shipboard Science Party, 1997).
3. Results
3.1. ROV observations of the NE Mound
Awater column acoustic plume was initially identified on the ROV's 330 kHz scanning sonar at 550 m water depth during the
s at a contour interval of 1 m. Parts A and B are at the same scale, although the the SWand NEMounds and surrounding swells. Both are views are facing 354°
first descent (dive T-785) to explore the NE Mound in the Santa Monica Basin from which gas hydrate has been recovered previously. ROV video observations confirmed that the plume was associated with rising gas bubbles, like those documented from other locations (e.g., Paull et al., 2007). The bubble plume was also detected as a mid-water sonar target below 500 m water depths on subsequent dives (T-792 and T-799). The plume was followed downward to the seafloor where continuous streams of bubbles were emanating from two vents near the crest of the mound at ~800 m water depth.
Gas captured with a funnel and directed into an overturned graduated cylinder was observed to quickly form a mushy mass of gas-hydrate-coated bubbles (Fig. 2A). Gas bubbles were not observed to be venting from the seafloor elsewhere, nor were acoustic plumes encountered in other locations during these ROV dives.
The shallowest area on this mound (800 mbsl) consists of a ~20 m long NW–SE oriented ridge. This ridge is composed of sediment-bare slabs of massive carbonate that form a nearly continuous pavement that extends down the mound's northeast flank. This carbonate pavement is broken by cracks into pieces that are 1–5 m across. These cracks form openings that are up to 20 cm wide and cut through the carbonate mantle suggesting that the slabs once formed a continuous layer that has been uplifted and cracked by expansion (Fig. 2A). Microbial mats cover much of the surface of themassive carbonate slabs. The centers of themats and the open cracks between the carbonate slabs are coated with orange-colored
Fig. 6. High-resolution seismic-reflection profiles of the Santa Monica mounds and extends NE–SWacross the SWMound (see Fig. 4) and illustrates that this topograph and the basin. Profile B extends NW–SE along the crest of the swell and crosses the S the mound. Profile C is an expanded view of part of profile B (see Fig. 4 for locatio crossing of the NE Swell and shows faulting (F) on its southwest limb. Seismic reflec E crosses the NE Mound. Irregular reflectors to the southwest of the NE Mound are
microbial mat (Fig. 2B), while the edges of the patches of mat are fringed with a white mat. No surface stains, shimmering water, or sediment accumulation was observed around the gas vents, which suggests that neither sediment nor water with a salinity or composition different from seawater is entrained in the venting gas.
The sediment-bare pavement of carbonate extends down the northeast side of the mound to where the topography merges with the surrounding sediment of the broad topographic swell. Below ~810 m water depth where the slope of the seafloor diminishes, the seafloor is comprised of apparently typical soft sediment characteristic of the deposits on the adjacent seafloor. Carbonates were not observed away from the mound flanks and the scanning sonar indicated that seafloor further to the northeast is featureless.
To the southwest of the summit is a N30 m wide terrace at ~802 m (Fig. 5B). This terrace is covered with extremely dense beds of small (~25 mm) living clams (Vesicomya elongata) and orange and white microbial mats that overlie a veneer of soft sediments (Fig. 2C). Blocks of carbonate are scattered around this terrace as well. Although the majority of the surface on this terrace and the mound's southwest flanks was sediment covered, coring attempts showed that hard layers were encountered at shallow depths (10–100 cmbsf) suggesting that layers of authigenic carbonate occur under the sediment cover. A transect down the southwest side of the mound showed that while beds of living V. elongata were not seen below 810 m water depths, scattered shells of dead Vesicomya, numerous gastropods, and small pieces of carbonate rubble extended out
swells collected using the chirp system on MBARI's mapping AUV. Profile A ic high is associated with an anticline. S/B indicates boundary between the swell WMound (see Fig. 4). The reflection profile does not resolve internal structure of n) and shows several small normal faults (F) on its NW limb. Profile D shows a tors traced within the basin thin and or pinch out on the flank of the swell. Profile interpreted to be associated with slumping.
Fig. 7. Photographs showing surfaces of cores (A–C) illustrating three common components of sediment alteration and an idealized stratigraphy in areas where methane is advecting toward the seafloor. Part A shows the surface of a vibracore between 0 and 20 cmbsf (from T-785 VC-11) containing shells and shell hash (sh) produced within seafloor chemosynthetic communities. Part B shows the surface of a vibracore that contains a tan colored authigenic carbonate (ac) nodule formed within the host sediment at modest subsurface depths (28 to 48 cmbsf in T-796 VC-31). Part C shows a white colored nodule of pure gas hydrate (gh) which was exposed on the surface of the piston core as the core catcher was removed (Normark et al., 2003; Hein et al., 2006; photograph taken
for over ~100 m on the down slope side. The topography on this flank suggests that part of the mound's surface on its southwest side has slumped away (Figs. 5D and 6E).
3.1.1. ROV observations of the swell to the NW of the mound Two ROV dives explored the topographic swell to the
northwest of the NE Mound (T-793 and T-796) where offsets associated with the San Pedro Basin Fault Zone have been identified in the regional survey data (Fisher et al., 2003; Gardner et al., 2003). The ROV's scanning sonar showed that live Vesicomya form dense, elongated beds (1–5 m wide and in places over 25 m long) aligned along a subtle linear depression on the side slope of the NE Swell (Fig. 4). No authigenic carbonates were observed on the seafloor in this area. ROV transects perpendicular to and on a zigzag pattern run along this linear feature showed open Vesicomya shells scattered on the seafloor and living gastropods are common on the down-slope side, but are rare farther up the slope of this linear feature.
3.2. Samples from NE Mound
During dives on the crest of the NE Mound sediments were inspected with special interest in whether extruded autochtho- nous solids were present. The mound crest was a continuous, hard surface that was difficult to sample because of the lack of loose pieces or exposed edges. Nevertheless, two samples that are clearly part of the outcropping carbonate pavement were recovered (T-785 R01 and T-799 R06). One half of sample T-785 R01 is composed of carbonate-cemented fine-grained sediment and the other half contained a cemented cluster of V. elongata shells like those living in this environment today (Fig. 2D) indicating these are locally formed authigenic carbonates.
The δ13C values of 15 sub-samples taken from the carbonate mantle on the NE Mound during ROV dive T-785 range from −35.73‰ to −54.98‰ (PDB) with a mean value±1σ−50.41± 4.88‰ (PDB) indicating they are composed of methane-derived cements (Table 1). Measurement of the δ18O values of carbonate in the same samples range from 2.85 to 3.52‰ (PDB) with a mean value±1σ 3.21±0.17‰ (PDB).
Table 1 Carbon and oxygen isotope composition of carbonate samples from the NE Santa Monica Mound
Sample Sample # Sample # δ13CPDB δ18OPDB
T785 R01 A −52.711 3.310 T785 R01 B −52.314 3.349 T785 R01 C −51.602 3.270 T785 R01 D −51.538 3.170 T785 R02 A −52.689 3.107 T785 R02 B −51.740 3.103 T785 R03 A −51.194 3.301 T785 R03 B −49.519 3.259 T785 R03 C −35.730 2.849 T785 R04 A −52.524 3.366 T785 R04 B −52.174 3.515 T785 R05 A −54.977 3.106 T785 R05 B −42.665 2.938 T785 R05 C −53.138 3.251 T785 R05 D −51.686 3.249
by Jamie Conrad). This section came from ~2 mbsf, below the level at which authigenic carbonate precipitation occurs. To the right is an idealized stratigraphy that illustrates how these three facies (i.e., A–C) occur within methane-rich, near-surface sediments on the NE Mound and Swell.
Eight vibracores, four from the flanks of the NE Mound (VC-2, −11, −36, and −37) and four from along the linear zone of scattered CBC along the projected trace of the San Pedro Basin Fault Zone ~1 km to the northwest of the mound (VC-27, −28, −31, and −33) were collected. Near-seafloor sediment in all eight cores contains a mixture of shell hash and dark gray to olive sediment (Fig. 7). Open shells with a noticeable nacreous luster were observed within the upper 10 cmbsf, however shells from greater depths had only a dull luster. The cores contain variable amounts of authigenic carbonate with depth, suggesting that this carbonate develops within the shell-hash matrix in the mound cores, but within hemipelagic sediment of other cores. Sediment in mound cores had a mousse-like texture. Similar mousse-like textures are commonly observed in
sediment that has undergone gas exsolution during core recovery (Paull and Ussler, 2001). The four cores from the fault trace are similar especially at their tops, but contain little or no shell hash at depth.
Table 2 Composition of pore waters samples from the NE Santa Monica Mound and Swell
Sample SO4 CH4 DIC
(mM) (uM) (mM)
T785 VC2 bottom 0.86 1,322 19.58 T785 VC2 bottom rep 19.44 T785 VC11 71–88 cm 0.54 1,020 21.47 T785 PC62 4–8 cm 22.24 102 7.44 T785 PC62 12–16 cm 5.99 555 16.65 T785 PC70 8–12 cm 3.77 1,488 19.59 T785 PC68 16–20 cm 0.6 1,016 21.09 T785 PC52 4–8 cm 18.55 137 9.29 T785 PC67 8–12 cm 0.31 1,451 23.09 T785 PC52 12–16 cm 1.15 1,340 22.10 T785 PC67 16–20 cm 0.48 1,455 18.45 T793 PC59 4–8 cm 26.2 13 3.81 T793 PC59 16–20 cm 23.08 17 6.97 T793 VC28 109–121 cm 22.99 9.8 6.67 T793 VC28 103–109 cm 21.29 11 6.43 T793 VC27 119–131 cm 17.19 33 10.13 T793 VC27 115–119 cm 16.83 40 10.44 T793 PC45 4–8 cm 27.57 0.9 T793 PC45 13–18 cm 27.1 4.8 T793 PC70 15–20 cm 25.26 3.3 T793 PC66 4–8 cm 27.77 0.1 T793 PC66 12–16 cm 27.49 0.4 T793 PC73 12–16 cm 26.84 0.3 T793 PC55 4–8 cm 27.71 0.2 T793 PC55 12–16 cm 27.65 0.7 T796 PC67 4–8 cm 27.54 0.5 2.85 T796 PC67 12–16 cm 26.5 2 3.20 T796 PC73 4–8 cm 27.5 0.7 3.45 T796 VC31 67–83 cm 0.6 1,576 18.03 T796 VC31 61–67 cm 1.91 720 19.32 T796 VC33 91–103 cm 0.85 1,522 18.15 T796 VC33 85–91 cm 0.59 1,020 19.33 T796 PC73 12–16 cm 14.69 142 12.14 T796 PC66 4–8 cm 27.8 0.1 3.23 T796 PC66 16–20 cm 27.42 0.4 3.02 T796 PC57 4–8 cm 27.89 0.1 T796 PC57 12–16 cm 27.67 0.1 T796 PC70 4–8 cm 27.3 0.1 T796 PC70 12–16 cm 26.69 0.2 T796 PC42 4–8 cm 27.65 0.5 T796 PC42 12–16 cm 26.07 2.2 T799 VC36 52–69 cm 0.48 2,128 20.92 T799 VC36 46–52 cm 1.09 1,899 21.95 T799 VC37 76–93 cm 0.67 1,193 21.61 T799 VC37 70–76 cm 0.47 4,555 21.07 T799 PC71 4–8 cm 2.63 327 21.43 T799 PC71 12–16 cm 0.6 1,388 19.96 T799 PC71 16–20 cm 0.8 977 26.34 T799 PC63 4–8 cm 3.2 388 19.71 T799 PC63 12–16 cm 0.67 1,541 19.91 T799 PC44 4–8 cm 1.02 1,576 20.88 T799 PC44 12–16 cm 0.36 405 20.35 T799 PC68 16–20 cm 26.44 4.60 T799 PC66 16–20 cm 27.33 3.93
m–mound, ns–north swell fault zone, bg–background.
Larger Vesicomya (Calyptogena) spp. and tubeworms, which are commonly observed at other CBC sites on the Pacific margin, were noticeably absent (e.g., Barry et al., 1996; Levin, 2005). A few Lucinoma shells were recovered in the
DIC CH4 Water Water I
δ13C δ13C δ18O δD
−43.74 −74.0 −0.27 −0.4 m −43.84 −35.74 −80.0 −0.33 −43.76 −93.2 −0.29 −1.1 m −58.67 −100.9 −0.33 −1.6 m −55.64 −95.2 −0.45 m −44.61 −75.6 −0.30 m −49.77 −92.4 −0.33 m −50.74 −81.2 −0.46 m −55.86 −99.4 −0.29 m −47.62 −78.7 −0.35 m −22.85 −0.31 m −42.13 −0.27 ns −42.81 −0.27 ns −4.2 2.00 ns −52.1 0.69 ns −52.45 −0.24 ns
ns-bg ns-bg ns-bg ns-bg ns-bg ns-bg ns-bg ns-bg
−6.81 −0.22 −2.1 ns −20.27 −0.32 −1.9 ns −19.81 −0.30 ns −55.77 −84.3 −0.24 −1.9 ns −58.68 −91.3 −0.26 ns −57.89 −96.2 −0.30 ns −56.29 −95.9 −0.19 ns −56.78 −78.0 −0.28 ns −5.93 −0.25 ns −11.03 −0.31 ns
ns-bg ns-bg ns-bg ns-bg ns-bg ns-bg
−40.79 −82.3 −0.40 m −42.09 −82.4 −0.30 m −41.39 −80.5 −0.28 m −41.3 −81.7 −0.29 m −59.48 −98.5 −0.30 m −48.74 −85.8 −0.41 m −44.27 −84.1 −0.30 m −59.5 −95.2 −0.28 m −49.02 −85.1 −0.33 m −40.73 −81.8 −0.40 m −38.74 −80.1 −0.37 m −12 −0.32 m −9.19 −0.34 m
Table 3 Thermal gradients measured on the NE Santa Monica Mound and Swell
Station Latitude Longitude Water Gradient±1σ Location
(°N) (°W) Depth (mK/m)
(m)
T785 HF1 33.799798 118.646477 803 144±14.9 m T785 HF2 33.799492 118.646494 803 67.1±8.8 m T785 HF4 33.798849 118.647659 822 143±8.4 m T785 HF6 33.798963 118.646118 804 65.1±1.8 m T785 HF8 33.800036 118.646531 814 101.5±6.6 m T785 HF9 33.801768 118.647599 819 90.7±9.0 bg T793 HF2 33.801623 118.658543 847 73.7±2.7 bg T793 HF3 33.802476 118.657942 846 93±3.3 bg T793 HF5 33.803762 118.656601 833 87.2±2.1 ns T793 HF6 33.804138 118.656029 823 78±2.8 ns T793 HF7 33.805339 118.655342 826 88.3±2.0 bg T796 HF1 33.803381 118.655723 821 69±7.2 ns T796 HF2 33.803547 118.655372 819 69.2±2.5 ns T796 HF3 33.803566 118.655304 819 71.7±1.9 ns T796 HF4 33.800937 118.652822 822 73.8±1.9 bg T799 HF1 33.797770 118.644042 823 82.8±8.9 bg T799 HF2 33.798406 118.645146 822 78±0.8 bg T799 HF3 33.798799 118.645904 818 77.7±3.4 m T799 HF5 33.799158 118.646514 801 35.4±3.1 m T799 HF6 33.799527 118.646695 801 90.2±4.1 m
m–mound, ns–north swell fault zone, bg–background.
Table 4 14C content of Vesicomya shells and tissues collected from the NE Santa Monica mound
Sample material δ13C %modern±1σ Δ14C±1σ CAM #
‰
PDB
T785 PC51 A. S −0.8 82.78±0.26 −177.8±2.6 121599 T785 PC51 A, T −37 84.18±0.28 −163.8±2.8 121769 T785 PC51 B, S −0.3 82.79±0.32 −177.7±3.2 121600 T785 PC51 B, T −37 85.20±0.28 −153.8±2.8 121770 T785 PC55-2, S −0.5 82.68±0.30 −178.8±3.0 123838 T785 PC55-2, Tf −37.1 83.61±0.27 −169.5±2.7 123685 T785 PC58-1, S −1.9 82.90±0.28 −176.6±2.8 123839 T785 PC58-1, Tf −38.7 83.54±0.27 −170.3±2.7 123686 T785 PC58-2, S −0.3 82.81±0.28 −177.5±2.8 123840 T785 PC58-2, Tf −36.9 84.23±0.35 −163.3±3.5 123687 T785 PC58-3, S −0.4 83.08±0.28 −174.8±2.8 123841 T785 PC58-3, Tf −37.2 83.77±0.27 −168.0±2.7 123688 T785 PC58-4, S −0.8 83.15±0.28 −174.1±2.8 123842 T785 PC58-4, Tf −37 82.86±0.33 −177.0±3.3 123689 T785 PC62 A, S −1.1 83.22±0.28 −173.5±2.8 121492 T785 PC62 A, T −37 84.18±0.32 −163.9±3.2 121767 T785 PC62 B, S −1.3 83.09±0.26 −174.7±2.6 121491 T785 PC62 B, T −37 84.80±0.28 −157.8±2.8 121768 T793 PC70 A, S −0.9 82.88±0.31 −176.8±3.1 121493
S–shell, T–tissue, Tf–foot tissue, assumed value for δ13C correction.
scoop samples and within push cores taken from the V. elongata beds, but none were observed on the seafloor.
3.3. Pore water geochemistry
A total of fifty-two pore water samples were extracted from 23 push cores and 8 vibracores (Table 2). The seafloor environments from which they came include sediment on the NE Mound, background sediment on the swell, and cores from ~1 km to the NW of the NE Mound where CBC were also encountered.
The chloride concentrations in these samples ranged from 510 to 544 mM with a mean±1σ of 537±6 mM. These values are similar to seawater. The limited range of chloride concentration in the samples collected around and over the mound suggest that gas hydrate was not present in the sediments and that there is no local salt source.
Pore water sulfate concentrations range from 27.9 mM down to 0.3 mM. Although the sulfide concentration was not measured, many samples initially had an intense sulfide smell. Even though the doors to the shipboard lab were kept open, people processing the samples became insensitive to the odor, suggesting that the initial pore water concentration of sulfide in some of these cores was significant.
Methane concentrations measured on 51 samples range from 0.07 to N4500 μM. The higher methane concentrations occurred in samples that were nearly sulfate-depleted. Methane δ13C values measured on 26 of these pore water samples range from −100.9‰ to −74.0‰ (PDB) with a mean±1σ of −86.7±8.0‰ (PDB). Forty DIC concentrations and δ13C values in selected samples ranged from 2.88 to 26.3 mM and δ13C values of −59.5‰ to −4.2‰ (PDB). Ten of these DIC measurements
yielded δ13C values more 13C-depleted than the authigenic carbonate samples. The δ18O values of thirty-nine water samples ranged from −0.46 to 2.00‰ (SMOW) with a mean±1σ of −0.22±0.40‰ (SMOW).
3.4. Vent gas sample
The sample of the venting gas collected underneath an inverted funnel was composed primarily of methane (99.22%) with trace amounts of ethane (0.0029%) and carbon dioxide (0.093%). The methane has a δ13C value of −70.8‰ (PDB) and a 14C content of b0.4% modern carbon.
3.5. Thermal measurements
Bottom water temperatures during the 5-day deployment of the ACM-CTD on the crest of the mound ranged from 5.21 °C to 5.55 °C with a mean±1σ 5.26±0.02 °C. The ACM was placed on the crest ridge ~3 m from and midway between the two bubbling vents. The ACM recorded oscillating northwest and southeast currents of b20 cm/s with a net movement to the northwest. Although the temperature also oscillated subtly on a tidal frequency, no significant spikes were seen that might suggest the instrument was exposed to venting waters of a contrasting temperature. Benthic bottom water temperature measurements made with the ROV CTD indicate that the flanks (830 m) were colder than the crest by 0.05 °C.
Temperature gradients measured using the ROV-deployed Alvin-style thermal probe range from 35.4 to 144 °C/km with a mean±1σ of 84±25 °C/km (Table 3). Pervasive carbonate cementation in mound sediment made it difficult to obtain a good insertion near the crest of the mound and in the bubble streams.
Table 5 Carbon and nitrogen isotope composition and ratios of V. elongata tissues from the NE Santa Monica mound
Sample δ15N (Air) δ13C (PDB) C:N (Atomic)
T785 PC 51-1 add 2.87 −36.05 4.58 T785 PC 51-1 foot 3.50 −37.53 6.13 T785 PC 51-1 gill 3.34 −35.52 4.60 T785 PC 52-1 add 2.20 −35.90 4.17 T785 PC 52-1 foot 2.59 −36.42 4.98 T785 PC 52-1 gill 1.91 −35.76 4.84 T785 PC 55-1 foot 2.75 −35.91 4.82 T785 PC 55-1 gill 2.84 −26.23 6.34
Sites in normal hemipelagic sediment away from any known vents have the narrowest range of values (69.0 to 93.0 °C/km) compared to mound-associated and mound periphery measurements (35.4 to 144 °C/km). Values obtained with the Alvin-style thermal probe are comparable to 13measurements obtained using a 6.4-m outrigger-style probe (103±26 °C/km) in the adjacent San Pedro and Santa Catalina Basins (Lee and Henyey, 1975).
3.6. 14C content of Vesicomya shells and tissues
The 14C content of carbonate in the shells of 10 live V. elongata from the NE Mound range from 82.7% to 83.2% modern carbon with a mean±1σ of 82.9±0.2% modern carbon (or −178.6Δ14C to −173.5Δ14C with a mean±1σ −176.2±1.8 Δ14C) (Table 4). The δ13C values of these same shells ranged from −1.9 to −0.3‰ with a mean±1σ of −0.83±0.50‰ (PDB).
The 14C content of tissue samples from Vesicomya collected on the NE Mound range from 82.9% to 85.2% modern carbon with a mean±1σ of 84.0±0.7% modern carbon. Measurements of the δ13C organic carbon values of four samples range from −38.70‰ to −36.91‰ (PDB). The δ13C and δ15N values for an additional eight measurements of V. elongata (Table 5) have δ13C values that range from −37.53‰ to −25.23‰ with a mean±1σ of −34.90±3.5‰ (PDB) and δ15N values that range from 1.91‰ to 3.5‰ with a mean±1σ of 2.7±0.5‰ (air).
3.7. 14C sediment age measurements
Ten radiocarbon age measurements on planktonic foraminifera from 8 vibracores at selected intervals ranging from 20 to
Table 6 14C age determinations made on planktonic foraminifera from ROV-collected vibrac
Sample ROV Dive #, VC #, depth interval
δ13C‰ PDB Age 14C yr BP Age Error±y
T785, VC-11, 69–71 cm, M 0.51 2,160 ±30 T785 VC-02, 88–90 cm, M 0.34 2,920 ±30 T793 VC-27, 20–23 cm, Sw −9.99 2,770 ±30 T793 VC-27, 100–105 cm, Sw −0.50 4,625 ±30 T793 VC-27, 113–115 cm, Sw −1.16 5,280 ±35 T793 VC-28, 101–103 cm, Sw −1.43 5,000 ±35 T796 VC-33, 82.5–84.5 cm, Sw −2.44 5,240 ±35 T796 VC-31, 65–67 cm, Sw −3.82 5,490 ±40 T799 VC-36, 44–46 cm, M −1.16 1,560 ±30 T799 VC-37, 68–70 cm, M −1.74 3,490 ±50
T–Tiburon; VC–Vibracore; with δ13C correction; M–core from mound; Sw–core
115 cm depths yielded Holocene ages (Table 6). Four of these samples came from near the bottom of cores from the crest of the NE Mound, four from near the bottom of cores from the northwest flank of the NE Swell, and two additional samples from shallower in on core (VC-27) from the flanks of the NE Swell. Age-depth relationships (or accumulation rates, calculated assuming a zero age for the seafloor) allow comparisons to be made between mound and swell cores. These data indicate that the accumulation rates are the same or slightly higher in the mound cores (24–64 cm per thousand years) than those measured on the swell (10–24 cm per thousand years).
3.8. AUV surveys
AUV survey lines extend from the shelf edge to the two survey grids on the upper portion of the basin, and one survey line extends out to ODP Site 1015 in 917 m water depth near the center of the Santa Monica Basin for stratigraphic control (Fig. 3). The two AUV survey grids (Figs. 3, 4, and 5), with 100% multibeam bathymetric coverage, consist of 34 northeast–southwest oriented lines and 3 northwest–southeast tie lines on the NE Mound and swell, and 6 northeast–southwest and 3 northwest–southeast tie lines on the SW Mound and Swell.
3.8.1. Bathymetry Both mounds protrude from broad bathymetric highs,
referred to as the SW and NE Swells with crests that are 10– 30 m higher than the adjacent seafloor (Figs. 4, 5, and 6). The NE Swell is outlined by the 835 m contour and is ~2 km long and ~1 km wide with its southwestern flank slightly steeper (up to 3° slopes) than its northeastern flank (b2°). While the AUV surveys did not delineate the full extent of the SW Swell, existing data show that the SW Swell is an asymmetric feature, with its ridge narrowing from N600 m wide in 870 m of water depths on the eastern side to b300 m wide at 880 m water depth on its western side.
The comparatively steep lower slopes of the mounds (Fig. 5A and B) stick up from the crest of the swells another 10 to 15 m. The summit depths of these mounds are in 858 and 800 m water depths for the SW and NE Mounds respectively. The general shape of both mounds is similar. The NE Mound, however, is
ores from the NE Mound and Swell
r Reservoir Corrected Age yr BP
Calendar Age yr BP
AR cm/103 yr
NOSAMS or CAM #
1,360 1,306 54 OS-61311 2,120 2,197 41 OS-61315 1,970 1,995 11 CAMS-127572 3,825 4,328 24 CAMS-127573 4,480 5,199 22 OS-61263 4,200 4,822 21 OS-61264 4,440 5,145 16 OS-61265 4,690 5,455 12 OS-61312 760 703 64 OS-61266
2,690 2,863 24 OS-61314
slightly larger and more circular than the SW Mound (Fig. 5A and B). Both mounds have sides with slopes of 10° to 20° that decrease towards a horizontal plateau near the summit with slopes b1°. The near-summit plateau of NE Mound is an approximately circular area more than 60 m across and outlined by the 804 m contour (Fig. 5B). The shallowest spot is on the very northeastern side of the near-summit plateau and corre- sponds with the area that was observed to be a carbonate ridge during the ROV dives (see Section 3.1.1 above and Fig. 2A). The near-summit plateau on the SW Mound is outlined by the 860 m contour and is elliptical with maximum and minimum diameters of 80 and 40 m respectively (Fig. 5A).
The AUV surveys show that the seafloor away from the mounds is generally smooth and featureless, but a few additional subtle features were noted: (1) A tongue of comparatively rougher seafloor occurs on the southwest-side of the NE Swell, which is downslope from the NE Mound (Fig. 5D). This tongue extends out to 840 mwater depths; (2) The bathymetry of the NE Swell shows seafloor lineations occur on both its southeast and northwest flanks (Figs. 4 and 5D). Subtle secondary notches that are parallel to the lineation below and perpendicular to the local slope are also seen near the northwest crest of the NE Swell; and (3) The transition from the flat basin floor to the flank of the SW Swell forms a distinct ‘fault-like’ lineation on the seafloor (Figs. 5C and 6A).
3.8.2. Sub-bottom stratigraphy and structure The AUV chirp seismic-reflection profiles (Fig. 6) resolve
reflectors in the upper 20 to 40ms of the seafloor (e.g., upper 15 to 30 m of the seafloor assuming a sediment velocity of 1500 m/s). Laterally continuous reflectors can be traced easily throughout most of the survey and only slowly change character laterally, with these exceptions: (1) no reflectors were resolved underneath the mounds; (2) some units pinch out on the side scarps on the seaward side of the swells; and (3) units thickening downslope of the NE Mound suggest the existence of slump debris. Profiles from the floor of the basin (Fig. 6A) characteristically show stronger reflectors, greater acoustic penetration depths, and increased resolution of reflectors compared to those from shallower water depths (Fig. 6D). This may be associated with a shift from hemipelagic sediments on the swells and flanks of the basin to more reflective turbidite-dominated sequences in the basin.
The gently warped reflectors that characterize the crest of both broad swells can be traced to the base of the mounds, but their resolution is lost under the mounds (Fig. 6A, B, and E) and the internal structure of the mounds is not resolved. The profiles that cross the anticlines, but do not cross the either mound (e.g., Fig. 6D) lack indications of through-going structure in the subsurface that extends away from the mounds.
Seismic-reflection profiles show that both topographic swells are anticlines (e.g., Fig. 6D). The crest of the anticline that forms the NE Swell has a northwest–southeast orientation and is bounded by lineations seen in the bathymetry (Fig. 4); the eastern lineation is subtler in the seafloor contours. The crest of the NE Swell anticline plunges at both ends and becomes parallel with the seafloor lineations. The crest of the anticline
that forms the SW Swell is somewhat irregular and is oriented approximately east–west and plunges to the west toward the basin floor.
Several disruptions in near surface reflectors are interpreted as being faults (e.g., Fig. 6B, C, and D). These can be grouped as either strike–slip or normal faults.
3.8.2.1. Strike–slip faults. Some profiles show subsurface discontinuities suggesting strike–slip faulting. For example, on the western flank of the NE Swell (Fig. 6 D), coincident with the area where ROV Dives T-793 and T-796 encountered dense CBC (see Section 3.2 above), several profiles show abrupt lateral changes in the reflection characteristics, kinks in reflectors, and shallow seismic blanking zones. Because a series of adjacent profiles show similar features and vertical offsets in individual profiles are small to undetectable, these anomalies are interpreted to be related to strike–slip faulting. While the sense of offset cannot be determined from this data, the regional framework suggests that these are right-lateral strike–slip faults.
3.8.2.2. Normal faults. Sub-bottom reflectors on the flanks of both swells show offsets suggesting that the sediments are cut by numerous small normal faults (e.g., Fig. 6B and C). Some of these offsets can be traced upward to where they offset the seafloor. However, most of these faults end at a prominent reflector that is at 3 ms sub-bottom on the SW Mound. Their apparent dips are consistently away from the crests of the anticlines, but individual faults cannot be convincingly correlated between profiles.
3.8.3. Ties to ODP Site 1015 The 15.7 km long AUV chirp tie line that connects the SW
Swell survey to ODP Site 1015 along with additional lines from pre-existing surveys provide constraints on the ages of the reflectors and thus the timing of faulting. Ten 14C measurements made on foraminifera picked from the hemipelagic material from between turbidite sand horizons at ODP Site 1015 show that the Holocene section is more than 22 m thick and identifies the age of multiple horizons (Normark et al., 1998, 1999a; Fisher et al., 2003, 2006b; and unpublished data courtesy of B. Romans, written communication). Laterally continuous reflectors can be confidently traced across the basin floor to the edge of the SW Swell (899 m water depth). At the base of the SW Swell some layers thin and other layers pinch out altogether (Fig. 6A). While most reflectors could not be traced as continuous surfaces over the flanks of the SW Swell, their appearance and relative depth provides some age constraints. The first strong reflector on the side of the mound is associated with a turbidite that was deposited about 7.5 ka BP and the Holocene drape on the swells is estimated to be N8 m thick.
4. Discussion
Themethane emanating from the crest of theNEMound (C1/C2
of 34,000 and δ13C values of −70.8‰ PDB) appears to be
Fig. 8. Plot of δ13C versus Δ14C content of shell and tissues of V. elongata specimens collected from the NE Mound. For comparison, the estimated value of the DIC for seawater at 800 m depth in the eastern Pacific and for the venting methane are also shown and connected with a line. Inset plot on the upper left expands the data in the region outlined with the dashed box for V. elongata samples and the estimated seawater DIC.
microbial in origin, based on conventional interpretation (Bernard et al., 1978). This methane must be migrating from a gas reservoir at greater depth because the continuous flow observed coming from the crest of the NE Mound could not be sustained by local methane production and because the 14C in the methane is predominately fossil (b0.4% modern carbon). Although previous measurements on gases from the sediment indicated the possibility of secondary amounts of thermogenic gas (Hein et al., 2006) and despite thesemounds being near the prolific oil producing fields of the Los Angeles Basin (Tennyson, 2005), no oil or other indication of thermogenic hydrocarbons was observed in our samples.
Gas bubbles were also released from the sediment beneath CBC on the near-summit terrace of the NE Mound when disturbed by the ROV's manipulator, indicating bubble saturation occurs within these sediments. The δ13C values for methane gas extracted from the pore water from sediment cores were distinctly more 13C-depleted [δ13C=−86.0±8.6‰ (PDB), n=27] than the methane gas that was captured bubbling from the vent. Methane samples from near the sulfate methane interface commonly have more negative δ13C values than methane at greater depths because of the effects of AOM (Borowski et al., 1997; Paull et al., 2000b).
4.2. Carbonate diagenesis
Elevated DIC concentrations (up to 26.3 mM) and 13C- depleted DIC δ13C values (as negative as −59.5‰ PDB) in pore waters obtained from sediment on and around the mound and along the presumed trace of the San Pedro Basin Fault Zone between 600 and 1250 m to the NW of the NE Mound indicate that conditions appropriate for the formation of methane-derived authigenic carbonates exist within the cored sediments. The rapid sulfate depletion in the upper sediment column (b150 cmbsf) also suggests AOM is active. Formation of methane-derived carbonates is favored in areas were AOM is occurring, because the oxidation of methane within the sediment produces pore water with high DIC concentrations and containing 13C-depleted carbon (Paull et al., 1992). Authigenic carbonate minerals attributed to AOM have now been found around numerous sea floor methane vents (e.g., Greinert et al., 2000; Peckmann et al., 2001; Roberts, 2001). The AOM is fueled by the supply of methane from below.
The δ13C values (as negative as −53.1‰ PDB) of the ‘rock’ samples collected from the surface of the NE Mound indicate these are methane-derived authigenic carbonates. Because they containV. elongata shells and sediment similar to the surrounding seafloor, they appear to have formed locally under present-day conditions.
The δ18O values of marine carbonates are controlled by the isotopic composition of the water from which they form, with some kinetic modifications associated with the temperature of formation and the particular carbonate minerals that initially form (Anderson and Arthur, 1983). The predicted δ18O value of the authigenic calcite formed under these conditions is 3.11‰ PDB [using a mean pore water δ18O value −0.22±0.40‰ SMOWand a mean temperature during the CTD deployment on the mound of 5.26±0.02 °C]. Samples identified as containing
authigenic carbonate have δ18O values of 3.21±0.17‰ PDB (Table 1), which is indistinguishable from the simple predicted equilibrium value. Apparently, these carbonates are formed under the present day conditions near the seafloor, rather than being pushed up from below.
4.3. Methane carbon isotope incorporation into shells
Hein et al. (2006) made the suggestion that V. elongata shells from the mounds contain 13C-depleted carbonate (−19 to −12‰ PDB) because significant amounts ofmethane-derived carbonwas incorporated into the shells, rather than exclusively seawater DIC. Hein et al. (2006) expended considerable effort trying to wash samples and acid rinse them to reduce the chances of contaminat- ing the original δ13C signal with secondary carbonate precipitated onto the shell from the pore water. Yet these samples were from piston and gravity cores that could have experienced some carbonate precipitation before post-cruise splitting and sampling.
The source of tissue and shell carbon (such as methane-derived pore water DIC or seawater DIC) can be distinguished using 14C as an independent tracer when the characteristics of both sources are known (Paull et al., 1989). Themethane emanating from themound is nearly 14C-depleted (b0.4%modern carbon orΔ14C~−1000‰) with a δ13C value of −70.8‰ PDB (Fig. 8). The 14C content and δ13C value of seawater DIC at the Station M site in the eastern Pacific west of Point Conception (Bauer et al., 1998;Masiello et al., 1998) provide a basis for predicting seawater DIC in the Santa Monica Basin. Interpolation between the measurements at 722 m and 1282 m water depth at Station M provide an estimate of seawater DIC isotopic characteristics at 800 mbsl in the Santa Monica Basin (Δ14C=−176.5‰ and δ13C DIC=−0.6‰ PDB).
The 14C content of the carbonate in the shells of V. elongata from the NE Mound (82.9±0.2% modern carbon or −176.2‰ Δ14C) are indistinguishable from the predicted 14C content of DIC in bottom waters in this area (Fig. 8). Our measurements of the δ13C values of carefully washed shells of living of V. elongata from the NE Mound yielded δ13C values (−0.8± 0.5‰) that are indistinguishable from the ambient seawater DIC. Together these data indicate that the V. elongata we collected from the Santa Monica Basin incorporated carbon into their shells from seawater DIC that was not diluted with a significant amount of methane-derived carbon, even though nearby pore waters have substantial amounts of methane- derived DIC carbon and the methane gas bubbling out of the mound is nearly 14C-depleted (~−1000 Δ14C).
The stable isotopic composition of tissues in organisms supported by local chemosynthetic production is known to reflect differences depending on whether the primary production is fueled by sulfide- or methane-oxidation (e.g., Rau and Hedges 1979; Paull et al., 1989). While fractionation during carbon assimilation at sulfide-rich environments is greater than typically associated with photosynthesis, the observed stable isotopic composition of the tissue samples of V. elongata from the NE Mound (Table 5) is similar to values measured from sulfide-rich communities elsewhere (e.g., Barry et al., 1996) and do not suggest that methane carbon is being incorporated into the tissue of the V. elongata at the NE Mound.
The tissue of nine individual V. elongata from the NE Mound (84.0±0.7% modern carbon or −165±0.3‰ Δ14C) are similar or slightly more enriched in 14C than the ambient DIC in bottom waters (Bauer et al., 1998; Masiello et al., 1998). The occurrence of slightly (~1.1%) more modern 14C in the tissues than shells suggests that some of the tissue carbon in the clams was fixed by photosynthesis rather than by chemosynthesis. However, because the 14C concentration of the tissues is similar or greater than the shell and estimated seawater DIC values, this suggests that methane carbon has not been incorporated into the tissue of these V. elongata. Apparently these tissues are constructed from carbon that was locally fixed utilizing carbon from ambient seawater DIC without detectible dilution with methane-derived carbon.
Carbonate samples obtained from methane venting environments, where high concentrations of 13C-depleted carbon occur,
Fig. 9. Multichannel seismic-reflection profile (A-1-98 L030) crossing the NE Swel 24-channel streamer with 10 m long groups (Normark et al., 1999a). Location of pro reflectors crossing BSR (B), blanking above the BSR (C), strong reflectors beneath B
are easily altered. We believe that the difference between our measurements and the Hein et al. (2006) measurements illustrate how difficult it is to identify carbonate diagenesis on samples from environments like this.
4.4. Gas hydrate stability and occurrence under and around the mounds
The seafloor surrounding these mounds (N800 m water depth and b5.5 °C) is well within the stability zone for the formation of methane gas hydrate (Sloan, 1998). While no gas hydrate was observed exposed on the surface of the mound or found in the short ROV-collected sediment cores (≤147 cmbsf), gas hydrate grew immediately upon trapping the rising gas bubbles (Fig. 2A) and was previously sampled at 2.12 m depths in a piston core (Hein et al., 2006). Based on the measured temperature gradients, the depth to the base of gas hydrate stability zone is between 50 and 100 mbsf on the flank of the NE Swell. This depth range is obtained using phase equilibria data for pure methane in seawater (Equation 7, Peltzer and Brewer, 2000), the bottom temperature of 5.2 °C and 820 m water depth.
Although the temperature of the venting gas could not be confidently measured, there is no indication that thermal anomalies adequate to move the near-seafloor sediment out of the gas hydrate stability field are associated with the vents. Apparently some gas is rising upward within the sediment column underneath the mound and passing through the gas hydrate stability zone without being trapped as gas hydrate, a phenomena that has been observed elsewhere (e.g., Paull et al., 1995; Torres et al., 2002). However, some gas hydrate may be forming as the gas passes through this stability zone.
A multi-channel seismic profile across the anticline associated with the NE Swell (Fig. 9) illustrates the subsurface conditions. This profile has a well defined reflector at ~100 ms two-way-travel time below seafloor that is interpreted as a bottom-simulating reflector (BSR) because it is associated with a phase reversal, crosses other reflectors, and separates a zone of relatively low reflectivity (e.g., a blanking zone) above from highly reflective potentially gas-charged sediment below. This BSR occurs between 75 and 80 mbsf (assuming velocities between 1500 and 1600 m/s), well within the depth range predicted for the base of gas hydrate stability. This profile
l. Profile was collected by the USGS using a 550-cm3 GI gun as a source and a file is indicated in Fig. 1B. Arrows indicate a reflector interpreted as a BSR (A), SR suggestive of trapped gas (D), and lack of obvious diapiric roots (E).
crosses over the crest of the anticline ~100 m to the west of the NEMound, but does not cross the flanking faults. While there is some bowing of reflectors under the anticline, there is no clear indication that a deeply-rooted diapir exists in this area (Fig. 9).
An idealized stratigraphy for the near-seafloor materials on the NE mound can be generated based on seafloor observations, pore water composition, and knowing that gas hydrates have previously been sampled at shallow depth in this area. CBC (primarily Vesicomya bivalves and microbial mats) supported by the sulfides produced as a result of AOM occur on the seafloor. These are underlain by sediments containing shells of dead Vesicomya bivalves. Beneath this surface layer of dead bivalves methane-derived authigenic carbonates have formed. The formation of these carbonates within the near surface sediment will cement the surface of the mound and significantly decrease the sediment permeability, thus slowing the migration of methane through the sediments and allowing sub-carbonate methane concentrations to increase. When concentrations are adequate, gas hydrate may form.
4.5. Tectonic movements
Both mounds protrude from crests of anticlines that form broad topographic swells on the upper basin seafloor. The position and orientation of the strike–slip faulting observed in the subsurface on the flanks of the NE Swell observed in the AUV reflection profile data and the lack of faulting on the crest of these structures suggests that these anticlines formed between fault offsets. Apparently, the anticline that forms the NE Swell accommodates compression between right-lateral stepover faults along the San Pedro Fault Zone (Fig. 4).
Evidence of faulting can be seen in surface sediments. Thinning of units and distortion of inferred Holocene layers are observed within the AUV seismic-reflection profiles at depths less than 4 mbsf (Fig. 6), but the lack of reflectors at shallower depths prevents finer resolution. The seafloor along the flanks of the NE Swell shows a distinct break in slope (Fig. 5D) that may be associated with right-lateral faults. Elevated methane concentrations, beds of living V. elongata and obvious alteration
Fig. 10. Cartoon showing three possible mechanisms for producing seafloor mounds. P material extruded as a slurry from the subsurface. Part B represents a diapir where the expansion associated with the addition of gas hydrate in the subsurface.
of the seafloor near the SE fault trace suggest that this fault is still active. Recent activity is also suggested by the numerous small normal faults within the Holocene sediments on the flanks of the SW Swell. This suggests that these fault systems are potential geo-hazards, especially considering their proximity to Los Angeles.
4.6. Potential for gas hydrate in sand-rich sediment
ODP Site 1015, which is located on the floor of Santa Monica Basin (Fig. 1), shows that that the basin fill is predominantly composed of turbidite sand (Shipboard Science Party, 1997). Horizons can be traced on seismic-reflection profiles from ODP Site 1015 to themounds (Normark et al., 1998, 1999a; Fisher et al., 2003, 2006b). Thus, gas hydrate underlying these anticlines may be hosted within turbidite sands. Most of the known marine gas hydrate occurrences are within fine-grained sediments (Ginsburg and Soloviev, 1998). Although rarer, current research on the potential of recovering gas frommarine gas hydrate is increasingly focused on sand horizons because of their high porosity and permeability (Max et al., 2006). Thus, this area should be considered as a potential research site to evaluate gas hydrate processes within marine sands.
4.7. Possible origins of the mounds
Because the mounds are elevated above the surrounding seafloor, a mechanism to either accumulate material on the pre- existing seafloor or to uplift the existing sediment is required. These features were previously described as diapir-like or mud volcanoes, based in part on their shape, lack of internal structure, and occurrence within a compressional tectonic regime (Normark and Piper, 1998; Normark et al., 2003; Hein et al., 2006). The topography associated with mud volcanoes is generated by accumulation of extruded material. The 14C measurements of the sediments found on the crest of the NE Mound and NE Swell yielded only Holocene ages (Table 6). The Holocene ages and similar sediment accumulation rates in the sediments from the NE Mound and NE Swell suggest that
art A represents a mud volcano where the topography is composed of reconstituted solids have flowed upward and deform the seafloor. Part C illustrates the effect of
these features are still active, but provide no evidence that the sediment from which the mound formed comprises older materials extruded from the subsurface.
Although the sediment and authigenic carbonate found on the seafloor of the NE Mound and adjacent to the trace of the fault to the north are different from what is found on the typical hemipelagic seafloor, the diagenetic processes associated with AOM within near-seafloor sediments can produce all these differences. No evidence was found during the ROV dives that extruded materials accumulated to form the mound, as would be the case for a mud volcano (Fig. 10A). Moreover, the lack of variation in the chloride concentration of the pore water samples from the mounds and absence of obvious diapiric structures in the regional seismic data are inconsistent with the mounds being the surface expression of deep-seated salt diapers (Fig. 10B). No evidence for mud diapirism exists in this region (Fisher et al., 2003).
The occurrence of these mounds on the crest of the young and apparently actively forming anticlines within the gas hydrate stability zone, the presence of methane venting through one of these features, and the inherent need for expansion to form gas hydrate lenses, suggest that these uplifted features may be blisters formed by expansion associated with net gas hydrate accumulation in the subsurface (Fig. 10C). The steps in a conceptual model for mound formation by subsurface gas hydrate accumulation involve focused gas migration to the crest of an actively forming anticline, formation of gas hydrate in the shallow subsurface, and upward expansion associated with continued gas hydrate growth and accumulation supported by the flux of methane through this open system which blisters the seafloor, generating the mounds.
Subsurface gas migration is presumably focused towards the crests of these anticlines. Gas hydrate formation within the subsurface sediment is predicted because the temperature and pressure conditions are appropriate, and the observed venting indicates that there is a sustained supply of methane. Thus, some gas hydrate may be forming along the migration pathway that extends from the base of gas hydrate stability (~80 mbsf) to the seafloor. Gas hydrate may accumulate along this pathway as long as the flux of methane continues. Lateral gas migration may be enhanced and focused toward these structures by the gently-dipping sand-rich turbidite horizons on the flanks of these anticlines.
The habits of gas hydrate crystal clusters formed within these sediments are poorly known andmay include pore-filling hydrate. However, most samples of sediment-hosted gas hydrate are nearly pure. Growth of pure gas hydrate requires exclusion of sediment. Accumulation of gas hydrate at these shallow depths may cause inflation of enclosing sediments. To blister the seafloor 15 m would require that approximately 20% of the sediment in the originally 80 m thick section be filled by pure gas hydrate.
Expansion associated with gas hydrate formation was recently hypothesized as a mechanism to explain 1 to 2 m high seafloor blisters on the Norwegian margin (Hovland and Svensen, 2006). The effects of gas hydrate growth on sediment are similar to those associated with ice formation and decomposition in permafrost regions. The expansion of ice within the subsurface in permafrost regions is known to alter the land surface and generate pingos, which are blister-like topographic features similar in size to the
mounds in the Santa Monica Basin. Shallow subsurface features elsewhere in the California Borderland have been attributed to the growth of pure gas hydrate lenses (Normark et al., 1999b). While the available data do not prove this hypothesis, we believe that in the case of the Santa Monica mounds, it fits the existing observations as well or better than the other hypotheses for the origin of these features. Thus we encourage other investigators to consider this mechanism for the formation of similar mounds in other areas.
5. Summary and conclusions
(1) The occurrence and impact of venting methane on the NE Mound in the Santa Monica Basin is obvious as a plume of gas was observed to be emanating from the top of NE Mound, chemosynthetic biological communities supported by dissolved hydrogen sulfide generated by anaerobic oxidation of methane cover much of the surface of the NE Mound and methane-derived carbonate cements pre-existing sediments that mantle the mounds. The same biological and diagenetic effects also are seen along the inferred trace of a fault on the northwest side of the NE Swell.
(2) The venting methane gas has elemental and isotopic composition indicating a microbial origin.
(3) While methane-derived carbonates appear to be actively forming in the near subsurface on the mound, the shells of the V. elongata we sampled reflect the δ13C and 14C composition of the DIC in ambient seawater and not the methane-derived carbon.
(4) The seafloor swells are anticlines formed between right- lateral offsets in regional transpressional faults and have been active in the Holocene.
(5) Themounds occur on the crests of these growing anticlines. (6) The surface of the NE Mound is composed of materials
that appear to have formed in place as a consequence of methane-induced diagenesis, but lack evidence of injec- tion and extrusion of material and thus do not appear to be true mud volcanoes.
(7) We suggest that these features are formed in ways that are analogous to open system ice-cored pingos in permafrost areas, but instead of ice, expansion is associated with the formation and accumulation of gas hydrate in the subsurface.
(8) The Santa Monica Basin has the potential to contain massive gas hydrate occurrences within sand-rich sediment sequences.
Acknowledgements
The David and Lucile Packard Foundation provided support. Special thanks to the crews of the R/V Western Flyer and R/V Zephyr, pilots of the ROV Tiburon and AUV operators. We thank Pete Darnell (USGS) for obtaining the additional multibeam data showing the SW Mound, Ray Sliter (USGS) for processing the multichannel profiles, Mary McGann (USGS) for help with radiocarbon dating of ODP Site 1015 core material, Ginger Barth (USGS) and Dave Valentine (UCSB) for helpful reviews.
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