Elsevier

Marine Geology

Volume 250, Issues 3–4, 1 May 2008, Pages 258-275
Marine Geology

Association among active seafloor deformation, mound formation, and gas hydrate growth and accumulation within the seafloor of the Santa Monica Basin, offshore California

https://doi.org/10.1016/j.margeo.2008.01.011Get rights and content

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 > 10 m high and > 100 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.

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 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 > 520 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 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).

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).

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 associated with 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 (SW Mound) was subsequently identified 2.3 km away in newly acquired surface-ship multibeam swaths since the Hein 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 these Monterey Bay 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.

Section snippets

ROV observations, sampling and sample processing

Five dives of the ROV Tiburon (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 Santa Monica Basin (Fig. 1B). The existence of the SW Mound was not known at the time of the ROV surveys. During this effort 20 h of video observations of the

ROV observations of the NE Mound

A water column acoustic plume was initially identified on the ROV's 330 kHz scanning sonar at 550 m water depth during the 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

Venting gas characteristics and source

The methane emanating from the crest of the NE Mound (C1/C2 of 34,000 and δ13C values of − 70.8‰ PDB) appears to be 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 (< 0.4% modern carbon). Although previous

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

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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