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Expedition 311 Scientists2
A number of cold vents characterized by near-vertical seismic blank (or wipeout) zones have been identified near Ocean Drilling Program (ODP) Sites 889/890 (Wood et al., 2000; Riedel, 2001; Riedel et al., 2002). The 2 km x 4 km cold vent field consists of at least four vents associated with near-surface faults. In addition to the main vent field, other vents have been identified by the presence of seismic blank zones ~4 km northwest of Site 889 and on the southeastern slope of an accreted ridge 8 km south-southwest of Site 889 (Riedel, 2001). The most prominent vent in this vent field, called Bullseye vent, has been the subject of intensive geophysical and geochemical studies since 1999, including
Blank zones associated with vents were first identified on Deep-Towed Acoustic Geophysics System (DTAGS) data in 1997 using intermediate frequencies from 250 to 650 Hz. Details of this study are described in Chapman et al. (2002) and Wood et al. (2002). The cold vent field associated with Integrated Ocean Drilling Program (IODP) Site U1328 (proposed Site CAS-06A; Collett et al., 2005) is characterized by numerous blank zones (Fig. F1). These subvertical zones of reduced seismic reflection amplitude have since been studied using different seismic recording systems and source frequencies, where the degree of blanking increases with higher seismic frequencies (Riedel et al., 2002). Several blank zones have been recognized around Site U1328. The blank zones range from 80 m to several hundreds of meters in width, and blanking seems to be restricted to the upper slope sediment section above the bottom-simulating reflector (BSR). However, the majority of the deeper accreted sediments lack coherent reflectivity and blanking cannot be distinguished from the generally chaotic low reflectivity of the sediments at these depths.
The distinct surface expression of Bullseye vent (blank Zone 1) is evident, whereas blank Zones 2–4 have no equivalent surface expression (Fig. F1). A prominent high-amplitude layer at ~10 meters below seafloor (mbsf) is an apparent upward barrier for blank Zones 2 and 4 as identified in the 3.5 kHz data (Fig. F1A). A slight surface expression can be seen at blank Zone 2 but does not exceed 1 m. Blank Zone 3 extends to the seafloor but has little seafloor topographic expression. Bullseye vent has a near-vertical boundary to the southwest, but the zone widens with depth and its boundary is more diffuse to the northeast (Fig. F2).
The vents appear to be underlain by a continuous BSR (Figs. F1, F2), but the BSR is generally weak except for a bright spot associated with Bullseye vent (Fig. F3A). Reflection amplitudes beneath the BSR bright spot are enhanced, suggesting the existence of free gas. The location of the BSR bright spot also coincides with a deep trough of slope basin sediments between buried ridges of accreted material (Fig. F3B). The boundary between slope and accreted sediments is the strongest lithologic boundary observed in this area and could potentially be a barrier in upward fluid migration and in this case acts as a trap for free gas, creating the observed BSR bright spot.
All of the blank zones are observed to have an east–west trend, as identified from 3-D seismic time-slice analyses (Fig. F4), which is interpreted to be the result of the major margin-perpendicular compressional stress regime (Riedel et al., 2002). However, the area of blanking at blank Zone 1 is also elongated but trends more in an northeast–southwest direction. At Bullseye vent, 3-D time slices of instantaneous amplitude show an apparent ring structure that is interpreted to be the result of seismic diffractions at the shallow gas hydrate cap (Fig. F5). In 3-D, the gas hydrate is seen to form a cap, which dips away from the center of the vent (Fig. F6). Below the gas hydrate cap the amplitude of the seismic data is reduced.
A total of 24 piston cores were collected over the area of the vent field in the year 2000, focusing on Bullseye vent and blank Zone 4 but also including reference locations outside the vent field (Novosel, 2002). The cored sediment mostly consists of finely laminated glaciomarine clays and silts with occasional layers of sands and silts. Occurrence of pyrite in the form of disseminated grains, framboidal fillings of small cavities, and complete replacement of foraminifers was limited to the cores located within the vents. Physical property analyses (P-wave velocity, density, magnetic susceptibility, and thermal conductivity) showed signs of sediment alteration from methane advection by the presence of increased reflection coefficients as well as higher thermal conductivities for cores inside the vent. Magnetic susceptibility was also significantly reduced by several orders of magnitude for cores taken inside the vent field relative to cores from outside the blank zone (Novosel et al., 2005). Similar observations were made on gas hydrate–bearing cores from the Mallik well site in the Mackenzie Delta, Northwest Territories, Canada (Lowe et al., 2005).
Coring investigations were complemented by heat flow measurements over the vent area to examine the thermal effect of fluid expulsion. Heat flow and thermal conductivity were measured at 11 core sites around Bullseye vent and at six sites at blank Zone 4 (Riedel et al., 2006). Heat flow is uniform over the vents with values of ~60 mW/m2.
During piston coring operations, massive gas hydrate was recovered in the center of Bullseye vent from 1 to 8 mbsf. Often, gas hydrate stopped core penetration and was found in the core catcher. All recovered gas hydrate samples contained mostly CH4, some H2S and CO2, and <0.5% of higher hydrocarbons (Solem et al., 2002; Riedel et al., 2006). The CH4 isotopic 13C values were measured in two gas hydrate samples and two sections of free gas within the cores. Gas hydrate samples ranged from –65 to –70, and the void gas sampled had a slightly lighter isotopic composition, from –71 to –77. Studies using powder X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and Raman spectroscopy showed that all samples recovered were Structure I gas hydrate, and hydration numbers were ~6 ± 0.2 (Lu et al., 2005).
Two dives with the remotely operated vehicle ROPOS were conducted in 2000 and 2001 at Bullseye vent. The second dive was extended to cover the cold vent field along seismic Inline 27 from the 3-D MCS survey, which also was imaged with DTAGS in 1997. Large outcrops of carbonates were found at several locations (Fig. F7A, F7B, F7C, F7D). Typically, the 10–15 cm thick carbonate sheets cover an area of >10 m2 of the seafloor and were located around relatively steep ridges ~3 m high. The carbonates often outcrop along multiple lineaments. The surface of the rocks is smooth and does not show any indication of alteration. The isotopic composition indicates that the carbonate is formed mainly by oxidation of biogenic methane (carbon isotopic values are about –45). Only a single, isolated living clam community (vesicomyids) associated with worms and bacteria mats was observed in the area around Bullseye vent in 2001 (Fig. F7G, F7H). The presence of these fauna indicates active methane venting and related sulfide emissions. The clams formed small colonies (1–2 m diameter) aligned around small carbonate ridges. Several water samples were taken with ROPOS in a vertical profile immediately above the clam colony (Solem et al., 2002; Riedel et al., 2006). Peak methane concentrations of ~20 nM were found at the seafloor and 100–200 m above the clam colony. Additional carbonate outcrops were observed at blank Zone 3, with carbonate formation being mainly flat, thin sheets embedded in the sediments (Fig. F7E, F7F).
Gas hydrate that forms in sediments may have a cementing effect on grains and could significantly impact the shear properties of the sediment column. Seafloor compliance is the transfer function between pressure variations (from ocean surface, long-period gravity, or infragravity waves) and the associated displacement of the seafloor. The data are most sensitive to the shear modulus as a function of depth. Shear modulus is converted to shear wave velocity by assuming a density function that increases linearly with depth. Willoughby and Edwards (2000) published results of compliance studies near Site 889 and the implications for the shear wave velocity structure in the region. Newly gathered data over two of the gas hydrate cold vent structures show significant shear modulus anomalies (Fig. F8). Unlike reflection seismic data, compliance data are not hindered by the possible presence of free gas or other sources of scattering of seismic energy. The anomalous compliance results over the two vent sites indicate a large increase in shear modulus in the gas hydrate stability zone (GHSZ), particularly near the base, suggesting an increase in gas hydrate concentration with depth (Willoughby et al., 2005). There is no observed increase in velocities at the top of the section just below the seafloor; however, the sensitivity of the data to the uppermost sediments is limited by the cut-off frequency of gravity waves.
The seafloor-towed CSEM system was developed at the University of Toronto and successfully deployed several times on the northern Cascadia margin over the past several years (Edwards, 1997; Yuan and Edwards, 2000; Schwalenberg et al., 2005). The CSEM array consists of a weight attached at the leading end of the cable followed by a 124 m long transmitter dipole and two receiver pairs (each 15 m long) that are located at 174 and 292 m from the transmitter, respectively. Received signals relate to depths comparable to half the transmitter–receiver separation.
Data from deployments of the CSEM system around Site 889 showed an average resistivity of ~2 m for the 100 m thick layer overlying the BSR, which is in very good agreement with the downhole electrical resistivity logs from ODP Leg 146 (Yuan and Edwards, 2000). More recent deployments of the array over the cold vents showed dramatic anomalies associated with the vents (Schwalenberg et al., 2005), as illustrated in Figure F9A. The resistivity values within the anomalous zones are higher for the larger receiver separation and rise locally to >5 m over the regional background resistivity of 1.1–1.5 m. Like compliance data, this suggests that gas hydrate content may increase with depth. The measured resistivity values were then converted to gas hydrate concentrations (Fig. F9B) using different assumed Archie parameters by Hyndman et al. (1999) and Collett (2000). For both sets of coefficients, comparable amounts of gas hydrate are required within the entire blank zone (from seafloor to the BSR) to explain the observed resistivity anomalies and reach 50% of the pore space. It should be noted that these are average concentrations for the entire blank zone from the BSR to the seafloor and therefore even higher concentration could be encountered locally in small pockets.
Several different mechanisms have been proposed for the origin of the seismic blanking observed in the Bullseye vent area and for the associated implications concerning the nature of the fluid venting. Riedel et al. (2002, 2006) suggested that much of the blanking is caused by near-surface carbonate or a massive shallow gas hydrate layer. Additional blanking may be caused by scattered accumulations of gas hydrate in fractures and veins in the deeper subsurface. Zühlsdorff and Spiess (2004) proposed that the oblong shape of the blank zone and its associated surface mound were created by natural hydraulic fracturing induced by a local elevation in the seafloor and BSR; the blanking is produced by free gas bubbles within the fractures, particularly in the period following a fluid expulsion event. Wood et al. (2002) argued that the entire blank zone is a gas chimney produced by rising warm fluids that significantly perturb the base of the GHSZ upward. Seafloor compliance and CSEM surveys gave additional constraints to the nature of the cold vents and the possible cause of blanking as observed in the seismic data. The increased shear modulus and electrical resistivity suggest higher gas hydrate concentration over the entire depth of the blank zone and support the model proposed by Riedel et al. (2006), contradicting those by Zühlsdorff and Spiess (2004) and Wood et al. (2002). Even with the new constraints from seafloor compliance and CSEM studies, no final conclusion can be made if free gas is actually present in conjunction with gas hydrate.
Site U1328 is unique relative to the other sites along the Expedition 311 coring transect in that it represents a location of focused fluid flow. Massive forms of gas hydrate were expected within the top few meters below the seafloor and sporadically throughout the depth of the blank zone in fractures and veins. Drilling and coring at this location will test the different models for the cold vent structure and associated causes of seismic blanking. It was important to obtain a high-resolution temperature profile for this area to assess any evidence of active fluid flow. Pressure coring using the Pressure Core Sampler (PCS) system will help answer the question if free gas is occurring within the GHSZ in this vent and contributing to seismic blanking. Pressure coring using the HYACINTH Fugro Pressure Corer (FPC) and HYACE Rotary Corer (HRC) recovered high-quality gas hydrate–bearing core for shore-based analyses.
The operational plan to achieve these objectives was based on a general three-hole concept, which included:
An additional hole was drilled for near-seafloor high-resolution geochemical and microbiological studies of the sulfate/methane interface (SMI).