Integrated side-scan, sub-bottom profiler and seismic signatures of methane seepage from Omakere Ridge on New Zealand’s Hikurangi margin

Omakere Ridge is one of a series of prominent northeast-southwest orientated anticlinal ridges associated with major thrust faults on New Zealand’s Hikurangi margin. The Hikurangi margin is an extensive gas hydrate province and recent marine surveys have confirmed that the mid-slope Omakere Ridge is a zone of methane-rich seabed seepage. Acoustic flares initially observed in the area by fishermen, were imaged in the water column at Omakere Ridge during a 2006 RV Tangaroa survey (TAN06-07). Anomalous methane concentrations (up to 165 nM) were detected by a methane sensor (METS) attached to a conductivity-temperature-depth-optical backscatter device (CTD) on TAN06-07 and a 2007 RV Sonne survey (SO-191). Six seep sites have been identified at the southern end of Omakere Ridge, where it bifurcates into two parallel ridgelines. All sites are located towards the crests of the two ridgelines in approximately 1150 m water depth. The seabed seeps were identified acoustically with an EdgeTech Deep-Tow side-scan operating at 75 kHz, and are shown as high backscatter intensity areas on processed side-scan data, which are interpreted to be methane derived authigenic carbonate hardgrounds. Acoustic shadows behind hardgrounds in the side-scan far range suggest the seabed features have moderate relief. Sub-bottom profiles acquired with an EdgeTech Deep-Tow chirper system, operating at 2-10 kHz, identified numerous signatures of shallow gas in the near subsurface. These signatures include zones of acoustic turbidity and gas blanking, interpreted to mark shallow gas fronts. The evidence for shallow gas in the subsurface from the sub-bottom profiler displays a marked spatial correlation with seabed expressions of seepage. The seepage sites also correspond to potential gas indicators in multi-channel seismic data, such as interpreted amplitude anomalies. Enigmatic subsurface features in the subbottom profiler data, such as potential amplitude anomalies and gas blanking, which are below the depression that bifurcates the ridge and are not associated with surface expressions of seepage, may represent lithological and topographic features or may be a component of the gas migration pathway which feeds the seeps on the ridge crest. Underwater video and still camera images show seabed seepage sites of high backscatter intensity represent widespread authigenic carbonate concretions and chemoherms associated with biological assemblages including siboglinid tube worms, vesicomyid clams, bathymodiolin mussels, and bacterial mats. A high backscatter intensity site of similar acoustic character to, and directly adjacent to, seep sites on the southern part of the ridge does not contain seep fauna and is interpreted to be a cold-water reef. While this feature may represent a relict seep, this finding highlights the fact that present day seepage cannot be identified with acoustic techniques alone

Gas hydrate and P-Wave Velocity Distribution in the Yaquina Basin at the Peruvian margin

The lower boundary of the methane hydrate stability zone in continental margin sedi-mentsis often marked by a strong, phase reversed reflection subparallel to the seafloor,called the bottom simulating reflector (BSR). High resolution multichannel seismic(MCS) data from the Yaquina Basin offshore Peru at 8 deg S show a BSR that is vary-inglaterally in amplitude as well as in continuity. The amplitudes of the reflectionsabove the BSR also vary with the appearance of the BSR. Where the BSR is strong,the reflections above it are weaker compared to areas where the BSR is weak. Andalthough the strong part of the BSR is underlain immediately by strong reflections,reflections several hundred meters beneath the BSR appear weaker than those wherethe BSR is weak. This variation indicates significant heterogeneity in the distribu-tionof gas and gas hydrate in this area. Chemoherms observed at the Yaquina Basinsea floor indicate the presence of free gas in the sediments up to the seafloor. Thepresence of gas and gas hydrate within the sediment sequence significantly influencesthe P-wave velocity in the affected layers. Therefore a detailed analysis of velocityvariations enables to understand the apparently different conditions for the formationof gas hydrate along the BSR and the migration paths of the free gas. Ocean bot-tomseismometer (OBS) data from profiles coincident with the MCS data can providesuch detailed velocity depth information. Velocity analysis from OBS data included2D-ray tracing and 1D-interval-velocity analysis by means of DIX-inversion. In orderto find a trade-off between vertical resolution and minimization of errors caused bythe sensitivity of the DIX formula to velocity variations in thin layers, the data haveundergone a Kirchhoff wave-equation datuming and adjacent coherence filtering wasapplied to the data to eliminate the one sided travel path through the water columnof the OBS-observations. The derived velocity structure confirms the interpretation ofthe reflection pattern in terms of gas and gas hydrate distribution

Seismic indications for free gas within the gas hydrate stability zone in the Yaquina Basin off Peru

MCS data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Intricate lateral variations of the reflection pattern at the assumed base of the GHSZ in terms of continuity, reflection amplitude, and signal attenuation underneath are observed, as well as the occurrence of paleo-BSR. Phase reversed reflections at an erosional unconformity above the BSR indicate free gas within the GHSZ. In order to further constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. The results from 2D-forward modelling and interactive velocity analysis show consistentresults. They exhibit a low velocity layer almost directly beneath the seafloor. Another low velocity layer with less than 1.5 km/s is present between the unconformity and the BSR.. In the vicinity and beneath prominent chemoherms, high velocities have been observed between the BSR and seafloor. Heat flux values calculated on the basis of the velocity-depth functions increase with decreasing amplitude of the bottom simulating reflector and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is present under parts of the BSR, and within the hydrate stability zone, particularly under the sea floor and under the erosional unconformity. The higher interval velocities near and beneath the chemoherms are suspected to be caused either by thick gas hydrate lenses or a significant amount of precipitated carbonate within the sediment or a combination of both. The hypothesis of a paleo-BSR that reflects the uplift of the base of the GHSZ caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values

Deep crustal refraction and reflection seismics. Crustal and sedimentary structures and geodynamic evolution of the West Antarctic continental margin and Pine Island

Accurate models of the geodynamic-tectonic evolution contain some of the most important parameters for understanding and reconstruction of the palaeo- environment. Geophysical surveys of the sedimentary sequences and the underlying basement of the shelf and slope of the southern Amundsen Sea, Pine Island Bay and its adjacent continental rise allow reconstructions of the formation of the tectonic and older sedimentary processes. The following objectives are addressed as part of a cooperative project between the Vernadsky Institute in Moscow (Dr. Gleb Udintsev) and AWI:• Identification of the boundaries between suspected crustal blocks and volcanic zones in Pine Island Bay. The glacier troughs and Pine Island Bay are thought to have developed along such tectonic boundaries.• During and after separation from the Chatham Rise and Campbell Plateau (New Zealand), the continental margin of Marie Byrd Land developed as a passive margin, probably accompanied by intensive volcanism. The question is whether this volcanism occurred mainly during the rifting process or during post-rift phases, or if it developed in relation to the West Antarctic rift system.• Recording of the sedimentary sequences across the shelf, slope and the continental rise, using deep reflection seismics, sub-bottom profiler (Parasound) and swath-bathymetry (Hydrosweep) in order to derive a sedimentation model.• Mapping of the acoustic basement and its structure with deep seismic reflection methods to obtain the tectonic geometries and boundary conditions necessary to understand sediment transport and depositional processes.info:eu-repo/semantics/publishe

Crustal and Sedimentary Structures and Geodynamic Evolution of the West Antarctic Continental Margin and Pine Island Bay

Since the last glacial maximum the West Antarctic Ice Sheet (WAIS) with a base mostly beneath the present-day sea-level has experienced dramatic volume changes within short periods of time. Studies are urgently required to show how these short-term variations are related to volume changes in the older geological past. Next to the ice drainage basins of the Weddell Sea and the Ross Embayment, Pine Island Bay forms the third-largest outflow area for the West Antarctic ice-shield. The main ice streams from the WAIS into Pine Island Bay flow through the Pine Island and Thwaites Glacier systems, through which most of the glacial-marine sediments onto the shelf of Pine Island Bay and across the continental slope into the deep sea have been transported. Geophysical surveys of the sedimentary sequences and the underlying basement of the shelf and slope of the southern Amundsen Sea, Pine Island Bay and its adjacent continental rise would allow reconstructions of the formation of the tectonic and older sedimentary processes as well as to find out about the history of large-scale glaciation in West Antarctica. Accurate models of the geodynamic- tectonic evolution contain some of the most important parameters for understanding and reconstruction of the palaeo-environment.info:eu-repo/semantics/publishe

Crustal and Sedimentary Structures and Geodynamic Evolution of the West Antarctic Continental Margin and Pine Island Bay

Since the last glacial maximum the West Antarctic Ice Sheet (WAIS) with a base mostly beneath the present-day sea-level has experienced dramatic volume changes within short periods of time. Studies are urgently required to show how these short-term variations are related to volume changes in the older geological past. Next to the ice drainage basins of the Weddell Sea and the Ross Embayment, Pine Island Bay forms the third-largest outflow area for the West Antarctic ice-shield. The main ice streams from the WAIS into Pine Island Bay flow through the Pine Island and Thwaites Glacier systems, through which most of the glacial-marine sediments onto the shelf of Pine Island Bay and across the continental slope into the deep sea have been transported. Geophysical surveys of the sedimentary sequences and the underlying basement of the shelf and slope of the southern Amundsen Sea, Pine Island Bay and its adjacent continental rise would allow reconstructions of the formation of the tectonic and older sedimentary processes as well as to find out about the history of large-scale glaciation in West Antarctica. Accurate models of the geodynamic- tectonic evolution contain some of the most important parameters for understanding and reconstruction of the palaeo-environment.info:eu-repo/semantics/publishe