ALBERT

Description: Submarine currents are a principal factor in controlling seafloor geomorphology. Herein, we investigate the role of dynamic current systems associated with the Subtropical Front in the formation and modification of seafloor depressions off the east coast of New Zealand’s South Island. Seafloor depressions are widespread in this region, with a diverse range of morphologies and sizes. We focus on two ‘end-member’ classes of depressions; densely spaced decametre-scale structures and more isolated ‘giant’ depressions of up to 12 km in diameter. Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions. We present a model to illustrate the role of submarine currents in shaping the morphology of these enigmatic seafloor depressions. This model demonstrates how contour currents, and potentially eddy currents, have extensively modified seafloor structures, resulting in elongate, asymmetrical depressions, partially infilled by sediment drift deposits.

Description: Porangahau Ridge, located offshore the Wairarapa on the Hikurangi Margin, is an active ocean-continent collision region in northeastern New Zealand coastal waters. Bottom simulating reflections (BSRs) in seismic data indicate the potential for significant gas hydrate deposits across this part of the margin. Beneath Porangahau Ridge a prominent high-amplitude reflection band has been observed to extend from a deep BSR towards the seafloor. Review of the seismic data suggest that this high-amplitude band is caused by local shoaling of the base of gas hydrate stability due to advective heat flow and it may constitute the location of elevated gas hydrate concentrations. During R/V Tangaroa cruise TAN0607 in 2006 heat flow probing for measurements of vertical fluid migration, sediment coring for methane concentrations, and additional seismic profiles were obtained across the ridge. In a subsequent 2007 expedition, on R/V Sonne cruise SO191, a controlled source electromagnetic (CSEM) experiment was conducted along the same seismic, geochemical, and heat flow transect to reveal the electrical resistivity distribution. CSEM data highlight a remarkable coincidence of anomalously high resistivity along the western, landward flank of the ridge which point to locally higher gas hydrate concentration above the high amplitude reflection band. Measured sediment temperature profiles, also along the western flank, consistently show non-linear and concave geothermal gradients typical of advective heat flow. Geochemical data reveal elevated methane concentrations in surface sediments concomitant with a rapid decline in sulfate concentrations indicating elevated methane flux and oxidation of methane in conjunction with sulfate reduction at the landward ridge base. Together, these data sets suggest that the western rim of Porangahau Ridge is a tectonically driven zone of rising fluids that transport methane and cause an upward inflection of the base of gas hydrate stability and the formation of locally enriched gas hydrate above the reflective zone.

Description: We present recently-acquired high-resolution seismic data and older lower-resolution seismic data from Rock Garden, a shallow marine gas hydrate province on New Zealand's Hikurangi Margin. The seismic data reveal plumbing systems that supply gas to three general sites where seeps have been observed on the Rock Garden seafloor: the ‘LM3’ sites (including LM3 and LM3-A), the ‘Weka’ sites (including Weka-A, Weka-B, and Weka-C), and the ‘Faure’ sites (including Faure-A, Faure-B, and Rock Garden Knoll). At the LM3 sites, seismic data reveal gas migration from beneath the bottom simulating reflection (BSR), through the gas hydrate stability zone (GHSZ), to two separate seafloor seeps (LM3 and LM3-A). Gas migration through the deeper parts of GHSZ below the LM3 seeps appears to be influenced by faulting in the hanging wall of a major thrust fault. Closer to the seafloor, the dominant migration pathways appear to occupy vertical chimneys. At the Weka sites, on the central part of the ridge, seismic data reveal a very shallow BSR. A distinct convergence of the BSR with the seafloor is observed at the exit point of one of the Weka seep locations (Weka-A). Gas supply to this seep is predicted to be focused along the underside of a permeability contrast at the BGHS caused by overlying gas hydrates. The Faure sites are associated with a prominent arcuate slump feature. At Faure-A, high-amplitude reflections, extending from a shallow BSR towards the seafloor, are interpreted as preferred gas migration pathways that exploit relatively-high-permeability sedimentary layers. At Faure-B, we interpret gas migration to be channelled to the seep along the underside of the BGHS — the same scenario interpreted for the Weka-A site. At Rock Garden Knoll, gas occupies shallow sediments within the GHSZ, and is interpreted to migrate up-dip along relatively high-permeability layers to the area of seafloor seepage. We predict that faulting, in response to uplift and flexural extension of the ridge, may be an important mechanism in creating fluid flow conduits that link the reservoir of free gas beneath the BGHS with the shallow accumulations of gas imaged beneath Rock Garden Knoll. From a more regional perspective, much of the gas beneath Rock Garden is focused along a northwest-dipping fabric, probably associated with subduction-related deformation of the margin.

Description: The southern Hikurangi Subduction Margin is characterized by significant accretion with predicted high rates of fluid expulsion. Bottom simulating reflections (BSRs) are widespread on this margin, predominantly occurring beneath thrust ridges. We present seismic data across the Porangahau Ridge on the outer accretionary wedge. The data show high-amplitude reflections above the regional BSR level. Based on polarity and reflection strength, we interpret these reflections as being caused by free gas. We propose that the presence of gas above the regional level of BSRs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids, although we cannot entirely rule out alternative processes. Simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low-permeability slope basins into the Porangahau Ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability. Focussing of fluid flow is predicted to take place deep in the accretionary wedge and/or the underthrust sediments. Above the high-amplitude reflections, sediment reflectivity is low. A lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas-hydrate-related amplitude blanking. Structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge. A gap in the BSR in the southern part of the study area may be caused by a loss of gas during fluid expulsion. We speculate that gaps in otherwise continuous BSRs that are observed beneath some thrusts on the Hikurangi Margin may be characteristic of other locations experiencing focussed fluid expulsion.

Description: Regional erosion of the Rock Garden ridge top, a bathymetric high within New Zealand’s Hikurangi Subduction Margin, is likely associated with its gas hydrate system. Seismic data reveal gas pockets that appear partially trapped beneath the shallow base of gas hydrate stability. Steady-state fluid flow simulations, conducted on detailed two-dimensional geological models, reveal that anomalous fluid pressure can develop close to the sea floor in response to lower-permeability hydrate-bearing sediments and underlying gas pockets. Transient simulations indicate that large-scale cycling of fluid overpressure may occur on time scales of a few to tens of years. We predict intense regions of hydro-fracturing to preferentially develop beneath the ridge top rather than beneath the flanks, due to more pronounced overpressure generation and gas migration through hydrate-bearing sediments. Results suggest that sediment weakening and erosion of the ridge top by hydro-fracturing could be owed to fluid dynamics of the shallow gas hydrate system.

Description: This paper is an introduction to and an overview of papers presented in the Special Issue of Marine Geology “Methane seeps at the Hikurangi Margin, New Zealand”. In 2006 and 2007, three research cruises to the Hikurangi Margin at the east coast of New Zealand's North Island were dedicated to studying methane seepage and gas hydrates in an area where early reports suggested they were widespread. Two cruises were carried out on RV TANGAROA and one on RV SONNE using the complete spectrum of state-of-the-art equipment for geophysics (seismic, sidescan, controlled source electromagnetics, ocean bottom seismometers and hydrophones, singlebeam and multibeam), seafloor observations (towed camera systems, ROV), sediment and biological sampling (TV-guided multi-corer, gravity-corer, grab, epibenthic sled), deployment of in-situ observatories (landers) as well as water column sampling and oceanographic studies (CTD, moorings). The scientific disciplines involved ranged from geology, geophysics, petrography, geochemistry, to oceanography, biology and microbiology. These cruises confirmed that a significant part of the Hikurangi Margin has been active with locally intense methane seepage at present and in the past, with the widespread occurrence of dead seep faunas and knoll-forming carbonate precipitations offshore and on the adjacent land. A close link to seismically detected fluid systems and the outcropping of the base of the gas hydrate stability zone can be found at some places. Pore fluid and free gas release were found to be linked to tides. Currents as well as density layers modulate the methane distribution in the water column. The paper introduces the six working areas on the Hikurangi Margin, and compiles all seep locations based on newly processed multibeam and multibeam backscatter data, water column hydroacoustic and visual data that are combined with results presented elsewhere in this Special Issue. In total, 32 new seep sites were detected that commonly show chemoherm-type carbonates or carbonate cemented sediment with fissures and cracks in which calyptogenid clams and bathymodiolid mussels together with sibloglinid tube worms live. White bacterial mats of the genus Beggiatoa and dark gray beds of heterotrophic ampharetid polychaetes typically occur at active sites. Bubble release has frequently been observed visually as well as hydroacoustically (flares) and geochemical analyses show that biogenic methane is released. All seep sites, bubbling or not, were inside the gas hydrate stability zone. Gas hydrate itself was recovered at three sites from the seafloor surface or 2.5 m core depth as fist-sized chunks or centimeter thick veins. The strong carbonate cementation that in some cases forms 50 m high knolls as well as some very large areas being paved with clam shells indicates very strong and long lasting seep activity in the past. This activity seems to be less at present but nevertheless makes the Hikurangi Margin an ideal place for methane-related seep studies in the SW-Pacific.

Description: The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for highresolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.