Investigation of Seafloor Depressions East of New Zealand’s South Island to Explore Their Potential Link to Methane Transport Processes Between the Seafloor and Ocean.

Numerous seafloor depressions, ranging in size from 20 to 700 m in diameter, have been identified off the east coast of New Zealand’s South Island. Depressions or pockmarks are a commonly observed seafloor expression of focused fluid flow or gas venting. The positions of these structures correlate well with the expected gas hydrate stability zone in the region; however, limited geophysical or geochemical evidence exists to support the hypothesis that gas hydrate dissociation is the primary mechanism responsible for their formation. The purpose of this project is to investigate these depressions, focusing on their geomorphology and underlying shallow subsurface structures, in order to determine the most probable mechanism of formation.

In addition to recently acquired data from the CHatham RIse Mega Pockmarks (CHRIMP) project, existing data sets from research and industry sources have been used in this study. One of the aims of the project is the use of multibeam bathymetry and backscatter data to characterise the lithology of the seafloor in the vicinity of these seafloor depressions. This work has been done by correlating automated supervised segmentation of backscatter data to sediment samples, underwater images, and subsurface profiles from seismic and parasound data.

The morphometric variability of these structures indicates that multiple processes were involved in their formation over a period of time. Shallow gas release and gas hydrate dissociation may have been a factor in the initial formation of some of the larger structures on the Chatham Rise; however, they subsequently have been modified and maintained by the action of strong current systems in the region, e.g. the northeasterly flowing Subtropical Front in the region. Along the Canterbury Shelf, the interaction of current systems and complex seafloor canyon systems has strongly influenced the geomorphology of the seafloor.