The objective of this research is to determine the extent and dynamics of gas hydrate deposits and their relation to areas of focused fluid flux at and beneath the seafloor. Specific objectives include: a). Refine geophysical, geochemical and microbiological technologies for prospecting hydrate distribution and content; b). Contribute to establishing high-priority geographical regions of prospective interest, in terms of methane volume estimates; c). Prediction of environmental effects and geologic risks at the continental margin associated to the natural resource occurrence and resource exploitation; and d). Expand understanding of the biogeochemical parameters and associated microbial community diversity in shallow sediments that influence the porewater sulfate gradient observed through anaerobic oxidation of methane. To accomplish these objectives, the Naval Research Laboratory (NRL) collaborated with New Zealand’s Institute of Geological and Nuclear Sciences (GNS) in a research cruise off the coast of New Zealand. NRL has conducted similar research cruises off the west coast and east coast of the United States, in the Gulf of Mexico and off the coast of Chile.
Marine Biogeochemistry Section, Naval Research Laboratory, Washington, D.C. 20375
The Hikurangi Margin lies along the east coast of New Zealand’s North Island and is an active continental margin where the Pacific Plate is being subducted beneath the Australian Plate. Its geologic similarity to the Nankai Trough (Japan’s focus area for future production of gas from hydrates) combined with its proximity to major population centers (Auckland, Wellington), make this margin the most promising gas hydrate province off New Zealand for possible future gas extraction.
Ubiquitous BSRs (bottom-simulating reflectors) indicate the wide-spread presence of gas hydrates over a large area. The strong variability of BSR strength suggests that in some locations there may be a focusing of gas being supplied into the gas hydrate zone, a potential contributor to the formation of “sweet spots”, or areas of highly concentrated gas hydrate. A state-of- the-art seismic transect was acquired by GNS in 2005 with the M/V “Pacific Titan” across several areas that may be candidates for the discovery of such sweet spots. For this survey geochemical and heat flow data were obtained to confirm the presence of “sweet spots”.
The collaborative study focused on two locations in this region and conducted coring and heat flow along three transects. During the cruise, shallow geochemical and heatflow data were collected with the goal of testing whether such data has any meaningful correlation to geophysical attributes such as BSR nature or evidence of focused vertical methane flux and/or contribution to detection/characterization of deep marine hydrates. Previous work on the mid Chilean margin resulted in a large contrast of data interpretations of a large seismic “wipeout” region where low vertical methane fluxes were measured. This survey on the Hikurangi Margin provides further calibration of geochemical and seismic data integration.
Vertical fluid diffusion and seismic surveys were coupled with shallow sediment geochemical and biogeochemical data to provide a thorough interpretation of deep sediment hydrate deposits and also to address basic research topics pertaining to the microbial role in methane production and fate. Associated with the biogeochemical surveys, anaerobic methanotrophy is predominately a microbial phenomenon. Sampling and identification of the microbes present will provide further characterization of the nature of anaerobic methanotrophy in seep areas. Data developing in the survey of the microbial community diversity in sediments around the world at the sediment sulfate-methane interface shows one organism is globally ubiquitous.
Data from this cruise, compared with other surveys, will also provide important information on the environmental role of marine methane and hydrates. Gas hydrate provinces on active continental margins may pose a significant tsunami risk, in that they may facilitate submarine slides triggered by minor earthquakes. The interaction of gas hydrates and seafloor weakening, however, is not yet well understood.
Data from this cruise provides for further characterization of methane hydrate deposits on continental margins. NRL has collected data now from several locations around the Pacific Ocean (Chile, New Zealand, Cascadia Margin), in the Atlantic Ocean at the Blake Ridge off North Carolina, and in the Gulf of Mexico. Geochemical, microbial, geophysical and thermal data collected by NRL at these sites is improves understanding of the nature of methane hydrate deposits, developing integrated geochemical and seismic data for prediction of deep sediment hydrate deposits, the process of methanotrophy, methane’s importance in the carbon cycle, and its influence on sediment and slope stability as well as its contribution to or influence on climate.
The cruise took place from June 20 to July 3, 2006. Twenty-three cores were collected across three transects; two transects across Ridge I selected in the Study Area A is located offshore of the Wairarapa and one transect across a ridge on Rock Garden. Piston coring and heat flow probe locations were based upon seismic profiles provided by GNS. From these cores, 250 geochemical samples and 73 microbial samples were collected. Heat flow penetrations were conducted at 38 stations. The results of the cruise were presented at the 5th International Workshop on Methane Hydrate Research and Development as well as at the American Geophysical Union’s Fall Meeting in San Francisco in December 2006.
A draft of the Final Report has been reviewed and some additional microbial analytical work is still ongoing.
This cruise off the coast of New Zealand was the first task carried out under a new Interagency Agreement between NETL and NRL. Future tasks may encompass additional cruises that will further research efforts into the characterization of methane hydrate deposits along continental margins.
This project has been completed. The final report [PDF-3.50MB] is now available.
Other U.S. Federal Government and international contribution: $738,000