Marine CSEM study of the southern Hikurangi Margin: A first step towards estimating the global gas hydrate carbon budget

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Gas hydrate is a solid ice-like mixture of water and gas, usually methane, that forms in seafloor sediments when pressure is high enough and temperature is cold enough. Such conditions are found in water depths greater than about 500 m, and gas hydrate is found on continental slopes world-wide wherever there is methane produced from the decay of organic matter or leaking from hydrocarbon systems. Vast amounts of carbon have accumulated this way, but estimates of the total quantity vary by several orders of magnitude, mainly because of limitations in using seismic methods to assess hydrate concentration, especially at lower concentrations. This gap in knowledge is a problem, because hydrate can have a big impact on human activity and well-being. Changes in sea level or temperature can cause hydrate to decompose, possibly contributing to seafloor landslides that can trigger tsunamis. Loss of sediment stability from hydrate decomposition can damage man-made seafloor infrastructure. Methane leaking from the seafloor is used as energy by some biological systems, but if too much escapes it can add to acidification and impact the health of the oceans. This project will begin to plug the gaps in knowledge about offshore hydrate by using a novel non-seismic geophysical method to image hydrate under hundreds of square kilometers of seafloor off the southern margin of New Zealand's North Island, where abundant seismic data have been collected and hydrate is inferred to exist in both low and high concentrations over broad regions. The project supports the training of an early career scientist. The electrical resistivity of methane hydrate is four orders of magnitude greater than seafloor sediment, and sediment resistivity varies smoothly and monotonically with hydrate saturation. We have developed a deep-towed controlled source electromagnetic (CSEM) system to image seafloor resistivity and have validated its use for quantifying seafloor hydrate concentrations off California, Japan, and the Gulf of Mexico. Typically CSEM studies target known methane seeps and hydrate deposits, but in this project data will be collected on a basin-scale to capture the broad distribution, using the Hikurangi subduction zone as a model to develop an understanding of the global hydrate inventory by extrapolation to subduction zones worldwide. By combining existing seismic data with the new CSEM data, through a collaboration with colleagues in New Zealand, the processes that concentrate hydrate will be examined, how much hydrate sits at the upper edge of the stability field (and thus vulnerable to dissociation) will be assessed, the relationships between free gas and gas hydrate (both resistive but easily distinguished in seismic data) will be studied, and the background levels of hydrate and the total hydrate inventory of the Pegasus Basin will be estimated. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.