Geophysical characterisation of gas hydrate systems on the southern Hikurangi Margin of New Zealand

Gas hydrates act as an efficient natural sequester of large amounts of carbon, and it is estimated that approximately 15% of Earth's total mobile carbon could be stored in gas hydrate provinces. They form complex systems in the shallow sediments of deep-sea regions, where there is sufficient supply of natural gas along with the stability conditions of moderately low temperatures and high pressures. Most gas hydrate provinces are found on continental margins. The southern Hikurangi subduction margin, o_ Wairarapa (New Zealand), contains a large gas hydrate province, inferred by the presence of widespread bottom simulating reflections (BSR) in shallow sediments. Locally intense fluid seepage at the seafloor associated with methane hydrate has also been observed and documented. Understanding the complexity of hydrate systems is valuable for a range of scientific issues related to climate change and ocean chemistry, geological hazards, deep-sea ecology, and energy supply. A quantitative approach to the characterisation of gas hydrate systems in the region is an essential step towards the estimation of the local carbon budget, especially in terms of the total volume of gas hydrate in the region and estimation of gas fluxes through the seafloor at cold seep locations. In this thesis, I combine a range of geophysical data and theoretical models to identify, map and quantitatively characterise gas hydrate deposits and cold seeps on the southern Hikurangi Margin. Multi-channel seismic (MCS) data and methods form a large part of the basis of the studies presented in this thesis. Two MCS datasets were used: long-offset, lower resolution petroleum industry data APB13 (R/V Duke, 2013) and higher resolution, short-offset academic data TAN1808 (R/V Tangaroa, 2018), acquired as a densely spaced grid of lines over five target sites. The synthesis of these datasets provides a complementary basis for characterising concentrated gas hydrate deposits. The long-offset data allow retrieval of P-wave velocity information of the subsurface, whereas the higher resolution data enable detailed imaging of geologic features associated with gas hydrates and fluid flow. Hydroacoustic data were used to characterise the water column and to estimate methane fluxes from gas seeps related to the gas hydrate systems