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Abstract:
International Ocean Discovery Program (IODP) Expeditions 372 and 375 targeted the Northern Hikurangi subduction margin east of New Zealand. These expeditions, combined with a number of site surveys, provided us with a vast dataset that sheds new light on the evolution of gas hydrate systems in subduction-zone settings. Our studies focus on the region near the up-dip limit of the gas hydrate stability field where hydrates are particularly susceptible to changes in pressure or temperature.
Double-BSRs are encountered at several locations in the study area. From pore water chemistry and head space-gas analysis, there are no indications for any thermogenic gases, making it unlikely that double-BSRs form from fractionation of a thermogenic gas mix. Beneath a ridge in the northern part of our study area, double-BSRs are thought to be linked to depressurization from tectonic uplift. Further south near the IODP drilling transect, double-BSRs may be the response to the thermal signal from a sedimentation pulse. Modelling suggests gas hydrate dissociation in conjunction with these double-BSRs to take place over thousands of years. Actively dissociating gas hydrates may therefore be present between the double-BSRs.
Furthermore, results from a heat flow transect near the up-dip limit of the hydrate stability field indicate that gas hydrates are actively dissociating. Anomalies in thermal gradients suggest that latent heat from ongoing gas hydrate dissociation may act as a buffer to transient advective heat flux.
Our findings suggest that gas hydrate systems that respond to subduction-related processes such as uplift or fluid pulses may often be in a transient state leading to on-going hydrate dissociation over thousands of years. Such ongoing gas hydrate dissociation could affect a number of seafloor processes, e.g., causing overpressure, affecting pore water chemistry, and distorting thermal and hydraulic regimes.
