Tracking the relict bases of marine methane hydrates using their intersections with stratigraphic reflections

We describe a three-dimensional seismic interpretation approach for identifying the past bases of the methane hydrate stability zone in marine settings. The bases commonly crosscut stratigraphic reflections, and these lines of intersection can be revealed by maps of seismic amplitude. Maps for four reflections that are crosscut by the base of the present-day offshore Mauritania methane hydrate reveal extraordinary curvilinear changes of amplitude that are parallel to one another and extend for as much as ~20 km. They are interpreted as marking the relict intersections of the bases of the hydrate stability zone, and formed by its upward resetting. We hypothesize that this was caused by pulsed sedimentation during repeated glacial-interglacial cycles over the past ~1.25 m.y. Localized deflections in the lines of intersection are indicative of local changes in sediment temperature. This technique could provide a wealth of information on local and regional changes in ambient conditions and better estimates of the volumes of methane being released through time. It therefore should help test for interdependencies between hydrate dissociation and climate change.