Observation and tentative interpretation of a double BSR on the Nankai slope

Seismic data collected during the French–Japanese KAIKO-Tokai cruise of R/V L’Atalante on the upper slope of the eastern Nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector (BSR)-type reflectors. The upper BSR is traced as a continuous reflector over about 10 km. As water depth decreases from 850 m to 550 m, its depth below seafloor decreases from 200 m to 40 m. The lower BSR is traced at 50–100 m below the upper one. The two BSRs end abruptly near the summit of the Daichii-Tenryu Knoll into an area where the 3.5-kHz record suggests active gas expulsion through the seabed. The observed depth of the upper BSR fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments. Thus, we interpret the upper BSR as an active methane hydrate BSR. We further suggest that the lower BSR is a residual hydrate-related BSR. This could have followed a recent migration of the base of the methane hydrate stability zone from the lower BSR to the upper one. As possible causes for this migration we discuss sea bottom warming and tectonic uplift. The BSR migration could have occurred as a response to a 1–2°C sea bottom warming or, with an equivalent effect, an event of fast uplift of the seafloor by about 90 m. We do not discard other interpretations of the lower BSR, such as an active hydrate-related BSR formed from a mixture of gases.