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

机译：南开斜坡双BSR的观测与初步解释

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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 deg 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.
机译：法国/日本KAIKO-Tokai巡洋舰R / V L'Atalante在南海东部边缘上坡上收集的地震数据显示，在两个底部模拟反射器（BSR）型反射器的海底以下两个不同深度处同时存在。上方的BSR被跟踪为约10 km的连续反射器。随着水深从850 m减小到550 m，其在海底以下的深度从200 m减小到40 m。较低的BSR追溯到较高的BSR下方50-100 m。两个BSR在Daichii-Tenryu Knoll山顶附近突然终止，进入3.5 kHz记录表明有活跃气体通过海床排出的区域。根据当前海底和斜坡沉积物的温度和压力条件估算，上层BSR的观测深度与甲烷水合物稳定区底部的预测深度吻合。因此，我们将上层BSR解释为活性甲烷水合物BSR。我们进一步建议较低的BSR是残留的水合物相关BSR。这可能是因为甲烷水合物稳定区的基底最近从下部BSR迁移到上部BSR。作为这种迁移的可能原因，我们讨论了海底变暖和构造隆升。 BSR迁移可能是对1-2摄氏度海底变暖的响应，也可能是海底快速上升约90 m的事件，具有同等作用。我们不会放弃对较低BSR的其他解释，例如由气体混合物形成的与水合物有关的活性BSR。

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