Computed tomography scan analysis of Site 941 cores, western mass-transport deposit, Amazon Fan

Abstract

X-ray computed tomography (CT) and pore-water chemistry analyses were performed using whole round-core samples from the regionally extensive Western Mass-Transport Deposit (WMTD), Site 941 of Ocean Drilling Program (ODP) Leg 155, on the Amazon Fan. Here, the CT value has a good correlation with the bulk density and water content of the sediment. The CT image shows a plume-shaped, fluidized structure with gas bubbles in the mud sediment, Sample 155-941B-6H-4, 0−20 cm, at ~47 meters below seafloor, at the top of a 2-m-thick soupy and gassy sediment interval. Significant increase in the water content was recognized in the fluidized sediment when compared to those of the surrounding unfluidized sediments. The content coincided with the freshwater dilution estimated from the anomalous pore-water chemistry. The fluidization was thus caused by the dissociation of gas hydrate. The quantitative CT value analysis allows us to estimate that the percentage of gas hydrate content reaches ~10 vol% of the fluidized sediment (17.5 vol% of the pore space). This content seems to be equivalent to the highest contents of the gas hydrate above bottom-simulating reflectors (BSRs) reported from other hydrate regions. BSRs underlie parts of the continental slope updip from the WMTD. The 2-m-thick soupy and gassy sediment is inferred to be derived from a part of "missing" sediment interpreted from seismic-reflection profiles of the source area of the WMTD that is bounded by scarps up to 200 m high. Furthermore, the disassociation of gas hydrate should be the most important trigger mechanism for causing the regionally extensive slope failure that produced the WMTD.