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 fresh-water 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.

Review of the Timing and Causes of the Amazon-Fan Mass Transport and Avulsion Deposits during the Latest Pleistocene

“…The BSR has been observed in acoustic data of the Amazon Fan and the Brazilian continental slope, indicating the presence of gas hydrates (Manley and Flood, 1988;Flood et al, 1995). In addition, Soh (1997, figure 11) showed the location of the BSR above the Western Debris Flow. The presence of gas hydrates has been reported in other deep-sea fans such as the Congo Fan (Uenzelmann-Neben and Spiess, 1996).…”

“…Whatever the cause of these two near-surface MTDs, there is evidence that they caused a secondary effect of releasing significant amounts of gas hydrates. First there is evidence for significant quantities of gas hydrates within the Amazon Fan MTDs (e.g., Figueiredo et al, 1996;Soh, 1997). For example, Soh (1997) demonstrates clearly from X-ray computed tomography (CT) and pore-water chemistry the presence of gas hydrate in the Western Debris Flow sediments recovered at Site 941.…”

“…First there is evidence for significant quantities of gas hydrates within the Amazon Fan MTDs (e.g., Figueiredo et al, 1996;Soh, 1997). For example, Soh (1997) demonstrates clearly from X-ray computed tomography (CT) and pore-water chemistry the presence of gas hydrate in the Western Debris Flow sediments recovered at Site 941. Using CT, Soh (1997) showed that the gas hydrate content reached ~ 10 vol% (or ~ 17% of the pore space) of the fluidized sediment, which is equivalent to the highest content of gas hydrate above the bottom-simulating reflectors (BSRs) reported from other gas-hydrate regions, e.g., ~ 18% of the pore space at the Chile Triple Junction (Bangs et al, 1993) or ~ 8.4% of the pore space at Site 997 on Blake Outer Ridge, Leg 164 Scientific Party (Paull et al, 1996).…”

Data report: descriptive evaluation of consolidated sedimentary rock structures with X-ray imaging: examples from Site 1276 core samples

“…The X-ray beam passing through a sample is attenuated by interaction with the various constituents in the target, and an X-ray CT image is obtained by integrating the X-ray attenuation through 360°. This imaging technique is popular in medical usage and has also become popular in geological usage because it is a nondestructive investigation technique (e.g., Holler and Kögler, 1990;Soh, 1997). X-ray CT imaging can reveal important information about sedimentary structures (e.g., Boespflug et al, 1995), deformation structures (e.g., Soh, 1997), and gas hydrate potential of unconsolidated sediments (Shipboard Scientific Party, 2002).…”

Identification of a Soil Liquefied Layer due to the 2011 off the Pacfic coast of Tohoku Earthquake Using X-ray CT Scan Imaging:

“…Asahiko Taira 1,5 , 2011a, b, c; , 2011; , 2011 , , 1986; , 1988; , 1991; , 2009 1994 2003 1987 13 m X CT X CT , Soh et al, 1993;, 1997;Soh, 1997;, 2005;Sakaguchi et al, 2011 Fig. 1a 40 Table 1 3 .…”