Physical properties of gas hydrate related sediments, offshore Vancouver Island
Date
2002
Authors
Novosel, Ivana
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Abstract
A gas hydrate vent field on the South Vancouver Island continental slope was the target of a series of seafloor core sampling programs in years 2000 and 2001. The vent structures had been previously imaged on the Cascadia accretionary margin as prominent blank zones in seismic reflection data. In 2000, 26 piston cores, with maximum recovered lengths of approximately 8 m, were collected in the vent field region.
Extensive physical property measurements were carried out on these cores. The objectives of the analyses were to: (1) provide ground-truth calibrations for seismic impedance and electrical resistivity remote surveys, and (2) examine the effect of high methane fluxes on the physical, chemical, and mineralogical character of sediments within the region of a vent field.
Gas hydrate was recovered in 4 cores located within the most prominent seismic blank zone (Blank Zone I). Cores located within the blank zones contained low magnetic susceptibility authigenic pyrite, rather than high magnetic susceptibility magnetite and thus created a low magnetic zone within the vent. Leather clams (genus acharax) were recovered in several cores located inside the vent, indicating an environment high in sulfide. Increased seismic reflection coefficients and sample thermal conductivity values within the topmost sediments of Blank Zone 1 correlated with the
presence of authigenic carbonate. All observations are consistent with high methane fluxes from seafloor sediments within a cold seep environment.
The comparison of core measured porosity, thermal conductivity, and velocity with physical properties from deeper in the stratigraphy from the Ocean Drilling Program (ODP) clearly indicates that within ~ 10 m of the seafloor sediment compaction is distinctly different from that indicated by deeper trends. Within these cored sediments porosity decreases from as much as 75% at the seafloor to 55% at 8 m below; in contrast, the porosities over this depth range extrapolated from the deeper ODP trends are consistently 60%. Thermal conductivity, being highly dependent on density and porosity, also increases dramatically within the top few metres of the sediment. However, no decrease in sample seismic velocity is observed over the same depth range. This particular behaviour of shallow sediments may be significant for: 1) heat flow studies.