Chemical composition of sediment samples from ODP Leg 164 sites, Blake Ridge

2019-11-22T00:39:17Z (GMT) by Gerald Roy Dickens
Ocean Drilling Program (ODP) Sites 994, 995, and 997 were drilled into a large gas hydrate deposit on the crest of the Blake Ridge (southeast U.S. margin) where upward CH4 fluxes (F_OUT) are related to depths of pore water SO4**2- depletion. High-resolution pore water SO4**2- and sediment Ba profiles have been constructed at these sites to assess present and past F_out. Pore water SO4**2- profiles are linear with zero SO4**2- concentration occurring at 21.4, 21.6, and 22.8 mbsf at Holes 994A, 995A and 997A, respectively. Using steady state solutions to diffusion equations with appropriate parameters, the steep SO4**2- gradients support upward CH4 fluxes between 7.2 and 8.6 mol/m**2/ky at present-day, with the range primarily reflecting different approaches for incorporating porosity (phi). Taking into account the generally decreasing phi with depth and the high clay content of the sediment, the best estimates for F_out are 7.9, 7.6 and 7.2 mol CH4/m**2/ky at Sites 994, 995 and 997, respectively. However, non-steady state solutions to diffusion equations show that the SO4**2- gradients do not imply steady state conditions. Elevated Ba concentrations (530–1410 ppm) exist in sediment between 18.23 and 20.65, between 17.31 and 20.31, between 19.40 and 21.80, and between 19.58 and 21.91 mbsf at Holes 994A, 994C, 995A, and 997A, respectively. These Ba fronts coincide with highs in bulk sediment Ba/Al (to 0.025) and are caused by Ba cycling just above time averaged depths of SO4**2- depletion. Because the Ba fronts lie immediately above the present-day depths of pore water SO4**2- depletion, and because no other Ba fronts are found in the upper 25 m at the three sites, the depth of SO4**2- depletion beneath the seafloor has been nearly constant for considerable time (>18000 years). Thus, CH4 fluxes can be determined through SO4**2- gradients and steady state solutions to diffusion equations. More importantly, F_out through the crest of the Blake Ridge has not varied significantly across major changes in sea level and hydrostatic pressure.

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