Diffusional methane fluxes within continental margin sediments and depositional constraints on formation factor estimates

Diffusion and reaction of chemical species in ocean sediments are important processes in global cycles of carbon, sulfur, and other elements. Chapter 1 explores the processes in continental margin sediments that supply a diffusional flux of methane to the upper sediment column, where methane is consumed in a net reaction with sulfate. Data obtained from the Ocean Drilling Program (ODP), and Integrated Ocean Drilling Program (IODP) show that, in general, in subduction zone sediments, more methane is being consumed for the amount of methane being microbially produced in situ than in divergent margin and non- subduction convergent margin (such as the California margin south of Mendocino) sediments. Chapter 2 provides a new way to decrease the uncertainty involved in estimating sediment tortuosity using porosity, lithology, and depositional environment. By compiling ODP formation factor and porosity data from sites with differing lithologies and depositional environments, it is shown that depositional environment may provide an additional constraint when estimating the relationship between porosity and formation factor using Archie's formula (Archie, 1942). Reducing the uncertainty of formation factor estimates can reduce the uncertainty of tortuosity and molecular diffusional flux calculations for natural sediment