Computer simulation of deep sulfate reduction in sediments of the Amazon Fan

Abstract

Pore water concentration profiles of sediments at a site on the Amazon Fan were investigated and simulated with the numerical model CoTReM (column transport and reaction model) to reveal the biogeochemical processes involved. The pore water profiles for gravity core GeoB 4417-7 showed a distinct sulfate-methane transition zone in which deep sulfate reduction occurs. Only a small sulfide peak could be observed at the reaction zone. Due to high amounts of iron minerals, the produced sulfide is instantaneously precipitated in form of iron sulfides. We present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate. Furthermore, sulfide, iron, pH, pE, calcium and total inorganic carbon (TIC) were included in the simulation. The program calculated mineral equilibria to mackinawite, iron sulfides (more stable than mackinawite), iron hydroxides and calcite via saturation indices (SI) by a module incorporating the program PHREEQC (Parkhurst 1995). The measured sulfide and iron profiles are obtained in the simulation output by using a constant SI (p0) for mackinawite and calcite, while a depth dependent SI distribution is applied for the PHREEQC phases "Pyrite" and "Fe(OH)3(a)", representing a composition and the kinetics of different iron sulfides and iron hydroxides. These SI distributions control the results of sulfide and iron pore water profiles, especially conserving the sulfide profile at the reaction zone during the simulation. The results suggest that phases of iron hydroxides are dissolved, mackinawite is precipitated within, and other iron sulfides are precipitated below the reaction zone. The chemical reactivity of iron hydroxides corresponds to the rate of sulfide production. The system H 2 O-CO 2 -CaCO 3 is generally successfully maintained during the simulation. Deviations to the measured pH profile suggest that further processes are active which are not included in the simulation yet.

Cretaceous black shales as active bioreactors: A biogeochemical model for the deep biosphere encountered during ODP Leg 207 (Demerara Rise)

“…50% of the diffusive sulfate flux is consumed due to AMO in Cariaco Trench sediments. Niewö hner et al (1998) and Adler et al (2000) calculated complete consumption of downward diffusing sulfate during AMO in sediments from the upwelling area off Namibia and the Amazon Fan, respectively. Generally one can assume that depth-integrated methane oxidation rates in the sulfate methane transition zone equal ca.…”

Groundwater – the disregarded component in lake water and nutrient budgets. Part 2: effects of groundwater on nutrients

“…Horner et al () list a number of reactive transport models, which are available to simulate processes at aquifer‐surface water interfaces: RT3D (Clement, ), COTREM (Landenberger et al, ; Adler et al, ), STEADYSED (van Cappellen and Wang, ), HYDROGEOCHEM (Yeh et al, ), TBC (Schäfer et al, ), MIN3P (Mayer and Frind, ), PHAST (Parkhurst et al, ) and PHT3D (Prommer, ). COTREM and STEADYSED are mainly used for marine environments.…”

Interaction between hydrocarbon seepage, chemosynthetic communities, and bottom water redox at cold seeps of the Makran accretionary prism: insights from habitat-specific pore water sampling and modeling

“…The software has been described in great detail elsewhere (e.g. Adler et al, 2000Adler et al, , 2001Wenzhöfer et al, 2001;Pfeifer et al, 2002). Notably, CoTReM has already been successfully applied to model transport processes and geochemical reactions at two mud volcanoes of the Eastern Mediterranean (Haese et al, 2003(Haese et al, , 2006.…”