Numerical modeling of biogeochemical processes in gas hydrate bearing sediments at Hydrate Ridge Roger Luff and Klaus Wallmann GEOMAR Research Center for Marine Geosciences Kiel, Germany rluff@geomar.de

Determination of: Introduction/Goals main biogeochemical processes sediments above gas hydrate layers velocity of advective fluid transport through the sediment response of the system to changes in pore water flow velocities benthic fluxes of dissolved species between sediment and bottom water

Cascadia Margin TECFLUX SO143 55-2 (SHR) July 1999

(bottom boundary layer) Model set up C. CANDI Water column SO42- CO2 ..... Sulfate Methane CH4 OM 790 m 0.1- 1 cm Bacterial mat Sediment (simulation area) 15 cm fluid flow Gas hydrate (bottom boundary layer) Transport: 16 solute, 6 solid species advection, diffusion, bioturbation, bioirrigation Reactions: 6 prim. redox reactions, 17 sek. redox reactions, 4 precipitation and dissolution reactions

Results STEADY STATE SIMULATIONEN

Results: concentrations 0 cm a-1 10 cm a-1 70 cm a-1 TVM 55/2 Pore water flux is lower than the extreme value of up to 1065 m a-1 Linke et al. (1994) for Hydrate Ridge, but falls into the range of fluxes (0 - 1000 cm a-1) measured by Tryon and Brown (2001) at bacterial mat sites situated on the northern and southern summits of Hydrate Ridge. (kCH4SO4 = 100 cm3 mmol-1 a-1) falls into the range of values found in for other sedimentary environments (0 - 10 000 cm3 mmol-1 a-1, Iversen and Jørgensen, 1985; Van Cappellen and Wang, 1996) ARAPPT > CALPPT: Die Bakterien die Methanoxidation machen scheinen die Fällung von Aragonit zu katalysieren! Da die gemessenen Daten (Mg/Ca Verhältnis, große SO4 und kleine PO4 Konz) eher für calcite Fällung sprechen 10 mmol-1 cm–3 a-1 Kinetic const. calcite prec. 100 mmol-1 cm–3 a-1 Kinetic const. aragonite prec. 100 cm3 mmol-1 a-1 Kinetic const. CH4 oxidation 10 cm a-1 Prescribed advection velocity

Relative Ratentiefenprofile: Results: rates SO42- reduction CH4 oxidation aragonite prec. calcite prec. aragonite diss. calcite prec. prim. sek. diss./prec.

Results: carbon budget Source [µmol cm-2 a-1] Sink [µmol cm-2 a-1] Water column Carbon turnover: 1043 [µmol cm-2 a-1] POM 1 CO32- 86 HCO3- 2 CO2 CH4 873 81 210 CO32- HCO3- CO2 CH4 POM Aut. CaCO3 120 22 1 48 843 9 TCO2 1030 Bacterial mat Sediment Gas hydrates

Results NON-STEADY STATE SIMULATIONEN

Results: fluxes Pore water fluxes, (Tryon et al., 2001) NHR/SHR (Suess et al., 1999) Benthic chamber 4.560-13.700

Conclusions Methane flux from below is the main source of carbon at cold vent sites (83%). Methane oxidation using sulfate is the dominate process in this environment (84%). Methane is almost completely oxidized by anaerobic micro-organisms using dissolved sulfate as electron acceptor at low fluid flow rates. Methane charged fluids have a significant influence on the biogeochemical processes in the sediment column but only a minor influence on the total methane flux into the ocean (~5.000 : 117.000 µmol cm-2 a-1).

Thank you! Questions? rluff@geomar.de