Volatiles and Fluids in Subduction Zones

Major research objectives

The central objectives of our investigations are to decipher the control parameters of forearc fluid discharge in different subduction zone settings, i.e. to determine the fluxes of volatiles from the seafloor and the microbial turnover of reduced compounds (e.g. methane and sulphide) at the sediment-water interface, to improve global estimates on benthic methane fluxes. In the third phase of the SFB, existing data sets from the extensive fieldwork of phase 2 will be compiled and compared with data from other active margin settings (e.g. Hydrate Ridge, Hikurangi Margin, Aleutean Trench and the mud volcanoes in the Gulf of Cadiz) to better constrain the volatile emission from the sea floor on a global scale. In addition, we plan to support these synthesis efforts by final field work offshore Costa Rica and new field studies offshore Chile. Pending on available ship time with RV Meteor in 2009, we will combine our proven techniques with microbial investigations in the sediments and in the water column.

Fig. B3-1: Scheme illustrating the transfer of inorganic substrates, which fuel the benthic methane filter. Methane advecting with fluids from below becomes gradually consumed by microbial processes in the sediment and in the water column until it is finally emitted into the atmosphere. AOM is the most significant process in this consumption chain. During AOM, methane is transferred to bicarbonate, which may precipitate as authigenic carbonates. Sulphate diffusing into the sediment is reduced to sulphide. Chemoautotrophic communities gain energy by oxidizing the sulphide with nitrate or oxygen. Oxygen is further used by aerobic methanotrophs at the sediment-water interface and in the water column.

Major questions addressed in this project are:

1. What is the amount of water and chemical species within fluids that is recycled into the ocean at different submarine venting sites?
2. Can we correlate external forcing, including tectonics, tides and other abiotic control parameters, with fluid expulsion rates?
3. What is the role of fluid flow and the chemical composition of fluids at subduction zones in the formation of individual benthic communities?
4. What is the impact of the benthic biological filter on the ratio of uptake (consumption) and emission (escape into the water column without consumption) of reduced, energy-rich fluid components?
5. How is this filter controlled by changes in fluid-flow?
6. Is a considerable amount of methane released into the water column already consumed within the benthic boundary layer?
7. Is the expulsion of methane-rich fluids at the upper slope mirrored in the surface waters, i.e. is there a contribution to the sea-air flux of methane?
8. Do we find distinct differences between benthic fluxes, biological communities, and methane release into the water column between the erosional margin of Middle America and the accretionary margin of northern Chile?
9. Are other transport mechanisms for methane (i.e. bubble transport and gas hydrate release), which could not be detected along the Middle American margin, of importance in the accretionary margin of northern Chile?