Volatiles and Fluids in Subduction Zones

Theme B: Forearc volatile turnover, fluid flow, and subduction input

Volatile fluxes related to mud extrusion and fluid flow through the continental slope wedge in Central America have been the central goal of Theme B during phases I and II. More than 100 vent sites were discovered along the Central American forearc including mud diapirs, mud volcanoes, carbonate mounds, fracture zones, and slope failure scars resulting from seamount subduction and tectonic erosion. Fluid flow and methane release were found to be active at all sites visited as indicated by the widespread occurrence of methane and sulfide-enriched fluids, authigenic carbonates, methane plumes in the bottom water, and vent macro-fauna. The chemical and isotopic compositions of fluids and carbonates strongly suggest that mid-slope venting is mainly induced by fluids diagenetically released from subducted sediments at elevated temperatures. Major goals comprise an improved understanding of structural and geochemical processes enabling fluid mobilisation and transport, the establishment of first order mass balances for fluid turnover in the forearc, the mechanisms of mud extrusion and mass wasting processes, the quantification of focused methane release into the water column (importance of the benthic filter), and the characterisation of authigenic carbonates that serve as an archive for variations in fluid geochemistry on time scales of 10-100 kyr.

In the final phase of the SFB, theme B projects will adjust their focus to a more fundamental understanding of fluid mobilisation processes in the forearc and volatile input to the subduction zone, and towards the global projection of water and element budgets. This will imply a systematic geochemical analysis of fluids emanating at cold vents and obtained by drilling efforts at various accretionary and non-accretionary subduction zones and complementary field work offshore central Chile and Costa Rica (proposed for 2009). In addition, new methods will be applied to enable more realistic estimates on in- and output fluxes of volatile elements in the forearc and on subduction zone input. At present, a comprehensive data analysis is lacking and seems to be the only viable approach to better understand the functioning of the hydrologic system in the forearc and hence, improve currently available mass balances.

Overall, a number of specific goals have been defined that shall be addressed in 3 subprojects,

which comprise

• the quantification of fluid exchange and volatile turnover in the forearc of subduction zones,
• the determination and characterisation of the sedimentary volatile input to the subduction factory, in particular their halogen contents,
• the investigation of fluid formation processes using isotopic tracers and numerical models in order to gain a systematic understanding of fluid recycling pathways in the forearc on a global scale,
• the estimation of volatile fluxes (CH4, CO2, SO4) from the seafloor to the water column in order to improve e.g. global estimates of benthic methane fluxes and emissions of methane from the ocean floor into the hydrosphere and atmosphere,
• the investigation of geochemical and (micro)-biological parameters effecting the efficiency of the benthic filter for the recycling of key elements (O, C, S, N),
• the investigation of cold seep carbonates as archives to record variations of environmental conditions and changes in fluid geochemistry at cold vent sites.

The basic concept is to extend the available data set from Central America by specific investigations of vent sites and incoming plate sediments along the central Chilean margin and the acquirement of comparable data sets from other convergent margin systems. The comparison of contrasting settings will improve current hypotheses and concepts on fluid recycling in the forearc at accretionary and erosive convergent margins. The objectives of subprojects and their contribution to the overall task are briefly summarized below.

Subproject B2, which will be concluded in 2008, is concerned with the dynamic forces driving mud diapirism and mass wasting processes on the continental margin. Geophysical characterization of mud diapirs, originally carried out within former subproject B1, are performed using high resolution surface seismics, Parasound, and imaging with the deep-tow streamer system. Mapping of the lateral extent of the BSR in the vicinity of mud diapirs and mass wasting structures, seismic stratigraphy, geotechnical and sedimentological work, and newly developed in situ observation tools are applied to identify the source region and extrusion dynamics of mud diapirs and the boundary conditions of slope stability.

Subproject B3 focuses on the role of the benthic filter on volatile recycling between forearc sediments and the deep ocean water. Specifically, turnover rates of reduced compounds at the sediment-water interface and the release of volatiles at active vent sites will be determined. The project will concentrate on the compilation of existing data and comparisons with other active margin settings in addition to finalizing field work off Costa Rica and conducting field work off Chile, specifically in order to fill gaps in the global methane budget. Long-term mesocosm studies with sediments from fluid discharge areas of the forearc are envisioned to study control parameters of the benthic methane filter. In addition, data gained from field work and in-vitro studies will be used in numeric models to assess the impact of transport processes on biomass distributions, microbial turnover and methane release fluxes from the seafloor.

Subproject B5 addresses the diagenetic turnover of volatiles in forearc sediments and the quantification of the sedimentary volatile input to subduction zones. Complementary to investigations offshore Central America, pore fluids and sediments will be recovered and analyzed from the central Chilean margin in order to determine fluid formation processes, the turnover of volatiles, and the input into the subduction zone. These efforts will be supported by a global data analysis of vent fluids in order to improve current estimates on forearc fluid flow. In addition, numerical modelling is applied to improve our current understanding of mineral transformation and fluid formation processes in deeply buried sediments, and to constrain volatile turnover rates and upward fluid flow in surface sediments affected by active fluid venting.

Subproject B6 investigates the formation processes, ages and the composition of authigenic carbonates which serve as important carbon reservoir and archive of fluid flow and methane cycling. The mineralogical and petrographic characterisation of these samples are accompanied by advanced mass spectroscopic U-series age dating and analysis of their isotopic and trace element composition. These studies help to constrain the genesis and source depths of vent fluids, the history of fluid venting, the mechanisms of carbonate precipitation, and the long-term integrated accumulation rates and inventory of these important carbon reservoirs.

Figure B-1: Methane plume in lower water column and seismic reflection line across northern summit (OR-8) of the Cascadia subduction zone. This image exemplifies the domed structure, extensional faulting of a typical accretionary ridge build by plate convergence, and the pattern of the bottom simulating reflector (BSR). The BSR is off-set and anomalously shallow in several places below the ridge crest where methane plumes originate; it responds to subsurface temperature anomaly pattern related to ascent of warm methane-rich fluids and indicates the transition between hydrate-containig sediments above and gas-containing sediments below. Two methane plumes, with concentrations >10,000 nl/L, are projected onto the trace of the seismic line. The plume maxima are off the seafloor indicating bubble transport.