WP2 - deliverable No. 2: GIS-map of methane flux and distribution in sediments
Deliverable
Michael Schlüter, Torben Gentz, Roi Martinez, Jørn B. Jensen, Laura Lapham.
Biogeochemical investigations about fluxes of methane within the sediment or along the sediment-water-interface (SWI) are mainly based on the sampling of sediment cores and subsequent analysis of the pore water composition. This provides very detailed information about fluxes and turnover processes for the specific sampling site.
Generalization of such site-specific data would support the identification of biogeochemical provinces as well as basin-wide estimates about inventories or fluxes of methane. For these purposes, we investigated the spatial relationship between indicators for the flux and turnover of methane with data sets available for large areas like bathymetry, sediment type, bottom morphology and chemical composition of the bottom water (Fig. 1).
- Figure 1
Fig 1: Examples of geodata applied for GIS mapping and spatial analysis. Left: Seismic lines and regions where free gas is observed (R. Endler, IOW). Right: Sediment types (raster data) at the seafloor of the Baltic Sea (BALANCE).
GIS-mapping for consideration of spatial distributions requires that all data sets are projected according to the same cartographic projection system. For this purpose, we applied the Lambert azimuthal equal-area projection, which supports calculations of spatial budgets. Therefore, all data sets were projected or re-projected to Lambert azimuthal and converted to same spatial resolution (e.g. raster size). This allows application of GIS techniques like buffer, overlay, clipping etc. designed for spatial modeling (Fig. 2A).
Besides the flux through the sediment-water interface (SWI), the depth layer as well as the flux of methane into the sulfate-methane-transition-zone (SMTZ) provides very useful information about the formation of methane and its potential release from surface sediments (Fig. 2B).
Since the sediment depth of the SMTZ provides a very robust proxy in terms of analytical accuracy and data availability, we applied the SMTZ as an indicator to identify regions where enhanced methane fluxes within sediments and probably into the water column might be expected. For example, low depths of the SMTZ (methane formation starts close to the sediment-water interface) suggest high methane concentrations (gas and dissolved phase) and an enhanced potential for high methane fluxes within sediments.
- FIGURE 2 A and B
Fig 2A: Sketch of different information layers, referenced to the same map projection, raster size etc. which allows application of different GIS techniques for spatial analysis.
Fig 2B: Sketch of a pore water profile in surface sediment and location of the Sediment-Water-Interface (SWI) as well as Sulfate-Methane-Transition-Zone (SMTZ).
For GIS-mapping of methane fluxes and distribution in sediments of the Baltic Sea, we considered the spatial distribution of the depth of the SMTZ in relation to the bathymetry, bottom water chemistry (e.g. oxygen, sulfate), sediment type (e.g. mud, sand), mass accumulation of particulate organic matter as well as seafloor morphology.
For example, we considered the depth of the SMTZ with respect to the sediment type. This reveals that low SMTZ depths are preferably observed for mud deposits. A more detailed consideration is achieved by considering different bed forms like plains, valleys, troughs, or basins (Fig. 3). The bed form categories were defined within the BALANCE project. Analyzing the SMTZ data for different bed forms reveals that low median values of 0.35 m and 0.6 m where observed for basin and plains, respectively.
Figure 3: Spatial distribution of the depth of the SMTZ in muddy surface sediments deposited within different bed forms like plains, valleys, troughs, or basins.
The compilation and integration of site specific datsets of qualifying fluxes and turnover rates with geodata available for large areas is considered a step towards improved computation of biogeochemical budgets and seafloor classification. On a smaller scale, GIS techniques as well as geostatistic methods were applied for the investigation of spatial patterns and biogeochemical provinces at pockmarks or mud volcanoes (e.g. Schlüter et al., 2004; Jerosch et al., 2007; Peesch et al., 2008).
References
Al-Hamdani, Z. and Reker, J. (2007). Towards marine landscape in the Baltic Sea. Balance Interim Report, No. 10, 115p.
Jerosch, K., Schlüter, M., Foucher, J., Allais, A., Klages, M. and Edy, C. (2007). Spatial distribution of mud flows, chemoautotrophic communities, and biogeochemical habitats at Håkon Mosby Mud Volcano, Marine Geology, 243 , pp. 1-17, ISSN: 0025-3227.
Leipe, T, Tauber, F, Vallius, H. Virtasalo, J., Uścinowicz, S., Kowalski, N., Hille, S., Lindgren, S. Myllyvirta, T. (2010) Particulate organic carbon (POC) in surface sediments of the Baltic Sea; Geo-Marine-Letters.
Pesch, R., Pehlke, H., Jerosch, K., Schröder, W. and Schlüter, M. (2008). Using decision trees to predict benthic communities within and near the German Exclusive Economic Zone (EEZ) of the North Sea, Environmental Monitoring and Assessment, 136(1-3)325, 313.
Schlüter, M., Sauter, E., Andersen, C., Dahlgaard, H. and Dando, P. (2004). Spatial Distribution and Budget for Submarine Groundwater Discharge in Eckernförde Bay (W-Baltic Sea), Limnology and Oceanography, 49 (1), pp. 157-167.