ALBERT

Description: The Chilean methane seepage area (CMSA) is characterized by patchy islands of chemosynthetic communities embedded in carbonate outcrops or plain sediment areas. The communities were often found to be dominated by one type of chemosynthetic organism: bacterial mat (Beggiatoa Sp.), clam fields (Calyptogena Sp., Archivesica Sp.), tubeworms (Lamellibrachia), and pogonophorans. Some samples revealed chemosynthetic organisms below the surface such as the clam Tyosira Sp. and the sulfur bacteria Thioploca. Other sites revealed indications of successions from a less to a more active seepage site (e.g., high-sulfidic sediments containing clam shells) or vice versa (e.g., sulfide-free sediments containing clam shells). This presentation will provide a preliminary overview of chemosynthetic communities and geochemical profiles at the CMSA and their implications for methane seepage and consumption activity in sediments.

Description: The overarching goal of SFB 574 is to understand the role and fate of volatiles and fluids in subduction zones. Both components have a major influence on, e.g., short- and long-term climate change, the geochemical evolution of the hydrosphere and atmosphere, as well as subduction-related natural hazards, such as earthquakes, volcanic eruptions and tsunamis, because they are cycled through the entire subduction system. The Chilean margin, which switched from erosion to accretion within the last several million years, has been chosen for the remainder of the SFB. Major goals of cruise SO-210 are: 1) to investigate and quantify the dewatering processes in the forearc of the central Chilean subduction zone, in particular the origin and output flux of vent fluids and volatiles, 2) to study biological processes fuelled by this discharge, 3) to use cold seep carbonates as a geochemical archive of cold seep activity, 4) to evaluate the role of forearc fluids in triggering mass wasting events that could generate tsunamis, 5) to characterize geochemically the subducting sediments to determine the input flux of climate-relevant volatiles and a variety of trace elements, and 6) to investigate the distribution of volcanic ashes to improve estimates on the volume of material emitted by volcanic eruptions and to date distinct events within the sedimentary sequence. Preliminary investigations by Chilean colleagues revealed definite indications of fluid venting which together with recent geophysical investigations are the basis for the present cruise. Here, we will present preliminary results and new discoveries made during the cruise.

Description: As a consequence of sediment compaction at the forearc of subduction zones, fluids ascend in sediments and are discharged in cold seeps. These fluids include the greenhouse gas methane, which is to a larger proportion depleted by anaerobic oxidization of methane (AOM) next to sediment-water interface. This microbial methane filter consists of archaea (methane oxidizers) and bacteria (sulfate reducers) that are living in syntrophic consortia. During AOM, the microbes produce hydrogen sulfide, which feeds chemoautotrophic “seep-organism”. A new sediment-flow-through-system was developed, to investigate the efficiency of AOM, the range of methane turnover rates, as well as key parameters controlling microbial filter. The system includes two different seawater media. The first medium supplies the microbes with sulfate from the top, (transported by diffusion); the second carries methane and is supplied from the bottom by advective transport. Sampling holes, sealed with silicon, allow the measurement of key parameters such as sulfate, sulfide, pH, redox potential, and total alkalinity as well as sediment sub-sampling to determine the abundance, e.g., of methanotrophic microbial groups. In this presentation the technical details of the sediment-flow-through-system will be introduced. Preliminary results from the first experiments with seep sediments and insights into ongoing studies will be provided.

Description: The microbial benthic methane filter of the ocean floors globally retains approximately 80-90% of the ascending greenhouse gas methane through anaerobic oxidation of methane (AOM). However natural and catastrophic fluctuations of methane fluxes (caused e.g. by gas hydrate melting, earthquakes, slope failure) can challenge the capability of this greenhouse gas sink. We ask: How efficient can the methanotrophic community adapt its activity to methane flux changes, what is its response time and what is the efficiency of the benthic filter in this time. To answer these questions, a new sediment-flow-through-system was developed. The system holds intact sediment cores and simulates natural condition of seepage with a diffusive supply of sulfate from the top and an advective transport of methane from the bottom. Sampling holes allow monitoring the key parameters (sulfate, sulfide, pH, Redox, Total Alkalinity) over the entire sediment depth. For our experiments, sediment from three different methane-rich environments were used: (1) gassy sediments from Eckernförde Bay (German Baltic) without naturally occurring advective fluid transport, (2) sediments with high advective transport from a methane seep within an oxygen minimum zone on the continental margin (Quepos Slide, Costa Rica), and (3) methane-seep sediments from the center of a mud volcano (North Alex Mud Volcano, Eastern Mediterranean Sea). Two different advective methane flow rates (15.3 and 153 mmol CH4 cm-'yr-1, fluid flow 10.9 and 109 cm yr-1) were applied for replicate sediment cores (upper 20cm) of the respective environments. The poster will present results of the long-term experiment and compare the response of the different sediment types to the varying methane and fluid flow rates.

Description: Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. The recently discovered Concepción Methane Seep Area (CMSA, water depths between 600 to 1100 mbsf) is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past. During the SO210 research expedition “Chiflux” (Sept-Oct 2010), sediment from the CSMA at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed. The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these fluids, which is supported by the biogenic origin of the methane (!13C -70‰ PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, black sediment spots, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots. Abundant massive carbonate blocks and plates hint to a very old seep system with a probably much higher activity in the past. The U-Th age record of these authigenic carbonates reach back to periods of venting activity with more than 150 ka ago. Carbon isotopic signatures of authigenic carbonates (!13C -50 to -40 ‰ PDB) suggest a biogenic carbon source (i.e. methane), also in the past. We found several indications for the impact of recent earthquakes within the seep area (cracks, shifted seafloor), which could be an important mechanism for the triggering of new seepage activity, change in fluid expulsion rates or colonization patterns of the cold seep fauna.

Description: Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. A recently discovered seep area located off Concepción (Chile) at water depth between 600 to 1100 mbsl is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past. During the SO210 research expedition “Chiflux” (Sept-Oct 2010), sediment from the Concepción Methane Seep Area (CSMA) at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed. The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these fluids, which is supported by the biogenic origin of the methane (�13C -70h PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, a black sediment spot, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots. Abundant massive carbonate blocks and plates hint to a very old seep system with a probably much higher activity in the past. The U-Th age record of these authigenic carbonates reach back to periods of venting activity with more than 150 ka ago. Carbon isotopic signatures of authigenic carbonates (�13C -50 to -40hPDB) suggest a biogenic carbon source (i.e. methane), also in the past. We found several indications for the impact of recent earthquakes within the seep area (cracks, shifted seafloor), which could be an important mechanism for the triggering of new seepage activity, change in fluid expulsion rates and colonization patterns of the cold seep fauna.