Biogeochemical characterization of newly discovered seeps in the Concepción Methane Seep Area, Chile.

Treude, Tina, Steeb, Philip, Linke, Peter, Schmidt, Mark and Liebetrau, Volker (2012) Biogeochemical characterization of newly discovered seeps in the Concepción Methane Seep Area, Chile. [Talk] In: The Lübeck Retreat, Collaborative Research SFB 574 Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters. , 23.-25.05.2012, Lübeck . The Lübeck Retreat: final colloquium of SFB 574; May 23-25, 2012: program & abstracts. ; pp. 33-34 .

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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.

Document Type: Conference or Workshop Item (Talk)
Keywords: Geodynamics
Research affiliation: OceanRep > SFB 574 > B6
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 574
OceanRep > SFB 574 > B5
OceanRep > SFB 574 > B3
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Date Deposited: 21 Sep 2012 10:09
Last Modified: 17 Jan 2013 13:16

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