Microbial activity and carbonate isotope signatures as a tool for identification of spatial differences in methane advection: a case study at the Pacific Costa Rican margin.

Krause, Stefan, Steeb, Philip, Hensen, Christian , Liebetrau, Volker, Dale, Andrew W. , Nuzzo, Marianne and Treude, Tina (2014) Microbial activity and carbonate isotope signatures as a tool for identification of spatial differences in methane advection: a case study at the Pacific Costa Rican margin. Open Access Biogeosciences (BG), 11 . pp. 507-523. DOI 10.5194/bg-11-507-2014.

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Abstract

Subduction of the oceanic Cocos plate offshore Costa Rica causes strong advection of methane-charged fluids. Presented here are the first direct measurements of microbial anaerobic oxidation of methane (AOM) and sulfate reduction (SR) rates in sediments from the two mounds, applying radiotracer techniques in combination with numerical modeling. In addition, analysis of carbonate δ18O, δ13C, and 87Sr / 86Sr signatures constrain the origin of the carbonate-precipitating fluid. Average rates of microbial activities showed differences with a factor of 4.8 to 6.3 between Mound 11 [AOM 140.71 (±40.84 SD); SR 117.25 (±82.06 SD) mmol m−2 d−1, respectively] and Mound 12 [AOM 22.37 (±0.85 SD); SR 23.99 (±5.79 SD) mmol m−2 d−1, respectively]. Modeling results yielded flow velocities of 50 cm a−1 at Mound 11 and 8–15 cm a−1 at Mound 12. Analysis of oxygen and carbon isotope variations of authigenic carbonates from the two locations revealed higher values for Mound 11 (δ18O: 4.7 to 5.9‰, δ13C: −21.0 to −29.6‰), compared to Mound 12 (δ18O: 4.1 to 4.5‰, δ13C: −45.7 to −48.9‰). Analysis of carbonates 87Sr / 86Sr indicated temporal changes of deep-source fluid admixture at Mound 12. The present study is in accordance with previous work supporting considerable differences of methane flux between the two Mounds. It also strengthens the hypothesis of a predominantly deep fluid source for Mound 11 versus a rather shallow source of biogenic methane for Mound 12. The results demonstrate that methane-driven microbial activity is a valid ground truthing tool for geophysical measurements of fluid advection and constraining of recent methane fluxes in the study area. The study further shows that the combination of microbial rate measurements, numerical modeling, and authigenic carbonate analysis provide a suitable approach to constrain temporal and spatial variations of methane charged fluid flow at the Pacific Costa Rican margin.

Document Type: Article
Research affiliation: OceanRep > SFB 574
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: Yes
Open Access Journal?: Yes
Publisher: Copernicus Publications (EGU)
Related URLs:
Projects: FLOWS, SFB574, Future Ocean
Contribution Number:
Project
Number
SFB 574
270
Expeditions/Models/Experiments:
Date Deposited: 13 Jan 2014 09:53
Last Modified: 23 Sep 2019 20:54
URI: https://oceanrep.geomar.de/id/eprint/22495

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