Efficiency and adaptability of the benthic methane filter at Quepos Slide cold seeps, offshore of Costa Rica

P Steeb,S Krause,T Treude,C Hensen,M Nuzzo,P Linke,A W Dale

doi:10.5194/bg-12-6687-2015

Abstract

Abstract. Large amounts of methane are delivered by fluids through the erosive forearc of the convergent margin offshore of Costa Rica and lead to the formation of cold seeps at the sediment surface. Besides mud extrusion, numerous cold seeps are created by landslides induced by seamount subduction or fluid migration along major faults. Most of the dissolved methane migrating through the sediments of cold seeps is oxidized within the benthic microbial methane filter by anaerobic oxidation of methane (AOM). Measurements of AOM and sulfate reduction as well as numerical modeling of porewater profiles revealed a highly active and efficient benthic methane filter at the Quepos Slide site, a landslide on the continental slope between the Nicoya and Osa Peninsula. Integrated areal rates of AOM ranged from 12.9 ± 6.0 to 45.2 ± 11.5 mmol m−2 d−1, with only 1 to 2.5 % of the upward methane flux being released into the water column. Additionally, two parallel sediment cores from Quepos Slide were used for in vitro experiments in a recently developed sediment-flow-through (SLOT) system to simulate an increased fluid and methane flux from the bottom of the sediment core. The benthic methane filter revealed a high adaptability whereby the methane oxidation efficiency responded to the increased fluid flow within ca. 170 d. To our knowledge, this study provides the first estimation of the natural biogeochemical response of seep sediments to changes in fluid flow.

Highlights

Subduction zones represent large-scale systems of sediment and element recycling
3.1 Ex situ profiles and numerical models. Both multicorer cores from station SO206-31 (MUC) cores (SO206-29 MUC and SO206-31 MUC) were sampled at ∼ 400 m water depth from sediments covered with sulfur bacteria mats, which are indicative of high methane fluxes (Torres et al, 2002; Treude et al, 2003)
In the high-flow regime core (HFC, 106 cm yr−1), the fluid flow was 2 to 10 times higher compared to our modeled data and higher than other values published for Quepos Slide (1–40 cm yr−1, Table 5; Karaca et al, 2012); the flow was still in the range of neighboring seeps (0.1–200 cm yr−1; Hensen et al, 2004; Linke et al, 2005; Karaca et al, 2010; Krause et al, 2014)

Summary

Introduction

Subduction zones represent large-scale systems of sediment and element recycling. Organic carbon accumulation at continental margins can lead to the formation of large methane reservoirs through its biological or thermogenic breakdown (Judd et al, 2002; Schmidt et al, 2005; Hensen and Wallmann, 2005; Crutchley et al, 2014). Produced methane gas may be transported upwards in solution by molecular diffusion or by ascending fluids, mobilized by, for example, sediment compaction or clay mineral dehydration (Hensen et al, 2004; Tryon et al, 2010; Crutchley et al, 2014). When the fluids are highly enriched in hydrocarbon gases, gas hydrates may precipitate depending on the pressure–temperature conditions (Hensen and Wallmann, 2005). Dissociating gas hydrates can act as additional sources of methane and fluids (Kvenvolden, 2002) or dilute fluids when they dissolve (Hesse et al, 2000; Hensen et al, 2004)

Methods

Results

Discussion

Conclusion