In situ benthic fluxes from an intermittently active mud volcano at the Costa Rica convergent margin

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Abstract

Along the erosive convergent margin off Costa Rica a large number of mound-shaped structures exist built by mud diapirism or mud volcanism. One of these, Mound 12, an intermittently active mud volcano, currently emits large amounts of aqueous dissolved species and water. Chemosynthetic vent communities, authigenic carbonates, and methane plumes in the water column are manifestations of that activity. Benthic flux measurements were obtained by a video-guided Benthic Chamber Lander (BCL) deployed at a vent site located in the most active part of Mound 12. The lander was equipped with 4 independent chambers covering adjacent areas of the seafloor. Benthic fluxes were recorded by repeated sampling of the enclosed bottom waters while the underlying surface sediments were recovered with the lander after a deployment time of one day. One of the chambers was placed directly in the centre of an active vent marked by the occurrence of a bacterial mat while the other chambers were located at the fringe of the same vent system at a lateral distance of only 40 cm. A transport-reaction model was developed and applied to describe the concentration profiles in the pore water of the recovered surface sediments and the temporal evolution of the enclosed bottom water. Repeated model runs revealed that the best fit to the pore water and benthic chamber data is obtained with a flow velocity of 10 cm yr 1 at the centre of the vent. The flux rates to the bottom water are strongly modified by the benthic turnover (benthic filter). The methane flux from below at the bacterial mat site is as high as 1032 μmol cm 2 yr 1, out of which 588 μmol cm 2 yr 1 is oxidised in the surface sediments by microbial consortia using sulphate as terminal electron acceptor and 440 μmol cm 2 yr 1 are seeping into the overlaying bottom water. Sulphide is transported to the surface by ascending fluids (238 μmol cm 2 yr 1) and is formed within the surface sediment by the anaerobic oxidation of methane (AOM, 588 μmol cm 2 yr 1). However, sulphide is not released into the bottom water but completely oxidized by oxygen and nitrate at the sediment/water interface. The oxygen and nitrate fluxes into the sediment are high (781 and 700 μmol cm 2 yr 1, respectively) and are mainly driven by the microbial oxidation of sulphide. Benthic fluxes were much lower in the other chambers placed in the fringe of the vent system. Thus, methane and oxygen fluxes of only 28 and 89 μmol cm 2 yr 1, respectively were recorded in one of these chambers. Our study shows that the aerobic oxidation of methane is much less efficient than the anaerobic oxidation of methane so that methane which is not oxidized within the sediment by AOM is almost completely released into the bottom water. Hence, anaerobic rather than aerobic methane oxidation plays the major role in the regulation of benthic methane fluxes. Moreover, we demonstrate that methane and oxygen fluxes at cold vent sites may vary up to 3 orders of magnitude over a lateral distance of only 40 cm indicating an extreme focussing of fluid flow and methane release at the seafloor.

Introduction

Active convergent margins are important regions for element recycling between crust, ocean and atmosphere [1]. The input into subduction zones is by sediments and altered oceanic crust and the return flow of volatile elements may take three transport pathways: through the subduction zone vents at the deformation front, the mud diapirs at the mid slope, and through subarial volcanoes at the arc. Mass transport through mud diapirism by ascending muds and fluids is being increasingly recognized as a potentially important, but largely unquantified process [2], [3]. For example, the Mariana fore-arc serpentinite mud diapirs are spectacular features, that transport fluids and material from up to 50 km of depth to the surface [4]. Other well-known subduction-related mud diapirs are found off the Indonesian island arc, Barbados, Costa Rica, Pakistan, and on the Mediterranean Ridge (e.g. [5], [6], [7], [8]). Mud diapirs at the erosive margin off Costa Rica have been known from extensive surveys previous to ODP Leg 170 which included a seismic [9] and an Alvin diving program [10], [11]. More recently, numerous geophysical and mapping surveys combined with video observations revealed a great number of mud diapirs at mid slope depths between 1000 to 3000 m [12], [13], five of which have been studied in detail so far [14], [15], [16], [17].

This study aimed at establishing benthic fluxes from one of these mounds, Mound 12, located in ∼1000 m water depth on the mid slope off Costa Rica (Fig. 1, Fig. 2). It is essentially round with a diameter of 800 m and features an irregular higher pinnacle at its NE end and a lower profile ridge towards the SW [18]. A widespread occurrence of chaotic mud flows interlayered with stratified slope sediments indicate frequent mud eruptions that alternate with periods of quiescence when only fluid venting occurs [18]. Video surveys revealed that the pinnacle top and its SW flank are partially covered with authigenic carbonates and typical cold vent fauna communities (Mytilid mussels, pogonophoran tubeworms and bacterial mats) [15], [17]. These indicators suggest strong venting activity which is supported by the highest bottom water methane concentrations of all surveyed mounds at Mound 12 (> 30 times the regional background of 1–2 nmol/l) [17].

During Meteor cruise M54/3a we obtained data from video-guided deployments of a Benthic Chamber Lander and a Multiple Corer, as well as by recovering standard gravity cores to evaluate the role of the benthic filter to the flux into the bottom water at Mound 12. To our knowledge this is the first integrative study encompassing pore water and sediment–water interface data, aimed at quantifying the in situ fluid flow and benthic turnover at a vent site. The interrelationship between pore water chemistry, biological activity, and production rates of mud and fluid is crucial for our understanding of vent systems at active margins. On one hand, the development of chemoautotrophic microbial and symbiotic communities is dependent on the transfer rates of reduced inorganic substrates [19], [20], and on the other hand this mass transfer is modified and filtered by their activities. Therefore, aqueous flow rates, the controlling parameters of fluid expulsion and resulting transient flow patterns, the concentrations of methane and reduced sulfur compounds delivered by the aqueous flow, and their transformation rates are important parameters for the determination of mass balances in various subduction zone settings.

Section snippets

Benthic lander deployment

In situ flux measurements were performed at Mound 12 with the Benthic Chamber Lander (BCL) [21]. The basic frame of the lander is a stainless-steel tripod that carries 21 Benthos glass spheres for buoyancy and ballast attached by release toggles to each leg. The ballast is controlled by two acoustic release units that provide redundancy. A radio beacon and strobe aid in location and recovery at the surface and an ARGOS system is used to track the lander in case of a premature release. The frame

Pore water composition

Pore fluids below bacterial mats have a strongly reducing character with high concentrations of dissolved sulfide, methane, barium and enhanced total alkalinity values (Fig. 3, Fig. 4). Calcium concentrations are low indicating carbonate precipitation at depth. Sulfate is depleted but not completely consumed at the base of the investigated cores. The presence of both sulfate and methane in millimolar concentrations at the core base is surprising because these chemical species should be

Conclusions

In situ measurements with chambers placed at the seafloor have to be evaluated carefully to avoid erroneous results. Thus, concentration changes recorded immediately after the deployment of the instrument may be too high because of the disturbance and suspension of surface sediments. Moreover, fluxes of reduced chemicals (sulfide, methane) depend on the concentration of dissolved oxygen in the chamber water. As oxygen is rapidly depleted during the deployment, the flux data recorded towards the

Acknowledgements

Many thanks are due to the vessel's master Hening Papenhagen and crew of R/V Meteor. Without the dedication and support of Bernhard Bannert, Michael Poser, Wolfgang Queisser and Matthias Türk, the deployment and functioning of our equipment would not have been possible. We are grateful for the analytical work of Bettina Domeyer, Karen Stange, Kristin Nass and Anke Bleyer at sea and in the shore-based laboratory. We appreciate the help of Christine Utecht in improving the figures and thank the

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