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