Fluid seepage at the continental margin offshore Costa Rica and southern Nicaragua

Abstract

A systematic search for methane‐rich fluid seeps at the seafloor was conducted at the Pacific continental margin offshore southern Nicaragua and northern central Costa Rica, a convergent margin characterized by subduction erosion. More than 100 fluid seeps were discovered using a combination of multibeam bathymetry, side‐scan sonar imagery, TV‐sled observations, and sampling. This corresponds, on average, to a seep site every 4 km along the continental slope. In the northwestern part of the study area, subduction of oceanic crust formed at the East Pacific Rise is characterized by pervasive bending‐induced faulting of the oceanic plate and a relatively uniform morphology of the overriding continental margin. Seepage at this part of the margin typically occurs at approximately cone‐shaped mounds 50 ‐ 100 m high and up to 1 km wide at the base. Over 60 such mounds were identified on the 240 km long margin segment. Some normal faults also host localized seepage. In contrast, in the southeast, the 220 km long margin segment overriding the oceanic crust formed at the Cocos‐Nazca Spreading Centre has a comparatively more irregular morphology caused mainly by the subduction of ridges and seamounts sitting on the oceanic plate. Over 40 seeps were located on this part of the margin. This margin segment with irregular morphology exhibits diverse seep structures. Seeps are related to landslide scars, seamount‐subduction related fractures, mounds, and faults. Several backscatter anomalies in side‐scan images are without apparent relief and are probably related to carbonate precipitation. Detected fluid seeps are not evenly distributed across the margin but occur in a roughly margin parallel band centered 28 ± 7 km landward of the trench. This distribution suggests that seeps are possibly fed to fluids rising from the plate boundary along deep‐penetrating faults through the upper plate.

Expedition 344 summary

“…Mud volcanoes and mud diapirs have also been found, particularly across the middle slope, and are associated with a high density of chemosynthetic vents. The chemistry of the pore fluids sampled at these midslope features is indicative of clay dehydration reactions at depth (Shipley et al, 1992;Bohrmann et al, 2002;Grevemeyer et al, 2004;Hensen et al, 2004;Sahling et al, 2008).…”

“…Along the entire Costa Rica margin, active fluid venting is indicated by elevated methane concentrations in the bottom water McAdoo et al, 1996;Bohrmann et al, 2002;Sahling et al, 2008). Chemoautotrophic and methanotrophic communities mark cold vents at numerous localities, but higher community abundances have been found where subducted seamounts have triggered fractures, slides, and slumps that break a low-permeability, shallow sediment carapace, allowing ascending fluids to feed the communities.…”

“…Fluid flow pathways through the Costa Rica margin include the upper plate basement and overlying sediment, the décollement, and the underlying oceanic crust (e.g., Silver et al, 2000;Fisher et al, 2003;Hutnak et al, 2007;Sahling et al, 2008;Harris et al, 2010b). Along the entire Costa Rica margin, active fluid venting is indicated by elevated methane concentrations in the bottom water McAdoo et al, 1996;Bohrmann et al, 2002;Sahling et al, 2008).…”

“…Mud volcanoes and mud diapirs have also been found, particularly across the middle slope, and are associated with a high density of chemosynthetic vents. The chemistry of the pore fluids sampled at these midslope features is indicative of clay dehydration reactions at depth (Shipley et al, 1992;Bohrmann et al, 2002;Grevemeyer et al, 2004;Hensen et al, 2004;Sahling et al, 2008).Along the décollement and the upper prism fault zone, coring and sampling during Ocean Drilling Program (ODP) Legs 170 and 205 offshore the Nicoya Peninsula revealed freshened pore waters containing elevated Ca, Li, and C 3 -C 6 hydrocarbon concentrations and low K concentrations (Kimura, Silver, Blum, et al, 1997;Silver et al, 2000;Chan and Kastner, 2000;Morris, Villinger, Klaus, et al, 2003;Kastner et al, 2006). These fluids contrast with pore fluids from below the décollement and between the décollement and upper fault zone that have near-seawater chemistry (Kimura, Silver, Blum, et al, 1997;Morris, Villinger, Klaus, et al, 2003).…”

A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems

“…We present observations made during exploratory dives near the base of Jaco Scar, formed from a subducting seamount on the convergent Costa Rica margin. This region hosts over 40 cold seeps with diverse morphologies [13], but hydrothermal activity has not been reported previously. Here, we use geochemical observations and subsequent analyses of microbes, macro-and megafaunal biota at this site to conduct in-depth exploration of the co-occurrence of hot vent and cold seep organisms from the perspective of ecological assemblage, phylogenetic affinities and trophic structure.…”