Multifrequency geoacoustic imaging of fluid escape structures offshore Costa Rica: Implications for the quantification of seep processes

Abstract

[1] Quantification of fluid fluxes from cold seeps depends on accurate estimates of the spatial validity of flux measurements. These estimates are strongly influenced by the choice of geoacoustic mapping tools. Multibeam bathymetry, side-scan sonar, and Chirp subbottom profiler data of several mound-shaped cold seeps offshore central Costa Rica show great variety in morphology and structure although the features are only a few kilometers apart. Mound 11 (a 35 m high and 1000 m in diameter structure), situated in the SE of the study area, has an irregular morphology but a smooth surface on side-scan sonar data, while mound 12 (30 m high, 600 m across) is a cone of more regular outline but with a rough surface, and mound Grillo (5 m high, 500 m across) shows the same rough surface as mound 12 but without relief. Video observations and sediment cores indicate that the structures are formed by the precipitation of authigenic carbonates and indications for extensive mud extrusion are absent, except for one possible mudflow at mound 11. Different sonar frequencies result in variable estimates of the extent of these mounds with low frequencies suggesting much wider cold seeps, consequently overestimating fluid fluxes. The absence of mud volcanism compared to accretionary prisms where mud volcanism occurs is related to different tectonic styles: strong sediment overpressure and thrust faulting in typical accretionary prisms can generate mud volcanism, while subduction erosion and normal faulting (extension) of the overriding plate at the Costa Rican margin result in fluid venting driven by only slight fluid overpressures.

Carbonate-hosted methanotrophy represents an unrecognized methane sink in the deep sea

“…Porosity and permeability heavily influence the microbial habitability of rock-hosted environments by determining both the habitable volume and the dynamics of chemical substrate delivery (CH 4 , SO 4 2 À and nutrients) and end-product removal (e.g., H 2 S and HCO 3 À ). The measured permeability values, which indicate connected pore spaces within rock (Supplementary Table 2), ranged over three orders of magnitude.…”

“…The resulting precipitation products may vary in size, ranging from microcrystals 15 to extensive structures of pavements, slabs and mounds produced by upward growth (known as chemoherms) or post-formation winnowing 14,16,17 . Carbonate volumes have been estimated for a few regions of active seepage and may exceed a million cubic metres at areas such as the Costa Rica margin 4 , Hydrate Ridge 6 and the Chilean subduction zone 5 , suggesting that these methane-derived structures have the potential to serve as a significant habitat within the seep ecosystem.…”

“…Microbiological study of AOM at methane seeps has focused almost exclusively on sediment-based habitats, where the most energetically favourable geochemical horizon, the sulphate-methane transition zone, typically occurs in the top several centimetres 3 . However, sediment accounts for only a portion of the seafloor cover at methane seep structures: based on sidescan sonar imagery and seafloor video observations, substantial portions of actively seeping areas are capped by carbonate rock precipitate [4][5][6][7] . The presence and potential activity of extant microbial communities within these carbonate rocks and their impact on global methane dynamics have so far been unexplored.…”

“…When subseafloor environments are considered, well-lithified rock constitutes the volumetrically dominant physical substrate at seep-associated mounds [4][5][6]14 . Although some endolithic organisms appear to gain nutrients from their host rock 22 , the self-sealing effect of microbially accelerated authigenic carbonate formation implies that the supply of energy and nutrients may be limited in the seep-associated endolithic habitat, offering a valuable comparison with the better-studied sediment-hosted methane seep communities.…”

“…Biologically mediated, sulphate-coupled anaerobic oxidation of methane (AOM) is the dominant sink for methane in the oceans, consuming an estimated 382 Tg CH 4 per year in seafloor sediments, a quantity equivalent to B88% of the methane released from subsurface reservoirs 2 . Microbiological study of AOM at methane seeps has focused almost exclusively on sediment-based habitats, where the most energetically favourable geochemical horizon, the sulphate-methane transition zone, typically occurs in the top several centimetres 3 .…”