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In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.

[1] Submersible investigations of the Cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence. Acoustic profiles of these bubble plumes in the water column in the vicinity of Hydrate Ridge offshore Oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column. The gas vent sites remained active over the span of two years, but varied dramatically on time scales of a few hours. All plumes emanated from local topographic highs near the summit of ridge structures. The acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m, independent of the seafloor depth. This coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m, suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution. The upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone.

Active fluid and gas transport were measured and observed along more than 200 km of the convergent margin of Costa Rica during cruise SO144-2 aboard RV Sonne. Ten profiles were run with the TV-sled OFOS, eight of which detected the dense occurrence of cold vent sites. This discovery shows that seafloor fluid expulsion is widely spread along the Pacific margin of Costa Rica. Surficial evidence of fluid expulsion is indicated by the appearance of chemosynthetic vent organisms such as bacterial mats, vesicomyid, solemyid and mytilid bivalves and tubeworms. Numerous active vents were indicated by elevated methane concentrations (£ 200 nmol L -1 ) in the bottom water. Although fluid-venting activity was known previously from a small area south of Nicoya Peninsula, the present study documents active seepage at landslides, headwall scarps related to seamount subduction, morphological intersections of faults and mid-slope mud volcanoes.

This work is distributed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Contents 1 Background and objectives 3 Operations 7 Lithostratigraphy 21 Biostratigraphy 24 Paleomagnetism 27 Structural geology 33 Geochemistry 37 Physical properties 40 Downhole measurements 42 Logging while drilling 49 Core-log-seismic integration 56 Observatory 61 References

[1] To constrain the fluxes of methane (CH 4 ) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (d 13 C-CH 4 ). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH 4 has a light d 13 C-CH 4 biogenic signature (À63 to À66% PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L À1 , vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH 4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (À56 to À28% PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (À25 to À6% PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of ''downcurrent'' plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 Â 10 4 mol h À1 for the working area (1230 km 2 ), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 Â 10 4 mol h À1 , and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally.

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The crest of Hydrate Ridge harbors a variety of gas hydrates in near-surface sediments. Hydrate formation and destruction continuously shape the ridge topography. Interstitial Cl-anomaly patterns in conjunction with video-guided sampling have established that the uppermost sediment column contains several distinct layers of gas hydrate which are exposed at the sea floor. A methaneoxidizing bacterial consortium populates the exposures of hydrate; colonies of vent macro-fauna are abundant as well. Discharge of methane from destabilized hydrate at the seafloor stimulates high rates of benthic oxygen consumption. These rates, however, vary by many orders of magnitude spatially and temporally, highlighting the need for implementing seafloor observatories at gas hydrate sites. Two types of hydrate fabrics were observed: A highly porous fabric with an estimated pore space of approx. 60 vol.-% and a massive type, with no visible pore space. Both types contain varying amounts of chloride, which need to be taken into account when estimating hydrate volumes from Cldepletion of pore waters. The porous hydrate has low bulk density, which may cause periodic release of large chunks of hydrate from the sea floor. They float to the surface and leave behind a chaotic topography of mounds and depressions. These pieces of floating hydrates constitute an important transport mechanism for methane from the seafloor directly to the atmosphere.