Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30% of the energy released from plate cooling, and for 70% of the global heat flow anomaly (the difference between observed thermal output and that predicted by conductive cooling models). Hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 Myr old. But in view of the relatively impermeable sediments that cover most ridge flanks, it has been difficult to explain how these fluids transport heat from the crust to the ocean. Here we present results of swath mapping, heat flow, geochemistry and seismic surveys from the young eastern flank of the Juan de Fuca ridge, which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km. Our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops. We find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies.

[1] The nature of ridge-flank hydrothermal circulation guided by basement outcrops protruding through thick sediments is constrained on the eastern flank of the Juan de Fuca Ridge using combined bathymetric, seismic, and thermal observations and analytical and numerical calculations of coupled fluid-heat flow. Observational data near the western edge of the survey area indicate that young, cool hydrothermal fluids circulate rapidly through upper basement, probably both across-strike and along-strike of dominant structural trends. Data from the eastern end of survey coverage (Second Ridge (SR)) indicate that upper basement is regionally nearly isothermal. A small number of basement outcrops in this area host focused hydrothermal discharge, as do additional basement outcrops to the north and south of the SR area. Numerical models of individual recharging and discharging outcrops, patterned after the Baby Bare and Grizzly Bare outcrops, suggest that local convection alone cannot explain observed patterns of seafloor heat flux near these features. Forced-flow simulations show that reasonable rates of hydrothermal recharge and discharge, inferred from independent observations, can explain nearby seafloor heat flux, provided that upper basement permeability within and near the outcrops falls within a range of 10 À13 to 10 À11 m 2 . Freeflow simulations of fluid circulation between paired outcrops separated by 50 km, as are Baby Bare and Grizzly Bare outcrops, are most consistent with observations when regional basement permeability is 10 À11 to 10 À10 m 2 . These simulations illustrate how sensitive these systems are to selection of appropriate properties and boundary and initial conditions.

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[1] Evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 kHz echosounder records ('flares') was obtained at the convergent Makran continental margin. The hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column. Dives with the remotely operated vehicle 'Quest 4000 m' revealed that several individual bubble vents contributed to one hydroacoustic anomaly. Analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin. Bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume. We found that a low volume flux (Flare 1 at 575 mbsl: 90 ml/min) caused a weak hydroacoustic signal in echograms whereas high volume fluxes (Flare 2 at 1027 mbsl: 1590 ml/min; Flare 5 C at 2870 mbsl: 760 ml/min) caused strong anomalies. The total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies. An order-of-magnitude estimate further considers the temporal variability of some of the flares, assuming a constant flux over time, and allows a large range of uncertainty inherent to the method. Our results on the fate of bubbles and the order-of-magnitude estimate suggest that all of the $40 AE 32 Â 10 6 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean.Citation: Römer, M., H. Sahling, T. Pape, G. Bohrmann, and V. Spieß (2012), Quantification of gas bubble emissions from submarine hydrocarbon seeps at the Makran continental margin (offshore Pakistan),

Shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep. We investigated the controls on methane production, methane accumulation, and gas bubble formation along a 400 m seismo-acoustic transect across a sharp transition from gas-free into gas-bearing sediment in Aarhus Bay (Denmark). Twelve gravity cores were taken, in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by 35 SO 4 2radiotracer method. The thickness of organic-rich Holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate-methane transition from >4 m to 2.5 m. In spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect, there were only small differences in total mineralization, and methanogenesis was only equivalent to about 1 % of sulfate reduction. Shallow gas appeared where the mud thickness exceeded 8-9 m. Rates of methanogenesis increased along the transect as did the upward diffusive flux of methane. Interestingly, the increase in the sedimentation rate and Holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments. This increase in methane flux, however, triggered a shallowing of the sulfate-methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone. Thus, our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic-rich mud is exceeded.