When wind speeds are 2–10 m s −1 , reflective contrasts in the ocean surface make oil slicks visible to synthetic aperture radar (SAR) under all sky conditions. Neural network analysis of satellite SAR images quantified the magnitude and distribution of surface oil in the Gulf of Mexico from persistent, natural seeps and from the Deepwater Horizon (DWH) discharge. This analysis identified 914 natural oil seep zones across the entire Gulf of Mexico in pre‐2010 data. Their ∼0.1 µm slicks covered an aggregated average of 775 km 2 . Assuming an average volume of 77.5 m 3 over an 8–24 h lifespan per oil slick, the floating oil indicates a surface flux of 2.5–9.4 × 10 4 m 3 yr −1 . Oil from natural slicks was regionally concentrated: 68%, 25%, 7%, and <1% of the total was observed in the NW, SW, NE, and SE Gulf, respectively. This reflects differences in basin history and hydrocarbon generation. SAR images from 2010 showed that the 87 day DWH discharge produced a surface‐oil footprint fundamentally different from background seepage, with an average ocean area of 11,200 km 2 (SD 5028) and a volume of 22,600 m 3 (SD 5411). Peak magnitudes of oil were detected during equivalent, ∼14 day intervals around 23 May and 18 June, when wind speeds remained <5 m s −1 . Over this interval, aggregated volume of floating oil decreased by 21%; area covered increased by 49% ( p < 0.1), potentially altering its ecological impact. The most likely causes were increased applications of dispersant and surface burning operations.

Natural hydrocarbon seeps are ubiquitous along continental margins. Despite their significance, we lack a basic understanding of the long-term temporal variability of seep dynamics, including bubble size, rise velocity, composition, and upwelling and entrainment processes. The shortcoming makes it difficult to constrain the global estimates of oil and gas entering the marine environment. Here we report on a multi-method approach based on optical, acoustic, satellite remote sensing, and simulations, to connect the characteristics of a hydrocarbon seep in the Gulf of Mexico to its footprint on the sea surface. Using an in-situ camera, bubble dynamics at the source were measured every 6 h over 153 days and the integrated total hydrocarbon release volume was estimated as 53 m 3. the vertical velocity was acoustically measured at 20 m above bed (mab) and found to be approximately 40% less than the dispersed-phase at the source, indicating that the measured values are reflecting the plume continuous-phase flow. Numerical simulations predict that the oily bubbles with diameters larger than 8 mm reach the surface with a small footprint, i.e. forming an oil slick origin, deflection of which with wind and surface current leads to the formation of an oil slick on the surface. Nineteen SAR images are used to estimate the oil seepage rate from GC600 for 2017 giving an average discharge of 14.4 cm 3 /s. Natural hydrocarbon seeps have been reported from a broad range of oceanographic settings from the coast to the deep ocean over a wide variety of geological environments that affect the biosphere, the hydrosphere, and the atmosphere 1. Beside the anthropogenic sources, hydrocarbon seeps are the single most important natural source of oil and methane (CH 4) that enter the ocean 2. Best estimates indicate that 3 to 48 Tg/y of geo-CH 4 is released from marine seeps alone 3,4. Recent global estimates of crude oil seepage range from 0.2 to 2 Mt with a best estimate of 0.6 Mt that accounts for 47% of the global estimate. The large uncertainties in estimates of hydrocarbons released from offshore marine sources are due to a wide range of factors including size, rise velocity, contamination by surfactants such as gas hydrates, composition, upwelling and entrainment effects, and interaction of gaseous bubbles and oil droplets with the local ocean environment that vary constantly with time. To quantify the bubble/droplet size and rise velocity and the corresponding emission rate of hydrocarbons from the seafloor, investigators have primarily relied on snapshot acoustic 5-9 or optical 10-13 measurements. Based on these measurements, significant changes in hydrocarbon seepage and bubble venting have been inferred to occur over intervals of months to years 14. Sufficiently long time series of seepage characteristics rarely exist to conclusively address the day-today variability of bubble size and rise velocity or the spatial changes in vent location over an extended period. Time variation in seep emissions is of particular interest. It impl...

of natural Hydrocarbon seepage in Mississippi canyon, gulf of Mexico, DeepSea Research Part II, http://dx. AbstractAnalysis of the magnitude of oil discharged from natural hydrocarbon seeps can improve understanding of the carbon cycle and the Gulf of Mexico (GOM) ecosystem. With use of a large archive of remote sensing data, in combination with geophysical and multibeam data, we identified, mapped, and characterized natural hydrocarbon seeps in the Macondo prospect region near the wreck site of the drill-rig Deepwater Horizon (DWH). Satellite image processing and the cluster analysis revealed locations of previously undetected seep zones. Including duplicate detections, a total of 562 individual gas plumes were also observed in multibeam surveys. In total, SAR imagery confirmed 52 oil-producing SZ in the study area. In almost all cases gas plumes were associated with oil-producing seep zones. The cluster of seeps in the vicinity of lease block MC302 appeared to host the most persistent and prolific oil vents. Oil slicks and gas plumes observed over the DWH site were consistent with discharges of residual oil from the wreckage. In contrast with highly persistent oil seeps observed in the Green Canyon and Garden Banks lease areas, the seeps in the vicinity of Macondo Prospect were intermittent. The difference in the number of seeps and the quantity of surface oil detected in Green Canyon was almost two orders of magnitude greater than in Mississippi Canyon. Garcia et al. Hydrocarbon Seepage in Mississippi Canyon

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