The Distribution of Dissolved Methane and Its Air-Sea Flux in the Plume of a Seep Field, Lingtou Promontory, South China Sea

Di, Pengfei; Dong, Feng; Chen, Duofu

doi:10.1155/2019/3240697

Cited by 4 publications

(4 citation statements)

References 56 publications

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“…However, methane in significant concentrations might also prevail in oilcoated bubbles up to the sea surface. The steep decrease of CH 4 concentrations from 30,000 nmol/L at 0.5 m above seafloor to 5 nmol/L at 72 m above seafloor illustrates the rapid dissolution of CH 4 from bubbles and its dispersion in the ocean, as similarity reported for a seep field in the South China Sea (Di et al, 2019). Detection of an additional methane concentration peak with much lower concentrations (13 nmol/L) at 2300 mbsl (∼1100 m above the seep sites) might result from sampling of water closer to the gas bubble stream ascending through the water column than for the samples above and below.…”

Section: Dissolved Methanesupporting

confidence: 72%

Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

et al. 2019

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This multidisciplinary study of the hydrocarbon seepage system at Tsanyao Yang Knoll (TYK) in the southern Gulf of Mexico illustrates the amount and fate of hydrocarbons (mainly oil and methane) emanating from the seafloor structure and rising through a 3400 m water column. TYK forms part of the Campeche Knolls and was found to be one of the most active seepage structures at such an exceptional depth. Combining ship-based and AUV-based hydroacoustic mapping with direct seafloor observations and investigations, which used a TV-sled and a remotely operated vehicle with gas and water sampling devices provided an integrated view for the various transport pathways of hydrocarbons from the seafloor to the sea surface. In total, 32 acoustic 'flares,' indicative of gas bubble emission sites, were detected emanating from depressions on top of the knoll. Most of the emission sites were concentrated in two depressions that comprised a main seep field. An estimated volume of 550-4650 L of hydrocarbons per hour (or 8300-70,600 mol CH 4 per hour) are released in the form of gas bubbles, which dissolve almost entirely during their rise in the water column. However, echograms showed gas anomalies to about 500 m below sea surface and some bubbles were seen to burst at the sea surface. Concentrations of dissolved methane were highly elevated (∼30,000 nmol/L) directly above the seafloor emission site, but decreased to background concentrations (3-5 nmol/L) within the lowermost 100 m. Smaller volume flow rates of oil also escaped from the seafloor, rose to the sea surface and generated natural oil slicks visible from the ship and in satellite images. This study shows that hydrocarbon seepage at ∼3400 m water depth can be followed to the sea surface. However, most of the methane dissolves in deeper waters, whereas oil reaches the sea surface.

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Section: Dissolved Methanesupporting

confidence: 72%

Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

et al. 2019

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“…Bubbles were observed by Di et al (2019); however, the seeps studied were much shallower (<20 m) than those in our study. The levels of dissolved methane measured by our study may be different than those measured by Di et al (2019) due to proximity to a plume, currents, depth, and plume intensity.…”

Section: Coupled Approach For Resolving Emissionscontrasting

confidence: 89%

Observations of Shallow Methane Bubble Emissions From Cascadia Margin

et al. 2021

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Open questions exist about whether methane emitted from active seafloor seeps reaches the surface ocean to be subsequently ventilated to the atmosphere. Water depth variability, coupled with the transient nature of methane bubble plumes, adds complexity to examining these questions. Little data exist which trace methane transport from release at a seep into the water column. Here, we demonstrate a coupled technological approach for examining methane transport, combining multibeam sonar, a field-portable laser-based spectrometer, and the ChemYak, a robotic surface kayak, at two shallow (<75 m depth) seep sites on the Cascadia Margin. We demonstrate the presence of elevated methane (above the methane equilibration concentration with the atmosphere) throughout the water column. We observe areas of elevated dissolved methane at the surface, suggesting that at these shallow seep sites, methane is reaching the air-sea interface and is being emitted to the atmosphere.

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“…In the Black Sea, ambient emissions arise from microbially produced CH 4 in shelf and slope sediments (Reeburgh et al, 1991). Di et al (2019) estimated 7.7 nmol m −2 s −1 for the shallow South China Sea based on an air-sea gas transfer model. If we disperse COP seep field atmospheric emissions of 1.15 × 10 9 M yr −1 over the ∼ 6.3 km 2 of 25 × 25 m 2 bins with emissions, we find 5.7 µM m −2 s −1 , 3 orders of magnitude greater.…”

Section: Methane and Nonmethane Hydrocarbon Emissionsmentioning

confidence: 99%

Long-term atmospheric emissions for the Coal Oil Point natural marine hydrocarbon seep field, offshore California

2021

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Abstract. In this study, we present a novel approach for assessing nearshore seepage atmospheric emissions through modeling of air quality station data, specifically a Gaussian plume inversion model. A total of 3 decades of air quality station meteorology and total hydrocarbon concentration, THC, data were analyzed to study emissions from the Coal Oil Point marine seep field offshore California. THC in the seep field directions was significantly elevated and Gaussian with respect to wind direction, θ. An inversion model of the seep field, θ-resolved anomaly, THC′(θ)-derived atmospheric emissions is given. The model inversion is for the far field, which was satisfied by gridding the sonar seepage and treating each grid cell as a separate Gaussian plume. This assumption was validated by offshore in situ data that showed major seep area plumes were Gaussian. Plume total carbon, TC (TC = THC + carbon dioxide, CO2, + carbon monoxide), 18 % was CO2 and 82 % was THC; 85 % of THC was CH4. These compositions were similar to the seabed composition, demonstrating efficient vertical plume transport of dissolved seep gases. Air samples also measured atmospheric alkane plume composition. The inversion model used observed winds and derived the 3-decade-average (1990–2021) field-wide atmospheric emissions of 83 400 ± 12 000 m3 THC d−1 (27 Gg THC yr−1 based on 19.6 g mol−1 for THC). Based on a 50 : 50 air-to-seawater partitioning, this implies seabed emissions of 167 000 m3 THC d−1. Based on atmospheric plume composition, C1–C6 alkane emissions were 19, 1.3, 2.5, 2.2, 1.1, and 0.15 Gg yr−1, respectively. The spatially averaged CH4 emissions over the ∼ 6.3 km2 of 25 × 25 m2 bins with sonar values above noise were 5.7 µM m−2 s−1. The approach can be extended to derive emissions from other dispersed sources such as landfills, industrial sites, or terrestrial seepage if source locations are constrained spatially.

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The Distribution of Dissolved Methane and Its Air-Sea Flux in the Plume of a Seep Field, Lingtou Promontory, South China Sea

Cited by 4 publications

References 56 publications

Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

Observations of Shallow Methane Bubble Emissions From Cascadia Margin

Long-term atmospheric emissions for the Coal Oil Point natural marine hydrocarbon seep field, offshore California

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