High Resolution Measurements of Methane and Carbon Dioxide in Surface Waters over a Natural Seep Reveal Dynamics of Dissolved Phase Air–Sea Flux

Abstract

Marine hydrocarbon seeps are sources of methane and carbon dioxide to the ocean, and potentially to the atmosphere, though the magnitude of the fluxes and dynamics of these systems are poorly defined. To better constrain these variables in natural environments, we conducted the first high-resolution measurements of sea surface methane and carbon dioxide concentrations in the massive natural seep field near Coal Oil Point (COP), California. The corresponding high resolution fluxes were calculated, and the total dissolved phase air-sea fluxes over the surveyed plume area (∼363 km(2)) were 6.66 × 10(4) to 6.71 × 10(4) mol day(-1) with respect to CH4 and -6.01 × 10(5) to -5.99 × 10(5) mol day(-1) with respect to CO2. The mean and standard deviation of the dissolved phase air-sea fluxes of methane and carbon dioxide from the contour gridding analysis were estimated to be 0.18 ± 0.19 and -1.65 ± 1.23 mmol m(-2) day(-1), respectively. This methane flux is consistent with previous, lower-resolution estimates and was used, in part, to conservatively estimate the total area of the dissolved methane plume at 8400 km(2). The influx of carbon dioxide to the surface water refutes the hypothesis that COP seep methane appreciably influences carbon dioxide dynamics. Seeing that the COP seep field is one of the biggest natural seeps, a logical conclusion could be drawn that microbial oxidation of methane from natural seeps is of insufficient magnitude to change the resulting plume area from a sink of atmospheric carbon dioxide to a source.

The interaction of climate change and methane hydrates

“…Constraining sea-air methane fluxes is the second component necessary for evaluating the interplay between climate warming and release of methane by marine gas hydrate dissociation. Scientists measure dissolved CH 4 concentration in surface waters using discrete [Reeburgh et al, 1991] or continuous [Du et al, 2014;Hu et al, 2012] sampling techniques and then apply the method of Wanninkhof [1992] to estimate the flux of CH 4 to the atmosphere at the sea-air interface. A major challenge in drawing conclusions about hydrate dissociation from these studies is the difficulty of tracing CH 4 in near-surface waters to a hydrate source.…”

Shallow-ocean methane leakage and degassing to the atmosphere: triggered by offshore oil-gas and methane hydrate explorations

“…Its reliability lies not only on gas transfer velocity and hydrodynamic conditions (Wanninkhof et al, 2009;Johnson et al, 2011), but also on the spatial resolution of sample collections. The surface CH 4 emission hot spots (i.e., areas of high flux) need to be covered (Du et al, 2014;Zhang et al, 2014). Compared to diffusion, ebullition of CH 4 -containing bubbles, especially a small fraction of the largest bubbles, may release larger amounts of CH 4 and exhibits much greater spatiotemporal variation (Greinert and Nützel, 2004;McGinnis et al, 2014;Xiao et al, 2014;DelSontro et al, 2015).…”

“…Nowadays preventing CH 4 release from vented and flared natural gas has received considerable attention (e.g., Elvidge et al, 2009;U.S. GAO, 2010), while the CH 4 degassing associated with episodic CH 4 leakage from seafloor is also identified (e.g., Du et al, 2014;Zhang et al, 2014). This mini review summarized recent progresses regarding the CH 4 leakage from seafloor caused by offshore oil-gas and marine methane hydrate explorations and its degassing to the atmosphere.…”

Global ocean methane emissions dominated by shallow coastal waters

“…The global ocean is a highly uncertain term in the atmospheric CH 4 budget, emitting 5–25 Tg of CH 4 per year (hereafter Tg yr −1 ) or 1–13% of all natural emissions 4 . The dominant source of this methane is traditionally thought to be the sea floor, where it is produced biologically in anoxic sediments 5 or released from geological reservoirs at hydrocarbon seeps 6 and degrading methane hydrate deposits 7 . Methane is emitted to the atmosphere by two processes: diffusive gas transfer and ebullition (i.e.…”