Researchers find global ocean methane emissions dominated by shallow coastal waters

"Methane is a potent greenhouse gas that is being added to the atmosphere through both natural processes and human activities, such as energy production and agriculture.

To predict the impacts of human emissions, researchers need a complete picture of the atmosphere's methane cycle. They need to know the size of the inputs—both natural and human—as well as the outputs. They also need to know how long methane resides in the atmosphere. [...]"


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Global ocean methane emissions dominated by shallow coastal waters


"Oceanic emissions represent a highly uncertain term in the natural atmospheric methane (CH4) budget, due to the sparse sampling of dissolved CH4 in the marine environment. Here we overcome this limitation by training machine-learning models to map the surface distribution of methane disequilibrium (∆CH4). Our approach yields a global diffusive CH4 flux of 2–6TgCH4yr−1 from the ocean to the atmosphere, after propagating uncertainties in ∆CH4 and gas transfer velocity.  [...]"

Source: Nature Communications
Authors: Thomas Weber, Nicola A. Wiseman & Annette Kock 
DOI: 10.1038/s41467-019-12541-7

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Proterozoic seawater sulfate scarcity and the evolution of ocean–atmosphere chemistry


"Oceanic sulfate concentrations are widely thought to have reached millimolar levels during the Proterozoic Eon, 2.5 to 0.54 billion years ago. Yet the magnitude of the increase in seawater sulfate concentrations over the course of the Eon remains largely unquantified. A rise in seawater sulfate concentrations has been inferred from the increased range of marine sulfide δ34S values following the Great Oxidation Event and was induced by two processes: enhanced oxidative weathering of sulfides on land, and the onset of marine sulfur redox cycling. [...]"

Source: Nature Geoscience
Authors: Mojtaba Fakhraee et al.
DOI: 10.1038/s41561-019-0351-5

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Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane


"Continued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 106 tons) of methane from thawing subsea permafrost on shallow continental shelves and dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO2) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous sea−air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (−33,300 ± 7,900 μmol m−2⋅d−1) twice that of surrounding waters and ∼1,900 times greater than the diffusive sea−air methane efflux (17.3 ± 4.8 μmol m−2⋅d−1).  [...]"

Source: Proceedings of the Nathional Academy of Sciences of the United States of America (PNAS)
Authors: John W. Pohlman et al.
DOI: 10.1073/pnas.1618926114

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