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δ(13)C-CH4 reveals CH4 variations over oceans from mid-latitudes to the Arctic.

Yu J, Xie Z, Sun L, Kang H, He P, Xing G - Sci Rep (2015)

Bottom Line: There were complex mixing sources outside and inside the Arctic Ocean.A keeling plot showed the dominant influence by hydrate gas in the Nordic Sea region, while the long range transport of wetland emissions were one of potentially important sources in the central Arctic Ocean.Experiments comparing sunlight and darkness indicate that microbes may also play an important role in regional variations.

View Article: PubMed Central - PubMed

Affiliation: Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 230026.

ABSTRACT
The biogeochemical cycles of CH4 over oceans are poorly understood, especially over the Arctic Ocean. Here we report atmospheric CH4 levels together with δ(13)C-CH4 from offshore China (31°N) to the central Arctic Ocean (up to 87°N) from July to September 2012. CH4 concentrations and δ(13)C-CH4 displayed temporal and spatial variation ranging from 1.65 to 2.63 ppm, and from -50.34% to -44.94% (mean value: -48.55 ± 0.84%), respectively. Changes in CH4 with latitude were linked to the decreasing input of enriched δ(13)C and chemical oxidation by both OH and Cl radicals as indicated by variation of δ(13)C. There were complex mixing sources outside and inside the Arctic Ocean. A keeling plot showed the dominant influence by hydrate gas in the Nordic Sea region, while the long range transport of wetland emissions were one of potentially important sources in the central Arctic Ocean. Experiments comparing sunlight and darkness indicate that microbes may also play an important role in regional variations.

No MeSH data available.


Variations of sunlight intensity in July and September over the OC, JS, NPO, BS, and CS regions.The error bars represent the positive standard deviation.
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f5: Variations of sunlight intensity in July and September over the OC, JS, NPO, BS, and CS regions.The error bars represent the positive standard deviation.

Mentions: The phase of δ13C-CH4 in the seasonal cycle is consistent with the kinetic isotope effect (KIE), which is due to OH and/or Cl radicals oxidizing δ12C-CH4 faster than δ13C-CH4, resulting in atmospheric methane enriched in δ13C-CH49. From mid- to high latitudes, the δ13C-CH4 showed a slight decreasing trend with latitude in sunlight. The latitudinal loss of δ13C-CH4 may be due to decreasing enriched input of δ13C or chemical oxidation. Considering the potential fuel source influences, the variation of CO with latitude and the air masses of all the samples are shown in Figure S1. The CO concentrations showed a decreasing trend up to about 50°N, indicating a decrease in local contribution by anthropogenic sources, such as fossil fuels, north of 50°N. The back-trajectories of the air masses further confirmed the influence of continental sources of CO in the OC and JS areas (<50°N). However, the latitudinal decreasing trend of δ13C-CH4 remained under background air, suggesting the potential role of oxidation. It has been reported that OH radicals in the troposphere are the primary sink for global atmospheric CH421. Cl radicals may also contribute to CH4 loss over oceans. To determine the potential reactive process for CH4, the contributions of the OH and Cl radicals were calculated as follows. The average concentrations of the OH and Cl radicals in the marine boundary layer are about 7 × 105–2.9 × 106 molecules·cm−3 and 1.8 × 104 molecules·cm−3, respectively22232425; mean CH4 concentration is 1.88 ppm; and the rate constant at 8 °C based on the mean sampling temperature for OH and Cl radicals is 4.42 × 10−15 and 7.59 × 10−14 cm−3·molecules−1·S−1, respectively26. Assuming the reactive height is 25 m, based on the sampling height, the CH4 consumption for OH and Cl radicals is 8.72 × 10−3–3.61 × 10−2 mg·m−2·d−1 and 3.85 × 10−3 mg·m−2·d−1, respectively. The effect of Cl radicals on CH4 is similar to that of the OH radicals. It is unclear if the level of Cl radicals varies with latitude. However, OH radicals can decrease from low to high latitudes2327. In addition, sunlight intensity at the high latitudes is lower than in the mid-latitudes. Thus, reduced oxidation in the high-latitude region might result in depleted δ13C-CH4. A similar principle could explain higher values of δ13C-CH4 in July compared to September over the CS area. The significantly higher sunlight intensity over the CS area in July compared with September indicated stronger oxidation potential (Fig. 5). However, the variations between July and September over the OC, JS, BS and NPO regions were not consistent with oxidation results. Most regions outside the Arctic Ocean were influenced by continental sources, and complex sources inputs may influence the oxidation results (Figure S1).


δ(13)C-CH4 reveals CH4 variations over oceans from mid-latitudes to the Arctic.

Yu J, Xie Z, Sun L, Kang H, He P, Xing G - Sci Rep (2015)

Variations of sunlight intensity in July and September over the OC, JS, NPO, BS, and CS regions.The error bars represent the positive standard deviation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4555175&req=5

f5: Variations of sunlight intensity in July and September over the OC, JS, NPO, BS, and CS regions.The error bars represent the positive standard deviation.
Mentions: The phase of δ13C-CH4 in the seasonal cycle is consistent with the kinetic isotope effect (KIE), which is due to OH and/or Cl radicals oxidizing δ12C-CH4 faster than δ13C-CH4, resulting in atmospheric methane enriched in δ13C-CH49. From mid- to high latitudes, the δ13C-CH4 showed a slight decreasing trend with latitude in sunlight. The latitudinal loss of δ13C-CH4 may be due to decreasing enriched input of δ13C or chemical oxidation. Considering the potential fuel source influences, the variation of CO with latitude and the air masses of all the samples are shown in Figure S1. The CO concentrations showed a decreasing trend up to about 50°N, indicating a decrease in local contribution by anthropogenic sources, such as fossil fuels, north of 50°N. The back-trajectories of the air masses further confirmed the influence of continental sources of CO in the OC and JS areas (<50°N). However, the latitudinal decreasing trend of δ13C-CH4 remained under background air, suggesting the potential role of oxidation. It has been reported that OH radicals in the troposphere are the primary sink for global atmospheric CH421. Cl radicals may also contribute to CH4 loss over oceans. To determine the potential reactive process for CH4, the contributions of the OH and Cl radicals were calculated as follows. The average concentrations of the OH and Cl radicals in the marine boundary layer are about 7 × 105–2.9 × 106 molecules·cm−3 and 1.8 × 104 molecules·cm−3, respectively22232425; mean CH4 concentration is 1.88 ppm; and the rate constant at 8 °C based on the mean sampling temperature for OH and Cl radicals is 4.42 × 10−15 and 7.59 × 10−14 cm−3·molecules−1·S−1, respectively26. Assuming the reactive height is 25 m, based on the sampling height, the CH4 consumption for OH and Cl radicals is 8.72 × 10−3–3.61 × 10−2 mg·m−2·d−1 and 3.85 × 10−3 mg·m−2·d−1, respectively. The effect of Cl radicals on CH4 is similar to that of the OH radicals. It is unclear if the level of Cl radicals varies with latitude. However, OH radicals can decrease from low to high latitudes2327. In addition, sunlight intensity at the high latitudes is lower than in the mid-latitudes. Thus, reduced oxidation in the high-latitude region might result in depleted δ13C-CH4. A similar principle could explain higher values of δ13C-CH4 in July compared to September over the CS area. The significantly higher sunlight intensity over the CS area in July compared with September indicated stronger oxidation potential (Fig. 5). However, the variations between July and September over the OC, JS, BS and NPO regions were not consistent with oxidation results. Most regions outside the Arctic Ocean were influenced by continental sources, and complex sources inputs may influence the oxidation results (Figure S1).

Bottom Line: There were complex mixing sources outside and inside the Arctic Ocean.A keeling plot showed the dominant influence by hydrate gas in the Nordic Sea region, while the long range transport of wetland emissions were one of potentially important sources in the central Arctic Ocean.Experiments comparing sunlight and darkness indicate that microbes may also play an important role in regional variations.

View Article: PubMed Central - PubMed

Affiliation: Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 230026.

ABSTRACT
The biogeochemical cycles of CH4 over oceans are poorly understood, especially over the Arctic Ocean. Here we report atmospheric CH4 levels together with δ(13)C-CH4 from offshore China (31°N) to the central Arctic Ocean (up to 87°N) from July to September 2012. CH4 concentrations and δ(13)C-CH4 displayed temporal and spatial variation ranging from 1.65 to 2.63 ppm, and from -50.34% to -44.94% (mean value: -48.55 ± 0.84%), respectively. Changes in CH4 with latitude were linked to the decreasing input of enriched δ(13)C and chemical oxidation by both OH and Cl radicals as indicated by variation of δ(13)C. There were complex mixing sources outside and inside the Arctic Ocean. A keeling plot showed the dominant influence by hydrate gas in the Nordic Sea region, while the long range transport of wetland emissions were one of potentially important sources in the central Arctic Ocean. Experiments comparing sunlight and darkness indicate that microbes may also play an important role in regional variations.

No MeSH data available.