Postglacial response of Arctic Ocean gas hydrates to climatic amelioration

Serov, Pavel; Vadakkepuliyambatta, Sunil; Mienert, Jürgen; Patton, Henry; Portnov, Alexey; Silyakova, Anna; Panieri, Giuliana; Carroll, Michael L.; Carroll, JoLynn; Andreassen, Karin; Hubbard, Alun

doi:10.1073/pnas.1619288114

Cited by 110 publications

(146 citation statements)

References 68 publications

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“…This concentration is higher than what was observed in surface waters over shallow (50-120 m depth) seep sites west of Svalbard where Graves et al (2015) report surface water CH 4 concentrations < 52 nmol L −1 . Very high fluxes of CH 4 from sub-seabed sources to the atmosphere have also been reported for the East Siberian Arctic Shelf (ESAS) (Shakhova et al, 2014), with flux values of ∼ 70-450 nmol m −2 s −1 under stormy conditions with surface water concentrations of the order of 450 nmol L −1 . However, the emissions reported for ESAS were over shallow water, and bubble dissolution, gas exchange, water column stratification, and microbial oxidation would significantly diminish CH 4 concentrations in the surface mixed layer above bubble emission sites in water depth > 100 m (McGinnis et al, 2006;Graves et al, 2015;Mau et al, 2017).…”

Section: Ocean-atmosphere Emissions North Of Svalbardmentioning

confidence: 91%

“…In a widely used GH sampling technique, small hydrate pieces are transferred into glass vials containing an aqueous sodium hydroxide (NaOH) solution and sealed with a rubber stopper (e.g. Smith et al, 2014;Serov et al, 2017). Overpressure due to gases released from the sediments is reduced by exposing the sample to the atmosphere.…”

Section: Collection Of Gas Hydrate Samplesmentioning

confidence: 99%

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Methane at Svalbard and over the European Arctic Ocean

et al. 2018

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Methane (CH 4 ) is a powerful greenhouse gas. Its atmospheric mixing ratios have been increasing since 2005. Therefore, quantification of CH 4 sources is essential for effective climate change mitigation. Here we report observations of the CH 4 mixing ratios measured at the Zeppelin Observatory (Svalbard) in the Arctic and aboard the research vessel (RV) Helmer Hanssen over the Arctic Ocean from June 2014 to December 2016, as well as the longterm CH 4 trend measured at the Zeppelin Observatory from 2001 to 2017. We investigated areas over the European Arctic Ocean to identify possible hotspot regions emitting CH 4 from the ocean to the atmosphere, and used state-of-the-art modelling (FLEXPART) combined with updated emission inventories to identify CH 4 sources. Furthermore, we collected air samples in the region as well as samples of gas hydrates, obtained from the sea floor, which we analysed using a new technique whereby hydrate gases are sampled directly into evacuated canisters. Using this new methodology, we evaluated the suitability of ethane and isotopic signatures (δ 13 C in CH 4 ) as tracers for ocean-to-atmosphere CH 4 emission. We found that the average methane / light hydrocarbon (ethane and propane) ratio is an order of magnitude higher for the same sediment samples using our new methodology compared to previously reported values, 2379.95 vs. 460.06, respectively. Meanwhile, we show that the mean atmospheric CH 4 mixing ratio in the Arctic increased by 5.9 ± 0.38 parts per billion by volume (ppb) per year (yr −1 ) from 2001 to 2017 and ∼ 8 pbb yr −1 since 2008, similar to the global trend of ∼ 7-8 ppb yr −1 . Most large excursions from the baseline CH 4 mixing ratio over the European Arctic Ocean are due to long-range transport from land-based sources, lending confidence to the present inventories for high-latitude CH 4 emissions. However, we also identify a potential hotspot region with ocean-atmosphere CH 4 flux north of Svalbard (80.4 • N, 12.8 • E) of up to 26 nmol m −2 s −1 from a large mixing ratio increase at the location of 30 ppb. Since this flux is consistent with previous constraints (both spatially and temporally), there is no evidence that the area of interest north of Svalbard is unique in the context of the wider Arctic. Rather, because the meteorology at the time of the observation was unique in the context of the measurement time series, we obtained over the short course of the episode measurements highly sensitive to emissions over an active seep site, without sensitivity to land-based emissions.

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Section: Ocean-atmosphere Emissions North Of Svalbardmentioning

confidence: 91%

Section: Collection Of Gas Hydrate Samplesmentioning

confidence: 99%

Methane at Svalbard and over the European Arctic Ocean

et al. 2018

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“…At this location, texturally heterogeneous marine sediments overlie glacigenic diamictite. A large quantity of methane escaping from the sediments to the water column was documented recently (Mau et al, 2017;Serov et al, 2017). Small mound features were observed at the seafloor with gas hydrate recovered from very shallow sediment depths (Hong et al, 2017).…”

Section: Study Areamentioning

confidence: 97%

“…1), is an area where the ice sheet carved the seafloor during the last glaciation and exposed Middle-Triassic bedrock (Long et al, 1998;Andreassen et al, 2017). Densely-distributed craters have recently been reported in this area with high methane concentration in the water column (Long et al, 1998;Andreassen et al, 2017). Only a thin layer of sediments above the bedrock is present on the seafloor.…”

Section: Study Areamentioning

confidence: 99%

Dynamic interactions between iron and sulfur cycles from Arctic methane seeps

Latour

Hong

Sauer

et al. 2018

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Bioavailable iron is an important micro-nutrient for marine phytoplankton and therefore critical to global biogeochemical cycles. Anoxic marine sediment is a significant source of Fe(II) to the ocean. Here, we investigate how the fluxes of Fe(II), both towards the sedimentary oxic layer and across the sediment-water interface, are impacted by the high concentration and flux of porewater sulfide in cold seep environments. We present new porewater data from four recently documented cold 15 seeps around Svalbard as well as from continental shelves and fjords in northern Norway. We quantitatively investigated porewater data first by calculating the Fe(II) fluxes towards oxidized surface sediments and bottom water and, second, applied a transport-reaction model to estimate the mass balance of several key chemical species. Sedimentary sulfur speciation data from two of the sites were used to constrain Fe(II) consumption in the shallow sediments. We showed that the iron reduction zone is usually confined to the top 10 cm of the sediments from our studied sites due to high sulfate 20 turnover and therefore high sulfide flux. Such a thin iron reduction zone allows proportionally more Fe(II) to reach the bottom water. Rapid precipitation of pyrite occurs at the base of the iron reduction zone, where the downward diffusing Fe(II) meets upward migrating hydrogen sulfide. Dissolved H 2 released during pyrite formation stimulates a small but significant rate of sulfate reduction in the same horizon, which results in faster production of hydrogen sulfide and a positive feedback for iron reduction in the shallow sediment. Deeper in the sediment, where sulfate is actively consumed due to 25 anaerobic methane oxidation, no apparent formation of pyrite is observed from the available measurements and our modeling results. This is mostly due to the relatively low availability of Fe(II) as a result of slower turnover of the less active iron mineral phases. Such an observation may contradict the use of pyrite abundance to deduce the sulfate-methanetransition-zone in past sedimentary records. A series of model sensitivity tests were performed to systematically investigate how the Fe(II) dynamics is impacted by higher deposition rate of iron (oxyhydr)oxides minerals on the seafloor and 30 intensifying methane supply. We showed that the increases in iron reduction rate, pyrite formation rate, and Fe(II) flux are expected with higher seafloor iron (oxyhydr)oxides deposition initially. However, complicated feedbacks between Fe(II) Biogeosciences Discuss., https://doi.2 production and sulfate reduction pose negative feedbacks to pyrite formation in the sediments. With a larger supply of methane, Fe(II) flux towards the oxic surface sediments is initially intensified by the higher production of hydrogen sulfide until such an interplay is too fast that essentially all reactive iron minerals settled on the seafloor dissolve immediately and dissolved iron is fixed through pyrite precipitation. Such an interplay between Fe(II) and sulfide determi...

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“…Several sites of methane seepage have been discovered on the continental shelf offshore Svalbard and in the northwest Barents Sea (Andreassen et al, 2017;Åström et al, 2016;Portnov et al, 2016;Sahling et al, 2014;). An abundance 20 of cold seeps in the Arctic is important, because the Arctic is connected to both the Pacific and the Atlantic Oceans.…”

Section: Introductionmentioning

confidence: 99%

Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep

Sen¹,

Åström²,

Hong³

et al. 2018

Preprint

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Cold seep communities around gas hydrate mounds (pingos) in the Western Barents Sea (76°N, 16°E, ~400 m depth) were investigated with high resolution, geographically referenced images acquired with an ROV and towed camera. Four pingos associated with seabed methane release hosted diverse biological communities of mainly non-seep (background) species including commercially important fish and crustaceans, as well as a species new to this area (the snow crab Chionoecetes 15 opilio). We attribute the presence of most benthic community members to habitat heterogeneity and the occurrence of hard substrates (methane derived authigenic carbonates), particularly the most abundant phyla (Cnidaria and Porifera), though food availability and exposure to a diverse microbial community is also important for certain taxa. Only one chemosynthesis based species was confirmed, the siboglinid frenulate polychaete, Oligobrachia haakonmosbiensis. Overall, the pingo communities formed two distinct clusters, distinguished by the presence or absence of frenulates. Methane gas advection through sediments 20 was absent below the single pingo that lacked frenulates, while seismic profiles indicated gas saturated sediment below the other frenulate colonized pingos. The absence of frenulates could not be explained by sediment sulfide concentrations, despite these worms likely containing sulfide oxidizing symbionts. We propose that high levels of seafloor methane seepage linked to sub-surface gas reservoirs support an abundant and active sediment methanotrophic community that maintains high sulfide fluxes and serves as a carbon source for frenulate worms. The pingo currently lacking a sub-surface gas source and lower 25 methane concentrations has lower sulfide flux rates and limited amounts of carbon insufficient to support frenulates. Two previously undocumented behaviors were visible through the images: grazing activity of snow crabs on bacterial mats, and seafloor crawling of Nothria conchylega onuphid polychaetes.Biogeosciences Discuss., https://doi

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Postglacial response of Arctic Ocean gas hydrates to climatic amelioration

Cited by 110 publications

References 68 publications

Methane at Svalbard and over the European Arctic Ocean

Methane at Svalbard and over the European Arctic Ocean

Dynamic interactions between iron and sulfur cycles from Arctic methane seeps

Geophysical and geochemical controls on the megafaunal community of a high Arctic cold seep

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