Methane at Svalbard and over the European Arctic Ocean

Abstract

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.

High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

“…Methane (CH 4 ) release from gas-bearing ocean sediments has been of high interest for many years (e.g. Westbrook et al, 2009;Ferré et al, 2012;Ruppel and Kessler, 2016;Jørgensen et al, 1990;Boetius and Wenzhöfer, 2013;Myhre et al, 2016;Platt et al, 2018). Once released and dissolved in the water column, the CH 4 gas diffuses and is partly oxidized in the water column (Reeburgh, 2007), contributing to minimum oxygen zones (Boetius and Wenzhöfer, 2013) and possibly to ocean acidification (Biastoch et al, 2011).…”

“…However, most studies of shallow CH 4 seepage sites have found no or little CH 4 flux to the atmosphere (e.g. Miller et al, 2017;Platt et al, 2018;Myhre et al, 2016;Gentz et al, 2014).…”

Biogeochemical Consequences of Nonvertical Methane Transport in Sediment Offshore Northwestern Svalbard

“…Over the past decade, several field studies reported methane release from the shallow seafloor of the Arctic Ocean (e.g., Mau et al, 2017; Shakhova et al, 2014, 2010; Thornton et al, 2016; Veloso‐Alarcón et al, 2019; Westbrook et al, 2009). Their sources, sinks, and potential connection to gas hydrate dissociation have been the focus of several follow‐up studies and discussions (Berndt, Dumke, et al, 2014; Berndt, Feseker, et al, 2014; Graves et al, 2017, 2015; James et al, 2016; Myhre et al, 2016; Platt et al, 2018; Riedel et al, 2018; Ruppel & Kessler, 2017; H. Sahling et al, 2014; Steinle et al, 2015; M Torres & Colwell, 2018; Wallmann et al, 2018). Different models have also been applied to investigate past, current, and future methane releases from the Arctic seafloor (Biastoch et al, 2011; Crémière et al, 2016; Marín‐Moreno et al, 2015, 2013; Reagan & Moridis, 2009; Thatcher et al, 2013).…”

High-resolution under-water laser spectrometer sensing provides new insights to methane distribution at an Arctic seepage site

“…Methane (CH4) release from gas bearing ocean sediments has been of high interest for many years (e.g. Jørgensen et al, 1990;25 Westbrook et al, 2009;Ferré et al, 2012;Boetius and Wenzhöfer, 2013;Myhre et al, 2016;Ruppel and Kessler, 2016;Platt et al, 2018). Once released and dissolved in the water column, the CH4 gas diffuses and is partly oxidized in the water column (Reeburgh, 2007), contributing to ocean acidification (Biastoch et al, 2011) and minimum oxygen zone formation (Boetius and Wenzhöfer, 2013).…”

“…However, most studies of shallow CH4 seepage sites have found no or little CH4 flux to the atmosphere (e.g. Gentz et al, 2014;Myhre et al, 2016;Miller et al, 2017;Platt et al, 2018).…”