River Inflow Dominates Methane Emissions in an Arctic Coastal System

Abstract

We present a year-round time series of dissolved methane (CH 4 ), along with targeted observations during ice melt of CH 4 and carbon dioxide (CO 2 ) in a river and estuary adjacent to Cambridge Bay, Nunavut, Canada. During the freshet, CH 4 concentrations in the river and ice-covered estuary were up to 240,000% saturation and 19,000% saturation, respectively, but quickly dropped by >100-fold following ice melt. Observations with a robotic kayak revealed that river-derived CH 4 and CO 2 were transported to the estuary and rapidly ventilated to the atmosphere once ice cover retreated. We estimate that river discharge accounts for >95% of annual CH 4 sea-to-air emissions from the estuary. These results demonstrate the importance of resolving seasonal dynamics in order to estimate greenhouse gas emissions from polar systems.Plain Language Summary The primary cause of recent global climate change is increasing concentrations of heat-trapping greenhouse gases in the atmosphere. Ongoing rapid Arctic climate change is affecting the annual cycle of sea ice formation and retreat; however, most published studies of greenhouse gases in Arctic waters have been conducted during ice-free, summertime conditions. In order to characterize seasonal variability in greenhouse gas distributions, we collected year-round measurements of the greenhouse gas methane (CH 4 ) in a coastal Arctic system near Cambridge Bay, Nunavut, Canada. We found that during the ice melt season, river water contains methane concentrations up to 2,000 times higher than the wintertime methane concentrations in the coastal ocean. We utilized a novel robotic kayak to conduct high-resolution mapping of greenhouse gas distributions during ice melt. From these data, we demonstrate that the river water containing elevated levels of methane and carbon dioxide (CO 2 ) flowed into the coastal ocean, and when ice cover melted, these greenhouse gases were rapidly emitted into the atmosphere. We estimate that in this system, more than 95% of all annual methane emissions from the estuary are driven by river inflow.

Observations of Shallow Methane Bubble Emissions From Cascadia Margin

“…The gas extraction technique does not completely separate gas from a water sample, so we apply a compensating extraction efficiency correction. For the DGEU used in this study, an extraction efficiency of 5.09% was determined by a series of validation tests previously performed in the Arctic (Manning et al, 2020) and applied to all data. When this extraction efficiency was applied to the data reported here, it resulted in consistent values for the baseline dissolved methane concentrations that are in equilibrium with atmospheric values.…”

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…For example, transcripts encoding for N 2 O consumption (nosZ) have repeatedly been identified in the oxic water column, despite denitrification being an anaerobic metabolic process (Wyman et al, 2013;Sun et al, 2017). The transcription of nosZ has also been located in highly dynamic O 2 permeable coastal sediments (Marchant et al, 2017). Denitrification under aerobic conditions is attributed to fluctuations in O 2 , NO − 3 , organic matter, and other parameters that affect the availability of electron donors and acceptors, which ultimately influences whether a coastal environment is a net source or sink of N 2 O, as discussed in the next section.…”

River inflow dominates methane emissions in an Arctic coastal system

“…During 2017 and 2018, we collected a time-series of dissolved CH4 and N2O measurements in the estuary and river (Figures 1 and S1). Additionally, in 2018, we used a remotely operated robotic kayak, the ChemYak (Kimball et al, 2014;Nicholson et al, 2018), to characterize fine-scale spatiotemporal changes in dissolved CH4 and CO2 in the estuary during peak river inflow (Figures 2-3), and collected water samples from Greiner Lake. Methodological details for bottle samples and the ChemYak are provided in the Supporting Information.…”

“…The DGEU does not perfectly extract all CH4 and CO2 from a water sample, and so we apply an extraction efficiency correction (Nicholson et al, 2018) by inspection of results from physical bottle samples collected contemporary to the field campaign with the ChemYak (Figure S3). The bottle samples are reported in molar and atmospheric quantities for CH4 and CO2 respectively.…”

“…Methane (CH4) emissions from Arctic waters and sediments may accelerate in the future as part of positive feedback from ongoing climate change (Biastoch et al, 2011;James et al, 2016;Shakhova et al, 2010). Landscapes that were once permanently frozen are now seasonally thawing, and the ice-free season is lengthening in freshwater and marine systems (Magnuson, 2000;Stroeve et al, 2012;Zona et al, 2016).…”