Abstract
Open questions exist about whether methane emitted from active seafloor seeps reaches the surface ocean to be subsequently ventilated to the atmosphere. Water depth variability, coupled with the transient nature of methane bubble plumes, adds complexity to examining these questions. Little data exist which trace methane transport from release at a seep into the water column. Here, we demonstrate a coupled technological approach for examining methane transport, combining multibeam sonar, a field-portable laser-based spectrometer, and the ChemYak, a robotic surface kayak, at two shallow (<75 m depth) seep sites on the Cascadia Margin. We demonstrate the presence of elevated methane (above the methane equilibration concentration with the atmosphere) throughout the water column. We observe areas of elevated dissolved methane at the surface, suggesting that at these shallow seep sites, methane is reaching the air-sea interface and is being emitted to the atmosphere.
Highlights
Methane (CH4) seeps are found throughout the ocean at continental margins, geologically active sites, and in hydrate fields (McGinnis et al, 2006; Reeburgh, 2007)
We demonstrate our technique at two shallow (
Multibeam sonar acoustic surveys of the Yachats area resulted in the identification of 92 seep sites with bubble plumes (Figure 3, Supplementary Figure 2 and Supplementary Table 1)
Summary
Methane (CH4) seeps are found throughout the ocean at continental margins, geologically active sites (e.g., mud volcanoes), and in hydrate fields (McGinnis et al, 2006; Reeburgh, 2007). A central question has been whether CH4 from these seeps reaches the sea surface and impacts the global atmospheric carbon budget (e.g., James et al, 2016; Ruppel and Kessler, 2017). Recent work suggests that CH4 gas emitted in the deep ocean does not regularly reach the atmosphere because of gas exchange during bubble ascent, methane dissolution, and aerobic microbial oxidation of gas (e.g., Ruppel and Kessler, 2017). Methane emitted from shallow sites has the potential to reach the ocean surface (McGinnis et al, 2006) and has a disproportionate impact on gas flux across the air-sea interface. Thorough assessments of shallow seep sites have not been completed in part because of the difficulty associated with studying bubble plumes due to their transient nature, stochastic release, and variable vigor. We contribute new observations from two shallow seep sites offshore Oregon that
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