Chemical distribution and range of the Cascadia Margin methane seep bubbles

Recent findings show that the storage and release of methane on the Cascadia continental margin in the northeast Pacific Ocean is much more widespread than previously realized, suggesting that the role of methane seeps and hydrates in the carbon cycle and in marine ecosystems is clearly underestimated. To better understand the distribution and range of the chemical composition of these seeps, 47 gas bubble samples were collected over a large range of depths and latitudes along the Cascadia Margin during four research cruises (NA072, NA095, NA098, FK190612) between 2016 and 2019. The bubbles were captured into gas-tight sampling bottles at the seafloor, subsampled on board using a high-vacuum extraction line, and subsequently analyzed on shore for total gas composition, carbon and hydrogen isotopes, as well as helium and neon isotope composition. Here, we present the results of the geochemical analyses of the gas bubbles. The methane concentrations were mostly consistent along the margin with only a few exceptions having less than 99 % methane of the total gas content. Methane isotope compositions are more variable and represent both microbial and thermogenic sources as well as mixtures thereof. Most of the seep gases have radiogenic helium isotopes with R/Ra < 0.1. In contrast, a mantle helium anomaly of R/Ra = 2.3 has been identified at the southern end of the Oregon margin and will be further elaborated on here. We will put these chemical findings in context of their location along the margin and start to define the chemical baseline for the Cascadia Margin methane seeps.