Methane is a potent greenhouse gas, and is released into the ocean from the seafloor as gas bubbles via methane seeps. Despite the prevalence of seeps along continental margins, data is limited, and their impacts on the ocean and atmosphere, both positive and negative, are poorly understood. To advance our knowledge about methane seeps, we are pursuing an approach to map and characterize methane seeps over wide areas. We are using easily deployable and relatively inexpensive autonomous underwater vehicles (AUVs) equipped with imaging sonars and custom sensors to find and map seeps and measure associated bubbles and dissolved methane right at the source and up through the water column.

Recent efforts to map seafloor methane seeps have shown them to be ubiquitous at widely varying depths along the coastal zone from Vancouver Island to northern California – a region known as the Cascadia Margin. More than 1300 emission sites have been discovered to date.

Autonomous undersea vehicles (AUVs) will be deployed on the coastal ocean off Oregon and Washington to characterize several methane emission sites at depths up to 500 m.

• Where and a what rate does methane enter the water column from Cascadia Margin seeps?
• How many sites release methane that reaches the air-sea boundary?
• How important are tides in modulating methane release from seeps?
• Do the rising bubbles have the potential to transport nutrients from the seafloor to the shallow ocean?
• do geological features control the location of bubble streams?
• How can AUV characterization of seep fluxes along the Cascadia Margin be used to improve the regional methane flux estimates made by shipboard surveys?

This research is funded by NOAA Ocean Exploration beginning in fiscal year 2022. The research team responded to a proposal solicitation to conduct or support ocean exploration resulting in outcomes that provide or enable initial assessments about unknown or poorly understood regions of U.S. waters.

Methane bubbles rising through the ocean and entering the atmosphere as a powerful greenhouse gas is not yet well understood. What is know well right now is that methane remaining in the ocean oxidizes, consumes oxygen, and produces carbonic acid. The oxidation of seep methane is an important contributor to hypoxia (low oxygen dead zones) and ocean acidification.

Methanogenesis

More than half of the global organic matter produced in the ocean's photic zone by biological production falls to the seafloor in the coastal ocean. This organic matter is buriede and forms sediments that accumulate over time. Beneath the seafloor where oxygen is depleted, the organic matter is transformed via microbe into reduced carbon compounds, including methane. Methane produced in the deeper sediment layers then migrates upward towards the seafloor. Under certain conditions, methane is released from the seafloor as bubbles and in dissolved form at methane seeps.

The Cascadia Margin presents an excellent opportunity to assess the role that methane seeps have in a changing ocean. Shipboard surveys have mapped nearly 40% of the approximately 800 km long and 100 km wide Cascadia Margin, identifying nearly 3500 methane bubble streams clustered into more than 1300 methane emission sites.

Upon arrival at a targeted seep site, the activity of that seep will be evaluated using a custom multibeam echosounder. We will confirm there is a relatively large and persistent flare observed on the ship's sonar. Then, AUVs will be deployed witha relatively short reconnaissance grid-pattern mission. The AUVs will then be recovered and the data reviewed, establishing a more accurate position for the bubble stream's source on the seafloor. Building on this information, longer duration missions will characterize:

We will interchange AUV tracking for CTD sampling and water-column multibeam sonar mapping when the AUVs are recharging. We will conduct vertical CTD casts over the seep sites that are identified with bubbles reaching high in the water column, and potentially making it all the way to the surface. Water samples for chemical analysis will be collected using Niskin bottles mounted on the CTD rosette.