Award: OCE-1851402

The work conducted here quantified methane emissions from coastal marine environments to the atmosphere, which comprised one of the most uncertain natural sources of methane to the atmosphere. This work was achieved through a unique and equal combination of observational and statistical modeling work, leveraging the unique expertise of two investigators. First, this project refined and reengineered a new autonomous analytical technique to measure and map the sea-to-air emission of methane with unparalleled spatial resolution. Second, this updated technique was applied to measure methane emissions along the coastal US Pacific and Atlantic margins. Additionally, the natural radiocarbon content of methane was measured to partition its source between ancient geologic and modern microbial endmembers. Biogeochemical variables including chlorophyll, nutrient and dissolved oxygen concentrations, temperature, salinity, depth, and windspeed were also measured alongside methane.  Third, the newly collected data was used to train machine learning models to predict the quantity of, and uncertainty in, surface methane supersaturation as a function of biogeochemical variables. Not only were region-specific statistical models established, but interregional differences were uncovered. Having established the fidelity with which this approach can generalize between different coastal environments, our results were extrapolated to map methane supersaturation and estimate regional-scale coastal methane emissions, while also quantifying the uncertainty of these final results.

INTELLECTUAL MERIT: Methane is a potent greenhouse gas whose role in future climate warming will hinge both on anthropogenic emissions and perturbation of the natural methane budget. It is therefore critical to understand the baseline rates and environmental sensitivity of natural methane sources to the atmosphere. While open ocean surface waters are thought to contribute a minor source in the atmospheric budget, concentrations – and thus emissions – can rise by orders of magnitude over a few kilometers when approaching coastlines. At the start of this project, a paucity of observations that accurately captured coastal methane gradients hinted that coastal ocean methane emissions were significant, albeit highly uncertain. Furthermore, the relative contributions of geologically-sourced and biologically-sourced methane in coastal surface waters was not well known, limiting our ability to predict future changes due to the different environmental sensitivities of these different methane sources. The work conducted here helped to close these gaps in our knowledge of the natural methane budget, yielding the most robust estimates to date of coastal ocean emissions and a new understanding of the mechanisms that sustain them. In particular, our work revealed significant roles for river plumes and aerobic biological production in driving methane emissions, and only a minor role for seafloor hydrocarbon seeps, given that seep methane is rapidly oxidized before it reaches the surface ocean.

BROADER IMPACTS:  Beyond simply constraining the flux of methane from the coastal ocean to the atmosphere, the results established here can be used as a first step in predicting how coastal ocean methane emissions may respond to future environmental changes in temperature, nutrient cycling, and hypoxia. In addition to these societal impacts of the completed work, this project also engineered a new autonomous measurement technique with potential commercial applications and uses in other marine and freshwater research projects. Finally, this project had strong educational impacts as it provided undergraduate students the opportunity to experience the entire scientific process from idea conception to publication of the final results. These students were not passive observers of this scientific process, but active participants in this research, and were empowered to pursue their own scientific interests that complemented the primary objectives. This led to two of the students pursuing graduate careers in the ocean sciences. This project also supported a Ph.D. student in a truly unique experience whereby he had opportunities to conduct meaningful research in both sea-going measurement as well as modeling laboratories, and thus integrate into two often disparate communities.

 

Last Modified: 03/20/2025
Modified by: John D Kessler