Spatiotemporal signature of methane venting from lake sediments: from lab to field scale

Abstract:

Methane is a potent greenhouse gas, and the production and emission of methane from sediments in inland waters and shallow oceans both contributes to and may be exacerbated by climate change. In some of these shallow-water settings, methane fluxes are often controlled by episodic free-gas venting. The fraction of the methane released from the sediments that bypasses dissolution in the water column and reaches the atmosphere impacts the magnitude of the climate forcing, and this fraction depends critically on the mode and spatiotemporal characteristics of the bubble releases. Here, we present measurements of the episodicity, spacing and persistence of ebullition from the laboratory scale (1-50 cm) to the field scale (0.5-20 m). Field observations were made using a fixed-location Imagenex DeltaT 837B multibeam sonar, which was calibrated to quantify gas fluxes with unprecedented spatial and temporal resolution (~0.5 m, 1 Hz). The field scale results show a pattern of short range spatiotemporal clustering (radius<2 m) in ebullition events that dissipates over time to a spatially homogenous process at the resolution of the sonar. The lack of persistent hotspots suggests a limited role for lateral methane transport within the sediments, and the spatiotemporal clustering implies a mechanism for triggering nearby aftershock ebullition episodes. The fine-scale (1-50 cm) experiment recorded ebullition from sediments that were dredged from the field site, reconstituted and incubated in the laboratory to generate methane. This experiment shows the degree of re-use of specific outlets, with implications for the scale of lateral methane transport and the role of hysteresis on sediment cohesion (healing of closed conduits). The details of the short range clustering process helps to identify the mechanism by which gas venting triggers nearby “aftershock” episodes of gas release. Taken together, these results point towards a better understanding of the microscale processes controlling methane venting from deformable sediments, as well as their impact on large-scale methane fluxes from shallow-water bodies.

Figure: Short-range spatial clustering, quantified with the Radial Distribution Function (RDF>1, r<2), dissipates to a homogeneous signature (RDF = 1) over long observation periods.