Subsurface sediment mobilization and active pockmarks from sublacustrine ground-water seepage

Abstract:

Lakes can be used as “model basins” to study fluid-flow processes with a multi-method approach in a well-defined environment. We present unprecedented insight into newly discovered pockmarks and associated subsurface sediment-mobilization structures in Lake Neuchâtel, Switzerland. A geophysical approach using multiple tools provides precise high-resolution bathymetric data and subsurface information of the sedimentary infill. We combine geophysical (300 kHz Kongsberg EM2040 multibeam, 3.5 kHz pinger seismic, deep-towed multi-frequency chirp seismic, mounted on an AUV), sedimentological (piston cores), hydrological (CTD), geochemical (methane, δ¹⁸O) and visual (ROV survey) data and observations. The data show several circular, crater-shaped pockmarks of up to 160 m in diameter and up to 30 m depth. The pockmarks are partially filled with mud in a fluid-like state. It is hypothesized that this mud is a result of active fluid flow within the pockmark. The levees of the pockmarks are characterized by high-amplitude wedge-shaped seismic reflections being intercalated with the background sediments. They are interpreted as overflow deposits originating from episodic increases in fluid flow from inside the pockmarks, causing sediment to be spilled over the margin and deposited on the levees. Data show multiple phases of sediment expulsion during discrete periods throughout the Holocene. Geochemical sediment analyses of headspace methane indicate the presence of purely microbial methane at low concentrations, thus no indications of active gas seepage. Elevated temperature values and depleted δ¹⁸O signals within the pockmark, compared to the reference sites, hint towards different water sources. We interpret these data to show two water bodies: (i) lake bottom-water, and (ii) groundwater entering as focused fluid flow through the pockmark. This multi-proxy approach shows that the newly discovered pockmarks of Lake Neuchâtel are sublacustrine springs, possibly related to regional karst structures and/or tectonic fault systems. The subsurface sediment-mobilization structures of these sublacustrine springs strongly resemble fluid-flow structures in the marine environment (e.g. mud volcanoes), thus linking processes within terrestrial hydrological systems and the marine realm.