Regensburg 2013 – scientific programme
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DY: Fachverband Dynamik und Statistische Physik
DY 27: Fluid Dynamics and Turbulence
DY 27.1: Talk
Thursday, March 14, 2013, 09:30–09:45, H48
Formation of Kinneyia via shear-induced instabilities in microbial mats — Katherine Thomas1, Stephan Herminghaus1, Hubertus Porada2, and •Lucas Goehring1 — 1Max-Planck-Institut für Dynamik und Selbstorganisation, Göttingen, Germany — 2Univeristy Göttingen, Goewissenschaftliches Zentrum, Göttingen, Germany
Kinneyia are microbially mediated sedimentary fossils with clearly defined ripple structures. They are generally found in areas that were formally littoral habitats and covered by microbial mats. To date there has been no conclusive explanation as to the processes involved in the formation of these fossils. Microbial mats behave like viscoelastic fluids. We propose that the key mechanism involved in the formation of Kinneyia is a Kelvin-Helmholtz instability induced in a viscoelastic film under flowing water. A ripple corrugation spontaneously forms in the film and grows in amplitude over time. Theoretical predictions show that the ripple instability has a wavelength proportional to the thickness of the film. Experiments carried out using viscoelastic films confirm this prediction. The ripple pattern that forms has a wavelength roughly three times the thickness of the film. Laboratory-analogue Kinneyia were formed via the sedimentation of glass beads, which preferentially deposit in the troughs of the ripples. Well-ordered patterns form, with both honeycomb-like and parallel ridges being observed, depending on the flow speed. These patterns correspond well with those found in Kinneyia fossils, with similar morphologies, wavelengths and amplitudes being observed.