Formation of Kinneyia via shear-induced instabilities in microbial mats

TL;DR

This study proposes that Kinneyia fossils form through shear-induced Kelvin-Helmholtz instabilities in microbial mats, supported by theoretical predictions and laboratory experiments that replicate natural ripple patterns.

Contribution

It introduces a novel mechanism explaining Kinneyia formation as a shear-induced instability in viscoelastic microbial mats, validated by experiments matching natural patterns.

Findings

Ripple wavelength proportional to film thickness

Patterns form independently of viscosity and flow conditions

Laboratory patterns resemble natural Kinneyia in morphology and scale

Abstract

Kinneyia are a class of microbially mediated sedimentary fossils. Characterised by clearly defined ripple structures, Kinneyia are generally found in areas that were formally littoral habitats and covered by microbial mats. To date there has been no conclusive explanation of 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 type instability induced in a viscoelastic film under flowing water. A ripple corrugation is spontaneously induced 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. This behaviour is independent of the viscosity of the film and the flow conditions. 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, with similar morphologies, wavelengths and amplitudes being observed.