Salt polygons and porous media convection

Graz University of Technology     |     Complexity Science Hub Vienna

Jana Lasser     |     Email: lasser@csh.ac.at     |     Twitter: @janalasser

slides available at
https://janalasser.at/talks/salt_polygons_convection/

Salt polygons occur in many regions around the world

What drives their formation?

Overview

  • Characteristic features of salt polygons.
  • Existing explanations for salt polygon formation.
  • Porous media convection as a new mechanism.
  • Evidence for the new driving mechanism.
  • Characteristic features of salt polygons

    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.

    Existing explanations for salt polygon formation

    (and why we believe they don't work)


    no closed polygons
    Wrinkling and cracking are definitely happening ...

    ... but they might be secondary mechanisms, occuring only after a different mechanism determined the scale and shape of the patterns first.

    Porous media convection as a new mechanism

    Idea: Connect surface pattern expression to dynamics in the underground.


    Saline water saturates the porous soil. Water evaporates, a vertical salinity gradient builds up. Density-driven convection arises.

    Saline water saturates the porous soil. Water evaporates, a vertical salinity gradient builds up. Density-driven convection arises.

    Saline water saturates the porous soil. Water evaporates, a vertical salinity gradient builds up. Density-driven convection arises.

    Saline water saturates the porous soil. Water evaporates, a vertical salinity gradient builds up. Density-driven convection arises.

    Upwelling and downwelling plumes cause horizontal salinity gradients. Salt precipitates preferentially in regions of high salinity.

    Upwelling and downwelling plumes cause horizontal salinity gradients. Salt precipitates preferentially in regions of high salinity.

    Upwelling and downwelling plumes cause horizontal salinity gradients. Salt precipitates preferentially in regions of high salinity.

    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.


    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.

    (1) We can measure soil permeability and porosity, and the density gradient to calculate Ra under field conditions.

    (1) We can measure soil permeability and porosity, and the density gradient to calculate Ra under field conditions.

    (1) We can measure soil permeability and porosity, and the density gradient to calculate Ra under field conditions.

    (1) We can measure soil permeability and porosity, and the density gradient to calculate Ra under field conditions.

    (2) We can measure the salinity distribution in the soil to check for gradients.

    Vertical salinity gradients
    Salinity over depth
    Salinity over depth

    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.

    Two-dimensional simulations of density-driven convection

    The simulation indicates strong horizontal inhomogeneities in the vertical salinity flux to the surface, tied to downwellings.

    The simulation predicts increased salinity flux (ridge growth) above downwellings.

    If this is the case, a horizontal salinity gradient should exist in the sediment below the crust.

    The compilation of salinity profiles
    There are horizontal salinity gradients in the field.
    Locations of high salinity in the sediment are correlated with locations of ridges at the surface.

    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.

    Details: Lasser et al. 2021, "Stability and dynamics of convection in dry salt lakes",
    J. Fluid Mech., accepted.

    Details: Lasser et al. 2021, "Stability and dynamics of convection in dry salt lakes",
    J. Fluid Mech., accepted.

    The coarsened wavenumber agrees with field observations. The wavenumber becomes independent of Ra.

    The coarsened wavenumber agrees with field observations. The wavenumber becomes increasingly independent of Ra.

    Preliminary 3D simulations show the formation of closed polygons.

    Image credit: C├ędric Beaume.

    Driving mechanism: necessary requirements

    The mechanism should ...

    ... occur in salt pan environments.

    ... be able to spatially modulate salt flux to the surface.

    ... generate closed polygons with a robust wavelength of 1-2 m.

    ... be robust to environmental variations.

    ... be stable for long enough to account for substantial ridge growth.

    Until now we have only considered the impact of dynamics in the soil acting on crust growth.


    But what if there is a feedback process and the crust also influences the dynamics underneath it?

    Temperature and humindity measurement in the field

    Low temperature and high humidity both reduce the evaporation rate E.

    Temperature and humindity measurement in the field

    Low temperature and high humidity both reduce the evaporation rate E.

    Temperature and humindity measurement in the field

    Low temperature and high humidity both reduce the evaporation rate E.

    Modulate the evaporation rate E at the top boundary

    Different modulation wavenumbers

    Low temperature and high humidity both reduce the evaporation rate E.

    Different modulation wavenumbers

    Low temperature and high humidity both reduce the evaporation rate E.

    Different modulation wavenumbers

    Low temperature and high humidity both reduce the evaporation rate E.

    Different modulation wavenumbers

    Feedback with the crust can ensure long-term stability of plume positions.

    Summary: Porous media convection


    ... is a mechanism that occurs in salt pan environments.

    Ra > Rac at all field sites

    Summary: Porous media convection


    ... is able to spatially modulate salt flux to the surface.

    Summary: Porous media convection


    ... generates closed polygons with a robust wavelength of 1-2 m and is robust to environmental variations.

    Summary: Porous media convection


    ... is stable for long enough to account for substantial ridge growth.

    More details at:


    Simulation code at:

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