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
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
... 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.
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.
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.
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: Cédric 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:
- Lasser et al. 2019, "Salt Polygons are Caused by Convection", https://arxiv.org/abs/1902.03600
- Lasser et al. 2021, "Stability and dynamics of convection in dry salt lakes", J. Fluid Mech., accepted, https://arxiv.org/abs/2004.10578
- Lasser et al. 2020, "Surface and subsurface characterisation of salt pans ex-pressing polygonal patterns", ESSD, https://doi.org/10.5194/essd-12-2881-2020
Simulation code at:
Find me me at:
- Email: lasser@csh.ac.at
- Twitter: @janalasser
- Website: www.janalasser.at