Laura Fernandez-Cascales PHD : Contribution of the mechanisms of dune growth to the resolution of planetary climates
Laura Fernandez-Cascales was recruited by UnivEarthS in October 2014 on the Interface project “From dust to planets“.
She presented her thesis defense on September 27, 2017 at the IPGP with the following jury:
Mr Pascal Allemand (University Lyon 1) Rapporteur
Mr Maciej Dłużewski (University of Warsaw) Rapporteur
Mr François Costard (Paris Sud University) Examiner
Ms. Laurie Barrier (Institut de Physique du Globe de Paris) Examiner
Mr. Clément Narteau (Institut de Physique du Globe de Paris) Director of thesis
Mr. Sébastien Rodriguez (Institut de Physique du Globe de Paris) Co-supervisor
Contribution of the mechanisms of dune growth to the resolution of planetary climates
Dunes are a dynamic interface between the solid and fluid parts of a planet. In the Solar
System, they are commonly observed on Earth, Venus, Mars and Titan. In this thesis work, we study the dunes at two length scales using two independent dune growth mechanisms : the bed and the fingering instabilities. The aim is to characterize different atmospheric flows properties.
At the elementary dune scale on Earth ( 20 m), the objective is to characterize the variation of the shear velocity along a dune to isolate the origin of its growing during the linear phase of the bed instability. At the scale of martian sand seas ( 105 m), the objective is to determine the multidirectional wind regimes which are responsible for the observed dune shape and orientation taking into account the sediment availability.
After flattening of two hectares of an experimental dune site belonging to the Chinese Academy of Sciences in the Tengger desert in April 2014, we have documented the emergence of dunes and their development during two years. This landscape-scale experiment presents the advantages to study dune dynamics from known initial and controlled boundary conditions by systematically measuring the winds at the origin of sediment transport. During the linear phase of the bed instability, which is responsible for the formation of a periodic train of linear dune, the goal is to evaluate for the first time the evolution of the shift between the maximum (minimum) of topography and the maximum (minimum) of wind speed. This offset results from a hydrodynamic coupling between the surface and the fluid that imposes perturbations of the flow upstream of the crest. During three field surveys (November 2014, April and November 2015), the measurements consisted of recording the wind speed in the inner layer (the first 15 centimeters above the surface) and in the outer layer (above) along different dune profiles. These velocities measurements are compared to the topography, in order to quantify the shift and evaluate its evolution according to the dune aspect ratio. These experiments allows us to study incipient dunes over an unexplored range of dune aspect ratio (< 0.03). Then, our measurements can be compared with the theoretical predictions suggesting that the offset decreases as dunes grow in amplitude.
On Mars, we study two dune fields located on the eastern edge of Olympia Undae. In these
areas, the albedo contrast between the dune material and the non-erodible surface is such that it is possible to extract the sediment cover from satellite images and directly compare it to dune orientation. At the length scale of these planetary dune fields, we can highlight for the first time the relationship between dune alignment and sediment availability. We show that this relationship results from the simultaneous expression of the two dune growth mechanisms under multidirectional wind regimes and variable sediment supply conditions. At the borders of major dune fields with high sedimentary cover, dune pattern reorientation depend on sediment flux direction. For outgoing flux, the reorientations are carried out over greater distances than in the case of incoming flux.
We explain these differences from the dynamics of dune interactions and transitions in dune shape between the two growth mechanisms. We also show that the theoretical understanding of the growth mechanisms is useful to solve the inverse problem of dune orientations which consists in assessing the distribution of sediment flux orientation (i.e. the wind regime) from dune alignment.
By solving this inverse problem under the hypothesis of bidirectional wind regimes, we show that a large diversity of dune patterns can be explained from very small variations in the distribution of sediment flux orientation. Our results also indicate that dune fields themselves may have feedback on winds.
Together, these studies provide new information on global climates and atmospheric flows that create dunes. Indeed, in areas where winds are poorly known, we show how the two dune growth mechanisms can more efficiently constrain past and present wind regimes. This approach can now be generalized on Earth, Mars, Venus and Titan to explain the dynamics of sand seas and predict their evolution in a context of climate change.