Q & A - Lucas Vimpere

Please tell us about your background and current PhD studies

I started my studies at the Earth and Environmental Sciences Department of the University of Geneva in 2010. After my Bachelor graduation, I began a master program focused on sedimentology and reservoir geology.

During this course, I specialised in carbonate sedimentology with a particular focus on the Quaternary period. I did my master thesis on the costal sediments and units buried within the Dean’s Blue Hole on Long Island in the Bahamas. The goal was to dive in the blue hole ( the deepest in the world at the time, being a depth of 202m), collect samples with a jackhammer, and use it as a natural exposure of the stratigraphy of the island, constrain its dimensions for the first time, and finally, try to explain its odd depth ( lower than the lowest sea level at - 120m).

I pursued my studies with a PhD focusing on the relationship between aeolian sedimentation, both in coastal and continental settings, with climate and global atmospheric circulation. We first compared the dunes ( chevrons) of the Bahamian isolated carbonate platforms with the dunes of the carbonate ramp of Shark Bay in Western Australia. We then looked globally with inland parabolic dunes and the specific climate factors triggering their formation/ migration. The overall goal was to assess the desertification hazards and identify the regions most at risk with respect to global climate change.

What are chevrons and when were they first identified?

The term “chevron” has first been used to describe structural folds ( see Ramsay, 1974) or laminae in oscillatory ripples ( see Allen, 1982). In 1989, Maxwell & Haynes applied this term to describe the morphology of low- lying V- shape aeolian ridges in the Selima Sand Sheet ( the Egyptian Sahara). This definition has then been reused by other geoscientists to describe similar landforms in the Bahamas, South Madagascar, and Western Australia.

Are they reasonably common or are they particularly well presented in specific locations, like in the Bahamas, South Madagascar and Australia?

If we look at the morphology, these ridges are very common along the world coastlines but their dimensions in the Bahamas, South Madagascar and Australia make them particularly extensive deposits. The difference is that the term “chevron” has been applied in these three locations whereas elsewhere they are called parabolic dunes.

Why has there been controversy and debate about the origin of these structures?

Since different groups of research have worked in these three places, different interpretations on their depositional process were made. In Western Australia and South Madagascar, the explanation of tsunamis generated by a meteorite impact in the Indian Ocean is based on:

• their large size and the fact that they develop on top of high cliffs where sediment supply is rather limited

• the fact that ( according to the authors) they are not aligned with the prevailing winds

• their association with big boulders

• oral traditions and mythologies of indigenous people mentioning giant waves and/ or flood

In the Bahamas, the giant waves generated by giant storms is based on:

• a “trilogy” of deposits observed on one location on Eleuthera: boulders - chevrons - run up deposits

• the presence of fenestrae ( type of porosity usually found in beach settings) within chevrons

• the fact that they date from the Last

Interglacial when the global temperature was higher by about 2° C than today. Specific climate conditions during this period ( e. g., changes in north Atlantic circulation, collapse of Antarctic ice sheet) would have triggered giant storms.

• the fact that these features cannot be observed during other periods of the Quaternary

It is hard to differentiate between beach- to- intertidal and aeolian deposits because the latter is very often composed of reworked sediment that formed in a beach- to- intertidal environment. This is even truer for carbonates since they mainly precipitate and form underwater.

In your study published in Sedimentology in 2020, why do you advocate for a windblown ( aeolian) origin of the chevrons in the Bahamas?

We started our study by assuming that both wind and waves could have been valid depositional processes for chevrons formation. The idea was to carry out a comparative study on chevrons, parabolic dunes ( wind), and storm deposits ( waves) and proceed by exclusion to provide a final explanation for their formation.

We introduced new quantitative data on their morphology, sedimentology, stratigraphic position, and grain size composition that we compared with the other types of deposits.

Overall, all elements pointed towards an aeolian origin. We then explained why they were only deposited during the Last Interglacial by explaining their relationship with global atmospheric circulation, climate, and sea level variations.

“... these ridges are very common along the world’s coastlines, but their dimensions in the Bahamas, South Madagascar and Australia make them particularly extensive deposits.”