Marine oolites as proxies for palaeoenvironmental reconstructions during extinction events.

At present time, it has been stated that “Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with
global surface temperature reaching 1.1°C above 1850–1900 in 2011–2020”. Therefore, understanding the consequences of human activities on present and
future climate is one of the most important issues of this century. It is not the first time that extreme climate conditions are recorded in the Earth geologic record.
Sometimes they affected the whole planet so deeply that they led to vast biodiversity breakdowns both in the oceans and on the continents, causing the disap-
pearance of most living species. That is the case of mass extinction events. Looking for reliable proxies to better reconstruct climate conditions during extreme
past events could help to predict climate evolution in high-stressed conditions, such as the one of the last decades. The present dissertation aims at digging into
the broad field of palaeoenvironmental reconstruction using a potentially successful proxy: marine ooids and their lithified equivalent, oolites. Ooids are tiny
(less than 2 mm) carbonatic marbles made of exceptionally thin, concentric layers around a nucleus. They accumulated in anomalously thick deposits after major
biodiversity crises. The scope of this dissertation is to recover specific chemical and physical conditions of the ocean water after two mass extinction events, the
end-Permian and the end-Triassic. For the former, I described ooids from the Italian Dolomites and for the latter from the United Arab Emirates, using several
advanced analytical techniques. By using optical and scanning electron microscopy, I carefully described their internal structure and monitored how different
types and sizes of ooids co-varied in time in the immediate aftermath of the extinction events. I also obtained chemical information from the ooids and other
components of the rock samples by using elemental mapping techniques with scanning electron microscopy and laser ablation coupled to mass spectrometry. In
order to widen our perspective on the environmental consequences of extinction events, I also describe a thick sedimentary section in Wadi Milaha (UAE) and
analyze its δ13Ccarb and δ18Ocarb isotopic record. I could demonstrate an original deposition in calcite of this oolite in contradiction with the accepted hypothesis
and give an explanation for this. I could show as well that the formation of ooids after the end-Triassic Mass Extinction was favored by the income of water
rich in phosphate on shallow environments. The end-Permian ooids could show the impact of the diagenesis on these proxies and which chemical elements are
preserved and which are not. The palaeonvironmental data I provided add new information on how the oceans reacted to extreme events in the past.