Past climate reconstructions at different time scales using marin deposits
Résumé de la communication de Marc de RAFELIS (Université Paul Sabatier, Toulouse, France) donnée à l’occasion du séminaire Les constructions en milieu marin / Constructions in marine environment du 14 au 19 mars 2016
In the context of climate change, the study of the current climate and its evolution are central to society’s concerns. One way to better understand and anticipate the response of the Earth system is to study past climates. Geological archives, such as sedimentary deposits (and fossils therein), provide accurate clues to better understand how our environment can react to such strong modifications. Indeed, during the past 50 million years, Earth climate history shows few warm periods that can be considered as good analogues of the ones expected in the next decades
This presentation shows how marine deposits can be used to reconstruct, at different time-scales, past environment and climate changes through a key period. First example focuses on Pliocene (-5.3 to -2.8 Ma) Mediterranean deposits that outcrop in Sicily. During Pliocene, marine sedimentation consists of regular alternations of marls and limestones made of nano-(coccoliths) and micro-plankton (forams) whose deposition is controlled by astronomical parameters such as precession, obliquity and eccentricity. And because those parameters are responsible of climate changes at millennium scales, reading of those deposits provides an interesting way of understanding climate and oceanographic changes during a period considered as the best analogue of what will be the tomorrow’s climate.
Another example, using oyster shells geochemistry, will show how geologist can reconstruct climate changes at seasonal scale whatever the period. Because bivalves build their shell with daily increments of calcium carbonate, fossils can be used as a book whose pages are thin mineralized layers. Using living experiments in monitored environments, an infra-daily age/growth model is obtained looking at bivalve shell deposits that can be successfully applied on fossil oyster shells. In addition, the use of very high-resolution geochemistry on shells provides a very powerful tool for paleoclimate and paleo-environment reconstructions. Such an approach will be illustrated with Eocene fossils from central Asia (Tarim Basin). Seasonal contrasts are studied in term of temperature and salinity variations at seasonal scale, to test the hypothesis of the onset of Asian monsoon 50 million years ago.
In both cases, data and results obtained from natural marine geomaterials (sediments and fossils) are compared to numerical climate modelling and the strong convergence of the two methods highlight the interest of marine deposits for past climate reconstructions.
Marc de Rafelis did also a presentation on marble statues, well illustrated by arts found in Les Treilles domain.