A 3.6km-wide crater: a unique opportunity to study climate over the past million years!
We can go back hundreds of thousands of years in the Earth’s history by looking at ice cores and marine sediment. In a few rare cases, we can get insight into insight in the to the past climates of temperate regions through lake sediment. But until now there have been no continental records on the southern hemisphere tropics that can be used to reconstruct climates over such time scales. Hence the interest in one remarkable site: the 300 metre-deep Colônia bowl-shaped depression in Brazil that has the makings of a gigantic crater. Geologists have not yet determined the precise origin of this formation, but the most popular assumption is that it is a meteorite impact. One thing they do know is that this depression has accumulated hundreds of thousands of years’ worth of sediment, proving a unique opportunity to go back far intime.
Researcher Marie-Pierre Ledru from the Institut des sciences de l’évolution de Montpellier (UM/CNRS/IRD/EPHE) in France has been analysing samples from this site since 2000. Thanks to drilling carried out in 2014 down to 14 meters, Ledru has been able to analyse the hydraulic changes, temperature variability and biodiversity of the last 250,000 years. But the tropical forests expert wants to go further by drilling up to 50 metres in order to research the last 800,000 years, which would cover the succession of multiple glacial/interglacialcycles.
“ The Colônia crater gives us the rare opportunity to explore past climate variability in the Brazilian tropical forest. By extracting a core from the heart of this geological structure, we hope to answer the question that has been pestering us for decades: why do tropical forests have such an abundance of species? ”
Institut des sciences de l’évolution de Montpellier (UM/CNRS/IRD/EPHE)
An extraordinary window on the past
“Up until now there has no continuous analysis of the last million years in the tropical regions of the southern hemisphere”, says Ledru. This core will provide the first insights into how a tropical forest - in this case the Atlantic Forest in Brazil - responds to big natural climate cycles.
Extraction of the core is planned for summer 2017. It will first be dated precisely to determine the chronological framework and thus identify the different climate cycles. Then the core will then be cut and sampled to allow a range of specialists to perform their analyses: paleomagnetic, geochemical (carbon, gas, trace elements, isotopes), mineralogical (sediments) and biological (pollen, diatoms).
until now there has no continuous analysis of the last 800,000 years in the tropical regions of the southern hemisphere.
A total of 11 laboratoriesfrom five countries (France,Germany, Brazil, Switzerland and theUnited Kingdom) will be taking partin these analyses. In addition to collectingdata on CO2 concentration,temperatures and rainfall, the analyseswill also shed light on variationsin insolation, and in plant species assembliesby analysing pollen.
The secret of rich biodiversity?“One of our assumptions is that the rich biodiversity of tropical forests is owed to the continuous reassembly of species over the big climate cycles. This reassembly is more a response to insolation changes that glacial/interglacial cycles”, Ledru explains. Accordingly, the researchers expect to find associations of species that evolve according to the insolation cycles (also known as Milankovitch cycles). By comparison, the plant assemblies of temperate forests appear to repeat from one glacial or interglacial cycle to the next.
The project will also compare in situ data with the results of different climate models. Can they accurately retrace the evolutions of this forest using mathematical equations? Comparing the two will ultimately help improve the reliability of models use to reconstruct the climate - both past and future - of the tropical regions.