A heritage disaster, the Notre-Dame fire in 2019 also allows us to increase our knowledge: the ruins of the famous cathedral are precious witnesses of the past! This series follows the scientific site of Notre-Dame, where charred wood and metal pieces reveal their secrets. For this first episode, we let the framework do the talking.
Notre-Dame is one of the most famous French monuments in the world. Its construction began in the reign of Louis VII, around 1160; spanned between the end of the twelfthand century, with emphasis on the construction of the choir, and the beginning of the 20th century.and century, with the construction of the nave and then its towers.
Also known as the “forest”, its structure was made up of a large number of oak beams. In reality, it is not a frame, but several that make up this forest. In fact, around 1225, part of the cathedral was dismantled to enhance the upper windows. Then, during the great restoration of Viollet-le-Duc in the 19th centuryand century, the tower was rebuilt, as well as part of the bell towers and the framing of the transepts.
From April 15 to 16, 2019, more than 800 years after its construction, the cathedral burned for one night, waking up without its tower, a hole in place of the roof.
First regarded as rubble, the remains of the cathedral, in particular the charred beams, were inventoried and their position in the cathedral recorded before being evacuated to a dedicated warehouse. The pieces of wood, for some blackened on the periphery, for others charred in the heart, are now accessible to researchers to unlock their secrets. A team of nearly 70 researchers came together to study them from all angles.
Read more: Science at the bedside of Notre-Dame
What the structure’s woods tell us is a millennium of shared history between men, society and forests. Trees felled to build the structure, for example, keep in their tree rings a record of what the weather was like when they grew up.
Thus, the – deplorable – destruction of the Notre-Dame structure offers a unique opportunity to access the climate of the Middle Ages on the Île-de-France.
What was the weather like in the Middle Ages?
The oaks in the frame of the nave and choir grew during the XIand-XIIIand centuries, a period concomitant with the “medieval climatic optimum” and for which climatic data are still lacking in northern France.
In the 1990s, about forty core samples (with a Pressler auger) were collected, but the methods are destructive and no further studies could therefore have been carried out since then.
This “optimal” climate phase corresponds to an increase in the relative frequency of warm episodes, mainly around the North Atlantic, identified in 1965 by historical documents (classical literature, administrative and ecclesiastical records) and by the study of ice cores, tree rings and crop yields.
Today, this period is called the “medieval climate anomaly”. This term, proposed by the American researcher Scott Stine in 1994, qualifies the first name: whether this period is well characterized by mild temperatures (a few tenths of a degree higher than those of 1960-1990) or greater aridity in Europe and other parts of the world, did not occur synchronously and uniformly in all these regions, with areas likely to be cooler or wetter.
In fact, the climate fluctuates under the influence of factors external to the climate system, also called “forcing” (intensity of solar radiation, volcanic activity), but also under the effect of its own dynamics. The medieval climatic anomaly is probably due to a combination of these causes of external and internal variations.
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These inherently complex changes may have led to the amplification or attenuation of certain climatic parameters depending on the region, such as relative humidity or temperature.
Tree rings, these natural archives
We have no meteorological records prior to pre-industrial times. To study the climate at the time of the cathedrals, we therefore use indirect records, from natural archives such as tree rings, corals or ice cores; then it is necessary to link observations from these natural archives to instrumental climate data.
In recent years, several temperature reconstructions have been made based on natural records such as tree rings. But the main climatic episodes of the last millennium are still not well described, as there are few natural archives with very fine temporal resolutions, on the scale of the year or month.
By studying the climatic signals recorded in the growth rings of Notre-Dame oaks (ring width, wood density, cellulose isotopic composition), we tried to reconstruct the local and regional climatic and environmental changes that occurred during tree growth – this discipline is called “dendroclimatology”.
These studies also provide a better understanding of the cathedral’s construction site, the social and economic contexts of Paris, and even the growth of oaks and forests in its environmental environment.
The growth of oaks under the magnifying glass
The width, density and chemical and isotopic composition of tree rings vary with climate. The isotopic compositions of carbon and oxygen from tree rings are classically used to study the evolution of past climates because they were fixed during the growing season. These compositions vary during the metabolic reactions in the tree and the intensity of the changes depends on the environmental and climatic conditions in which the growth takes place.
For example, the carbon that makes up the cellulose in trees comes from COtwo of the atmosphere. It is assimilated at the leaf level and integrated into sugars, which combine to form cellulose molecules. During a dry summer, the tree limits its gas exchange in order not to become dehydrated, which leads to an increase in the carbon isotope ratio of sugars and, consequently, of cellulose. A relatively high value of the carbon isotope ratio of the cellulose of a ring may therefore indicate that the summer during which the tree formed that ring was quite dry.
Depending on the species, region and growing environment of the tree, isotopic ratios of carbon and oxygen may reflect atmospheric temperature, water stress, amount of precipitation or cloud cover.
Understand how wood burned
If it is known that carbonization modifies the physical and chemical properties of wood, we also know that the degree of alteration of the material varies according to the combustion mode: temperature, duration of heating, availability of oxygen in the atmosphere, shape and size of the sample. of wood, the species and its water content can have an impact on the carbonization process.
It is difficult to reproduce frame fire in the laboratory, but the processes of combustion and pyrolysis can be reproduced and studied through experiments.
The recent development of analytical tools to characterize the char intensity of burned wood could make it possible to use the carbon and oxygen isotopes of the charred Notre-Dame beams – and more generally the coals – to reconstruct the climate.
For example, the “Raman Palethermometer” method allows estimating the maximum temperatures reached during the Notre-Dame fire around 1200°C. Below 400°C, the infrared spectroscopy method allows probing the different stages of thermal decomposition of wood components.
For each of these tools, a calibration curve between the assumed temperature to be reached and the measured difference in the isotopic signature of the wood can be produced experimentally from the burned wood. These calibrations will allow us, in the coming years, to “correct” the isotopic values and thus return to the natural variability of the climate during the long construction site of Notre-Dame.