Penchenat T.
Doctoral thesis at the University of Paris-Saclay, under the direction of V. Daux and I. Mundo
For the past hundred years or so, the western part of South America, from the Altiplano to northern Patagonia, has been experiencing increasingly frequent and longer periods of drought. This trend, which is probably partly linked to the expansion of the Hadley atmospheric cell associated with a dominant positive phase of the Antarctic Oscillation in recent decades, will continue according to climate modelling.
The aim of this thesis is to contribute to a better understanding of the processes responsible for this evolution by reconstructing past hydro-climatic variations in Northern Patagonia. The latter are reconstructed on the basis of temporal variations in the carbon (δ13C) and oxygen (δ18O) isotopic composition of the cellulose in the rings of Araucaria araucana, a species endemic to Patagonia whose range lies between 37°20‘S and 40°20’S.
The main results were as follows :
– The isotopic composition of A. araucana cellulose reflects the climatic conditions of the current growing season, whereas ring widths are more dependent on the climatic conditions of the previous growing season. Remobilisation of reserves is therefore not involved in the manufacture of the sugars used to produce the rings.
– Variations in δ13C and the physiological behaviour of trees are linked to the humidity of the environment in which they grow. There is therefore a difference between trees growing to the west of the Andes, where rainfall is abundant, which are more sensitive to variations in light levels, and those growing to the east in a drier environment, which are more sensitive to variations in humidity.
– Variations in δ13C and δ18O in the rings of A. araucana growing in a dry environment are linked to those in temperature and humidity, which are themselves controlled by the Antarctic Oscillation and the position of the descending branch of Hadley’s cell.
– The high potential of δ13C in the cellulose of A. araucana tree rings to record variations in temperature and summer aridity on a regional scale makes it possible to reconstruct climatic variations in Patagonia over several centuries (315 years) and highlights a sharp increase in maximum temperatures (+1°C) at the end of the 18th century (Figure). On a regional scale, climate reconstructions reveal a uniformity of trends (temperature and humidity) over recent decades, probably linked to global climate change, which is becoming the main factor controlling climate variability.
– The δ18O of cellulose, controlled by the δ18O of precipitation and soil, also recorded a climatic change at the end of the 18th century that would be attributable, at least in part, to the Antarctic Oscillation. The movements of the downward branch of Hadley’s cell were also recorded by the δ18O until the mid-1990s, by which time the cell had probably spread too far to affect A. araucana at these latitudes.
– Isotopic tools are therefore proving to be a promising way of gaining a better understanding of the spatio-temporal variability of phenomena affecting Patagonia, and in particular the contribution of the Hadley cell to current global climate change.
