Reconstruction of Holocene climate in southern France and eastern Spain using quantitative anatomy of olive wood and archaeological charcoal

Quantitative anatomical analysis of wild olive wood demonstrates that certain criteria are determined by climatic factors. Regression models, developed from modern calibration samples located in French and Spanish Mediterranean regions and charred under controlled thermal and atmospheric conditions, allow us to quantify the response of `vessel densityʹ and `hydraulic conductivityʹ to `mean annual temperatureʹ and `mean annual precipitationʹ, respectively. From these predictive models, climatic conditions are compared with data from wild olive charcoal from three different archaeological sites (La Cova de lʹEspérit and Montou, France; La Cova de les Cendres, Spain). Although archaeological charcoal results from the partial combustion of wood collected by man around settlements, the available samples are sufficiently numerous to infer regional palaeoclimatic conditions. They allow us to estimate the climatic changes that occurred in southern France (Roussillon) and eastern Spain (Levante) during the Holocene period. The reconstructed mean annual temperature conditions show low variations throughout each site. An increase from 1° to 1.5°C is recorded in southern France at the beginning of the Subatlantic chronozone and could be related to the expansion of thermophilous vegetation including Olea, which is traditionally attributed to anthropogenic influence. The mean annual precipitation curves fluctuate to a much greater extent. They show positive oscillations (wet phases) at the Atlantic chronozone, and at the transition between the Subboreal and the Subatlantic. In Spain, bioclimatic conditions comparable to those of the present day are recorded during the Subboreal chronozone. In southern France, the onset of modern thermic conditions seems to be reached only during the Subatlantic chronozone. Finally, at the latitudes 39–40°N and 43–44°N, the onset of the modern Mediterranean climate (in terms of temperature and precipitation) occurred simultaneously, about 3000 years ago.