Stable carbon isotopes in archaeobotanical remains and palaeoclimate

Abstract

The present report describes a novel approach to infer the amount of precipitation in antiquity based on the analysis of carbon isotope composition (δ13C) in archaeobotanical remains. After discussing the physiological background of the technique, we illustrate the usefulness of δ13C as palaeoclimate proxy by means of a case study from the NE Iberian Peninsula. The goal of the study was to quantitatively reconstruct the evolution of seasonal (spring) and annual precipitation during the last 4000 years based on δ13C evidence. The samples analysed were charcoals of Aleppo pine and charred grains of barley and wheat. Our findings indicate that estimated past precipitation was consistently higher than today, with a gradual trend towards a drier climate. This increase in aridity, however, did not develop uniformly; instead, two main phases of greater precipitation (1800900 BCE; 300 BCE 300 CE) alternated with drier periods (900300 BCE; 900 CE present). The relative significance of spring rainfall in the past was variable. From approximately 300 BCE onwards, spring accounted for a higher proportion of annual rainfall than is the case today. In contrast, during the period 1800800 BCE, the contribution of spring rainfall to annual precipitation was less. A transition phase occurred from ca. 800 to 300 BCE, a period marked by a sudden recovery in spring precipitation. Subsequently, the synchrony of δ13C changes in grains and charcoal points to the installation of the Mediterranean climate in the region.