Stable carbon isotopes in archaeobotanical remains and palaeoclimate

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.