The use of radiocarbon-derived Δ13C as a paleoclimate indicator: applications in the Lower Alentejo of Portugal

Values of δ13C are frequently reported with radiocarbon dates from organic materials. In C3 plants δ13C values have been linked to changes in water use efficiency as a response to arid conditions. By calculating 13C discrimination (Δ13C) from 13C isotopic composition (δ13C), archaeologists can gain potentially valuable inference into past climate conditions. Values of Δ13C reflect the process of discrimination against heavier 13C isotopes of carbon by comparing the δ13C of samples to that of the atmosphere, and can be calculated when records of atmospheric δ13CO2 are available. The present study examines a 1300 year history of radiocarbon-derived Δ13C from the Lower Alentejo of Portugal using charcoal recovered from excavations of a series of medieval habitation sites in the study area. To calculate Δ13C, the posterior means generated from Bayesian change-point analysis of δ13CO2 records were used. Archaeological data were then compared to contemporary ecological studies of Δ13C of the same taxa against instrumental records of climate. Values of Δ13C fell within mean ranges for the taxa through a period of population growth between the 7th and 10th centuries AD. During the height of the Medieval Warm Period in the 11th century AD Δ13C values frequently fell to low levels associated with arid conditions. At this time environmental degradation and erosion were documented. Values of Δ13C increased for a brief period in the early 12th century AD before the rural Lower Alentejo was largely abandoned for nearly two centuries. Another period of aridity occurred in the 16th and 17th centuries AD. Radiocarbon-derived Δ13C is a potentially useful paleoclimate proxy for archaeologists provided that results can be paired with observed Δ13C variation in studies that pair these data with instrumental climate records.

► Stable carbon isotopes from radiocarbon-dated charcoal can indicate paleoclimate. ► Bayesian change-point models can aggregate atmospheric δ13C records. ► Pairing radiocarbon-derived Δ13C with modern data improves interpretations. ► The Lower Alentejo may have been more arid in the medieval warm period. ► Aridity and erosion may contribute to the abandonment of rural Lower Alentejo.