The use of radiocarbon-derived Δ~(13)C as a paleoclimate indicator: applications in the Lower Alentejo of Portugal

摘要

Values of δ~(13)C are frequently reported with radiocarbon dates from organic materials. In C_3 plants δ~(13)C values have been linked to changes in water use efficiency as a response to arid conditions. By calculating ~(13)C discrimination (Δ~(13)C) from ~(13)C isotopic composition (δ~(13)C), archaeologists can gain potentially valuable inference into past climate conditions. Values of Δ~(13)C reflect the process of discrimination against heavier ~(13)C isotopes of carbon by comparing the δ~(13)C of samples to that of the atmosphere, and can be calculated when records of atmospheric δ~(13)CO_2 are available. The present study examines a 1300 year history of radiocarbon-derived Δ~(13)C from the Lower Alentejo of Portugal using charcoal recovered from excavations of a series of medieval habitation sites in the study area. To calculate Δ~(13)C, the posterior means generated from Bayesian change-point analysis of δ~(13)CO_2 records were used. Archaeological data were then compared to contemporary ecological studies of Δ~(13)C of the same taxa against instrumental records of climate. Values of Δ~(13)C 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 Δ~(13)C values frequently fell to low levels associated with arid conditions. At this time environmental degradation and erosion were documented. Values of Δ~(13)C 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 Δ~(13)C is a potentially useful paleoclimate proxy for archaeologists provided that results can be paired with observed Δ~(13)C variation in studies that pair these data with instrumental climate records.