Reconstructing Holocene evolution in the archaeological site of Campo Lameiro (NW Spain) : an interdisciplinary approach to geoarchaeology

Abstract

The objective of this research was to evaluate the potential of sedimentary deposits to be used for the reconstruction of Holocene environmental changes in the Campo Lameiro area (NW Spain). We focused on the evolution of landforms as a key factor in the configuration of alluvial and colluvial sequences. The geomorphological and sedimentological studies indicate that the distribution of landforms is a main factor to understand the formation of colluvial soils. Granitic macroforms dominate the present landscape, constituted by alveolar depressions surrounded by crests and slabs. The thickest sedimentary deposits were found in the depressions. We identified two main stratigraphic units: a basal inorganic layer represented by alluvio‑ colluvial sediments, formed in a highly energetic environment, probably dating to the Younger Dryas (>11000 years BP), and a younger unit of thick sandy, blackish, organic matter rich, colluvium. The oldest radiocarbon age obtained for this unit indicates that it may have started to form by 11240‑ 11130 cal. BP. The Holocene colluvial soils show discontinuities in grain size, soil reaction, elemental composition of the inorganic phase and molecular composition of the soil organic matter. These features are evidence of the occurrence of several phases of erosion/sedimentation (i.e. landscape instability), some of which were coeval with known periods of Holocene abrupt climate change – the 8.2 ka event, the beginning of the Neoglaciation (ca. 6 ka BP) or the 2.8 ka wet/cold event. But some of the most intense phases coincided with increased human pressure on landscape during the Neolithic, Bronze Age, Roman Period, and the Middle Ages. Charcoal layers, burnt soil layers and the highly aromatic nature of the soil organic matter point to frequent fire episodes. Pollen studies also indicated a sharp decrease in forest cover beginning by ca. 6000 cal BP, which seems to have been accompanied by a progressive soil acidification with time. Our research suggests that both climate and human activities played an important role in the formation of colluvial deposits in the area, confirming that they are valuable geoarchives of Holocene environmental change from a geoarchaeological approach.