Tufa buildups, landscape evolution and human impact during the Holocene in the Upper Ebro Basin

The presence of tufas is of outstanding significance due to their inter-relationship with human occupation and landscape changes. The impacts induced by human occupation in the tufa systems during the Holocene would influence the balance of their construction and destruction and the genesis of tufaceous sequences. In the studied area, a Mediterranean-transitional low mountain environment in North Iberian Peninsula, there is numerous tufa buildups scattered by bottom valleys and slopes. In this paper, a first diachronic synthesis of landscape changes during the Holocene is carried out based on construction and erosion of tufas, evolution of settlement and known environmental changes by pollen studies, to establish a landscape sequence in which assess the importance of the natural dynamics or human activity in shaping the landscape. The main components of the landscape in the Holocene, tufas, vegetation, and human intervention are analyzed from published references and our work on tufa. Reconstruction of landscape evolution is based on tufa morphosequence evolution and geomorphology, archeological, dating, and palynological data. Nineteen dated tufa buildups, ten archaeological sites of environmental interest, and two peat bogs are available.The evolution of the landscape involves human and natural changes during the Holocene. Nine different landscape stages have been differentiated from the Upper Pleistocene to historical times. The natural changes determine the landscape between 14 and 8.1 ka, after which the human occupation started to impact the natural environment. Approximately between 4.2 ka and 3.2 ka, the Millennium of Change occurred, when agriculture was consolidated, the population expanded, feeding habits and customs changed, the tufa system eroded, and intense landscape repercussions resulted. From 2 to 1.9 ka, a new constructive phase of tufas was generated that has also lasted to the present day. This new tufa construction stage is interpreted as the response to the re-balancing of the geosystem, now in balance with human use. This was the last phase of the anthropic-climate sequence that began 5.5 ka, determining the landscape evolution. The human intervention on the territory and consequent changes in the geomorphological processes on slopes and valley bottoms, altering tufa construction and generating intense erosion processes, soil loss and vegetation cover modification, permits us define the geomorphological and landscape Holocene evolution as a human-climate sequence.