Quaternary International 93–94 (2002) 13–18 Late-glacial and Holocene palaeoclimatic record from Sierra de Cebollera (northern Iberian Range, Spain) M. Jos ! e Gil Garc ! ıa*, Miriam Dorado Vali * no, Ana Valdeolmillos Rodr ! ıguez, M. Blanca Ruiz Zapata Departamento de Geolog ! ıa, Edificio de Ciencias, Universidad de Alcal ! a, 28871 Alcal ! a de Henares, Madrid, Spain Abstract The study of a pollen sequence obtained from a peat deposit in Hoyos de Iregua (Iberian Range, north-central Spain) has been carried out, revealing new information about the vegetation dynamics and climate changes since Late-glacial times. Pollen zones have been defined corresponding to successive phases in vegetation history during the end of the Late W. urm (Oldest Dryas, Late- glacial interstadial, Younger Dryas) and the Holocene. During the Oldest Dryas event, there was a sparse vegetation cover dominated by herbaceous taxa, with Pinus as the unique arboreal taxon with some importance. The vegetation landscape during the Late-glacial interstadial (13,000–11,000yr BP) corresponded to coniferous forest and mixed forests, including the presence of evergreen Quercus. Between the Late-glacial and the Holocene, a brief cooler and drier phase occurred, the Younger Dryas event (11,000–10,000yr BP). During this period an important reduction of the tree masses characterized the evolution of the landscape. The Holocene began with a period of increasing temperature that resulted in an expansion of the arboreal elements, which reached their highest representation during the Holocene Climatic Optimum. From 5060790 BP a change to more arid conditions occurred, favouring the development of the mediterranean vegetation. Within the Late Holocene, clear evidence of the human impact on the environment are detected, being more accentuated towards the uppermost levels. r 2002 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction This work presents the results of the palynological study of a core obtained from the peaty deposit of Hoyos de Iregua (Sierra de Cebollera) located at 1780 m asl (Fig. 1). The Sierra de Cebollera is in the northwest section of the Iberian Range. It is a mountain formation aligned in a general W–E direction, which for 30km forms the drainage divide between the Duero and Ebro rivers. It also largely coincides with the administra- tive boundary separating the provinces of La Rioja (to which the northern slopes pertain) and Soria (Ortigosa Izquierdo, 1985, 1986; Sanz P! erez and Pellicer Corellano, 1994). The highest sectors are characterized by a dense and varied glacial morphology. Cirques are the most common glacial formation. These morphologies that have formed in the Sierra de Cebollera present, in general terms, a similar configuration; they appear as hollowed-out basins, with roughly semicircular borders. From a climatic point of view, this area currently receives Atlantic influences from the Cantabrian Sea, which arrive after first passing over the Basque and Obarenses Mountains, and which weaken towards the southeast due principally to the Fo. ehn effect. For this reason and because of the high altitude, the headward portion of the Iregua River is the most humid area in the entire basin. The area belongs to the Oro- Mediterranean bio-climatic horizon, with an annual rainfall between 1000–1500mm, and an average annual temperature of 101C (N ! u * nez Olivera and Mart ! ınez Abaigar, 1991). 2. Methodology The analysed core (H1) was obtained from the peaty deposit of Hoyos de Iregua, in Sierra de Cebollera (42101 0 26 00 N, 2145 0 W) at 1780 m a.s.l., with a northerly orientation (Fig. 1). A manual Russian corer (Aaby and *Corresponding author. E-mail address: mjose.gil@uah.es (M.J. Gil Garc ! ıa). 1040-6182/02/$-see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII:S1040-6182(02)00003-4