Holocene environmental changes in the Gallocanta lacustrine basin, Iberian Range, NE Spain

The sedimentological and palynological study of the Gallocanta lake sediments, in the NE of Spain, has allowed the Holocene environmental evolution of this lacustrine basin to be established, and several changes in the hydrological balance to be interpreted. The chronology of the sequence is based on new radiocarbon datings and 210Pb radiometric techniques. Ten facies have been characterized, which have been grouped into five sedimentary units: two (Units A and B) are interpreted as deposited in alluvial areas, and three (Units C, D and E) in lacustrine environments. Moreover five pollen zones have been established for this succession. From the stratigraphic correlation of the cores extracted in this lake, three stages in the basin evolution have been interpreted. Stage 1 was characterized by a broad development of alluvial fans during the Fullglacial or Lateglacial times. The sediments of this stage are lacking in pollen as they were exposed for most of the time. Stage 2 started around 10 100 yr BP and is characterized by the establishment of a brackish lake linked to a decrease in alluvial supplies and a more positive water balance. The maximum lake level would have been reached around 8010 yr BP coinciding with the relatively moist period of the beginning of the Holocene recorded in some Spanish lakes, although this dating would not match that of other areas of the Iberian Peninsula, in which the maximum lake levels have been recorded around 6000 yr BP. A sharp decrease in the lacustrine level has been registered at 3405 yr BP, which may be correlated to a change to more arid conditions. Nevertheless, the lake level recovered later. Stage 3 was linked to the development of an ephemeral shallow lake. An age of AD 1840 yr has been calculated for this palaeogeographical change. The data obtained suggest that the main vegetation changes during stage 2 were more related to climate variability, whereas during stage 3 the anthropogenic impact became more significant.