Multiproxy study in two karstic lakes in the iberian ranges(Spain)climate variability and anthropogenic activities during the last millennium

Resumen

Present-day climate change strengthens the interest in studies combining climate variability and anthropogenic activities. In this thesis, we studied lacustrine sediments from two sites, (Lagunillo del Tejo and El Tobar), situated 60 km apart in the Iberian Ranges, to reconstruct climate and human activities during the last millennium. We used cladoceran subfossils, plant macrofossils and isotopes to investigate changes in lake levels and macrocharcoal analyses to infer fire history. Lagunillo del Tejo responds to rainfall variability with changes in its depth and littoral configuration, which determine the sedimentary inputs. More arid conditions are detected from the bottom of the cores that corresponded to the 11th century up to the 13th century coinciding with the end of the Medieval Climate Anomaly. The following period (AD 1300-1600) is characterized by alternation of arid and humid phases. A general increase in moisture conditions is recorded from AD 1600 to AD 1800 within the Little Ice Age (LIA) and a tendency towards marked dry events with some alternating wetter periods during the last centuries. Comparison of the inferred lake- level changes with historical/documentary sources shows that there is an obvious climatic forcing behind the lake-level changes recorded in Lagunillo del Tejo sediments. Additionally, there is a good chronological agreement between lowest fire activity and high lake- levels (ca. AD 1600 ¿ 1800), concurrent with the late LIA and the collapse of the Transhumance sheep breeding system. The reconstructed fire regime may therefore be natural (climate-induced), but can also be explained by important socio-economical events/changes, including wars against Muslims and the introduction of the Transhumance practices (AD 1273). In El Tobar, lake-level changes and trophic variations have been the main disturbances over the last millennium. During the Medieval Climate Anomaly relatively low water levels but continuous changes were registered and around AD 1550 lake level and productivity increased coinciding with the onset of the Little Ice Age. At that time, lake level was very likely controlled by rainfall variability and in particular by the inflow of the springs and seepage in the meromictic sub-basin. Higher lake levels were maintained until the end of LIA (ca. AD 1850). Then, an arid phase occurred until AD 1960 when a canal was built to drive water from a reservoir in a river from a neighboring valley to the lake. From that moment, the lake became man-regulated and therefore, it lost its condition as a climatic sensor. Although the transference of water led to a slightly increase in lake level, trophic conditions rose favoring the growth of planktonic populations. The most perceptible impact was the invasion of the planktonic cladocera Bosmina longirostris. Fires seem to have been caused by the socioeconomical changes cited above since there is no a good agreement between arid periods and high fire activity. Considering a multicentenial scale, sedimentary sequences of both lakes are consistent with the known major arid and humid phases during the last millennium: a dryer period during MCA, fluctuations in wetness from AD 1350 to AD 1600 and a humid phase around the Maunder Minimum. The variations of sedimentary proxies in the more recent part of the records were also concordant with the instrumental precipitation data: the transition between 19th and 20th centuries was marked by an arid period whereas the onward period (ca. AD 1930- 1980) was characterized by an increase precipitation. On the other hand, both fire histories were coincident from AD 1100 to AD 1550 and were provoked by regional events such as wars or transhumance. From AD 1500, there was a divergence of the signals in each lake caused by local activities such as charcoal production or sheep breeding. Both sediments revealed that human impact in that area was higher in the past than in the last centuries, which is concordant with population movements towards cities and to the coastlines. These sequences from Lagunillo del Tejo and El Tobar are good examples of the interplay between climate and human factors over the last millennium in the Iberian Ranges.