A climitic interpretation of 15, 000 years of environmental variability in the central Pyrenees
by
Jordi Catalan
Department of Ecology, University of Barcelona, Diagonal 645, Barcelona 08028, Spain
Coauthors: Lluís Camarero (Universitat de Barcelona), Sergi Pla (Universitat de Barcelona), Teresa Buchaca (Universitat de Barcelona), Maria Rieradevall (Universitat de Barcelona), Jaume Bordonau (Universitat de Barcelona), Joan Garcia (Universitat Politècnica de Catalunya), Josep Peñuelas (CREAF-CSIC), Ramon Pérez (Universitat Autònoma de Barcelona)

Holocene climatic variability in the Pyrenees has been inferred mainly from lake sediment pollen records. Although it has provided a clear sequence and chronology of changes, the climatic interpretation is still speculative, particularly in quantitative aspects. Here we present a summary of environmental changes throughout the last 15,000 years in the Central Pyrenees, based on multi-proxy analysis of records from two lakes of contrasting sedimentation rate. At present, one of the lakes (Redòn) is located above, and the other at tree line (Redó d'Aigüestortes). In both cases, the main features of the pollen record agree with already known sequences for the Central Pyrenees. In the longest record (Redó d'Aigüestortes), the Younger Dryas fluctuation is well recorded.

To independently estimate the climatic clues of the vegetation changes. Mean annual temperature fluctuations were inferred by means of several transfer functions based on chrysophytes, chironomids and diatoms. All proxies show the Younger Dryas fluctuation quite clearly, however, the rest of the Holocene was flat, but for Redòn, were a clear cold event around 8,000 years ago was recorded. The flatness of temperature fluctuations contrasts with the changes in pollen taxa and aquatic organisms throughout the Holocene. Human impacts in this area did not appear until late in the Holocene and it has been always low due the remoteness of the area. Therefore, we have to assume either that the resolution of our temperature transfer functions was too coarse (which is possible), or that the observed taxa fluctuations were induced by climatic components other than mean annual temperature.

We do not have moisture transfer functions yet, but we are able to reconstruct some chemical components of the waters (e.g. pH, alkalinity, sodium) using diatoms and chrysophyte cysts. Trends in diatom-inferred alkalinity agree well with independent geochemichal estimations of rock weathering. These patterns show significant higher chemical weathering in the first par of the Holocene, with a maximum from 6,000 to 8,000 years ago. This period corresponds to the maximum of deciduous trees according to the pollen record. The weathering signature is particularly clear in the lake located above tree line, in a catchment that probably has never been occupied by trees, since no macrofossil remains have been found in the sediment. The changes in chemical weathering suggests moisture fluctuations, particularly rainfall during summer. In that sense, changes in deposition described for Northwestern Africa may be extended, at least, up to the Pyrenees.

Reconstruction of sodium concentrations in lake water, which in this case cannot be checked against other proxies, indicate a change in the maritime nature of the atmospheric deposition occurring around the 8,200 cold event. Before the event, the inferred sodium was significantly higher in lake water, and after it the concentration dropped to present levels. The sodium fluctuations were independent from alkalinity changes, and they indicate a more maritime influence during early Holocene than after the 8,200 event.

Date received: April 27, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-39.