Modern-analog studies and high-resolution paleoenvironmental reconstruction of the last 500 years using the varved sediments of the Mediterranean Lake Montcortès (Central Pyrenees)

Abstract

[eng] Varved lake sediments also known as annually laminated sediments, are natural paleoenvironmental archives containing high-resolution proxy data and precise chronologies. They are one of the few natural archives that can provide enough time resolution (seasonal/annual) to bridge the temporal gap between past and present environmental data to ensure the continuity between climatic /ecological data and paleoclimatic /paleoecological data. However, it is not easily to manage due to the scarcity of this kind of archives and the lack of modern analog studies required for inferring reliable its seasonal signal This thesis focuses on the study of modern sedimentary analogs of a Pyrenean lake with varved sediments and into reconstruct the last 500 years of environmental change at high temporal resolution (sub-decadal). It is aimed to provide a tool for improving paleoecological reconstructions and to contribute to bridge the temporal gap between ecology and paleoecology by providing long-term high-resolution and continuous paleoenvironmental data. To do this, we performed a two-year of monthly limnological and sedimntological monitoring at lake Montcortès (Central Pyrenees) with special regard for biological and biological induced proxies ( calcite, diatoms and pollen). We reconstructed the last 500 years of lake-catchment system environmental history and its interactions by using fossil pollen and pigments as environmental indicators Data obtained during the modern analog study (2013-2015) revealed a strong seasonal trend for all studied proxies. Changes in calcite, pollen and diatoms were highly depending on seasonal succession of lacustrine and terrestrial life forms that, in turn, were modulated by environmental variables. There appeared clear dissimilarities in terms of timing and seasonal signal recorded in the three proxies between years that have been potentially related with changes in temperature and precipitation indicating sediment sensitivity to inter-annual variations. Pollen has been revealed as a most reliable indicator to track seasonality on the sediment record, being the one maintaining the same seasonal signal between years. While periods of major calcite precipitation can fluctuate within spring, summer and fall and diatoms may suffer breakage and dissolution depending of water conditions, which would truncate the final sedimentary signal. With pollen data in combination with independent evidence from historical sources we have documented in detail most important factors responsible for landscape modulation in Lake Montcortès during the last 500 years. Such factors were mainly human related namely cropping, livestock breeding, and hemp retting. Sedimentary pigment data in combination with pollen data showed that changes in land use greatly influenced aquatic photosynthetic community indicating lake-catchment connectivity. After 1850 CE, coinciding with the beginning of industrialization, vegetation and aquatic community showed a clear point of change although with opposed inferred signals. While vegetation changes indicate land abandonment and less human pressure in the area, aquatic community indicated a trend towards eutrophication. Such change on aquatic community could be an effect of non-point nutrient sources from historical legacies of intense land use joined to atmospheric deposition derived of the industrialization process. Overall, for both, catchment and lake, the main signal inferred during the last 500 years was related with human-pressure even during harsher climate conditions (LIA). The only climate related signal inferred were heavy rainfall episodes occurred during the last of half 19th century, indicated by both proxies. However, there is still many uncertainties and open questions to solve probably related with climatic and natural forcing not accounted for directly in this work. The results obtained in this thesis by combination of modern analog studies and high-resolution paleo environmental provide valuable long-term continuous data to contribute to understand current ecological changes and the past environmental history as part of a time continuum.

[cat] Els sediments lacustres varvats, també coneguts com sediments laminats anualment, són arxius paleoambientals naturals que contenen informació ambiental a molt alta resolució temporal i amb els quals es poden obtenir cronologies precises. Són un dels pocs arxius naturals que poden proporcionar una resolució temporal suficient (estacional /anual) per tal de tancar la bretxa que existeix entre les dades ambientals obtingudes de estudis pealeoambientals, és a dir del passat, i les dades obtingudes de l’estudi de l’ambient en el present. Aquesta tesi es centra en l'estudi dels anàlegs sedimentaris moderns d'un llac pirinenc amb sediments vavats i en la reconstrucció, a alta resolució temporal (sub-decadal), dels últims 500 anys de canvi ambiental. L'objectiu és proporcionar una eina per a millorar les reconstruccions paleoecològiques i contribuir a superar la bretxa temporal entre l’ecologia i la paleoecologia proporcionant dades paleoambientals continues i d'alta resolució. Les dades obtingudes durant l’estudi d’anàlegs moderns (anys 2013-2015) van revelar l’existència d’una forta tendència estacional per part de tots els indicadors estudiats. S’han observat diferències en tots tres indicadors estudiats en termes de temporalitat i senyal estacional que han estat potencialment relacionades amb canvis de temperatura i precipitació. Aquest fet ens indica la sensibilitat del sediment a enregistrar les variacions interanuals. La combinació de dades pol·líniques, juntament amb documentació històrica, ens ha permès documentar detalladament els factors més importants responsables dels canvis en el paisatge durant els darrers 500 anys. Aquests factors són principalment d’origen antròpic essent bàsicament cultiu, cria de bestiar i activitats relacionades amb el cànem. L’estudi de les dades de pigments fotosintètics de les comunitats aquàtiques en combinació amb les dades pol·líniques ens indiquen que els canvis en l'ús del sòl han estat els major responsables dels canvis ocorreguts en els productors primaris del llac indicant la connectivitat existent entre la conca i el llac. Els resultats obtinguts en aquesta tesi a partir de la combinació d’estudis d’anàlegs sedimentaris moderns i les reconstruccions paleoecològiques d’alta resolució proporcionen series de dades llargues i contínues que ens ajuden a entendre els canvis ecològics actuals i la història ambiental del passat com a part d’un continu de temps.