High-resolution (sub-decadal) pollen analysis of varved sediments from lake Montcortès (South-central pre-Pyrenees): a fine-tuned record of landscape dynamics and human impact during the last 500 years.

A high-resolution (average 6 years/sampling interval) palaeoenvironmental reconstruction using pollen, charcoal and non-pollen palynomorphs was carried out on annually laminated sediments of Lake Montcortès (southern Pyrenean flank). The results were combined with historical data to better understand landscape evolution and human interaction during the last 500 years. Our results show that human activities (cropping, livestock breeding and hemp cultivation and retting) have been the most important factors responsible for vegetation changes with highest intensity between 1530 and 1900 CE. By means of a sub-decadal study we have been able to evaluate short-lasting events at local and regional scales related to climate (heavy rainfall events and, high-land forest fluctuations) or to historical and well-dated and documented socio-economic events (i.e., crop promotions (hemp) or land abandonment-population emigration). The temporal extent (400 years) and continuity of Cannabis pollen peak have been confirmed, and new evidence of water quality changes, likely as a consequence of hemp retting practices between the mid-17th to late 19th century, are provided. This is the first high-resolution palaeoenvironmental study carried out in a varved lake on the Iberian Peninsula so far. With these data we hope to contribute to filling the gap in high-resolution palaeoenvironmental data

Seasonal patterns of pollen sedimentation in Lake Montcortès (Central Pyrenees) and potential applications to high-resolution paleoecology: a two-year pilot study.

Lakes with varved sediments are especially well suited for paleoecological study, from annual to even seasonal resolution. The interpretative power of such high-resolution paleoenvironmental reconstructions relies on the availability of modern analogs with the same temporal resolution. We studied seasonal pollen sedimentation in varved Lake Montcorte`s, Central Pyrenees (Spain), as a modern analog for highresolution reconstruction of Late Holocene vegetation and landscape dynamics. Seasonal samples were obtained from sediment traps that were submerged near the maximum water depth for a 2-year period (fall 2013 to fall 2015). Seasonal pollen sedimentation was compared with meteorological variables from a nearby weather station. Bulk pollen sedimentation, dominated by Pinus (pine) and Quercus (oak), followed a clear seasonal pattern that peaked during the spring/summer, coinciding with maximum temperature and precipitation, minimum relative humidity and moderate winds from the SSE. Pollen sedimentation lags (PSL) were observed for most pollen types, as substantial amounts of pollen were found in the traps outside of their respective flowering seasons. Two pollen assemblages were clearly differentiated by their taxonomic composition, corresponding to spring/summer and fall/winter. This pattern is consistent with existing interpretation of the sediment varves, specifically, that varves are formed by two-layer couplets that represent the same seasonality as pollen. We concluded that pollen sedimentation in Lake Montcorte`s exhibits a strong seasonal signal in the quantity of pollen, the taxonomic composition of the pollen assembalges, and relationships between the pollen and meteorological variables. Thus, varved sediments provide a potentially powerful tool for paleoecological reconstruction at seasonal resolution. This method could be used not only to identify paleoenvironmental trends, but also to identify annual layers and therefore date sediments, even in the absence of evident sediment laminations. A satisfactory explanation of PSL will require further studies that examine internal lake dynamics and pollen production/dispersal patterns

Modern Analogue Approach Applied to High-Resolution Varved Sediments-A Synthesis for Lake Montcortès (Central Pyrenees)

In Quaternary paleosciences, the rationale behind analogical inference presupposes that former processes can be explained by causes operating now, although their intensity and rates can vary through time. In this paper we synthesised the results of di erent modern analogue studies performed in a varved lake. We discuss their potential value to obtain best results from high resolution past records. Di erent biogeochemical contemporary processes revealed seasonality and year-to-year variability, e.g., calcite precipitation, lake oxygenation, production and deposition of pollen and phytoplankton growth. Fingerprints of the first two of these processes were clearly evidenced in the varve-sublayers and allow understanding related to past events. Pollen studies suggested the possibility of identifying and characterizing seasonal layers even in the absence of varves. Marker pigments in the water column were tightly associated with phytoplankton groups living today; most of them were identified in the sediment record as well. We observed that 50% of these marker pigments were destroyed between deposition and permanent burying. In another study, seasonality in the production/distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) and derived temperature estimates were investigated in catchment soils and particles settling in the lake. The signatures of brGDGTs in depositional environments mainly were representative of stable conditions of soils in the catchment that last over decades; no brGDGTs seemed to be produced within the lake. The main contribution of this review is to show the advantages and limitations of a multiproxy modern-analogue approach in Lake Montcortès as a case study and proposing new working hypotheses for future research

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

[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