Climate and environmental changes over the last 2000 years in the Serra da Estrela, Portugal.

The compound-specific signal of leaf wax n-alkanes can be used for reconstructing climatic and environmental changes. This work assesses the carbon and hydrogen isotopic compositions of sedimentary n-alkanes from a high-mountain lake record (Lake Peixão) in Serra da Estrela (Western Iberia, Portugal) over the last 2000 years.

Our interpretations are supported by the modern settings of the lake and the long-chain n-alkanes characterization of the modern vegetation in the lake’s watershed. The δ¹³C signal of long-chain n-alkanes (C₂₉ – C₃₃) suggests that terrestrial vegetation, dominated by C₃ heathlands and grass mosaics, did not change significantly since the last 2000 years. In contrast, δ¹³C signatures of C₂₅ and C₂₇, associated with aquatic plants inputs, reveal a more enriched and variable signal, suggesting these compounds as indicators of water availability in the studied area.  In this high-altitude setting, temperature significantly controls water availability and the δD signal of terrestrial plants (δD_terr), which shows major changes across the last 2000 years. In contrast, aquatic-derived n-alkanes (δD_aq) show a relatively constant and stable water source. These data enabled us to detect major climate shifts in the region and evaluate the role of the main drivers (solar activity and the North Atlantic Oscillation-NAO) in those long-term changes prior to intense human activities.

The Roman Period (0 – 500 AD) was relatively dry but shifting from a warm to a cold phase, under a predominant positive mode of NAO and a Grand solar maxima. From the Dark Ages until the Medieval Climatic Anomaly (500 – 1300 AD), the climate was generally mild and wet under a nonstationary mode of NAO and a gradual decrease in solar irradiance. The LIA (ca. 1350 – 1850 AD) was composed by two main phases: the first cold and wet followed by an extreme cold episode; both under a predominantly negative NAO mode. The extreme cooling (centered at 1700 AD), coincident with the Maunder Minimum, was driven by the southward displacement of the polar front, causing extended periods of ice cover on the lake. The climate became warm and dry since 1880 AD, coinciding decrease in vegetation capability to buffer the runoff energy and promote the observed high sedimentation rate during this period, probably due to increased anthropogenic impact.  This work also underlines the sensitive nature of the high-mountain lake ecosystems and contributes to the spatial coverage of paleoclimate studies in the Atlantic region of the Iberian Peninsula.

Acknowledgments

The authors are grateful to FCT (Fundação para a Ciência e a Tecnologia) for the financial support of this work through the projects: HOLMODRIVE—North Atlantic Atmospheric Patterns Influence on Western Iberia Climate: From the Late Glacial to the Present (PTDC/CTA-GEO/29029/2017). WarmWorld—Features, and lessons from Past Interglacials “warm periods” during the last 1.5 Ma (PTDC/CTA-GEO/29897/2017). RNS´s grant supported by Ultimatum—Understanding past climatic instabilities in the North Atlantic Region (IF/01489/2015)