Marta Garcia

Recent climatic and anthropogenic imprints on lacustrine systems in the Pyrenean Mountains inferred from minerogenic and organic clastic supply (Vicdessos valley, Pyrenees, France)

High-resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin: Lake Majeur and Lake Sigriou (1630 m a.s.l. and 1995 m a.s.l., respectively, Eastern French Pyrenees). Identifying specific minerogenic and organic markers of autochthonous and allochthonous supply, data allow documenting past climatic and anthropogenic pressures. Over the past century, the lacustrine sediment of Lake Majeur has been essentially composed of algae, drastically contrasting with the natural sedimentary infill of the basin, mainly resulting from soil erosion from the mid–late Holocene. Since ad 1907, the Lake Majeur has been used for hydroelectricity production. Human-induced lake-level regulations, affecting up to 37% of the lacustrine surface, have increased by fourfold the accumulation rate of the lake and favoured water enrichment. Rubidium abundance within the lacustrine sediments of the two lakes reflects the mid–late Holocene palaeohydrology. After dam construction in ad 1907, greater quantities of rubidium found in Lake Majeur sedimentary infills indicate drier climatic periods, such as from ad 1975 to ad 1982, during which water reservoirs were particularly in demand. Inversely, before the dam was built, rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation, synchronous with European climatic deterioration phases. We notably document that the Mediaeval Climate Anomaly was interrupted by some humid periods dated c. ad 940, ad 1080, ad 1100 and ad 1250. We also date the onset of the ‘Little Ice Age’ c. ad 1360 and identify that this period was wetter after c. ad 1500.

Sea surface temperature reconstructions over the last 70 kyr off Portugal: Biomarker data and regional modeling

This study aims at providing robust temperature reconstructions for a key oceanographic setting in the North Atlantic and at understanding the nature of the temperature signal recorded by the two biomarkers U-37(k) and TEX86, considering season and depth of production. To do so, high-resolution signals of U-37(k) and TEX86 are determined for the last 70kyr for core MD95-2042, located off Portugal. Signals of U-37(k) and TEX86 present a tight correlation, demonstrating a dominant temperature effect. U-37(k) signals correspond well to the annual mean sea surface temperature (SST), whereas TEX86H-derived temperatures are 5.6 degrees C higher, which is unrealistically warm for this area. Unsuitable TEX86 global linear calibrations on the Iberian Margin may suggest a possible occurrence of archaeal communities with specific temperature response. To assess the impact of different season or depth of production of the biomarkers on the recorded temperature in the sediment, modeled temperature proxies (Tproxies) are introduced in a Regional Oceanic Modeling System and tested for different seasons (annual/summer/winter) and depths (surface and 0-200m) of production for three climate modes (Present Day (PD), Last Glacial Maximum (LGM), and Heinrich Stadials (HS)). Similar temperature amplitudes between climate modes are found at MD95-2042 core site for observations, for both biomarkers, and for modeled annual surface production Tproxy: 5.5-7 degrees C for T(PD-LGM) and 3-4 degrees C for T(LGM-HS). Therefore, we propose a new TEX86H regional calibration to reconstruct present and past annual mean SSTs on the Iberian Margin.