Holocene environmental changes in lacustrine lake sediments from Northern ChileanPatagonia (Chile)

L. Nuttin L.1, N. Fagel N.1, G. Borgniet G1, S. Bertrand S.2, S. Schmidt3,

A. Arraneda A.4,5, F. Torrejon F.4,5 and R. Urrutia R.4,5

(1)AGEs -Clays, Sedimentary environments and Geochemistry, Department of Geology, University of Liege, Allée du 6 aout, B-4000, Liège, Belgique

(2)EU Marie Curie Postdoctoral Fellow, AWI, Bremerhaven, Germany

(3)Département de Géologie et Océanographie, Bordeaux, France,

(4)Aquatic Systems Research Unit, EULA – Chile Environmental Sciences Centre, Universityof Concepcion, Casilla 160-C, Concepcion, Chile

(5)PatagonianEcosystemsResearchCenter(CIEP), Coyhaique, Chile

The Southern Hemisphere has a crucial, but still not constrained, role in global climatic fluctuations. In order to improve our understanding of the past climate changes in southern South America and their impact on terrestrial and lake environments, we investigate the sedimentary record of three small lakeslocated between 45 and 47°S in the Northern Chilean North Patagonia. Our aim is to evaluate natural climate variability over the Holocene and to decipher between climate and anthropogenic factors during the Late Holocene. In each lake, the a coring site was chosen selected after a bathymetric survey using an echo-sounder. The short cores were retrieved using an Uwitec gravity corer: the length of the cores ranges between 60 and 140 cm depth. We conducteda multiproxy study combining sedimentological, mineralogical and geochemical analyses (LOI 105, 550 and 950°C, biogenic silica content, magnetic susceptibility, C/N ratio, bulk bulk mineralogy by X-ray diffraction, inorganic geochemistry by ITRAX datacore scanner and ICP-AES). The corechronologies arey is based on a combination of 210Pb and 137Cs radionuclides in the upper part and radiocarbon dating (is completed in the deepest part by 2 to 514C datingsdateson bulk sediment or plant macrofossils per core were obtained). According to theage-depth models thetwo southern lakes are characterised by very low sedimentation rates, averaging 0.1 mm/yr in Lago Larga (47°S) and 0.2 mm/yrin Lago Trapial (46°S). With an accumulation rate of 1 mm/yr,only Lago Thompson (45°S) can be studied with a will allow to reach a decennial resolution. The sSediment deposition in the three lakes is not affected by main disturbances. They however contain , except a few volcanic layers detected by macroscopic description and/or smear slide observation. that represent explosive eruptions of volcanoes from the southern volcanic zone of Chile (Hudson, Cay, Macá). In particular, the base of the core collected in Lago Larga contains one 4 cm-thick layer is observed at the base of Lago Larga. This layerthat probably corresponds to a major eruption of the Hudson volcano 8 kyr ago. Likely all the parameters depict a major change in the upper part of Lago Trapial. Characterised by a significant change in C/N ratio this event is related to a recent anthropogenic perturbation of the watershed, probably related to the road construction in the Eighties. Sediments from Lago Larga and Lago Trapial are mainly composed by of detrital componentsparticles, and their biogenic silica content is . Based on leachate experiment, the biogenic silica usually represents less lower than 5 %. In contrast,the biogenic silica content of Lago Thompson is characterised by a much higher biogenic fraction, rangesing between 40 to and 80%, which is likely due to the small catchment to lake surface ratio that reduces the supply of terrestrial particles to the lake. Only Lago Trapial shows a major change pointing towards an abrupt increase in terrestrial supply during the last century. This event most likely represents a recent anthropogenic perturbation in the lake watershed, which is associated with the construction of a main road on the southern shore of the lake in the Eighties. These preliminary results paleoproductivity profiles will later be later compared with the identification of biological assemblages proxies (diatom, chironomid, pollen) to reconstruct paleoenvironmental and/or paleoclimatic changes over the last milleniamillennia. This research is funded by Fondecyt project number 1070508 and by a CGRIWallonie-Chilecooperation project.