Palaeolimnology of Lake Hess (Patagonia, Argentina): multi-proxy analyses in short sediment cores.

P. GUILIZZONI, J. MASSAFERRO, A. LAMI, E. PIOVANO, S. RIBEIRO GUEVARA, S. FORMICA, R. DAGA A. RIZZO AND S. GERLI

HYDROBIOLOGIA

Springer

Año: 2009 vol. 631 p. 289 - 302

0018-8158

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0mm; margin-bottom:.0001pt; mso-pagination:widow-orphan; mso-hyphenate:none; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:IT; mso-fareast-language:AR-SA;} @page Section1 {size:612.0pt 792.0pt; margin:72.0pt 90.0pt 72.0pt 90.0pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Compared with North America and Europe, relatively few studies have quantified the anthropogenic and/or climate impacts in Patagonian lakes. We addressed these issues by analysing geochemistry, lithology, pigments and chironomid remains from sediment cores collected from Lake Hess (41° 22´ 20” S, 71° 44´ 0” W) located in the Nahuel Huapi National Park, in northern Patagonia. The aim of this study is to provide a palaeoenvironmental and climate reconstruction of the last ca. 4-5 centuries for the cold oligotrophic, quasi-pristine Lake Hess which receives melt waters from the Tronador ice cap. Chronology was based on 137Cs and 210Pb measurements of the upper core layers and the sedimentation rate of 23.2 mg cm-2 y-1 (0.148 cm y-1) was in good agreement between both techniques. Lake Hess was rich in tephra deposits particularly in the lower part of the cores. Peaks of ash layers were evident through magnetic susceptibility profiles. Tephra layers were used for core correlation. Results from the multiproxy analysis in the longest core of 80 cm allowed us to clearly identify three main periods. From the bottom up to 40 cm, the sediment shows light-grey clays. Both pigments and chironomids, suggest a fairly high productivity. From 40 cm to 22-25 cm, sedimentary record is composed of alternating black and dark clay layers with variable amounts of macrophyte remains. Chironomid head capsule abundance decreased and pigment concentrations were also very low. These facies are composed of very fine and plastic sediments with some faint laminated intervals with a organic matter composition gradually decreasing towards the end of the zone. We interpreted this period as cold and wet and probably synchronous with the Little Ice Age (LIA). A sharp change occurs at 25 cm showing a strong increase of organic matter, algal nutrients, and plant pigments together with a switch in the composition of chironomid assemblages. This, documents a change in the trophic condition of the lake. Although there are records of human impact in the study environment, particularly on extended fires, we believe that owing the lake location, most of the productivity changes in Lake Hess sediment sequence were firstly related to climate changes, specially to variations in moisture balance bring about by the westerlies