Diatoms, sedimentology and geochemistry study from a high altitude lake in the Fuegian Andes

FERNADEZ, M.; SAN MARTIN, C.; BIGLER, C.; RYDBERG, J.; PONCE J. F.

Congreso; IAL IPA 2022; 2022

In order to analyze environmental and climatic changes during the late Holocene, alacustrine sediment core was obtained from Laguna Ceniza (54˚ 40’ 38” S; 68˚ 13’ 13” O)located in a hanging valley in the Fuegian Andes (Argentina). The lake is fed by a streamoriginating from a small cirque glacier located in the valley head. The core (75 cm) wastaken close to a delta of the stream and covers the last 2775 cal years BP. Here, we presentthe preliminary results based on sedimentological, geochemical and diatom data from theupper part of the core (last 1500 cal years BP). The sedimentological analysis shows thepresence of six lithofacies: grey silty-sand, reddle silty-sand, laminated sandy-silt, laminatedsilt, grey clay-silt and white clay-silt. These lithofacies are likely associated to variations inthe relative importance of glacier dynamics and mass movement processes from the valleyslopes. The geochemical data (ITRAX core scanner) show a general decrease in the Al-Titowards the top, and throughout the core there are several distinct peaks in Ca-Sr (possiblyindicating input from the glacier). The diatom assemblages show a variety of species mainlydominated by Pinnularia microstauron (60 %) and P. interrupta, P. viridiformis, Aulacoseiraalpigena, Brachysira huitatorum, Encyonema difficilis, Frustulia aff fuegiana, F. rhomboidesand Veigaludwigia urbana in lower frequency (20%). In general, these taxa indicate afreshwater basin with oligotrophic conditions. At 16-17 cm of the core (900 cal years BP),Achnanthidium minutisimum reaches a frequency of 40%, together with an increase ofPsammothidium incognitum (28%) and P. confusum (9%). The clear rise in the mentioneddiatoms is associated to the reddle silty sand lithofacie, suggesting the occurrence of a highenergyflow event into the lake.