ENVIRONMENTAL MAGNETISM STUDY OF A HOLOCENE EOLIAN SEDIMENTS AND PALEOSOLS SEQUENCE IN THE NORTH OF TIERRA DEL FUEGO (ARGENTINA)

ORGEIRA, M. J.; CORONATO, A.; VASQUEZ, C.; PONCE, J. F.; MORETTO, A.; EGLI, R.; ONORATO, M. R.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Año: 2010 vol. 51 p. 191 - 193

0006-6729

The studied sequence is located in the northern region of Isla de Tierra del Fuego, Argentina (53° 42’ 48.6’’S, 68° 18’ 20.3’’W, Fig. 1), altitude 71 m asl (Coronato et al, 2009). The present mean annual rainfall in the area is around 380 mm and the mean annual temperature is 5.2° C. The sequence is located in a large area of low pressure under the effect of both the westerlies and the Polar Front. Wind frequency is daily, with and average rate of 25 km/h, with frequent periods of higher wind speeds. The influence of Antarctic air produces short periods of colder and drier climate. The studied sequence is located in the semiarid Fuegian steppe where Festuca gracillima (coirón) is the dominating specie. The sequence comprises a succession of 20 m of eolian silty-fine sand-clay sediments with 8 paleosols layers interbedded. Paleosols can be clearly identified on the basis of typical pedofeatures such as, coal grains, clay coatings, organic content and dark colour (10Y/R to 5YR). The paleosols layers contain abundant fossil record represented by Lama guanicoe and Ctenomys sp, and three or four different horizons have been identified in each paleosol. Radiometric data of the intermediate paleosoil 4 (5800 yr) and its overlying tephra layer (ca 3200 yr) allow to infer that the eolian deposition and the edaphic processes started during the Early/middle Holocene or earlier. The origin of the deposits is not clear yet. Two hypothesis are being analyzing: a) they could be the result of deflation during dissication periods of a shallow lake located near the eolic deposit, or b) they could be deposited forming perched dunes, ciclically eroded, edaphized and buried by new deposits. In case a) the source of the sediments must be the lake bottom formed by weathered marine Tertiary rocks and probably till; in case b) the source must be the weathered silty-sand bedrock which form the cliff over which de deposits lay. On going geomorphological studies will help to explain this depositional sequence. Additionally, this sequence is interesting due to archeological record. Abundant lithic and faunal archeological materials, formely buried are exposed by present eolian erosion. This contribution includes preliminary results, mainly focused in environmental magnetism measurements of 93 samples collected along the sequence and present soils of the same area (Fig. 2). The magnetic results include magnetic susceptibility measurements at room temperature and two frequencies (470 and 4700 Hz) (Bartington MS2), Vibrating samples magnetometer (Molspin VSM) measurements at room temperature and some measurements of susceptibility at high and low temperatures (Kappa bridge) in selected samples. The concentration of ferrimagnetic mineral recorded along the sequence (mass specific magnetic susceptibility, c between 17 and 7 E-7 m3/Kg) is similar to those of Pampean loess sequences. This fact allows to compare magnetic results from both areas in order to evaluate wind impact in the magnetic signal. The obtained magnetic results are used to prove, for high-mid latitudes (Tierra del Fuego and Alaska), the quantitative model for magnetic signal proposed by Orgeira et al (2010). Fundamentally, the model is based on the hypothesis of ultrafine magnetite precipitation during altenating wetting and drying cycles in the soil micropores. The rate at which this occurs depends on the frequency of drying/wetting cycles, and on the average moisture of the soil. In order to do the comparison cited above, the W values (moisture ratio = precipitation/evapotranspiration, Orgeira et al., 2010) were calculate for both areas (Figs. 3a and 3b). Therefore, the magnetic signal of the area and its W value were compared with those obtained in a loess deposit from Alaska (Lagroix and Banerjee, 2002). After the comparison above mentioned, the magnetic results obtained in the paleosols can be transformed in paleo-average moisture. Finally, these magnetic proxi plus the other multiproxi data obtained in the studied sequence define the Holocene climatic pattern for the area, wich is characterized by an important climate variability occurred in the southern extremity of the Americas. Based on this study we hope to contribute to the knowledge of the variability of the southern atmospheric circulation for the most recent geological times.