Geochemistry, Provenance and Environmental Conditions of Holocene Sediments of Upper Manso River Basin-Lake Mascardi, Argentina

Abstract

The Upper Manso River drainage basin and Lake Mascardi (c., 41~ 72~ are located in Patagonian Andes, Argentina. The very steep Upper Manso River is 23 km long, has its headwaters at the Manso glacier (Mount Tronador, 3554 m a.s.1.) and discharges into Lake Mascardi (793 m a.s.1.). Turbid glacial melt water feeds the system, as well as rain and snowfall between May and July. The geology of the area includes phillitic, porphyric and granitic rocks, as well as Tertiary volcanic and sedimentary rocks. Varves, tephras, and glacial/ glacio-fluvial deposits are widely distributed throughout the basin. Rocks, suspended and river bed sediments, as well as lake bottom sediments, and material from sediment traps deployed in Lake Mascardi, were collected in the area of study. Samples from an AMS radiocarbon-dated sediment core obtained in Lake Mascardi at 30 m water depth were also included. Lake Mascardi sediments record variations in the transport of glacially-derived clay and silt to the basin, which reflects variations in Southern Hemisphere climatic characteristics for the last 15.0 kyr BP (Ariztegui et al., 1997). All the samples were analysed for 25 major and trace elements using instrumental neutron activation (INAA) in the RA-6 research reactor (S.C. de Bariloche, Rio Negro, Argentina). In this paper we analyse the chemical variability of glacio-fluvial sediments in a pristine area, also examining inorganic inputs in a lake core since Late Pleistocene. Glacial environments are characterized by low chemical weathering rates. However, the production of fine-grained, easily weathered material, may compensate this well-known fact and introduce an important variable in global weathering. In this particular case, a regenerated glacier at the uppermost reaches, introduces an important debris load into the river. The mineralogical composition of such debris, derived from basalts, shows a dominance of feldspars, pyroxenes and amphiboles. Feldspars are sensitive to weathering and their transformation to clays is indicative of the reached degree of alteration. In order to quantify such process, we have used the chemical index of alteration (CIA) and the chemical index of weathering (CIW). CIA values for presentday sediments suggest a clear dominance of plagioclase. A relatively uniform variation of CIW/CIA also indicates a constant mineralogical composition. The geochemistry of the Upper Manso river drainage basin is controlled by a weathering limited regime (Stallard and Edmond, 1983). Hence, detrital materials exhibit a close relationship with source rocks (Tronador Fm. Miocene basalts). This is clearly shown by R E E normalized patterns, Eu/Eu* values and the relative distribution of immobile elements. The main deviations from this trend were observed for high-mobility elements (U, Na, Sr) and those selectively adsorbed in clay-size materials (Cs, Rb). The available evidence also suggests an Nd enrichment in the finer size fractions. Although the textural differences are favoured by selective sorting, the immobile element distribution of source rocks is preserved in sediments of Upper Manso drainage basin. Due to the presence of plagioclase-bearing basalts in the drainage basin, the sediments offer excellent conditions for the study of the Eu behaviour during sedimentary processes. Following the Albarbde and Semhi's approach, the modelling of the Eu fractionation in relation with trivalent R E E suggests little reworking of sediments, with a total loss of soluble or weathered material of 36%. Material collected in sediment traps moored off the mouth of the Manso River has exhibited features