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Jean-Frank Wagner, Trier University (Germany)
Oscar Baeza-Urrea, Trier University (Germany)
Tatjana Steinke, Trier University (Germany)
Rolf Kilian, Trier University (Germany)
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Characteristics and changes of the clay mineral transport pattern from the partly glaciated and superhumid mountain range of the Southern Andes at 53°S towards the westward Pacific and eastward foreland has been investigated for the period from the Late Glacial to Present day. This part of the Andes is affected whole year-round by southern hemispheric westerlies, producing precipitations up to 10 m/year. Ten 3-8 m long piston cores and numerous gravity cores were taken along a fjord transect which document the sedimentation of clay minerals over more than 120 km distance towards both side of the Andean mountain range. Age determinations (14C and 210Pb) and tephra chronology give an age frame for the sediment cores which were further investigated concerning magnetic susceptibility, grain sizes, geochemistry and XRD clay mineralogy.
Our results show that Late Glacial proglacial lakes on both sides of the Andes were characterised by very low clay settling times, resulting in a long distance transport of a chlorite-rich clay fraction, derived from mafic Jurassic to Cretaceous lithologies, exposed in the central part of the Andes. The amount of the chlorites in the sediment could be correlated with the Fe-Mg content of the clay fraction. The transport and deposition of clay minerals from partly exposed granodiorites of the Patagonian Batholith was characterised by higher illite contents and could be correlated with the potassium content in the clay fraction. The marine transgression to the western fjord system at around 14,000 years caused a more rapid clay flocculation and a decrease in the long distance transport, so that the sedimentation became autochtonous at most sites, associated by a strong decrease in the sedimentation rates. To the east of the Andes the long distance clay mineral transport was partly controlled by thermohaline characteristics of a semihaline fjord system, which was reconstructed based on the chlorinity of the pore water of the clayey sediments.
The eastern fjords are characterised by pronounced eastward superficial low salinity currents which can reach velocities of up to 20 km/day during stormy periods. A northward migration of the westerly wind belt during the Neoglacial produced a decrease in the average westerly wind component during the Neoglacial after 5000 years B.P in the southernmost Andes. Such a decreasing westerly influence can explain the drop in the long distance eastward clay mineral transport in the middle to late Holocene. Subordinate kaolinite contents (up to 10 vol.-%) in some sediments seem to be related to the Holocene alteration of granodiorites which were covered by peat with very acid (pH 3-4) soil water. Our results should help to improve paleoenvironmental and paleoclimate interpretations of sediment cores and to better trace sediment pathways in this very remote area at the southern tip of South America.
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