Climatic periodicity during the late Pleistocene from a loess–paleosol sequence in northwest Argentina

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A 42 m thick loess–paleosol sequence was studied in the pre-Andean intramountain valley of Tafi-del-Valle, 100 km west of the city of San Miguel de Tucumán, at an elevation of 2280 m asl, in northwest Argentina. The sequence contains 28 paleosols alternating with loess layers. The loess layers are coarse loamy and have large hexagonal polyhedra. The soil layers are restricted to Bt horizons, having fine loamy textures, prismatic structures and organo-argillans on the structural surfaces. Surface (A) and eluvial (E) horizons are absent from the whole sequence. Soil truncation is unlikely as no remains of these horizons or erosional unconformities were identified. The dated part of the sequence encompasses a time span of 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the base of the exposed sequence). The youngest strata are probably missing, either because of a depositional hiatus or because of post-depositional erosion, as dated paleosols in adjacent areas indicate that loess deposition continued during at least the early Holocene. The 20 Bt/C couplets identified in the dated sequence correspond to 20 climatic oscillations in 10,080 years. Thus, every 500 years on average there was a climatic change from dry–cool conditions, with loess deposition, to moist–warm conditions favouring soil development. This might have been related to recurrent northward shifts of the polar front, accompanied by weakening of the mid-latitude South-Pacific anticyclone. Similar short-term periodicity has been identified in the ice cap of Greenland and in tropical lake level records, so such frequent climatic changes during the late Pleistocene seem to have been global in character.