Loess–paleosol sequence of La Mesada in Tucuman province, northwest Argentina characterization and paleoenvironmental interpretation

A 42 m thick loess–paleosol sequence was studied in the dry pre-Andean valley of Tafi-del-Valle, at the La Mesada site (2280 masl), Tucumán Province in northwest Argentina. The sequence contains 28 paleosols interbedded with 26 loess layers. The latter are coarse loamy and have large hexagonal polyhedrons. The soil layers are restricted to fine loamy Bt horizons, with prismatic structure and organo-argillans on the structural surfaces. Surface (A) and eluviation (E) horizons are systematically absent from the whole sequence. Soil truncation is unlikely as no remains of such horizons or erosional uncomformities were identified. The dated part of the section encompasses a time span of 10,080 years, from 17,580 BP at 5.2 depth to 27,660 BP at 42.3 m depth. A total of 20 Bt/C pairs developed during this time interval, corresponding to a climatic change every 500 years on the average between dry–cool conditions, promoting loess influx, and moist–warm conditions favouring soil development. The depth function of the Bt/C clay ratios reflects the climate improvement during the soil formation interstadials, with an optimum in the upper part of the sequence. This might be related with recurrent northward shifts of the polar front, accompanied by the weakening of the mid-latitude South Pacific anticyclone. Similar short-term periodicity has been identified in the ice cap of Greenland, highlighting the global character of such frequent climatic changes during the Pleistocene.