Palaeoenvironmental use of silicophytoliths in soils and palaeosols associated with Holocene volcanic ash deposits in north-western Argentina

Abstract

We compare the silicophytolith record of current soils and Holocene palaeosols from two sedimentary sequences in a region between the Chaco-Pampean Plain and the Puna (Tolombon section in Santa Maria Valley, and Tafi del Valle section in Tafi Valley), NW Argentina. These two geographic areas are separated by the Aconquija-Cumbres Calchaquies ranges, a climatic barrier to the wet Atlantic winds. These sequences contain volcanic ash deposits recording two large eruptions occurred during Holocene in the Southern Puna in the Central Volcanic Zone of the Andes, from the Cueros de Purulla volcano (ca. 7820 BP) and the Cerro Blanco Volcanic Complex (ca. 4200 cal BP). These ash-fall deposits buried soils and preserved their silicophytolith record, allowing the use of this palaeoenvironmental proxy to clarify the vegetation prior these two volcanic events. The silicophytolith assemblages yield evidence for a desert shrubland when Tolombon palaeosol was formed, similar to the presentday environment in the Santa Maria Valley, with presence of C-4 grasses (Chloridoideae and Panicoidae). Instead, the climate was cooler and wetter ca. 4200 cal BP than currently in the Tafi Valley, increasing over time aridity and abundance of C-4 grasses (Chloridoidae, Panicoideae), and decreasing the quantity of C-3 grasses (Pooideae). Prehistorical and historical land use also contributed to this change. The impact of these large volcanic ash-falls on vegetation did not translate in an abrupt change on phytodiversity. Results point out a stable arid environment during Holocene in the Santa Maria Valley whereas the Tafi Valley was more sensitive to environmental changes due to its location in the eastern slopes of Aconquija-Cumbres Calchaquies ranges, more exposed to the wet Atlantic winds.