No abstract

Vegetation reconstructions rest on modern vegetation-pollen rain relationships and deductive reasoning. Establishing this relationship is a nontrivial task because differences among pollen assemblages are not necessarily proportional to differences in vegetation. This task is particularly challenging in Patagonia, where some tree taxa have indistinguishable pollen, and pollen grains can be transported long distances. In this study, we describe the modern pollen of 48 lake and wetland samples from northern Patagonia (40.5-44°S) to better discriminate the major vegetation zones of the region through pollen analysis. Specifically, we focus on the performance of three methodological approaches, namely, pollen indicators, classification trees, and optimal thresholds of dissimilarity. As a proof of concept, we use the modern pollen-vegetation relationships to reconstruct the vegetation history at Laguna el Trébol (41.07°S; 71.5°W). Our results revealed that (1) pollen sums exceeding 260 grains ensured replicable vegetation reconstructions, (2) modern vegetation zones could not be separated solely by visual inspection of their pollen spectra, (3) the classification tree and optimal thresholds of dissimilarity permitted discrimination of most vegetation zones, (4) detection of nonanalog communities required use of pollen indicators or optimal thresholds of dissimilarity, and (5) vegetation at L. el Trébol was likely dominated by late glacial shrubland with no modern analogs in the study area (15,000-12,180 cal. yr BP), modern shrubland (12,180-6500 cal. yr BP) and mixed forest (6500 cal. yr BP-present). This study allows a more realistic understanding of the pollen-vegetation relationship and provides new tools for interpreting past vegetation in northern Patagonia.

This study presents a pilot geoarchaeological investigation of terraced agricultural systems near San Francisco de Sangayaico, in the upper Ica catchment of the Southern Peruvian Andes. It aims to assess the evidence for soil fertility associated with agricultural strategies practiced throughout the Prehispanic, Spanish colonial and modern occupations in this region. A series of twenty-two test pits were hand excavated through two terraced field systems, and sampled to examine the changes in soil physical and chemical characteristics down-profile and downslope. This study provides the first geoarchaeological analyses of the agrarian soil system surrounding Viejo Sangayaico in the upper Ica catchment. Results demonstrate that the soil system was much modified prior to the creation of the terrace systems, probably about 900 years ago. This system was characterised by a weakly acidic to slightly calcareous pH, a consistent but low electrical conductivity, reasonable-butvariable phosphorus content, and a loamy soil texture with a component of weathered volcanic tonalite parent material. The shallow terrace soil build-up on the slopes investigated indicates that slope modification was as minimal as possible. Moreover, the relatively low frequencies of organic material and phosphorus suggest that the terraces were not heavily fertilised in the past, making the stability and management of the nutrient-rich topsoil vital. The results of these excavations and soil fertility analyses are situated within the context of the wider Andean ethno-historic and the archaeological record to address questions regarding how the terraces were built and maintained over time. Agricultural terraces undoubtedly mitigated the effects of slope erosion associated with cultivation. But, the terrace soil features observed at Sangayaico do not appear to be the same as those documented in other geoarchaeological studies of Andean terrace systems. These contrasts may be accounted for by a combination of differing geological substrate and hydrological conditions, as well as variable trajectories in past soil development, erosion factors, manuring/field management practices and crop selection.