Molecular features of organic matter in anthropogenic earthen mounds, canals and lagoons in the Pago Lindo archaeological complex (Tacuarembó, Uruguayan lowlands) are controlled by pedogenetic processes and fire practices

Abstract

The Pago Lindo site in the River Plate basin (Uruguay) is an important pre-hispanic mound settlement (ca. 3000-600 B.P.), of which the technologically and socially advanced nature have only recently been revealed. Different angles of pedogenetic and palaeo-ecological science are rapidly improving our understanding of the history of the site and the relationships between human activity and habitat alteration. Here we add to this progress by molecular characterization of soil organic matter (SOM) in anthropogenic earthen mounds, canals and lagoons from Pago Lindo, by pyrolysis-GC-MS. The results showed that the SOM in the earthen mound and the archaeological lagoon are composed of microbial (partially chitin), aliphatic (partially root-derived) and pyrogenic (from fireplaces or wildfires) materials, whereas the canal and younger lagoon contained SOM with larger proportions of relatively intact plant remains (lignin and polysaccharides from herbaceous species). In most systems, a clear degradation trend can be observed with increasing depth -either from intact plant remains to microbial tissues (canal, young lagoon), or from microbial tissues to recalcitrant aliphatic and pyrogenic sources (earthen mound, ancient lagoon)-which is indicative of a strong control of decay intensity on SOM composition. Multivariate statistics confirmed that most variability in pyrolysis fingerprints can be attributed to degradation/preservation dynamics, which probably erased most of the molecular information on habitat development under the influence of past societies, including nearby maize cultivation.

Geoarchaeological signatures of anthropogenic soils in southeastern Uruguay: Approaches to formation processes and spatial–temporal variability

“…The high positive correlation between Pav and SOM also supports the existence of burnt organic matter that could contribute to ash content, but scarce macro‐charcoal remains probably because of the use of non‐woody fuel (Kaal et al, 2019). The heat from the fire, combined with the ash content and the combustion of bones, could lead to the generation of high Pav values.…”

“…Fire played a central role in the anthropogenic sediment formation processes that make up the mounds, possibly evidencing a combination of cleaning and thinning activities in domestic and productive spaces, cooking food, technological production, and waste management. The intensive and prolonged addition of carbonized organic matter, as evidenced in cerritos in northeast Uruguay (Kaal et al, 2019), can contribute to melanization processes and to a stable SOM stock, with high nutrient retention capacity and remarkable recalcitrance, such that it may persist in the environment for centuries (Kern et al, 2017). Then, thermo‐alteration has promoted the fixation of nutrients and possibly the increase in pH through the contribution of ash, also affecting soil structure and microbial action (Bracco et al, 2019).…”

“…Beyond the conspicuous architectural features of cerritos , the archaeological record from these structures has evidenced diverse ancient soil and landscape management practices, including (1) the conditioning of space and planning by the construction and maintenance of the structures, generating the elevated mounds and surrounding depressed area used for the mound source material (e.g., Bracco et al, 2019; Castiñeira et al, 2016; Gianotti & Bonomo, 2013). These modifications promoted disturbances in soil water regime and natural drainage patterns; (2) the physical (grain size composition), compositional (mineral and micro‐artifacts), and geochemical features of mound matrices allow their identification as anthropogenic soils, with nutrient levels (mainly available phosphorus and organic matter) much higher than those of surrounding “natural” soils (Bracco et al, 2019; Castiñeira et al, 2016); (3) evidence of landform modification for water regulation include small lagoons and channels (Gianotti & Bonomo, 2013; Kaal et al, 2019); (4) earthworks aimed to generate raised land for cultivation (Gianotti et al, 2013).…”

“…According to Bracco et al (2019), low SOM percentages could be explained by the degradation caused by fire, with heat and the resulting ash content least partially responsible for high P values. Although the recovery of charred plant remains in high concentrations is far from common, the role of fire is evidenced by the presence of burnt earth fragments and faunal remains with clear signs of heat alteration, as well as by the luminescent spectra of the quartz fraction observed during thermoluminescence (TL) dating (Bracco et al, 2019; Duarte et al, 2017), and the presence of pyrogenic SOM from grass vegetation in all anthropogenic mound layers analyzed by pyrolysis (PY)–gas chromatography (GC)–mass spectrometry (MS) (Kaal et al, 2019).…”

Field burn versus fire-related cues: germination from the soil seed bank of a South American temperate grassland

“…The Río de la Plata grasslands are one of the largest areas of natural temperate sub-humid grasslands in the world, covering more than 700,000 km 2 distributed across eastern Argentina, the whole of Uruguay and southern Brazil (Soriano et al, 1991). The history of fire in the Río de la Plata grasslands is largely unknown, but palaeoecological data suggest that fire became frequent about 7000–3000 years ago, after the arrival of human populations in the region (Behling et al, 2004; Kaal et al, 2019). Currently, grazing by livestock is the main productive activity, and in certain areas, controlled burns are used to increase primary production and forage quality (Pillar and de Quadros, 1997; Laterra et al, 2003; López-Mársico et al, 2019).…”