Food and diet of the pre-Columbian mound builders of the Patos Lagoon region in southern Brazil with stable isotope analysis

Highlights

• Diet of Late Holocene mound builders in southern Brazil investigated.

• Stable isotopes and Bayesian Mixing Models used for the first time in the region.

• Aquatic resources played a major role as sources of dietary proteins and calories.

• Mixed economies, not agriculture, fuelled population growth in the region.

Abstract

A constant and intense debate in South American archaeology stands on the origin and patterns of food production of ancient populations and the correlation of both aspects with demographic growth and social complexity. Some studies associated population growth with the shift from foraging to agricultural practices in hotspots on the continent. This association has been confronted by a number of studies performed in recent years that reconstructed dietary patterns for several tropical and subtropical areas of eastern South America. However, there is still a lack of information on the diet for Late Holocene populations in the wetlands of the Pampa Biome. In order to access the paleodiet of earthen-mound builders from southern Patos Lagoon (Brazil), this study combined bulk collagen stable carbon and nitrogen isotopes analysis of faunal and human remains with Bayesian Stable Isotope Mixing Models. The paleodiet of 20 human and one dog remains from six sites was reconstructed. The Bayesian mixing model on collagen δ¹³C and δ¹⁵N values revealed variable subsistence strategies with high consumption of marine/estuarine protein for some individuals. Other individuals’ diets relied on terrestrial/freshwater resources as protein sources. In southeastern South America, such patterns reinforce the resource-rich aquatic environment as a facilitator for the endurance of mixed economies. In addition, our results suggest that the Patos Lagoon surroundings was a “hub of interactivity” characterised by a remarkable intra-site dietary variability.

Introduction

Differing positions on the origin and changing nature of food production economies spark some of the most contentious debates in South American archaeology. Middle-Late Holocene population growth has been often associated with the shift from foraging to predominant agricultural practices in cultural hotspots on the continent (Goldberg et al., 2016). However, in several tropical and subtropical areas of eastern South America, the importance of agriculture to demographic growth and the establishment of sedentary populations during the Late Holocene is subject to contention. A growing body of evidence suggests that in these regions agriculture, if defined as a “provisioning system based primarily on the production and consumption of domesticated resources” (Smith, 2001; Zeder, 2015), did not develop until recent times (Iriarte et al., 2017; Neves, 2013). Instead, the prevailing subsistence economies were mixed, based on varying combinations of plant gathering and cultivation (both wild and domesticated), forest management, hunting and fishing. This supported population growth, reduced territorial mobility and the development of long-lasting cultural practices during most of the Late Holocene (De Blasis et al., 1998; Fausto and Neves, 2018; Iriarte et al., 2017; Neves, 2013; Ottalagano and Loponte, 2017).

Subtropical and tropical coastal areas of southeastern South America preserve archaeological evidence of mixed economies that successfully persisted throughout the Middle and Late Holocene, and over extensive geographic areas. Shell mounds and middens of the Brazilian coast (locally known as sambaquis) are examples of sites built by coastal-adapted populations who attained relatively large demographic sizes and developed relatively complex social organizations without relying on domesticated plants. Their economic systems were based on the exploitation of aquatic organisms (Bastos et al., 2014; Colonese et al., 2014; Fossile et al., 2019; Pezo-Lanfranco et al., 2018b), along with a range of terrestrial plant resources, including some cultivars in more recent times (Pezo-Lanfranco, 2018; Scheel-Ybert and Boyadjian, 2020; Wesolowski et al., 2010), and the occasional hunting of land mammals (da Rocha Bandeira, 2015). In agreement with contemporary evidence from wetland areas in eastern South America (Iriarte et al., 2017), the picture that emerges from coastal sites in Brazil is one where aquatic environments offered a competitive edge to mixed economies during the Late Holocene. However, considerable uncertainty still remains about the extent that coastal resources were used by adjacent populations, such as the flora and fauna of the wetlands of the Pampa biome, and the relative contribution of marine and terrestrial food items to their overall subsistence economy.

The Pampa biome and the La Plata basin in Brazil, Uruguay, and Argentina were occupied by groups who built earthen-mounds during the Late Holocene, known as cerritos (Bonomo et al., 2011b, 2017; Iriarte, 2006; Iriarte et al., 2004). Archaeobotanical data from sites in the La Plata basin reveal that these groups cultivated domesticated crops (e.g. maize, beans, squash) ca. 4000 years ago (Iriarte et al., 2004). More recent groups also cultivated domesticated crops in the Paraná River delta and exploited wild faunal and plant resources (Bonomo et al., 2011a, 2011b). This is confirmed by recent stable isotope analysis on human individuals from earthen-mounds in this region, which indicated diets with a predominant consumption of C₃ plants complemented by the hunting of wild animals (Bonomo et al., 2017). By contrast, very little is known about the subsistence economy and diet of groups who occupied the Patos Lagoon in southern Brazil. This ecosystem is particularly productive in terms of aquatic and terrestrial animals and zooarchaeological analysis suggests that fish may have been a major source of protein for groups in this region (Chim, 2016; Milheira et al., 2019a; Schmitz, 1976).

To reconstruct the paleodiet of Late Holocene human remains from a number of earthen-mounds in the southern Patos Lagoon (Fig. 1), this study combined bone bulk collagen stable carbon and nitrogen isotope analysis with Bayesian Stable Isotope Mixing Models (BSIMMs). BSIMMs express dietary contribution probability distributions while accounting for multiple sources of uncertainty (Fernandes et al., 2014; Parnell et al., 2013; Phillips et al., 2014), therefore offering more robust dietary estimates than simple raw data comparison. Stable isotope analyses were also performed on dentine serial sections to obtain information on childhood and adolescent diets from a few individuals. Each section portrays the average diet of 6–8 months of the individual's life, making this technique a powerful tool for highlighting dietary variability with greater temporal resolution. We aimed to assess the relative contribution of marine/estuarine and terrestrial/freshwater food sources, including plants, to the diet of mound builders and to contribute to debates on the nature of subsistence strategies in eastern South America during a period of increased social complexity and cultural networking.

Cerritos in southern Brazil, Uruguay and Northeast Argentina have been associated with distinct activities, including temporary camps, residential households, and cemeteries (Fig. 2). The height of the mounds varies between 30 cm and 7 m, and they are commonly articulated to adjacent built structures, including micro-reliefs (mounds less than 30 cm tall), elongated platforms, borrow pits, tracks, pathways and artificial lakes that compose archaeological complexes situated in flooded environments (Bracco et al., 2008; López Mazz and Pintos, 2000; Milheira and Gianotti, 2018). The chronology of the sites spans the Middle Holocene to the colonial period, from ca. 4700 to 200 cal years BP (Bracco et al., 2000, 2008; Iriarte, 2003, 2006; Lópes Mazz, 2001; Gianotti, 2015; Milheira and Gianotti, 2018), but most sites in the Patos Lagoon region were formed between 2400 and 800 years ago (Schmitz, 1976; Milheira et al., 2017a, 2019a).

Beginning 2400 years ago, the sites in the region became more numerous, associated with elements of social complexity, such as clusters of monumental sites, technology changes, resource intensification and broad territoriality (Iriarte, 2006; Moreno, 2014; Bracco et al., 2008; del Puerto, 2015; Gianotti, 2015). This time interval coincides with more specialised hunting strategies and the management of wild species (Moreno, 2014). Domesticated and managed plants, including maize (Zea mays), squash (Cucurbita sp.), beans (Phaseolus sp.), tubers (Canna sp. and Calathea sp.), and palm nuts (Butia odorata and Syagrus romanzoffiana), have been recorded since ca. 4000 cal years BP (Iriarte et al., 2004), but became more intensively incorporated to the economy 1600 years ago (Iriarte, 2006; del Puerto, 2015; del Puerto and Campos, 1999; del Puerto et al., 2008; Milheira and Gianotti, 2018). The earliest evidence of domestic dog (Canis lupus familiaris) in the region, often associated with human burials, is also dated to around 1600 years ago (Loponte et al., 2021; Prates et al., 2010; López Mazz et al., 2018; Milheira et al., 2017a, 2017b).

Fish is typically considered a complementary source of animal protein, which was composed primarily of cervids and small rodents such as the Cavia aperea (Pintos, 2000; Moreno, 2014). However, in regions such as the Patos Lagoon, fishing is attested at many sites (Schmitz, 1976; Schorr et al., 1975; Milheira et al., 2019a; Chim, 2016; Calippo, 2000; Ulguim, 2010, 2018), and evidence of it dominates the zooarchaeological assemblages at Pontal da Barra (Milheira et al., 2019a). The most abundant taxa are marine catfish (Genidens planifrons, Genidens barbus, and others of the Ariidae family), drums, from the Sciaenidae family, notably the whitemouth croaker (Micropogonias furnieri) and the black drum (Pogonias cromis). Freshwater species are scarce and only represented by trahira (Hoplias sp.) and swamp eel (Synbranchus sp.). A great diversity of marine catfish and drums is also observed in the Patos Lagoon and adjacent marine areas today (Garcia et al., 2003). Aquatic birds, freshwater turtles and terrestrial mammals, mainly small rodents, carnivores and deer, were also exploited in the area.

The Patos Lagoon (30°S and 32°S) is the world's largest choked lagoon and one of the most voluminous water-bodies in southern Brazil, with an area of ca. 10227 km² (Garcia et al., 2003). It is located between Rio Grande do Sul state and the east coast of Uruguay and is connected with the Mirim Lagoon through the 75 km long São Gonçalo channel forming the Patos-Mirim complex or Patos Lagoon complex (Simon and da Silva, 2016). The lagoon is considered an important nursery and feeding ground for marine species and, despite the overexploitation of fish stocks, fisheries remain highly productive (Haimovici and Cardoso, 2016). The potential for catches attracted indigenous populations since pre-colonial times, as attested by more than 400 cerritos (Milheira et al., 2019b), shell mounds (Ribeiro and Calippo, 2000) and Guarani sites (Milheira and Wagner, 2014) with evidence of exploitation of aquatic resources.

In the municipalities of Pelotas and Capão do Leão, at the southern tip of the Patos Lagoon, the team of the project “Archaeology and Indigenous History of the Pampas” has surveyed 43 earthen-mounds in the wetlands of the Pontal da Barra, Fragata Lagoon, and Pequena Lagoon. The excavations at five of these sites provided the most robust chronological information for cerritos in southern Brazil to date, with radiocarbon dates ranging from 2177 to 754 cal years BP (Milheira et al., 2017a). These sites are circular and elliptical-shape mounds, no more than 1.20 m tall, usually located in riparian forests along rivers and lagoons (Milheira et al., 2019a). The combination of radiocarbon dating and analysis of the material culture and faunal remains suggests that the long-term occupation might be contemporaneous with the development of semi-sedentary villages. A complex stratigraphic record is a distinctive feature of these sites. The archaeological deposits are dark, rich in faunal and charred plant remains and artifacts including a vast number of pottery shards and at times human bones. Human remains always appear disarticulated, highly fragmented and spatially scattered, following the same distribution of other materials. Although the nature of these funerary patterns is unclear, the consistency of radiocarbon dates strongly indicates that in-situ manipulation of bodies or secondary burials occurred during the construction of the sites (Milheira et al., 2019a).