Challenging the Atacama desert: Agronomic and water conditions for pre-Hispanic maize agriculture in hyper arid environments inferred by δ18O isotopes

During the Formative Period (ca.2400–950 years BP), pre-Hispanic farmers in Pampa del Tamarugal developed a complex agricultural system in the hyper-arid Atacama Desert in Tarapacá, northern Chile. This system involved numerous agricultural fields congregated near the Tarapacá Valley’s perennial stream and the Guatacondo ravine’s ephemeral stream. Well-established villages such as Caserones, Pircas, Ramaditas, and Guatacondo accompanied these developments. However, the importance of understanding the water sources has been neglected despite the efforts to understand the relationships between pre-Hispanic agriculture and their living environment. Thus, we presumed the use of local water sources without a clear understanding of their exploitation methods or the associated technological implications. Furthermore, there is limited research on groundwater use in Tarapacá. This research aims to understand water sources used in the cultivation of maize (Zea mays) using δ¹⁸O isotope values obtained from pre-Hispanic maize kernels in Tarapacá. We compared these values with published δ¹⁸O values of water sources and applied a generalized linear model (GLM) with a Gaussian distribution, performing a Tukey’s post hoc test for multiple comparisons of means with heteroscedasticity-consistent covariance estimation. The best-fit model was identified using a stepwise model selection procedure based on the Bayesian information criterion (BIC). Our results indicate that mean δ¹⁸O values of organic matter from maize kernels range from 24.73 ‰ to 31.65 ‰. The best performing model on δ¹⁸O only included Group (BIC=298.2) as the explanatory variable as Period, Site, and Weight had no effects. These findings point towards a significant statistical relationship between the δ¹⁸O values of organic matter derived from maize kernels and the specific geographic regions they originate from. These values also show an enrichment of δ¹⁸O isotope in Tarapacá samples, except for Pica 8. These results indicate diverse agricultural strategies that utilized different water sources including perennial flow in the Tarapacá River, ephemeral runoff at Guatacondo, and groundwater in the Pampa del Tamarugal. Also, due to the enrichment of δ¹⁸O, we suggest that the circular structures found among the fields were used as water reservoirs leading to the observed enrichment of δ¹⁸O isotope values. We discuss the role of ancient agriculture technology in water management, the role of maize, cultural strategies, and the relationships with their environment. In conclusion, pre-Hispanic farmers managed limited water sources successfully despite intermittent drought for hundreds of years until the 1970 s, when agriculture was abandoned due to multiple factors.