Human resilience to Holocene climate changes inferred from rodent middens in drylands of northwestern Patagonia (Argentina)

This article presents a series of thermoluminescence dates obtained from archaeological sites in the Angostura micro region, at the southern edge of the Santiago basin, an area occupied by Bato and Llolleo groups during the Early Ceramic period (200 BC–AD 1300). This series of dates, which cover a time span of around 1500 years, shows moments when dates are abundant and others when they are remarkably scant. We propose that a comparison of the relative frequency of dates can be used as an indirect measure of variations in the intensity of occupation of each of these groups in the zone. We used a summed probability analysis by decade to evaluate sets of dates according to cultural context and sectors of the study area. The results show that while the Bato and Llolleo were irrefutably contemporaneous, the Bato emerged slightly earlier, and that both –but especially the Llolleo– coexisted in later times with Aconcagua groups. At a micro regional scale significant differences as well as changes in Bato and Llolleo population dynamics were made clear. While the Bato display a natural growth curve and then declines, Llolleo increased their population significantly after AD 700, probably because of their reliance on maize crops. Not only did the chronological curves differ; the sectors where they placed their settlements varied over time depending on their specific horticultural practices, local climatic fluctuations, and each other's proximity.

The regional archaeological discontinuity between ca. 11,761 ± 272 Cal BP and the beginning of the Holocene at Ultima Esperanza, Chile will be discussed. The absence of humans as measured by the deposition of artifacts on dated deposits was originally detected at the site scale, especially at Cueva del Medio and Cueva Lago Sofía 1, but recent excavations at several other sites extend it to the full scale of the Cerro Benitez locality. It is suggested that this is not a case of abandonment of a colonized region, but instead a case of discontinuation in the logistical use of a particular environmental patch. Accordingly, the Early Holocene human presence in the Cerro Benitez locality is seen as the result of a new exploration by human foragers adapting to the increasingly forested environments of Ultima Esperanza.

As with most living organisms, human populations respond to climatic, environmental, and population pressures by transforming their range and subsistence strategies over space and time. An understanding of human ecology can be gained when the archaeological record is placed within the context of dynamic landscape changes and alterations in natural resource availability. We reconstructed the landscape evolution of the Quebrada Maní fan complex, situated along the west-facing slope of the Central Andes in the hyperarid core of the Atacama Desert, an area that contains an archaeological record that spans almost 13,000 years. Surficial geologic mapping and dating of three 2–12 km² study sites, in conjunction with archaeological records and analysis of remotely sensed data for the ∼400 km² fan complex, was conducted to reconstruct the landscape evolution and the way of life of Paleoindian (ca. 12.8–11.5 ka) and early/late Formative (ca 2.5 to 0.7 ka) social groups.

Just prior to any known human occupation, a large pluvial event in the high Andes, regionally referred to as CAPE I, impacted the Quebrada Maní fan complex from ca.18–16.5 ka. During CAPE I, the Maní fan complex was dominated by perennial stream systems that deposited well-sorted conglomerates in the upper reaches of the fan (Unit T2) and perennial wetlands (Unit B1). This pluvial period was followed by the onset of an extreme drought sometime after 15 ka, but before 13 ka, when wetlands desiccated and the distal reaches of the fan deflated. Sand sheets and sand dunes were deposited across broad reaches of the landscape and Quebrada Maní incised 3–5 m into its floodplain. This drought had profound implications for the distribution of natural resources during the subsequent pluvial event (CAPE II) that ensued from ca. 12.5–9.5 ka. Incision along the upper reaches of the fan caused a more restricted floodplain and allowed the deposition of extensive wetlands along the more distal central reaches of the fan where groundwater emerged. Paleoindian residential open-air camps were placed in these areas. Wetlands were replaced by a tree-covered floodplain during the latter portion of this pluvial event (ca. 10.5–9 ka).

We found no archaeological evidence for human occupations between ∼8–2.5 ka, suggesting a lack of natural resources and/or very low hunter-gatherer population densities. During this time, Quebrada Maní incised up to 8 m into the floodplain. Mudflow deposition – typical of the present-day fan complex – initiated around 2.5 ka, likely responding to an increase in precipitation. This triggered a re-population of the fan surface by Formative agricultural groups that irrigated and extensively farmed these floodplains. By the end of the Formative, these socio-cultural groups became increasingly vulnerable to climatic changes as cut-and-fill cycles in the drainage necessitated major infrastructure adjustments, until the technologies and social-cultural convention of the epoch could not cope with environmental change and investments were abandoned by ∼0.8 ka.

This article discusses the distribution of radiocarbon age signatures obtained from archeological sites between 29° and 35° S in Central Chile and Midwest Argentina. The goal of this analysis is to establish bases from which to interpret regional trends in the distribution of the archaeological record that connect these areas, which have been traditionally considered to be geographically decoupled. We propose a standardized methodology for selecting ages that provide a reliable human signature. Variations in date frequencies in a regional scale are discussed with the use of summed probability distributions. Radiocarbon voids at the regional level previously identified in Midwest Argentina are explored. Regional chronological information is compared to the available paleoenvironmental records, thereby emphasizing the possible role of climate pulses in the spatial organization of human populations. Significant arid conditions between 7800 and 5700 cal BP are coincidental with a focused occupation of the Andes Mountains, an area which may have offered stable resources and thus was more effectively occupied than other environmental bands.

The Somuncurá Plateau is a Protected Natural Area located in the middle of the northern extra-Andean arid Patagonia. Inhabited by at least 20 small mammal species, is the place with the uppermost species richness in Patagonia. The aim of this study was to examine the parasite remains from micromammal coprolites collected in association with a bone sequence recovered at the east of the Somuncurá Plateau (site “Alero Las Lechuzas”). Coprolites came from the four temporal units previously defined: unit I (4790 ± 100 yrs. ¹⁴C B.P.), unit II, unit III (7840 ± 120 yrs. ¹⁴C B.P.) and unit IV. Each coprolite was processed, rehydrated, homogenized, processed by spontaneous sedimentation and examined using a light microscope. Coprolites and eggs were described, measured and photographed. Samples were positive for two nematode species: Helminthoxys caudatus Freitas, Lent & Almeida, 1937 (Oxyurida, Oxyuridae) and Trichuris spp. (Trichinellida: Trichuridae). This is the first paleoparasitological study developed for the Somuncurá Plateau Protected Area. Moreover, this is the first time that the genus Helminthoxys is reported from ancient times worldwide. Coprolites were attributed to the mountain cavy Microcavia australis (Rodentia, Caviidae).The presence of H. caudatus for the Middle Holocene of northern Patagonia contributes to the study of the history of the histricomorphs and pinworms relationships.

In the southern Andes, Holocene climate records show drastic changes in moisture during the early and middle Holocene. To generate a more refined chronology of climate changes in this region, we present a Bayesian model that combines published cosmogenic dates from the Encierro Valley (29.1°S) and radiocarbon dates on peat and soils from the western slope of the Andes (27–33°S). We compare this to a similar model from the high-altitude archaeological site ARQ-18 in the Las Taguas Valley (29.5°S), San Juan, Argentina. These chronologies indicate synchronous changes in climate and occupational intensity, which shed light on hunter-gatherer mobility decisions. This site was first occupied in the early Holocene, when nearby valleys were deglaciated by around 10,700 cal BP. ARQ-18 was occupied a few centuries later around 10,100 cal BP. The site was first colonized during a regional wet phase, probably by hunters from the highlands to north who moved quickly among humid high-altitude valleys. As regional moisture began dropping around 8700–7800 cal BP, occupational intensity at ARQ-18 reached a maximum as diverse groups gathered in the valley. At this point, an important environmental threshold was crossed as groups reversed their mobility patterns and decisions and did not occupy the site for 1700 years. This “archaeological silence” correlates closely with the middle Holocene's hyperaridity during 7800–5700 cal BP. As soon as humidity returned, groups began visiting the site again. From this point on, strategies increasingly incorporated herding in response to less stable environmental conditions.