Paleoenvironmental and paleoecological changes during the early GABI in the Argentine Pampas: a stable isotope approach

The analysis of stable isotopes in fossil mammals is a powerful tool to reconstruct paleoenvironmental and paleoecological conditions. Nevertheless, there are few works of this type focused on the Neogene of South America, specifically, on the Argentine Pampas. In this context, we perform an integrative approach for the Late Miocene-Early Pliocene of this region combining new U-Pb zircon dating and carbon and oxygen stable isotope analysis, to contextualize the paleoenvironmental and paleoecological evolution of the region. The δ¹³C values are used to reconstruct the diets and preferred habitats of the taxa, while changes in the δ¹⁸O values of animals forced to drink reflect variations in δ¹⁸O of meteoric water, controlled by temperature and evaporation rate. We selected a total of 270 bioapatite samples of δ¹³C_CO3 and δ¹⁸O_CO3 from eight localities of La Pampa and Buenos Aires provinces. Radioisotopic ages for six of them allowed us to pin down maximum deposition ages and a time interval of ca. 5 million years for our study (from 9.7±0.3 Ma at Arroyo Chasicó to 4.5±0.2 Ma at Farola Monte Hermoso), including the Chasicoan, Huayquerian, and Montehermosan stages/ages. We studied genera of Litopterna, Notoungulata, Rodentia, Pilosa, and Cingulata orders. During the Chasicoan Stage/Age, herbivore δ¹³C values point to mixed C₃–C₄ diets, evidencing the existence of favorable habitats for C₄ plants before their great expansion. By contrast, during the Huayquerian Stage/Age, taxa show values indicative of feeding preferentially on C₃ plants, except for some rodents that continued including C₄ plants in their diets (possibly related to an early specialization of this group). In the latest Huayquerian-Montehermosan stages/ages, herbivorous taxa incorporated a higher percentage of C₄ plants in their diets, coinciding with the global expansion of this type of vegetation. This change in δ¹³C values also reflects an increase in aridity and/or temperature since the Late Miocene-Early Pliocene in the area, coincident with results of other proxies. The δ¹⁸O values of the notoungulates support these interpretations, evidenced by higher δ¹⁸O values during the Chasicoan and latest Huayquerian–Montehermosan stages/ages; on the contrary, there is no trend recorded in litopterns, while only a slight decrease in δ¹⁸O values was obtained in rodents. These differences are possibly linked to the fact that each order records conditions at different scales; litopterns might register global hydrological conditions, notoungulates regional conditions, and rodents more local conditions. In addition, the difference with rodents is probably due to the fact that they obtained part of the water to cover their physiological needs by eating, in contrast to the notoungulates which needed to drink to meet these requirements.

This study was financed by the projects: PGC2018–094955–A–I00 and PID2022-138275NB-I00 (Ministerio de Ciencia e Innovación, Spain), 13G and 21G (FCEyN, UNLPam, Argentina), PGI 24 H/154 (Secretaría de Ciencia y Tecnología, UNS, Argentina). DSP acknowledges a predoctoral grant PRE2019–089848 and AEM and RLT funding from LA. TE. Andes – CONICET (2020-2021). This is a contribution of the research group UCM 910607 on Evolution of Cenozoic Mammals and Continental Palaeoenvironments.