The environmental context of Ambrona Middle Stratigraphic Member (∼350 ka): Insights from stable isotope analysis on mammalian tooth enamel

Abstract

The Middle Pleistocene was characterized by pronounced climatic oscillations that reshaped European ecosystems and influenced hominin settlement dynamics. This study reconstructs the paleoenvironmental context of the Middle Stratigraphic Member of Ambrona (Soria, Spain), dated to ∼350 ka (MIS 9), using stable carbon (δ13C) and oxygen (δ18O) isotope analyses of 56 tooth enamel samples from Equidae, Bovidae, Cervidae, and Elephantidae. The isotopic dataset is strongly dominated by equids, and paleoenvironmental interpretations primarily reflect equid ecology. The δ13C values indicate diets dominated by C3 vegetation and, when combined with body mass-corrected δ13Cecosystem estimates, support a mosaic landscape structured around pine-dominated woodland and open herbaceous sectors, rather than a dense closed-canopy forest. Intra-tooth δ18O profiles provide insights into seasonal to sub-annual variability, although their interpretation is constrained by temporal resolution and enamel mineralization processes. Overall, δ18O values suggest relatively stable hydrological conditions and sustained water availability. Comparison with equid isotopic data from the contemporaneous MIS 9 site of Schöningen (Germany) reveals significant differences in δ13C and δ18O values, indicating regional variation in vegetation structure and hydroclimatic conditions. These differences reflect spatial heterogeneity within European interglacial environments, with Ambrona characterized by a Mediterranean hydroclimatic regime and Schöningen by a temperate setting. Integrated with faunal, sedimentological, and paleoenvironmental evidence, these results define the Ambrona Middle Stratigraphic Member as a humid, environmentally structured woodland-wetland mosaic with persistent freshwater availability. While the isotopic data do not directly resolve hominin behavior, they provide a refined environmental framework within which hominin occupations can be contextualized and contribute to understanding the role of spatial climatic variability in shaping Middle Pleistocene European landscapes.