Dual role of human activities and climate in pre-industrial nitrogen shifts in Ireland

Past research has uncovered a shift in herbivore collagen δ¹⁵N values during the Middle to Late Bronze Age (3700–2750 BP) in Ireland, attributed to intensified land-use - another example in a growing body of evidence that suggests that the Anthropocene concept (whereby humans have made significant impacts to the Earth's systems) began far earlier than previously thought (the ‘Palaeoanthropocene’). While human activity was clearly an important driver of this shift, it remains possible that climate may also have played a role. In Ireland, this shift in δ¹⁵N appeared to be sustained despite subsequent reforestation, suggesting that Bronze Age and later peoples left a profound biogeochemical mark on the environment. Here, we revisit this topic, presenting new stable isotope (δ¹⁵N and δ¹³C) measurements from directly radiocarbon-dated wild and domesticated adult herbivorous ungulate bones from the Bronze and Iron Ages, with the aim of constraining the timing of this shift and disentangling the role of human land use from climatic influences. A sustained nitrogen isotopic shift was constrained to ca. 3000 cal BP. Notably, during the Iron Age, a time of widespread reforestation, wild and domesticated herbivorous ungulates exhibited δ¹⁵N values that were not significantly different from one another. This suggests that, while the δ¹⁵N shift was initiated by land-use change, climate variability, particularly shifts towards warmer and drier conditions during the Late Bronze/Iron Age transition and later Iron Age, may have contributed to its persistence. This study refines the chronology of nitrogen cycle changes in Ireland, demonstrating that human activity initiated a long-term shift in δ¹⁵N values during the Late Bronze Age, but that climatic factors may have played a role in maintaining elevated δ¹⁵N values during periods of reduced land use. These findings underscore the complexities of understanding the nitrogen cycle in ancient environments and highlight the potential interplay between anthropogenic and environmental drivers of biogeochemical change.