Tracing giants through time: stable isotope insights into the historical ecology of North Atlantic sperm whales (Physeter macrocephalus)

Introduction Sperm whales (Physeter macrocephalus) are deep-diving apex predators whose offshore ecology limits direct observation. Stable isotope analysis is widely used to infer cetacean diet and habitat use, but its application to archaeological whale remains remains rare despite the regular occurrence of sperm whales in zooarchaeological assemblages. Here, we combine stable carbon (δ¹³C) and nitrogen (δ 15 N) isotope analysis with zooarchaeological and palaeontological data to investigate the long-term foraging ecology of sperm whales in the North Atlantic. Methods Bone specimens identified as P. macrocephalus using morphological and palaeoproteomic methods, including Zooarchaeology by Mass Spectrometry (ZooMS), were sampled from Late Holocene contexts across northern and western Europe. Stable isotope values of bone collagen were measured using EA-IRMS. Three known-age specimens were used to calculate an empirically based, eastern North Atlantic sperm-whale-specific marine reservoir correction, which was applied when comparing the Late Holocene specimens with previously published radiocarbon-dated Late Glacial material. Published modern isotope data were corrected for the Suess effect before comparison. Results The three known-age specimens produced a provisional ΔR₂₀ estimate of −192 ± 40 ¹⁴C years. Within the Late Holocene, δ¹³C values showed modest temporal variation, including more depleted values during some earlier periods and relatively enriched values from the Medieval Climate Anomaly onward, whereas δ¹⁵N values showed no directional temporal trend. Late Glacial specimens had significantly higher δ¹⁵N values than Late Holocene specimens, by approximately 2.3‰, while δ¹³C values did not differ significantly between the periods. After Suess correction, modern oceanic North Atlantic sperm whales were isotopically similar to Late Holocene North Atlantic specimens, whereas Mediterranean sperm whales were isotopically distinct regardless of time period. Discussion The results suggest broad long-term stability in carbon isotope baselines or habitat use, alongside substantial reorganisation of nitrogen cycling or deep-pelagic food-web structure between the Late Glacial and Late Holocene. The distinct Mediterranean isotope values are also consistent with the known demographic separation of Mediterranean sperm whales from the wider North Atlantic population. These results provide rare historical isotopic baselines for sperm whales in the eastern North Atlantic and demonstrate that apparent stability in predator trophic ecology can coexist with substantial changes in underlying ocean biogeochemistry. Such baselines are essential for evaluating future responses of this deep-diving predator to anthropogenic pressures and climate-driven reorganisation of deep-ocean food webs.