Multi-Proxy Annual Profiles in Narwhal Tusks Reveal Biogeochemical Baseline Restructuring Under Rapid Arctic Change

Abstract

The Arctic is undergoing rapid physical and biogeochemical change, yet major gaps remain in our understanding of how these transformations propagate through marine food webs and influence the ecology of long-lived marine mammals. Climate-driven shifts in prey availability, sea-ice dynamics, and habitat structure shape diet, mobility, and contaminant exposure, but the ecological consequences of these interacting pressures remain difficult to disentangle. Similar isotopic and elemental responses can arise from changes in foraging behaviour, movement patterns, or environmental baselines, creating a central interpretive challenge for long-term geochemical records. This thesis uses incrementally growing narwhal tusks as natural archives to disentangle ecological change from climate-driven restructuring of baseline conditions. Multi-proxy geochemical profiles were measured along annual growth layer groups in two narwhal tusks from contrasting Canadian Arctic regions, Baffin Bay and Hudson Bay, spanning the late 1990s to early 2020s. Stable isotopes (δ13C and δ15N in collagen; δ13C and δ18O in carbonate), isotopic spacing between carbonate and collagen (∆13Ccarb-coll), mercury concentrations, lead isotope ratios (206Pb/204Pb, 208Pb/206Pb, 206Pb/207Pb), and selected elemental ratios (Pb:Ca, Cd:Ca, As:Ca) were integrated with regional time series of sea-ice extent, coastal ocean temperature and salinity, and Arctic climate modes. Additional population-level δ13C and δ15N data from Pond Inlet narwhal embedded teeth and Baffin Bay beluga teeth were used to evaluate whether inferred baseline shifts are expressed across individuals, populations, and species. The dominant isotopic signal was a pronounced decline in δ15Ncollagen in the Baffin Bay individual, whereas δ15Ncollagen remained comparatively stable in the Hudson Bay individual. Mobility-sensitive tracers showed strong interannual variability but no sustained directional trends, indicating no long-term relocation sufficient to account for the observed δ15N pattern. In both individuals, Ba:Ca, Sr:Ca, and ∆13Ccarb-coll displayed consistent early-life structure, supporting an ontogenetic transition toward an adult-like feeding regime and changes in macronutrient routing. Bulk collagen isotopes behaved differently: δ13Ccollagen covaried with δ13CCO3 in both tusks, indicating a shared sensitivity to baseline carbon-system variability, while the contrasting δ15N records may reflect population-specific baseline forcing, potentially limiting the use of bulk δ15N as a long-term trophic indicator. Toxicological proxies showed contrasting trajectories, with mercury increasing through time in both individuals, consistent with trophic biomagnification combined with climate-sensitive baseline forcing, whereas Pb:Ca, Cd:Ca, and As:Ca declined systematically, consistent with changing exposure pathways driven by sea-ice loss, altered primary production, circulation shifts, and reduced atmospheric inputs. Lead isotope variability further indicated changes in contaminant provenance, particularly in Baffin Bay. Independent multi-individual datasets revealed coherent shifts toward lower δ13C and δ15N beginning around the early 2000s at the population level in narwhals and also in belugas, demonstrating that the signal extends beyond single individuals to multiple populations and species. The concordance of these records is consistent with an ecosystem-scale baseline driver potentially linked to climate-driven loss of sea ice and associated reorganization of carbon and nitrogen cycling at the base of the food web in Baffin Bay. In this region, the population-level decline in δ15N is interpreted as reflecting increased reliance on regenerated nitrogen and enhanced N2 fixation under declining sea-ice conditions, processes that would lower baseline δ15N values in primary producers and propagate upward to apex predators. Concurrent declines in δ13C are consistent with shifts in dominant carbon sources associated with reduced ice-algal production, enhanced pelagic phytoplankton growth, and changing CO2 availability, indicating baseline restructuring within the Baffin Bay ecosystem. Together, these findings establish narwhal tusks as high-resolution archives of both ontogenetic ecology and multi-decadal baseline restructuring. The integration of isotopic and elemental tracers within annual growth layers provides a robust framework for disentangling ecological change from shifting environmental baselines and for improving understanding of how Arctic marine systems and the species inhabiting them respond to changes in primary production, reorganization of biogeochemical cycling, ongoing sea-ice loss, and evolving contaminant pathways.