Narwhal Tusks Are Climate Change Time Capsules

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The rapid increase in global warming has caused Arctic temperatures to rise at a startling rate. Thick, multilayered Arctic sea ice is melting and profoundly impacting the marine ecosystem (Ogawa, et al., 2018). To guarantee a sustainable future for Arctic predators like the narwhal, beluga whale, and polar bear, scientists need a sounder understanding of the mechanisms behind CO2‐forced warming. They begin this daunting challenge by assessing the impact climate shifts have already made and prepare for the future ecophysical hurdles temperature rises may bring.

The future of arctic marine life hinges on their ability to adapt to their habitats' recent changes. Large cracks in polar ice, known as leeds, create canyon-like channels of open water. Because these rapidly growing expanses of open water expose the dark ocean, leeds have a much higher ability to absorb light and heat than the neighboring ice. Warming in these waters causes drastic changes in large predators' food source and prey distribution. The phrase "you are what you eat" holds for arctic animals. As their dietary ecology changes, the aquatic predators begin to display changes in their chemical concentrations that parallel their prey's compositions. In addition to having a warming effect, arctic Leeds funnel upper-atmosphere and mercury-rich air to the water's surface. Neurotoxic mercury near the surface can settle in the water or ice and subsequently enter the marine food chain (Moore, et al., 2014). Changes in global temperatures and mercury-induced toxic stress are threatening the future of many arctic predators.

In general, Arctic pollution studies are limited to a single snapshot, like a tissue sample, of the wildlife's condition in the particular moment when the researcher has gathered their data. While this information can be helpful, it does not adequately display the stressors an aquatic creature faces during their lifetime. Researchers at Aarhus University in Denmark devised a brilliant plan to trace changes in more than 50 years of compositional and ecological changes in the Arctic ocean using the tusks of narwhals.

Similar to other continuously growing tissue like hair or teeth, a narwhal's tusk records the animal's physiological condition as it grows. The researchers examined varying isotope concentrations, known as biogeochemical tracers, of carbon and nitrogen found in the tusk's layers. These chemical markers can help researchers identify the narwhal's habitat preference, food source, and movement during its lifetime. Isotope levels can distinguish coastal from oceanic ecosystems as well as open-water versus ice-associated food webs. Because environmental and dietary information found in biogeochemical tracers is passed from prey to predator, the tusk's layers directly correlate to changes in the narwhal's feeding ecology throughout its life (Dietz, et al., 2021).

The narwhal tusk's unique bio-tracing ability can also effectively tracks contaminant trends. Like the isotopes, marine organisms accumulate mercury from their food. The researchers measured the yearly changes in carbon, nitrogen, and mercury concentrations found in the narwhal's tusk. High concentrations of this toxic element can cause detrimental increases in tissue concentrations. The information gathered about the narwhal's prey, and the type of marine environment each species prefers can help the researchers hypothesize temporal changes in sea temperature and pollution (Dietz, et al., 2021).

The researchers examined the tusks of ten narwhals whose lives collectively spanned from 1962-2010. The study showed drastic changes in the narwhals' diets throughout their lives. Early in the predators' lives, they primarily consumed a sympagic or ice-associated diet. Later, the researchers noted a shift to pelagic or open-water biogeochemical tracers. The trophic switch is likely due to diminishing Arctic sea-ice and smaller sympagic regions. The tusks also displayed an annual increase in mercury. Marine mercury prevalence is likely caused by increased emissions and quicker absorption in climate-induced warmer waters (Dietz, et al., 2021).

The narwhal's tusk makes it possible to investigate dietary changes and pollution levels caused by climate change. These biological time-capsules support studies claiming that sea-ice coverage is reduced by global warming and subsequently lowering marine biodiversity. Arctic ice melting impacts narwhal migration, contaminant exposure, physiology, and nutrition. While these incredible animals have survived ecological upsets to date, they are in great danger if their habitat continues to disappear.

Sources:

Dietz, R., Desforges, J. P., Rigét, F. F., Aubail, A., Garde, E., Ambus, P., & Sonne, C. (2021). Analysis of narwhal tusks reveals lifelong feeding ecology and mercury exposure. Current Biology, https://doi.org/10.1016/j.cub.2021.02.018.

Moore, C. W., Obrist, D., Steffen, A., Staebler, R. M., Douglas, T. A., Richter, A., & Nghiem, S. V. (2014). Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice. Nature, 506(7486), 81–84. https://doi.org/10.1038/nature12924.

Ogawa, F., Keenlyside, N., Gao, Y., Koenigk, T., Yang, S., Suo, L., & Semenov, V. (2018). Evaluating impacts of recent Arctic sea ice loss on the northern hemisphere winter climate change. Geophysical Research Letters, 45(7), 3255-3263. https://doi.org/10.1002/2017GL076502.