Ancient Marine Mammal Bones Record Dynamic Change in Mercury Concentrations Over the Last 3000 Years

The following post is one of a series previewing the research that will be presented at the SETAC North America 41st Annual Meeting (SciCon2), 15–19 November 2020.

A guest post by Julie P. Avery, Nicole Misarti, Todd M. O’Hara, and Lorrie D. Rea

Do you like fish?  Have you heard warnings from the FDA about consuming predatory fish such as tuna, swordfish, and shark due to high mercury? Fish are a healthy food and an excellent source of lean protein, healthy omega-3 fats, vitamin D, iron, selenium, zinc, and iodine. Fish provide all these nutrients, which are essential for human and animal health. Marine mammals in Alaska, like Steller sea lions and northern fur seals, thrive on a diverse fish diet. But what about the mercury? Since we eat some of the same fish as seals and sea lions, we can study them to understand how mercury might affect humans. 

Coastal and Arctic communities are especially vulnerable to the effects of mercury contamination due to their dependence on fish and marine resources for food and sociocultural needs. According to the World Health Organization, mercury is one of the top 10 contaminants of concern for human health.  

There are two primary sources of mercury in the Arctic: (1) natural inputs from volcanoes, melting permafrost, and river systems, and (2) human sources from distant industrial activities in human population centers that are transported through the atmosphere and deposited in the Arctic.

Mercury deposited in the environment is chemically modified by bacteria to its biologically active form, methylmercury. This form of mercury can then be easily incorporated into the marine food web affecting invertebrates, fish, and mammals, including humans. Chronic exposure to even low quantities of methylmercury can have long-lasting effects on humans and animals alike, including reduced health, impaired reproduction, and lower survival rates. Mercury is greater in older individuals because it accumulates over an individual’s lifetime (bio-accumulation). Mercury is also greater in predators like sharks and tuna because they eat other animals. Biomagnification is the term used to explain why predators at the top of the food web have greater levels of mercury than species at the bottom of the food web. Thinking of it in human terms, a vegetarian would be less likely to have high levels of mercury than someone who eats a lot of fish and meat; and even if exposed to mercury through their diet, a child would probably have lower concentrations of mercury than an adult or a senior citizen eating the same diet.

On a clear day looking out from the shore of Sanak Island in the Eastern Aleutian Islands, nearby volcanoes appear in the distance. Photo credit: Nicole Misarti

Our team of interdisciplinary scientists includes paleo-ecologists, archaeologists, volcanologists, physiologists, and toxicologists among others. We were interested in how mercury changed in the Arctic before the Industrial Revolution added human sources of mercury to our environment. The Aleutian are a chain of volcanic islands in the Northern Hemisphere spanning from Alaska to Russia, separating the Bering Sea from the North Pacific Ocean. The significant volcanic activity in the Eastern Aleutian Islands is well studied. For this reason, we chose to study Sanak Island located within the Eastern Aleutians. 

Archaeological middens are a treasure chest for scientists of all disciplines. These man-made mounds of discarded materials include animal bones, mollusk shells, plant material, artifacts and more. Middens can give us information about past ways of life, past ecosystems, and past climates.

Layers of sediment, rich with marine animal bone, are excavated from an archaeological midden on the Alaska peninsula. Photo credit: Nicole Misarti
An archaeological screen sifts out the soil but captures bones and artifacts from an excavation. This screen contains bones from multiple species of fish from a dig on the Bering Sea coast in 20214. Photo credit: Nicole Misarti

Archaeological excavations of midden sites on Sanak Island provided 4000 years of data, including the fish and marine mammal bones for our study.

Early in our investigation, our small sample size suggested a gradual increase in mercury over time; however, with an increase in sample size we have found some of the greatest concentrations of bone mercury from individuals that lived more than 3000 years ago (Image 4). According to our colleagues at the United States Geological Survey (USGS) 3000 to 4000 years ago was a very active volcanic period for the Eastern Aleutian Islands. Our data suggest that volcanic eruptions may have contributed to increased mercury concentrations in predators of the Eastern Aleutian Islands.    

These preliminary data are the first step in our understanding the natural processes that contribute to biologically active mercury in the Aleutian environment. Our next step is to use stable isotope analyses to determine the diets of each individual we sample. Top predators like Steller sea lions and northern fur seals vary in the amount of mercury they are exposed to depending on the types of fish they consume.  Eating more predatory fish will increase the amount of mercury in the body.

In addition to investigating what our mammals may have been eating, we will also evaluate mercury dynamics based on changes in climate (warm vs. cool years), volcanic eruptions, and other non-biological factors. Finally, we will expand this work to the Central and Western Aleutian Islands to examine regional variability in mercury trends.

Ancient bones from the fish-eating sea lions and fur seals of the Aleutians provide a unique opportunity to examine 3000 years of natural mercury dynamics. Understanding what factors contribute to mercury accumulation in top predators will help us protect communities that rely on coastal resources for food, commerce, and survival.

Presentation information for SciCon2:

1.09.24: Total Mercury, Total Selenium, and Monomethylmercury in Multiple Age Cohorts and Tissue Types in Alaskan Steller Sea Lions

1.09.36: Ancient Marine Mammal Bones Record Dynamic Change in Natural Levels of Mercury Over the Last 3000 Years

4.01.07: Significant Within-Decade Increase in Mercury Concentrations in Steller Sea Lion Pups at Agattu Island, Alaska

1 thought on “Ancient Marine Mammal Bones Record Dynamic Change in Mercury Concentrations Over the Last 3000 Years

  1. Ray Kinney

    Great line of investigation using shell middens! There is a lot of opportunity to include prehistoric and historic bioaccumulation of the toxicants, such as Pb, Cd, and N-MBAA synergistically damaging to many physiologic pathways in organisms. The upper layers of shell middens protect the layers below from most effects of leaching due to rainfall and snow melt acidification pressures. These toxicologic assessment techniques offer great opportunity for us to gain better understanding of anthropogenic pollution histories within salmon streams in the Oregon and Washington salmon conservation paradigm. Freshwater mussel shell middens can contribute knowledge of river pollution through time, and together with marine shell middens, perhaps bring better clarity of anthropogenic contributions of pollution to the nearshore marine critical habitat.

    Reply

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