Mercury, cod, and climate change

Hallo Bay Sunset Katmai NPS

It’s hard to imagine stalking the shores of Alaska hunting with spear or net more than four millennia ago. Harder still to know that the people living in that already-harsh time faced an even more insidious threat than hunger or the fierce elements. New archaeological findings show elevated levels of toxic mercury in Pacific Cod bones recovered from 4,000 – 5,000 year old settlements on Mink Island, one of many small islands in Amalik Bay, Katmai National Park and Preserve, Alaska.

The dangers of mercury

Mercury element
Mercury / Courtesy U.S. Environmental Protection Agency

Mercury is a toxic heavy metal. In high concentrations it causes serious damage to fish, ecosystems, fish-eating wildlife and humans.

When toxic mercury accumulates in organisms’ tissues it can cause severe harm. It damages the immune system and the nervous system, changes the expression of genes and enzymes, reduces reproductive success, impairs growth and development, and negatively influences behavior, coordination, and the senses. It is particularly harmful to developing fetuses.

A global problem

Mercury is called a global contaminant because it can enter the atmosphere and stay there for years, traveling extreme distances on atmospheric currents as tiny particles or as gas. Eventually it reaches the ground, deposited by rain or snow or simply settling down as dust particles. Even though the toxic contaminant may have been released far away, it can end up in remote and otherwise pristine places.

A National Park Service resource exploring the effects of mercury on ecosystems states: “Once airborne, these toxic contaminants are often carried towards polar or high elevation environments where, in cold conditions, they condense and are deposited. This is believed to account for the surprisingly high concentrations present in arctic environments, and in the indigenous peoples who live there.” (NPS, no author listed.)

Global contaminant sources

Mercury occurs naturally in extremely small amounts in all rock, soil, and water. Some parts of nature contain more mercury than others: volcanoes, thermal springs, and specific naturally occurring mineral deposits. In frozen soil like permafrost, or in dry soil, naturally occurring mercury can remain trapped underground for a long time where it does not pose a large danger to living things.

Human activity has aggressively increased the amount of circulating mercury since the Industrial Revolution. There is three times as much mercury in ocean surface waters and three to five times as much mercury in the atmosphere as there was before. These human-caused anthropogenic sources mainly include burning fossil-fuels like coal in coal-fired power plants, incinerating waste, the use of industrial boilers, smelting, and techniques used in mining operations.

The Bering Land Bridge

Beringia Bering Land Bridge
Green outlines what was once Beringia, or the Bering Land Bridge / Courtesy NPS

If mercury levels have increased in modern day due to human activities, why are archaeologists finding so much mercury in the bones of Pacific Cod fished and eaten by people living long before modern-day industry began?

We know that mercury is released when frozen or dry ground is inundated with water. Wetlands, coastal flooded areas, and land flooded on purpose (say, a reservoir for a new dam) release mercury. The people who lived 4,000 – 5,000 years ago in Alaska were exposed to heightened levels of mercury because the Bering Land Bridge was submerged beneath the ocean, a huge stretch of land covered with water and releasing previously land-bound reserves of mercury.

Levels of mercury

Today’s global sea level is 393.7 feet [120 meters] higher than it was during the last glacial maximum, an Ice Age when a lot of the world’s water was locked up and frozen in glaciers and immense ice sheets. As those ice sheets warmed and melted over a long period of time, the melt water covered over the Bering Land Bridge that once connected Asia and North America. At Alaska’s shoreline land was submerged beneath ocean waters. The rate of change was slow and the changes took place over a long time, but by the end the Bering Sea had nearly doubled in size.

The naturally occurring mercury unlocked when coastal land was submerged by water is what caused heightened levels of mercury in the bones found on Mink Island.

Pacific Cod junvenile
A young Pacific Cod / Courtesy

Mink Island archaeological Pacific Cod bones contain levels of mercury which match or exceed present-day levels of mercury in fish. The United States Food and Drug Administration limits acceptable mercury levels to 1 part per million (ppm) in fish muscle. Modern-day cod near Mink Island average 0.25 – 0.5 ppm in their muscle. The cod remains from the archaeological dig averaged 0.4 ppm and reached as high as 0.7 ppm in their bones, which suggests even higher concentrations would have been found in their muscle tissue.


When mercury (Hg) enters aquatic environments it can be transformed into methylmercury (CH3Hg), a highly toxic form which builds up or bioaccumulates in organisms’ tissues. Bacterial microorganism take up mercury and convert it to methylmercury as they ‘eat’ (perform metabolic processes). The methylmercury they create and release back into the water is used by i.e. phytoplankton, microscopic organisms which perform photosynthesis. These itty-bitty concentrations of methylmercury accumulate in larger and larger organisms; small fish eat plants, cod eat small fish, birds and humans eat cod. Methylmercury concentrations increase as you move up the food chain in a process called biomagnification, until levels are high enough to cause damage.

Mad Hatter
Illustration of Lewis Carroll’s Mad Hatter. Hat makers were exposed to mercury poisoning while shaping hats, and as a result developed neurological disorders. / (public domain)

Increasing the acidity of water also increases the levels of methylmercury in that water, making it more likely to enter the food chain. Global warming can force water to become more acidic because of heightened levels of carbon dioxide. Carbon dioxide dissolves in water to form an acid called carbonic acid (H2CO3).

Pristine sites, polluted

snow analysis investigators
Western Airborne Contaminants Assessment Project investigators collect snow samples for airborne contaminant analysis / Courtesy USGS, Don Campbell

A study performed by the U.S. Geological Survey (USGS) and National Park Service (NPS) and published April 2014 investigated mercury concentrations in more than 1,400 freshwater fish from 86 sites across 21 national parks in 10 western U.S. states. The fish were collected from high elevation, remote bodies of water in the parks.

At sites in two different Alaskan parks, the average concentration of mercury in sport fish exceeded EPA’s human health criterion.

Increasing global levels of mercury from industry are contaminating our fish. As warming temperatures cause sea-level rise and coastal flooding, we can expect levels of mercury to rise again and again. Climate change is also projected to thaw permafrost and increase precipitation in parts of Alaska, leading to more previously-land-bound mercury entering aquatic habitats. It endangers ecosystems, wildlife, and humans.

“This is a wake-up call,” according to NPS ecologist Colleen Flanagan Pritz. “We need to see fewer contaminants in park ecosystems, especially contaminants like mercury where concentrations in fish challenge the very mission of the national parks to leave wild life unimpaired for future generations.”

Laura Nielsen 2015

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

Watertracks project

  • ‘Biogeochemical analysis of ancient Pacific Cod bone suggests Hg bioaccumulation was linked to paleo sea level rise and climate change’ Maribeth S. Murray, C. Peter McRoy, Lawrence K. Duffy, Amy C. Hirons, Jeanne M. Schaaf, Robert P. Trocine, and John Trefry, Front. Environ. Sci., (February 2015) doi: 10.3389/fenvs.2015.00008
  • ‘Mercury in fishes from 21 national parks in the Western United States—Inter and intra-park variation in concentrations and ecological risk’, Eagles-Smith, C.A., Willacker, J.J., and Flanagan Pritz, C.M., U.S. Geological Survey Open-File Report 2014-1051, 54 p. (April 2014)
  • ‘Ancient cod bones carry modern warning about mercury, climate change’ Yereth Rosen, Alaska Dispatch News (March 2015)
  • ‘Effects of Air Toxics/Mercury on Ecosystems’ Explore Nature, Air Resources, National Park Service – no author listed (January 2013)
  • ‘Elevated Levels of Mercury Found in Fish in Western U.S. National Parks‘ National Park Service Press Release, Susan Kemp, USGS, Jeffrey Olson, NPS (April 2014)
  • ‘Mercury in the Environment – Fact Sheet 146-00’ U.S. Geological Survey – no author listed (October 2000)