Ancient cod bones unearthed at an Alaska archaeological site carry a very modern warning for a world with a rapidly changing climate -- as sea levels rise, so do levels of mercury in the food chain.

The bones, discovered at a coastal site in Katmai National Park and Preserve, date back to the early and mid-Holocene, a time when a warming climate melted glaciers and expanded the oceans. The rising seas inundating the Bering Land Bridge and other stretches of terrain caused some of the naturally occurring mercury that was locked in dry or frozen land to get free and disperse in the expanded marine waters.

The high levels of mercury in the cod bones are described in a study published in the journal Frontiers in Environmental Science.

After the end of the last Ice Age, sea levels inched up gradually, reaching their current level about 4,000 years ago. The cod bones, found among the trash dumped at an ancient human dwelling site, date back to that time and earlier.

Examination of the cod bones shows that mercury levels peaked as sea levels rose during the first half of the Holocene and mercury levels fell dramatically thousands of years later, when sea levels were stable and no new land areas were being conquered by water.

The study, conducted by scientists from the University of Calgary, UAF, the National Park Service and other institutions, is the first to examine mercury levels in a Gulf of Alaska marine species over that span of time. It describes just one aspect of the knowledge that has come from the Shelikof Strait archaeological site at a place called Mink Island.

Excavations began in the 1990s and were prompted by concerns that treasure hunters or curio seekers could damage the site, said study co-author Jeanne Schaaf, a retired National Park Service archaeologist who led the work there.

Long-term erosion and land changes also took a toll, but plenty of dwelling remains were left -- enough to provide a wealth of archaeological information about the people who were part of what is known as Ocean Bay culture.

"The erosion is more like big bites out of a cookie, but the layers of the cookie are still very well-preserved," Schaaf said.

The Mink Island site, now sealed off to protect against future erosion, holds a series of dwelling floors unearthed in the excavations, she said. The floors are covered with reddish ochre, a mineral that was used as a waterproofing material, she said.

The cod bones are among thousands of artifacts retrieved in the project, Schaaf said. While there were other types of fish bones, the bones of cod are a little larger and more solid and thus considered easier to test for mercury, she said.

Analysis at UAF found mercury levels that were generally above 0.4 parts per million in bones about 4,400 to 5,200 years old. The highest mercury level found in any of the cod bones was 0.7 parts per million. Much-younger cod bones extracted from the Mink Island site -- those less than 1,000 years old -- had much lower mercury levels, down to about 1/20th of the highest measurements found.

Modern-day Shelikof Strait-area cod, the study said, generally carry mercury levels in their muscles of 0.25 to 0.5 parts per million, according to the study.

The highest readings from the ancient cod bones are eye-opening because they come from a time well before the Industrial Revolution, but match or exceed present-day mercury levels in fish, which are known to be elevated by industrial activities around the world, said study co-author Lawrence Duffy, a UAF biochemistry professor and expert in contaminants.

It is not yet understood precisely how mercury levels in bone correspond to those in muscle – the part of the fish that people would eat – but scientists believe the cod muscle would generally have significantly higher levels, the study said.

If readings from a contemporary sea otter in the Mink Island area are any indication, Duffy said, some of the cod eaten by people thousands of years ago was laden with mercury in levels well above today's Food and Drug Administration's limit of 1 part per million for fish.

That sea otter had a mercury level of 9 parts per billion in its bones, but the level was more than six times as high -- 58 parts per billion -- in the animal's muscle tissue, Duffy said. If the ancient cod muscle similarly held six times the level of mercury found in bones, he said, eating that fish would have posed health risks for the people inhabiting the area 4,000 to 5,000 years ago, especially the youngest inhabitants, whose brains were just developing and were thus most vulnerable to contaminants, he said.

"The population, I'm comfortable in saying, would be more at risk than a population 1,000 years ago or 500 years ago when the levels had dropped," he said.

Mercury is a toxic element that can occur naturally and be sequestered in dry or frozen soils. But when those soils become wet or thaw, the mercury can be released and consumed by organisms. Mercury is most dangerous when it undergoes a chemical process to become methylmercury, the form that accumulates most readily in fish and the mammals that eat them.

Industrialization has added to the loads available to fish.

Human activities since the start of the Industrial Age (circa 1790), largely fossil-fuel combustion and mining, have tripled the levels of mercury in the surface waters of the world's oceans, according to a comprehensive study published last August in the journal Nature.

Now, present-day climate change is combining with modern industrial activities to increase mercury loads in ecosystems around the world. Effects are likely to be pronounced in Alaska, among other northern locations, Duffy said.

"With the predicted increase in rainfall along the Aleutians and in western Alaska in the next 50 to 100 years, with glaciers melting and ice levels melting, mercury will become more mobile and go into the food chain," he said.

Mercury levels in the far north are rising, scientists say, from permafrost thaw, increased freshwater runoff from rivers and other manifestations of climate change.

Added to that is long-range transport on atmospheric currents of distant mercury emissions from coal burning, mining and other industrial activities happening in latitudes far to the south.