The Arctic Ocean off Canada was previously considered largely unencumbered by human lead pollution. However, a new study shows that traces of lead from human sources are also found in this remote marine area. One of the main causes was therefore high lead emissions in the 20th century, which entered the Arctic with aerosols. The total lead exposure is low, but an average of 60 percent comes from human sources – significantly more than previously assumed. According to the researchers, lead levels could rise as more arctic ice, which has been reflected in historical lead emissions, melts.
Lead damages the nervous system, disrupts blood formation and is thought to be likely to cause cancer. The toxic heavy metal occurs only in very small quantities in natural environments. It is released, for example, in volcanic eruptions or through abrasion of particles from the earth’s crust. Today, however, the main source is human activity: most of the emissions in road traffic are caused by the abrasion of tires and brakes, others come from the combustion of fossil fuels and from industrial exhaust gases. Human lead emissions peaked in the 20th century, before leaded gasoline was banned.
Isotopes reveal the origin
Since aerosols containing lead can spread widely with the air currents, many remote natural areas are also polluted with them. The Canada Basin in the Arctic Ocean has so far been largely untouched. Previous sediment analyzes showed only a very low level of lead, which was believed to come from natural sources. A recent study by researchers led by Joan de Vera from the University of Toronto in Canada contradicts this finding. “Our study shows that anthropogenic lead is ubiquitous in the seawater of the western Arctic Ocean,” the authors report.
For their analyzes, they examined seawater samples that were collected from a depth of up to 3,500 meters in the Canada Basin. From the ratio of different lead isotopes in these samples, they were able to infer the origin of the lead. A low ratio of the isotope lead-206 to the isotope lead-207 is typical for lead emissions from European gasoline in the 20th century. The exhaust gases from Russian furnaces also produce a similar signature. In the case of natural lead sources in the region, however, the ratio is shifted in favor of lead-206.
Emissions from Europe and Russia
“The results of the isotope analysis show that historical aerosol lead from Europe and Russia, which was deposited in the Arctic in the 20th century and subsequently remobilized, is a significant source of lead in the water of the Canada Basin,” the researchers write. It has long been known that particulate matter from Eurasia reaches the Arctic. A similar lead isotope distribution has already been discovered in ice cores and in arctic snow.
Overall, de Vera and her colleagues found a very low lead concentration in the water samples, averaging five picomoles per kilogram, with the content in the upper water layers being slightly higher than in the lower layers. Depending on the depth, the human share of lead pollution fluctuated between seven and one hundred percent with an average value of 60 percent. “We have detected Eurasian lead from the 20th century at various depths, especially in water layers with higher lead concentrations above 1000 meters and near the shelf,” the researchers report.
Lead from the meltwater
The finding that the concentration of anthropogenic lead is highest near the shelf suggests, according to the researchers, that the lead emissions were initially deposited in the ice and from there entered the ocean with the meltwater. “Studies in the Arctic show that the shelves are important sources of trace elements such as iron, zinc and radium,” the authors explain. “This probably also applies to lead. Shelf and coastal processes, including coastal erosion and the melting of sea ice and permafrost, are not only a source of natural lead, but also appear to remobilize the previously deposited Eurasian lead contamination of the 20th century. “
According to the authors, further pollutants could be released in the future due to accelerated ice melt and coastal erosion as a result of global warming. Since the lead content in the Arctic Ocean is very low overall, even small changes in the concentration are noticeable. “This sensitivity makes lead and its isotopes in arctic seawater a useful indicator in identifying sources of particulate matter and pollutants,” write de Vera and her colleagues. In view of increasing human activities in the Arctic, the region can be seen as a kind of large-scale experiment. “By observing, documenting and using the changes in the region, we can understand how our activities affect the regional environment,” the researchers say.
Source: Joan de Vera (University of Toronto, Canada) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2100023118