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Chemistry

Lead Pollution in Antarctic Waters: Have We Cleaned Up Our Act?

Paper: Ndungu, K.; Zurbrick, C. M.; Stammerjohn, S.; Severmann, S.; Sherrell, R. M.; Flegal, A. R. “Lead sources to the Amundsen Sea, West Antartica.” Environ. Sci. Technol. 2016. DOI: 10.1021/acs.est.5b05151
 

This Study

The usage of leaded gasoline from the 1920s to 1970s emitted massive amounts of lead to the atmosphere. Much of this lead was eventually deposited in our oceans. In this study, researchers traveled to one of the least explored regions on Earth – the remote Amundsen Sea of Antarctica – to determine whether we’ve cleaned up our act since the ’70s – Are lead levels in this pristine region back to what’s natural?

Amundsen Sea

Map showing the Antarctic continent and general location of the Amundsen Sea (marked by the red star). Source: en.wikipedia.org

They measured lead levels in ocean water and used data on the isotopic composition of the lead to determine how much originated from human industrial activities, and how much was naturally occurring lead from sediments and glacial ice melt. Their findings suggest something unexpected: melting glaciers may be an increasingly significant source of lead to the water in polar regions.
 

Why is there lead in the water?

It’s no question that we don’t want lead in our water. However, we weren’t always quite so knowledgeable. Lead was previously extracted from the Earth’s crust, where it occurs naturally, and used in plumbing, paint, gasoline, and a wide variety of consumer products. Many of these applications have declined dramatically since the 1970s, when lead was found to be a potent toxin that is particularly disruptive to the brain development of young children.

Luckily, we stopped using leaded gasoline before levels in the open ocean reached a point where they would harm valuable ecosystems. Even though we’ve learned our lesson and halted widespread usage of lead, lead still lurks on land – particularly in older infrastructure, like the pipes in Flint, Michigan. Elevated lead levels also persist in environmental reservoirs like ocean sediments and polar ice deposits.

Researchers have reported that lead levels in the North Atlantic and many other ocean waters are declining, but that doesn’t mean they’re back to what they were before human perturbations. The oceans don’t respond to our whims overnight; it can take decades or even longer for the quality of the environment to be restored, even after emissions of a harmful chemical have ceased.
 

Research aboard an icebreaker

Scientists collected sediment and water samples while aboard the icebreaker Nathaniel B. Palmer as part of the Amundsen Sea Polynya International Research Expedition (ASPIRE). The study was a part of GEOTRACES, which is an international effort to better understand the distribution of trace elements in the world’s oceans.

The Nathaniel B. Palmer, the icebreaker used in this study. Source: commons.wikimedia.org

The Nathaniel B. Palmer, the icebreaker used in this study. Source: commons.wikimedia.org

Researchers took water samples at four different locations as the ship moved away from the Antarctic ice shelf and into the open Amundsen Sea. At each location, they took samples at different depths to construct what oceanographers refer to as a “depth profile” – a plot showing how levels of a certain chemical change as you travel down through the water column from surface to depth.
 

What did they find?

Lead concentrations were generally very low, as you’d expect in such a pristine, remote region. Researchers measured lead dissolved in the water at concentrations of 0.9 ng to 18 ng per liter of seawater (where ng stands for a nanogram, or 0.000000001 grams). This was not particularly surprising, but the study also resulted in two very interesting findings:

(1) Lead concentrations were greater in the open Amundsen Sea than along the coast.

Lead Depth Profile

Researchers measured concentrations of lead at four sites traveling away from the ice shelf towards the open sea (Site 50, 57, 66, and 4). At each site, they took samples at various depths. This figure shows how lead levels varied with depth at each site. The two sites nearest the ice shelf (50 and 57 – boxes and circles) had the lowest lead concentrations. Lead levels were greatest at site 4 (triangles), the farthest from the ice shelf. Source: Ndungu et al. 2016. Copyright 2016 American Chemical Society. 

Lead increased threefold as the scientists traveled away from the Antarctic coast into the open sea. The figure below shows depth profiles at each site, and it is clear that the site furthest from the coast (Site 4) contained the highest levels of lead.

The authors attributed these high lead concentrations in the open waters to two sources: aerosol deposition of industrial lead into surface waters, and leaching from sediments containing deposits of historical lead. They also observed that waters closer to the coast may be able to remove lead more efficiently – there is more biological matter in surface waters along the ice shelf, which leads to increased fluxes of particulate matter sinking from the surface to depth, bringing things like lead and other contaminants with it and possibly leading to a cleaner water column.

(2) Lead in samples closer to the coast originated from primarily natural sources, while more industrial lead was seen in the open Amundsen Sea. 

Previous studies have shown that natural lead from the Antarctic crust has a different isotopic signature than lead originating from industrial activity. The researchers measured isotopic composition of the lead in each sample to determine whether it was natural or anthropogenic (human-caused) in origin.

Percentage Natural Lead

The investigators found that the percentage of lead originating from natural sources (sediments and glacial melt) decreased as they moved away from the ice sheet and into the open Amundsen Sea. Source: Ndungu et al. 2016. Copyright 2016 American Chemical Society. 

They found that lead from the most distant offshore site had an isotopic signature more similar to industrial lead, while coastal samples were more similar to natural lead. The researchers estimated that more than 80% of lead in water from locations closer to the coast was derived from natural sources. In the open sea, ~ 60% of lead was estimated to be natural. They concluded that glacial ice melt is contributing natural lead to waters near the coast.

There are two primary types of water from different sources coming together in the Amundsen Sea: there is colder water on the surface of the sea, referred to as “Winter Water”, but there is also warmer (>33°F), saltier water deeper in the sea, referred to as “Circumpolar Deep Water”, or CDW. This deep water mass flows into cavities in the base of the ice sheets and is responsible for melting the underside of ice shelves along the Amundsen Sea’s coastline. As the water flows back out of these cavities, it brings with it glacial melt water and trace metals, including lead.
 

Why does it matter?

These findings suggest that our removal of lead from gasoline has led to declines in industrial lead in remote waters. The researchers predict that in general, lead concentrations will continue to decline as the Earth continues to respond to ever-lowering anthropogenic  lead emissions. As this occurs, a greater proportion of total lead in the sea will come from sediments and glacial melt, as opposed to industrial emissions.

We’ve stopped using leaded gasoline, but we’re still driving cars

The researchers also raised this interesting point: While humans may have stopped using leaded gasoline, we are still driving cars that run on fossil fuel consumption. Emission of anthropogenic lead has fallen, but increases in carbon dioxide emissions and subsequent global warming are causing increases in the release of natural lead by increasing the mobility of elements stored within ice shelves, and in sediments underneath them. In this way, increased carbon dioxide emissions could undermine some of the good we’ve done by halting our lead emissions.

Glaciers discharging into the Amundsen Sea are among the most vulnerable to climate change, and are responsible for significant discharges of ice melt into the ocean. Much of the melting in this region is occurring below sea level, meaning that underlying, older ice is being melted as opposed to newer, surficial ice. Increased melting of this old ice could cause increased inputs of natural lead to the Amundsen Sea over time.

This phenomenon could be even more of a concern in regions at lower latitudes than the Amundsen Sea, where increasing air temperatures are causing increased melting of ice at the surface of glaciers. Because this surficial ice was deposited during the industrial era, it is storing larger amounts of anthropogenic lead, and is expected to release more significant fluxes of industrial lead to the ocean as it melts.

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