Reviewing: Olivelli, A., Murphy, K., Bridgestock, L., Wilson, D. J., Rijkenberg, M., Middag, R., Weiss, D.J., van de Flierdt, T. & Rehkämper, M. (2023). Decline of anthropogenic lead in South Atlantic Ocean surface waters from 1990 to 2011: New constraints from concentration and isotope data. Marine Pollution Bulletin, 189, 114798.
Lead naturally exists in Earth’s crust and rocks, but burning fossil fuels and waste, and smelting release additional anthropogenic (human activities-derived) lead into the atmosphere. Once lead is released in the atmosphere, it stays there for 10 days or less as aerosols, then falls into the ocean. As lead accumulated in oysters, mussels and fish may build up in our bodies when we consume seafood, lead pollution in the ocean poses a potential health risk for us. Excessive exposure to lead can cause damage to our brains, livers and kidneys.
Since leaded gasoline was identified to be the primary source of anthropogenic lead into the environment, some countries banned the use of leaded gasoline in road vehicles starting in the 1970s. North American and Western European countries prohibited leaded gasoline usage in the 1970s and the 1980s, followed by Brazil and Argentina in 1991 and 1996 respectively. Many more South American and West African countries continued using leaded gasoline until 2005, but it wasn’t until July 2021 when Algeria last banned its use that leaded gasoline was no longer used anywhere in the world.
A team of scientists were interested in finding out if the ban on leaded gasoline in South American countries surrounding the South Atlantic Ocean helped to reduce lead pollution in the South Atlantic. In 2011, they collected seawater offshore of the South American coast, spanning from Punta Arenas, Chile to the equator, and measured how much lead was in these waters. They found that there was about a third less lead in these waters than there was in the 1990s. “But wait,” one might wonder, “there’s natural lead in rocks and dust. What if there’s less natural lead and not less anthropogenic lead in the ocean? How do we know that the decrease in lead levels is actually associated with anthropogenic lead, not natural lead?”
Well, turns out there is a way to discern between natural and anthropogenic lead. All elements, including lead, are made up of atoms, which in turn are made up of a certain number of protons and neutrons. The number of protons in an atom is what determines the identity of the element. For example, an atom with 82 protons is a lead atom, and an atom with 8 protons is an oxygen atom. While the number of protons determines the type of the element, the same element can have different numbers of neutrons. In the case of lead, most lead atoms have 82 protons and 126 neutrons, but some lead atoms have 82 protons and 125 neutrons, or 124 neutrons, or 122 neutrons. These different combinations of protons and neutrons are called “isotopes”. And lead from different sources have different amounts of lead isotopes. In other words, by measuring the abundance of each lead isotope, we can tell whether the lead is coming from natural land dust, lead ores, or coal. We can even tell if the natural dust or coal is from northern Africa, southern Africa or South America!
Using lead isotopes, the team of scientists found that 64% of lead in the South Atlantic Ocean surface waters in 2011 came from urban pollution in major South American cities such as Sao Paulo, Brazil, while a third came from natural dust from Patagonia, Argentina. They also found out that only 24% of lead in the South Atlantic Ocean in 1996 was natural. This means that banning the use of leaded gasoline in road vehicles was effective in reducing the amount of anthropogenic lead in the ocean over the past three decades. While this study shows that environmental policies can prove effective in reducing lead pollution in the ocean, it also indicates that urban pollution is still contributing to two-thirds of lead in the ocean, despite the worldwide ban on leaded gasoline. Future environmental policies must target sources of lead pollution to further lower ocean lead pollution and restore natural lead levels.
I am a PhD student in chemical oceanography at University of Washington. I am studying how different forms of metals in the ocean are shaping microbial communities in the North Pacific Ocean. When not working, I like going for a walk, visiting farmers’ markets and playing keyboard.