The chemistry of Sydney's 'tar balls' explained

Hundreds of mysterious sticky blobs washed up on Sydney’s shores last week, including at Coogee, Bondi, Bronte, Tamarama, Little Bay and Maroubra. Beaches within the Randwick and Waverley LGAs were swiftly closed, while analysis of the debris — deemed to be “consistent with the makeup of tar balls”, according to Randwick City Council — was conducted by scientists from UNSW Sydney.

“Australia’s beaches, including recently along Sydney’s coastline, have seen the arrival of tar balls — dark, spherical, sticky blobs formed from weathered oil,” said UNSW’s Professor Alex Donald, who helped to carry out an array of preliminary analyses of the debris. This ‘weathering’ process refers to the changes that occur to oil as it spends time in the environment.

Globally, tar balls have been appearing on shores for decades. On Californian beaches, tar balls form because of natural oil seeping from the sea floor. The 2010 Deepwater Horizon spill in the Gulf of Mexico left massive tar ball deposits along coastlines.

“In Australia, Golden Beach in Queensland and Ninety Mile Beach in Gippsland have seen similar incidents, often after oil spills or shipping discharges,” Donald said. “These sticky blobs typically point to larger environmental issues, whether caused by human activity or natural oil seepage.”

How do tar balls form?

The discovery of tar balls at Coogee Beach and elsewhere in Sydney is the latest in a series of similar incidents. As Donald explained, the key challenge is determining whether the tar balls originate from human activity like shipping spills, or natural oil seeps from the ocean floor.

“This distinction is important because a natural seep requires different environmental responses compared to an industrial oil spill, which calls for immediate clean-up and potential legal action,” Donald said.

Tar balls form due to supramolecular chemistry — chemistry of interactions between small molecules that self-assemble into larger, more complex entities — and are driven by intermolecular forces — attractive and repulsive forces that arise between the molecules of a substance. When oil enters the ocean, weathering processes cause the lighter components to evaporate, leaving behind heavier compounds. The relative amounts of maltenes — the lighter, fluid fractions — and asphaltenes — the heavier, solid fractions — play a key role in the formation of tar balls.

Asphaltenes, which don’t break down in water, aggregate and cluster together through an interaction known as van der Waals forces, which in turn leads to the sticky, cohesive blobs that eventually wash ashore. Crude oil is composed of both maltenes and asphaltenes, Donald explained, but in natural oil seeps, the slow weathering process often results in a higher concentration of asphaltenes.

“This means that in the occurrence of an oil spill, some maltenes can remain,” Donald said. “The balance of these components can help scientists trace the origin of tar balls, though it’s not always a definitive measure. But in this case, the samples contained mostly maltenes consistent with an oil spill.”

What have the tests revealed?

A team at UNSW Chemistry, working alongside UNSW’s Mark Wainright Analytical Centre, conducted a preliminary analysis of Coogee’s tar balls with approval from Randwick City Council, who provided the samples. The team carried out nuclear magnetic resonance (NMR) testing, which is like magnetic resonance imaging (MRI), to reveal the structure, identity, concentration and behaviour of molecules in solid or liquid samples.

“The NMR and other results indicated that the tar balls contained bituminous materials, primarily maltenes and asphaltenes, as well as some biological materials such as fatty acids,” Donald said. “While these findings confirm that the tar balls originated from weathered oil, it is still unclear whether they resulted from natural seepage or a crude oil spill. Further chemical analysis is needed to pinpoint the source.”

Further analysis using ultraviolet A (UVA) radiation revealed potential clues on the mystery origin of the tar balls, said UNSW’s Associate Professor Vinh Nguyen.

“Several parts of a cross section inside a tar ball were fluorescent under UVA,” Nguyen said. “If the inside of a tar ball is fluorescent, it indicates the presence of organic compounds that absorb and re-emit light, commonly aromatic hydrocarbons such as polycyclic aromatic hydrocarbons (PAHs).”

This fluorescence from the tar balls collected from Coogee Beach suggests that they likely originated from petroleum-based sources, as these materials naturally contain PAHs. According to Nguyen, “The source could be linked to oil spills, petrochemical processes or environmental pollution involving hydrocarbons. Fluorescence also provides insights into the aging and chemical transformation of the tar ball.

“Additionally, fluorescence can hint at a biological contribution, as some organic materials, like marine algae, also fluoresce under UV light.”

Why this matters for Australia

Tar balls aren’t just a cosmetic problem for beaches. The sticky material can cause harm to marine ecosystems and impact animals such as seabirds, turtles and fish by affecting their movement and feeding.

“Additionally, the toxic compounds in tar balls can leach into the water, posing long-term threats to marine biodiversity,” Donald said.

“The appearance of tar balls on beaches like Coogee not only damages Australia’s pristine coastal reputation but also threatens the health of ecosystems that support industries like fishing and tourism.”

While Sydney’s beaches have since reopened, following the removal of around 2000 balls and health advice that the balls are not highly toxic to humans, NSW Maritime Executive Director Mark Hutchings has emphasised that any incoming balls should not be touched or picked up.

“If you or your family accidentally touches one, wash your hands with soap and water or baby oil,” he said.

In the meantime, the NSW Environment Protection Authority is still examining the balls in the hope of discovering their source.

“Whether caused by human activity or natural processes, solving the mystery of these sticky invaders requires ongoing scientific inquiry,” Donald concluded.

Image caption: The research team looked for clues on the tar balls by studying them under ultraviolet light. Image credit: Associate Professor Vinh Nguyen.