Samples of microplastics. Microplastic samples collected from the oceans. Plastic fragments less than 5 mm in length are categorized as microplastics. The scale bar shows a length of 2000 µm or 2 mm. Credit: Kyushu University/Asahi Kasei Corporation
Key points:
- Researchers developed a method to estimate the age of microplastics in the ocean.
- Microplastics nearshore were found to be older than those collected offshore.
- Nearshore microplastics were aged as old as 5 years.
Researchers in Japan have developed a new way to estimate the age of microplastics found in the oceans. The method involves a combination of analyzing plastic oxidation levels and environmental factors, such as UV exposure and ambient temperature.
First, the researchers needed to determine what metrics could be used to measure microplastic age.
"The most common material in plastic is called polyethylene. We know that as polyethylene interacts with the environment it, oxidizes and degrades," said Rie Okubo, a researcher at Asahi Kasei Corporation and first author of the study. "This degradation level can be measured using the change in the material's molecular weight and something called the carbonyl index. When polyethylene degrades its carbonyl index increases and molecular weight decreases."
With the carbonyl index set, the team then needed to standardize how temperature and UV radiation affect plastic degradation.
Through a series of exposure experiments, the researchers found that UVER—ultraviolet erythemal radiation, a measurement of UV radiation at ground level—and seawater temperature were the two biggest contributors of plastic degradation.
Finally, the team was able to apply their data to the microplastic samples they collected from the North Pacific Ocean and Philippine Sea.
According to the study, published in Marine Pollution Bulletin, nearshore microplastics ranged in age from 0 to 5 years old, whereas offshore samples ranged from 1 to 3 years old.
"We hypothesize the reason why nearshore microplastics [have a longer range] is because they are being frequently washed ashore and surviving for a longer time,” said Okubo. “Offshore microplastics, on the other hand, take longer to reach that part of the ocean, hence why we didn't find microplastics over 3 years old. These offshore microplastics are also likely removed from the upper oceans by settling deeper into the waters.”
The researchers said they hope the new method will give them better insights into how microplastics are generated and spread in the environment. The data will also help in developing more accurate simulations to track microplastics across the ocean.