For decades, researchers believed that the carcinogenic pesticide dichlorodiphenyltrichloroethane, commonly known as DDT, was the primary contaminant leaking into the ocean from dumped barrels. Historical records indicate that DDT was disposed of in this area during the mid-1900s. However, the appearance of strange ghostly halos around these barrels prompted scientists to reconsider the source of contamination.
A significant breakthrough in understanding the halo phenomenon occurred when a dedicated team aboard the Schmidt Ocean Institute’s Research Vessel Falkor deployed the remotely operated vehicle (ROV) SuBastian to collect sediment samples from the sea floor. Acquiring these samples proved challenging, as the leakage from the barrels had transformed the surrounding sediment into a concrete-like texture.
Upon analysis of the collected samples, researchers discovered an unexpected finding: the levels of DDT did not increase as they approached the barrels. This crucial evidence suggested that these barrels were not the source of the DDT contamination. The team was left to ponder what other substance could have leaked from the barrels.
The analysis revealed traces of microbial DNA, albeit in low diversity. The bacteria present were predominantly extremophiles, organisms typically associated with the harsh conditions found near hydrothermal vents, which are among the most inhospitable habitats in the ocean. Additionally, testing the pH level of the sediment unveiled another surprising result: it measured around 12. This high pH indicates a highly alkaline environment, suggesting that the leakage was due to some form of caustic alkaline waste rather than DDT.
Johanna Gutleben, a postdoctoral scholar at Scripps and the first author of the study, stated, “One of the main waste streams from DDT production was acid, and they didn’t put that into barrels. It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?” This raises vital questions about the nature of the materials that were disposed of in these barrels.
The ghostly halos observed around the barrels result from a chemical reaction between the alkaline waste and seawater, which leads to the formation of the mineral brucite. This reaction has not only contributed to the cement-like texture of the sediment but also made the surrounding area more alkaline, which, when combined with seawater, has led to the creation of calcium carbonate deposits.
This discovery significantly recontextualizes the long-standing environmental disaster associated with the dumping of DDT. While the exact contents of the barrels remain unknown, researchers now have a clearer direction for future investigations. “DDT was not the only thing that was dumped in this part of the ocean, and we have only a very fragmented idea of what else was dumped there,” Gutleben remarked. “We only find what we are looking for, and up to this point, we have mostly been looking for DDT. Nobody was thinking about alkaline waste before this, and we may have to start looking for other things as well.”
The implications of this research indicate that alkaline waste should be considered as significant a pollutant as DDT in marine environments. Paul Jensen, an emeritus marine microbiologist at Scripps and senior author of the study, highlighted that alkaline waste typically would have been expected to dilute quickly in seawater; however, the ongoing presence of these ghostly halos suggests otherwise.
Jensen stated, “This adds to our understanding of the consequences of the dumping of these barrels. It's shocking that 50-plus years later, you’re still seeing these effects. We can’t quantify the environmental impact without knowing how many of these barrels with white halos are out there, but it’s clearly having a localized impact on microbes.”
As researchers continue to explore the depths of the ocean and the effects of human activity on marine ecosystems, this study underscores the importance of investigating all potential pollutants, including those that may have been overlooked in the past.
