In 2020, stark seafloor images of rusting barrels off Los Angeles, some ringed by ghostly white halos, reignited concerns about mid-century ocean dumping. Long blamed on lingering DDT residues, new work from Scripps Institution of Oceanography now shows that at least some of those haloed barrels held strongly alkaline waste. The caustic leakage has chemically transformed patches of seabed into cemented, high-pH microenvironments that favor extremophile microbes and exclude typical ocean floor life.

The discovery sprang from targeted diving campaigns aboard R/V Falkor using the ROV SuBastian. When coring tools struck halo zones, the seafloor behaved like concrete; scientists had to extract chunks of hardened crust for lab analysis. Measurements revealed sediments around halo barrels with pH values near 12, far above normal seawater. Mineralogy identified brucite, a magnesium-hydroxide precipitate, as the principal cementing agent. When the alkaline waste released from the barrels reacted with magnesium in the surrounding seawater it formed brucite, which glues sediment grains into a hard crust. As brucite dissolves at the halo margins, it sustains local alkalinity; where this plume meets normal seawater chemistry, calcium carbonate precipitates as the visible white dust of the halo.

Ecologically the consequences are localized but profound. Microbial DNA was scarce and communities were dominated by types adapted to extreme alkalinity, more commonly found in vents and alkaline springs. Earlier Scripps work also documented reduced small-animal biodiversity around haloed barrels, suggesting the chemical alteration reshapes food webs and biogeochemical cycling.

Pinpointing industrial sources is difficult: DDT production yielded alkaline as well as acidic wastes, and regional oil-refining and drilling also produced caustic residues. Historical disposal records are fragmentary, and the total number of barrels on the seafloor remains unknown.

Practically, the team proposes using white halos as a rapid screening tool to map alkaline-impacted sites and to prioritize sampling. They caution against disruptive sediment removal, which could release buried contaminants, and are instead exploring in-place options. That includes hunting for native microbes capable of degrading DDT.

The haloed barrels are a sober reminder: the ocean archives industrial practices in unexpected chemical forms. Addressing that legacy will require careful, multidisciplinary science, targeted monitoring, and remediation strategies that work to cleanup the polluting ways of the past.