The quantity and distribution of trace metals in the ocean impact marine ecosystems due to their roles as essential nutrients and potentially harmful toxins (1). Exchanges between dissolved and particulate phases are central for governing trace metal fluxes across ocean boundaries (2–4), and for processes that internally cycle trace metals between surface and deep ocean reservoirs (5–7)(Fig. 1). Traditionally, such dissolved–particulate exchanges have typically been viewed in unidirectional terms. For example, the abiotic transfer of dissolved trace metals to particulate phases that subsequently settle to the seafloor—termed “scavenging”—has long been recognized as a key output of trace metals from the ocean (8). However, it is increasingly recognized that continual, two-way exchanges of trace metals between dissolved and particulate phases are required to fully explain certain aspects of marine trace metal distributions (2–7). One such process is “reversible scavenging,” which involves the continuous, two-way exchange of trace metals between the dissolved pool and sinking particulate matter. It is invoked to play a key role in governing the marine distributions of radioactive isotopes (5), essential trace metal micronutrients (6, 7), and toxic trace metal contaminants (9). The exact impact of reversible scavenging on marine trace metal distributions, however, remains elusive due to difficulty in distinguishing between a multitude of other governing processes. In this issue of PNAS, Lanning et al.(10) provide clear evidence for the transport of the toxic trace metal, lead (Pb), into the deep Pacific by reversible scavenging. Not only do these findings provide a rare insight into an important, but enigmatic process, they also demonstrate how human contaminants can pervade even the most isolated parts of the environment.

Human emissions have significantly increased trace metal fluxes to the ocean, with Pb arguably being the marine trace metal cycle most heavily impacted by human activities. Emissions from vehicles, coal combustion, and metal smelting have dominated over natural Pb inputs to the ocean in the past century (11). Variability in the magnitude and geographic distribution of these emission sources has resulted in well-documented spatial and temporal variations in surface ocean–dissolved Pb concentrations (11–13). Ocean circulation transports these transient inputs of Pb contamination into the deep ocean, as is particularly apparent from dissolved Pb distributions from the recently ventilated North Atlantic Ocean (11, 14, 15). Waters of the deep North Pacific Ocean are among the “oldest” in the ocean, having been isolated from the surface prior to the onset of significant human Pb emissions. Consequently, these waters are expected to be free of human Pb contamination, which seems to be confirmed by their low dissolved Pb concentrations (9, 10). A previous study by Wu et al.(9), however, used Pb isotopes