Award: OCE-1850765

Tiny algae grow near the sunlit surface, die, and sink toward the seafloor as a steady shower of organic "marine snow." This sinking material carries carbon and nutrients into the deep sea, shaping Earth's climate and feeding deep-ocean life. This “top down” process explains reasonably well how macronutrients are distributed throughout the ocean and has for a long time been the paradigm for describing the geochemical cycles of elements in the oceans. However, many other elements in seawater – e.g., trace elements, including biologically important ones like iron, zinc, and cobalt, plus rare earth elements such as neodymium – don’t behave in a manner easily reconciled with the “top down” model. Instead, their concentrations are shaped heavily by wherever they input the ocean and by local processes removing them. Scientists have proposed many explanations to explain the distributions of these elements — selective uptake by particles, chemical bonding with organic molecules, volcanic vents, runoff from continents, dust falling from the atmosphere — but a clear, unifying picture has remained elusive.

Our research here challenges the top-down story at least for certain trace elements. By analyzing seawater, sediment pore water, and the sediment itself we have found that the seafloor is actively supplying elements back into the ocean; thus acting as a source and sink of these trace elements. Moreover a critical component of this “bottom-up” view of marine geochemistry is the minor mineral manganese oxide that has an unusually strong pull on neodymium and likely other rare earth elements. These sinking manganese oxide grains irreversibly scavenge trace elements and carry them all the way down to the seafloor, where they are then more prone to remineralization. This oxic sea floor remineralization -  oxic diagenesis -  gradually release trace elements back into the water just above the seafloor. This return flux has two sources: some of it is simply "recycled" material that had been scavenged out of the water column earlier and is now being released again, while a smaller portion is genuinely "new" material, freed by the slow weathering of silicate minerals buried within the sediment itself. Once released, this combined flux of recycled and new material spreads and mixes upward into the deep ocean, where it's eventually recaptured by sinking particles again, in an ongoing cycle. The scale of this effect is significant: because the abyssal Pacific is so vast, even tiny fluxes from the seafloor, multiplied across its enormous area, add up to a major influence on ocean chemistry.

This finding matters for a few reasons. First, it suggests the seafloor — long regarded as chemically inert and unimportant once particles settle there — is actually a site of ongoing, active chemical transformation. Second, it hints that other trace elements with short ocean lifespans, including biologically essential ones like iron, might follow similar rules, meaning scientists may need to reconsider how nutrients critical to marine ecosystems get distributed globally. Third, because the "new" component involves weathering of seafloor minerals, the results could affect estimates of how much this process contributes to long-term climate regulation over geological timescales. While early in understanding, this work offers a novel challenge to standing marine biogeochemical models and may potentially reshape our understanding of ocean chemistry from the bottom up.

[For a great summary please see review by Daniel Ohnemus, Nature 642, p.575]

Last Modified: 08/17/2026
Modified by: Brian A Haley