The world’s oceans are a vast and dynamic ecosystem, teeming with life from the tiniest microscopic algae to the largest marine mammals. At the very base of this immense marine food web are phytoplankton, microscopic photosynthetic organisms that, much like terrestrial plants, harness sunlight energy to manufacture organic matter essential for their growth. Occupying the uppermost 100 meters of the ocean—the sunlit zone—these organisms drive a process of carbon fixation comparable in magnitude to that performed by all land plants combined annually. Their survival and productivity hinge not only on sunlight but critically on the availability of various nutrient elements dissolved in seawater, such as nitrogen, phosphorus, and trace metals like iron and zinc, which form the biochemical building blocks of life in the ocean.
While the parallels with terrestrial ecosystems are strong, marine biogeochemical cycles diverge markedly in certain respects. Unlike on land, where organic material decomposes in soils and nutrients are recycled within the ecosystem, when phytoplankton die in the ocean, their remains sink into the dimly lit abyssal depths. Here, the detrital organic matter is subjected to bacterial decomposition, effectively returning vital nutrients to the seawater in the deep ocean but removing them from the surface waters where life thrives. This vertical transport and recycling of elements underpin the complex interplay between ocean chemistry, biology, and global climate processes. The central puzzle in ocean science has long been understanding how these essential nutrients, once exported to the deep ocean, are eventually returned to the surface to sustain ongoing biological productivity.
A recent revolutionary study led by geochemist Derek Vance and his team from ETH Zurich offers fresh insights into these underexplored mechanisms. Employing advanced chemical tracers and oceanographic measurements, the researchers discovered that many critical trace metals are rapidly and irreversibly removed from the seawater column through a non-biological process involving the formation of solid manganese-oxide particles. These mineral particles precipitate directly from seawater and, laden with incorporated metals, descend swiftly to the abyssal seafloor sediments. This discovery challenges long-held assumptions that trace metals dissolved in seawater are primarily cycled through biological pathways, revealing instead a significant abiotic sink shaping ocean chemistry on a global scale.
The implications of manganese-oxide mediated scavenging are profound. Metals such as iron, zinc, and others essential for phytoplankton growth become locked away in the sediment minerals, seemingly sequestered from the biologically accessible ocean reservoir. However, Vance’s team uncovered a crucial counterbalance: chemical reactions occurring within the sediments release these metals from their solid manganese-oxide hosts, freeing them back into seawater solution at the sediment-water interface. This newly soluble pool of metals then gently leaks from the sediments into the deep ocean, where physical ocean mixing transports them upward through thermohaline circulation and other oceanic currents, eventually replenishing nutrient levels in the sunlit surface waters.
To elucidate the scale and dynamics of this recycling process, the team paired their geochemical observations with comprehensive numerical models simulating oceanic transport and mixing. The models confirmed that metal fluxes from sediments provide an indispensable source of trace nutrients, effectively closing the loop on ocean trace-metal cycles. These findings refine our understanding of the ocean’s capacity to support phytoplankton productivity and, by extension, regulate atmospheric carbon dioxide concentrations. Since phytoplankton act as a critical sink for atmospheric CO₂—transferring carbon from the surface ocean and atmosphere into the deep ocean—their growth and nutrient supply have direct ramifications for Earth’s climate system.
Perhaps most strikingly, this research overturns the traditional view of the deep seafloor as a permanent repository that irreversibly traps bioessential elements. Instead, the abyssal seabed emerges as an active and essential driver of trace-metal biogeochemical cycles, regulating nutrient availability over vast temporal and spatial scales. This cycling process has likely influenced the oceans’ biological productivity and climate feedback mechanisms throughout geological history. The notion of sedimentary "leakage" of metals back into the ocean highlights new complexities in how scientists must approach marine nutrient budgeting and models of future climate scenarios.
Given the increasing interest in geoengineering approaches that leverage ocean ecosystems to mitigate climate change—such as fertilizing surface waters with nutrients to stimulate phytoplankton blooms—understanding the nuanced biogeochemical role of sediments and abiotic processes becomes imperative. Strategies aiming to increase carbon sequestration through enhancing phytoplankton growth must incorporate these findings to realistically estimate the availability and recycling rates of trace metals. Disregarding the sedimentary trace-metal source or solid-phase scavenging mechanisms could lead to overestimations of fertilization efficacy or unintended ecological consequences.
This work also opens fresh avenues for exploration in marine geochemistry, with manganese oxides identified as pivotal agents controlling the fate of trace metals across diverse oceanic regimes. Further investigation into how varying sediment compositions, redox conditions, and ocean circulation patterns affect metal liberation from abyssal sediments could unveil new controls over marine nutrient dynamics. Enhanced observational networks integrating chemical tracers, sediment analyses, and physical oceanography promise to disentangle these complex feedbacks with greater precision.
“The ocean’s biogeochemical cycles are far more intricate than previously believed,” Derek Vance reflects. “Recognizing the deep seafloor not only as a sink but also as an active driver of trace-metal cycles reshapes fundamental concepts about how marine ecosystems function and sustain themselves.” This paradigm shift propels us toward a more holistic appreciation of the ocean as a dynamic environment where chemical, biological, and physical processes intertwine to regulate life and climate on our planet.
In sum, the abyssal seafloor emerges not as a final resting place for crucial elements but as a vibrant and interactive interface that modulates the availability of metals indispensable for marine life. By mediating trace-metal cycling through mineral precipitation and sediment release, the sediment-ocean gateway intricately controls phytoplankton growth potential and, ultimately, Earth’s carbon balance. As climate change accelerates and human activities increasingly impact ocean chemistry, elucidating these deep-sea biogeochemical processes takes on ever-greater significance for predicting and managing future environmental change.
Subject of Research: Ocean trace-metal biogeochemical cycling and sediment-ocean exchange processes
Article Title: Abyssal seafloor as a key driver of ocean trace-metal biogeochemical cycles
News Publication Date: 11 June 2025
Web References: https://doi.org/10.1038/s41586-025-09038-3
References: Du J, Haley BA, McManus J, Blaser P, Rickli J, Vance D: Abyssal seafloor as a key driver of ocean trace-metal biogeochemical cycles, Nature (2025)
Keywords: Phytoplankton, Trace Metals, Manganese Oxides, Ocean Sediments, Biogeochemical Cycles, Carbon Sequestration, Nutrient Recycling, Ocean Chemistry, Climate Change, Deep Ocean, Marine Geochemistry
