Study Reveals Deep-Sea Sediments Play Role in Global Trace-Metal Cycling

By Muhammad OsamaReviewed by Laura ThomsonJun 17 2025

A recent article published in Nature highlighted the significant role of the abyssal seafloor in regulating the biogeochemical cycling of trace metals in the ocean. Researchers utilized field data and advanced modeling to comprehensively explore how sedimentary processes and particle interactions in deep-sea regions influence the distribution and behavior of key trace elements.

These findings challenge traditional top-down models and highlight the importance of benthic processes in maintaining marine ecosystem health and regulating the climate.

The Role of Trace-Metal Cycling in Marine Environments

Trace elements, present in small concentrations, are essential nutrients for marine life and play an important role in carbon cycling and climate regulation. Traditionally, their cycling has been explained by a top-down model focusing on surface input and biological uptake. However, this model often overlooks the influence of manganese (Mn) and iron (Fe) oxides.

Recent outcomes from the Global Ocean Tracers Experiment (GEOTRACES) program indicate that seafloor sediments can significantly affect the distribution of trace elements. It highlights the importance of "boundary exchange," which refers to interactions between particles and dissolved elements at ocean margins.

Investigating Abyssal Processes and Trace-Metal Cycling

Researchers combined field observations, laboratory analyses, and advanced modeling to explore trace-metal cycling in the abyssal Pacific Ocean. During the KM2012 Pacific cruise aboard the R/V Kilo Moana, they collected water-column, sediment, and porewater samples from three sites along 152°W at depths exceeding 5,000 meters.

Sampling methods included seawater filtration for dissolved trace elements, sediment coring, and pore water extraction via centrifugation and filtration. Rare earth elements (REEs), mainly neodymium (Nd) and its isotopes were measured using high-precision techniques at Oregon State University and ETH Zurich. Additional measurements of nutrients and dissolved organic carbon helped characterize the biogeochemical environment.

To determine how trace metals interact with various particle types, including biogenic, authigenic, and lithogenic, the authors used partition coefficients derived from global GEOTRACES datasets. Manganese oxides were identified as the main scavengers of REEs in deep waters.

A reactive-transport diagenetic model was developed to simulate the interactions of trace elements within sediments and pore waters. The study also employed a three-dimensional (3D) ocean circulation inverse model (OCIM2-48L) to simulate Nd cycling in the water column, accounting for particle scavenging and benthic fluxes.

Key Findings: Dominance of Oxide-Mediated Scavenging

Data analysis showed that manganese oxides, despite constituting less than 1% of particle mass below the surface, are responsible for 50-90% of Nd removal in the Pacific Ocean. This role extends into surface sediments, where REEs are predominantly found in manganese-rich layers. Reactive transport modeling demonstrated that as organic matter breaks down in sediments, it lowers the pH of porewater and promotes the release of REEs into the surrounding water, creating an upward benthic flux of dissolved trace metals.

Although this flux recycles only about 5% of the sinking particle flux, it significantly enriches deep ocean waters, particularly in the abyssal ocean due to the large seafloor area.

Mixing near the bottom caused by internal tides enhances the upward movement of these metals through the water column. The inclusion of manganese oxide scavenging and benthic fluxes into the 3D model of Nd cycling matched real Nd data, mainly in the deep Pacific.

The study also explored neodymium isotopes (ε_Nd), which become more radiogenic as deep water ages. The model included benthic inputs from recycled particles and weathering of volcanic sediments, successfully matching the observed isotope values in deep waters.

Applications for Climate and Marine Ecosystem Studies

This research has significant implications for understanding ocean chemistry and climate. It demonstrates that particle scavenging, and seafloor processes play crucial roles in the cycling of trace metals. Improved models of neodymium isotopes and trace metals can aid in studying past ocean circulation and climate changes. The findings suggest that chemical weathering on the deep seafloor may remove more carbon dioxide from the ocean than previously estimated, enhancing predictions of long-term carbon cycling.

In marine biogeochemistry, the study highlights how sediments influence the availability of trace metals, which are essential for ocean life, and explains the movement of nutrients in the deep sea. Incorporating seafloor mixing and metal fluxes into ocean models enhances their realism and predictive capability regarding ocean circulation and chemistry changes.

Future Directions in Marine Trace-Metal Research

This bottom-up approach challenges the traditional top-down view of how trace metals cycle in the ocean. It highlights the critical role of the abyssal seafloor in reshaping ocean chemistry and affecting isotopic patterns. The seafloor serves as a source of recycled metals and a contributor of new materials from the breakdown of rocks.

Future work should collect more data from different ocean regions to better estimate benthic metal fluxes and their isotopic signatures. Exploring how hydrothermal activity and varying oxygen levels affect trace-metal cycling will enhance the detailed understanding of ocean biogeochemistry, with key implications for climate research, ocean health, and the responsible use of deep-sea resources essential for clean energy technologies.

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