<p id="d1919800e182">Iron is an essential micronutrient for life. However, its scarcity
limits algae growth
in about one-half of the ocean. Its cycle is therefore linked to the global carbon
cycle and climate. We present an iron isotope section from the Southern Ocean. In
contrast to the common but oversimplified view, according to which organic matter
remineralization is the major pathway releasing dissolved iron below the surface layers,
these data reveal other dominant processes at depth, likely abiotic desorption/dissolution
from lithogenic particles. This suggests that the iron cycle, and therefore primary
production and climate, may be more sensitive than previously thought to continental
erosion, dissolved/particle interactions, and deep water upwelling. These processes
likely impact other elements in the ocean.
</p><p class="first" id="d1919800e185">As an essential micronutrient, iron plays a
key role in oceanic biogeochemistry. It
is therefore linked to the global carbon cycle and climate. Here, we report a dissolved
iron (DFe) isotope section in the South Atlantic and Southern Ocean. Throughout the
section, a striking DFe isotope minimum (light iron) is observed at intermediate depths
(200–1,300 m), contrasting with heavier isotopic composition in deep waters. This
unambiguously demonstrates distinct DFe sources and processes dominating the iron
cycle in the intermediate and deep layers, a feature impossible to see with only iron
concentration data largely used thus far in chemical oceanography. At intermediate
depths, the data suggest that the dominant DFe sources are linked to organic matter
remineralization, either in the water column or at continental margins. In deeper
layers, however, abiotic non-reductive release of Fe (desorption, dissolution) from
particulate iron—notably lithogenic—likely dominates. These results go against the
common but oversimplified view that remineralization of organic matter is the major
pathway releasing DFe throughout the water column in the open ocean. They suggest
that the oceanic iron cycle, and therefore oceanic primary production and climate,
could be more sensitive than previously thought to continental erosion (providing
lithogenic particles to the ocean), particle transport within the ocean, dissolved/particle
interactions, and deep water upwelling. These processes could also impact the cycles
of other elements, including nutrients.
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