Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage.

Dissolved iron (dFe) plays an important role in regulating marine biological productivity. In high nutrient, low chlorophyll (HNLC) regions (> 33% of the global ocean) iron is the primary growth limiting nutrient, and elsewhere can regulate nitrogen fixation and growth by diazotrophs. Overall, dFe supply potentially impacts half of global ocean productivity. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas, or cellular Fe:C ratios. This ratio can vary by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentration in sparse field observations. The Community Earth System Model (CESM) ocean component has been modified to simulate dynamic, group-specific, phytoplankton iron quotas (Fe:C) that vary as a function of ambient iron concentration. The simulated Fe:C ratios match the spatial trends in the observations and improve the correlation with global-scale, observed nutrient distributions. Acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition fluxes of soluble iron, compared to a model with fixed Fe:C. However, varying atmospheric soluble iron supply still has first order impacts on global carbon and nitrogen fluxes, and on the spatial patterns of nutrient limitation; both of which are strongly sensitive to changes in pyrogenic sources of iron. Accounting for dynamic, phytoplankton iron quotas is critical for capturing the ocean biogeochemical responses to varying atmospheric soluble iron inputs, including expected changes in both the mineral dust and pyrogenic sources with climate warming and anthropogenic activity.

Chromium (Cr) has shown promise as a paleoceanographic proxy due to the redox-driven control of dissolved Cr concentrations ([Cr]) and stable isotope composition (δ53Cr). However, substantial uncertainties in the biogeochemical Cr cycle have limited its paleoproxy application to date. To improve the mechanistic understanding of Cr cycling in the modern ocean and strengthen its potential proxy applications, we present new data from regeneration incubations, bottom and sediment pore waters, and a compilation of intermediate and deep water data. While Cr removal and biological export from the surface ocean is associated with organic carbon export, the deep water release of dissolved Cr from sinking particles is not directly dependent on organic carbon respiration, as indicated by differing trends between Cr, oxygen utilization and the regeneration of organic-associated macronutrients (e.g. N, P). Pore water and bottom water data demonstrate that benthic Cr fluxes are locally important and may be significant globally. The pore water dissolved Cr flux at our CaCO3-rich site is likely driven by the re-release of Cr scavenged from the water column by sinking particles, with minor contributions from lithogenic phases. We argue this is consistent with the highest open ocean [Cr] to date being found in the water column below oxygen minimum zones, likely reflecting the release of scavenged Cr in deep waters or surface sediments. Chromium released from suspended particles and surface sediments follows the global δ53Cr–[Cr] array, supporting the proposed role of biological export and regeneration in shaping global Cr and δ53Cr distributions. Global intermediate and deep water [Cr], δ53Cr and Cr:macronutrient relationships are thus shaped by a synergy of circulation patterns, water mass mixing, a deep Cr regeneration cycle, and benthic Cr sources. A biogenic control on global Cr distributions indicates that sedimentary Cr records may reflect biogenic as well as O2-dependent processes, while more research is needed to assess sediment Cr record fidelity based on an active diagenetic cycle.