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Primary production in the sunlit surface ocean is regulated by the supply of key nutrients, primarily nitrate, phosphate and iron (Fe), required by phytoplankton to fix carbon dioxide into biomass1–3. Below the surface ocean, remineralization of sinking organic matter rapidly regenerates nutrients, and microbial metabolism in the upper mesopelagic ‘twilight zone’ (200–500 m) is thought to be limited by the delivery of labile organic carbon4,5. However, few studies have examined the role of nutrients in shaping microbial production in the mesopelagic6–8. Here we report the distribution and uptake of siderophores, biomarkers for microbial Fe deficiency9across a meridional section of the eastern Pacific Ocean. Siderophore concentrations are high not only in chronically Fe-limited surface waters but also in the twilight zone underlying the North and South Pacific subtropical gyres, two key ecosystems for the marine carbon cycle. Our findings suggest that bacterial Fe deficiency owing to low Fe availability is probably characteristic of the twilight zone in several large ocean basins, greatly expanding the region of the marine water column in which nutrients limit microbial metabolism, with potential implications for ocean carbon storage.

Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ114Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ114Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ114Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO4maximum and is thus a consequence of an integrated regeneration signal of Cd and PO4at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol−1), rather than being related to in situ removal processes in low‐oxygen waters. Atmospheric inputs of isotopically light Cd play an important role in setting surface δ114Cd when surface Cd concentrations are lowStrong Southern Ocean control on subsurface Cd and δ114Cd distribution; Antarctic Intermediate Water influences δ114Cd of North Pacific intermediate watersA Cd* minimum at depth in the North Pacific is associated with the PO4maximum, a consequence of integrated regeneration Atmospheric inputs of isotopically light Cd play an important role in setting surface δ114Cd when surface Cd concentrations are low Strong Southern Ocean control on subsurface Cd and δ114Cd distribution; Antarctic Intermediate Water influences δ114Cd of North Pacific intermediate waters A Cd* minimum at depth in the North Pacific is associated with the PO4maximum, a consequence of integrated regeneration

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ30Si and δ56Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m−2day−1) compared to open ocean sediments (typically <0.001 mmol m−2day−1) is supported by both reductive and non‐reductive dissolution of glacially‐sourced reactive Fe phases, as reflected by the range of pore water δ56Fe (−2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02–0.05 mmol m−2day−1) is relatively low (typical ocean values are >0.1 mmol m−2day−1). High pore water δ30Si (up to +3.3‰) observed near the Fe(II)‐Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems. Through physical grinding and chemical reactions, glaciers are capable of releasing nutrients such as silicon and iron from the underlying rocks. In particular, there is a growing interest in the supply of these glacier‐sourced nutrients to sustain the growth of marine life in high‐latitude estuaries (fjords) and the adjacent coastal ocean. However, a substantial portion of the glacier‐sourced nutrients may be removed by rapid settling of debris (sediments) at the heads of these fjords, before they can be used by marine life. This study analyzes the chemistry of fluids within sediments collected from Patagonian fjord near river and glacial inputs to investigate the fate of glacier‐sourced nutrients (silicon and iron) that have been transferred to the fjord floor. Our laboratory analysis and calculations reveal that biological, chemical and geological processes in these sediments release significant amounts of iron back into the overlying fjord waters, but these high levels of iron chemically bind silicon, restricting its release into fjord waters. The disproportionate release of the two nutrients from fjord sediments found in this study advances our understanding of the modes of supply of glacier‐sourced nutrients to fjords and the coastal ocean. Schematic shows the coupling of benthic Fe and Si cycling, and the inferred spatial difference in the supply of easily reducible Fe (oxy‐)hydroxides and the intensity of pore water advection between the fjord sites. Adsorption of Si onto Fe (oxy‐)hydroxides and co‐precipitation of amorphous Fe‐Si phases occur under relatively oxic conditions in the upper sediments, while Si is released back to solution following the reductive dissolution of Fe in the ferruginous zone. Circles with brighter colors and solid outlines represent solid‐bound Si and Fe(III) phases, while circles with lighter colors and dashed outlines represent dissolved Si and Fe2+phases.