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  • AGU (American Geophysical Union)  (2)
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  • 1
    Publication Date: 2024-02-07
    Description: We present labile (L-pTM) and refractory (R-pTM) particulate trace metal distributions of Fe, Mn, Al, Ti, Co, Zn, Cd, Ni, Pb, Cu, and P for a transect along the southwest African shelf and an off-shore section at 3°S of the GEOTRACES GA08 section cruise. Particle sources and biogeochemical cycling processes are inferred using particle-type proxies and elemental ratios. Enhanced concentrations of bio-essential L-pTMs (Zn, Cu, Ni, Cd, Co, and P) were observed in the Benguela upwelling region, attributed to enhanced primary production. Bio-essential pTM stoichiometric ratios (normalized to pP) were consistent with phytoplankton biomass across the transect, except for Fe and Mn, which included adsorbed and labile oxide phases. Low pP lability (∼41%) suggests a potential refractory biogenic source on the Benguela shelf. Variable labilities observed between stations along the transect indicated potentially different biogenic pP labilities among different plankton groups. Benthic resuspension was prevalent in (near-)bottom waters along the transect and formed an important source of Fe and Mn oxides. Lithogenic particles along the entire shelf were Mn deficient and particles on the Benguela shelf were enriched in Fe, consistent with regional sediment compositions. Enhanced available-Fe (dissolved + labile particulate Fe) concentrations (up to 39.6 nM) were observed in oxygen-deficient (near-)bottom waters of the Benguela shelf coinciding with low L-pMn. This was attributed to the faster oxidation kinetics of Fe, allowing Fe-oxide precipitation and retention on the shelf, while Mn oxidation was slower. Enhanced L-pFe in the Congo River plume, which comprised as much as 93% of the available-Fe pool, was attributed to increased scavenging and formation of Fe oxides. Increased scavenging of other particle-reactive trace metals (TMs) (Mn, Al, and Pb) was also apparent in Congo-influenced waters. However, particles did not play a significant role in transporting TMs off-shelf within Congo plume waters. Key Points: • Different oxidation kinetics lead to decoupled Fe and Mn oxide redox cycling within oxygen-depleted waters on the Benguela Shelf • Lower lability of particulate phosphorus (∼41%) indicate potential refractory biogenic source on Benguela shelf • Nepheloid particles formed important sources of Fe and Mn oxides that adsorb trace metals (TMs), and serve as potential TM sources from shelf to open ocean
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
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  • 2
    Publication Date: 2024-02-07
    Description: Cyclonic ocean eddies drive upwelling of deep waters enhanced in nutrients, which can elevate phytoplankton productivity. At mid‐latitudes in the North Atlantic, satellite images show enhanced chlorophyll‐a associated with eddies. However, surface macronutrient concentrations are often not fully depleted in this region, implying enhanced macronutrient supply is not the primary control. We conducted high resolution sampling through two mid‐latitude Atlantic eddies in late spring, located 800 and 350 km east of the Newfoundland Grand Banks. Waters outside of both eddies had unused residual macronutrients, low dissolved iron, and iron‐stressed phytoplankton. Inside both eddies, plankton biomass was higher and macronutrient concentrations lower. However, full macronutrient drawdown and an absence of iron stress were only present in the eddy nearer the continental shelf. From these two examples, iron supply and proximity to shelf iron sources appear to be important factors regulating productivity and macronutrient utilization in mid‐latitude North Atlantic cyclonic eddies.
    Type: Article , PeerReviewed
    Format: other
    Format: other
    Format: text
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