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1. Phosphate Scavenging During Lava‐Seawater Interaction Offshore of Kīlauea Volcano, Hawaii

   https://doi.org/10.1029/2021GC009754  

   Foreman, Rhea K. ; Björkman, Karin M. ; Funkey, Carolina P. ; Hawco, Nicholas J. ; Wilson, Samuel T. ; Rohrer, Tully ; White, Angelicque E. ; John, Seth G. ; Karl, David M. ( July 2021 , Geochemistry, Geophysics, Geosystems)

   The 2018, subaerial eruption of Kīlauea volcano, Hawaii, resulted in a 5‐km‐long stretch of coastline that actively drained lava into the ocean. Nutrients were added to the surrounding ocean through the dissolution of basaltic rock and thermal upwelling of deep water, thereby fueling a large phytoplankton bloom. Lava‐impacted, surface seawater had high suspended particle loads, and concentrations of chlorophyll, silicic acid, phosphate (P_i), nitrate, and iron that were elevated up to 12, 36, 5, 960, and 1,400 times, respectively, above the background oligotrophic levels. Widespread precipitation of iron oxyhydroxides (Fe_ox) led to extensive scavenging of the dissolved P_ipool, similar to what occurs along mid‐ocean ridge hydrothermal systems. This scavenging transformed a “fertilization” event into a P_isink near the coast of the ocean entry; however, nutrient data from outside the bloom suggest that P_icould also desorb from the Fe_oxas it is dispersed into the open ocean. From lava quench experiments, we estimate that the hydration state of the Fe_oxprecipitate (H₂O/Fe) was 5.2–5.7, and that the equilibrium partition coefficient of P_iinto Fe_ox(solid/liquid) was 10⁶. In addition,³³P_iradiotracer incubations were used to differentiate between biotic and abiotic uptake of P_iat Kīlauea's ocean entry. These findings are important for understanding modern‐day volcanic fertilization events, modeling nutrient dynamics during major events in Earth history (such as oxygenation of the atmosphere and the formation of large igneous provinces), and predicting the marine response to greater continental weathering in a warming climate.

    

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2. On‐Shelf Nutrient Trapping Enhances the Fertility of the Southern Benguela Upwelling System

   https://doi.org/10.1029/2019JC015948  

   Flynn, R. F. ; Granger, J. ; Veitch, J. A. ; Siedlecki, S. ; Burger, J. M. ; Pillay, K. ; Fawcett, S. E. ( June 2020 , Journal of Geophysical Research: Oceans)

   The southern Benguela upwelling system (SBUS) supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fueled not only by upwelled nutrients but also by nutrients regenerated on the broad and shallow continental shelf. We measured nutrient concentrations and the nitrogen (N) and oxygen (O) isotope ratios (δ¹⁵N and δ¹⁸O) of nitrate along four zonal lines in the SBUS in late summer and early winter to evaluate the extent to which regenerated nutrients augment the upwelled nutrient reservoir originating offshore. During summer upwelling, a decrease in on‐shelf nitrate δ¹⁸O revealed that 0–48% of the subsurface nutrients derived from in situ remineralization. The nitrate regenerated on‐shelf in the more quiescent winter (0–63% of total nitrate) extended further offshore along the mid‐shelf. A shoreward increase in subsurface nitrate δ¹⁵N and a greater N deficit in on‐shelf bottom waters further indicated N loss to benthic (and at times, watercolumn) denitrification coincident with the on‐shelf remineralization. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer through early winter, enhancing the nutrient pool that can be upwelled to support surface production. We hypothesize that this process is aided by a number of equatorward‐flowing hydrographic fronts that impede the lateral exchange of surface waters. The extent to which nutrients remain trapped on the shelf has implications for the occurrence of hypoxic events in the SBUS.

    

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3. Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean

   https://doi.org/10.1029/2023AV000973  

   Marshall, Tanya A. ; Beal, Lisa ; Sigman, Daniel M. ; Fawcett, Sarah E. ( December 2023 , AGU Advances)

   The Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient‐limited. Episodic mixing events characteristic of WBC systems can temporarily alleviate nutrient scarcity by vertically entraining deep nutrients into surface waters. However, our understanding of these nutrient fluxes is lacking because they are spatio‐temporally limited, and once they enter the sunlit layer, the nutrients are rapidly consumed by phytoplankton. Here, we use a novel application of nitrate Δ(15–18), the difference between the nitrogen and oxygen isotope ratios of nitrate, to characterize three (sub)mesoscale events of upward nitrate supply across the Agulhas Current in winter: (1) mixing at the edges of an anticyclonic eddy, (2) inshore upwelling associated with a submesoscale meander of the Agulhas Current, and (3) overturning at the edge of the current core driven by submesoscale instabilities. All three events manifest as upward injections of high‐Δ(15–18) nitrate into the thermocline and surface where nitrate Δ(15–18) is otherwise low; these entrainment events are not always apparent in the other co‐collected data. The dynamics driving the nitrate supply events are common to all WBCs, implying that nutrient entrainment facilitated by WBCs is quantitatively significant and supports productivity in otherwise oligotrophic subtropical surface waters. A future rise in energy across WBC systems may increase these nutrient fluxes, partly offsetting the predicted stratification‐induced decrease in subtropical ocean fertility.

    

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4. The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric

   https://doi.org/10.1029/2019JC015052  

   Freeman, Natalie M. ; Munro, David R. ; Sprintall, Janet ; Mazloff, Matthew R. ; Purkey, Sarah ; Rosso, Isabella ; DeRanek, Carissa A. ; Sweeney, Colm ( July 2019 , Journal of Geophysical Research: Oceans)

   The Drake Passage Time‐series (DPT) is used to quantify the spatial and seasonal variability of historically undersampled, biogeochemically relevant properties across the Drake Passage. From 2004–2017, discrete ship‐based observations of surface macronutrients (silicate, nitrate, and phosphate), temperature, and salinity have been collected 5–8 times per year as part of the DPT program. Using the DPT and Antarctic Circumpolar Current (ACC) front locations derived from concurrent expendable bathythermograph data, the distinct physical and biogeochemical characteristics of ACC frontal zones are characterized. Biogeochemical‐Argo floats in the region confirm that the near‐surface sampling scheme of the DPT robustly captures mixed‐layer biogeochemistry. While macronutrient concentrations consistently increase toward the Antarctic continent, their meridional distribution, variability, and biogeochemical gradients are unique across physical ACC fronts, suggesting a combination of physical and biological processes controlling nutrient availability and nutrient front location. The Polar Front is associated with the northern expression of the Silicate Front, marking the biogeographically relevant location between silicate‐poor and silicate‐rich waters. South of the northern Silicate Front, the silicate‐to‐nitrate ratio increases, with the sharpest gradient in silicate associated with the Southern ACC Front (i.e., the southern expression of the Silicate Front). Nutrient cycling is an important control on variability in the surface ocean partial pressure of carbon dioxide (pCO₂). The robust characterization of the spatiotemporal variability of nutrients presented here highlights the utility of biogeochemical time series for diagnosing and potentially reducing biases in modeling Southern Ocean pCO₂variability, and by inference, air‐sea CO₂flux.

    

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5. Patterns of iron and siderophore distributions across the California Current System

   https://doi.org/10.1002/lno.11046  

   Boiteau, Rene M. ; Till, Claire P. ; Coale, Tyler H. ; Fitzsimmons, Jessica N. ; Bruland, Kenneth W. ; Repeta, Daniel J. ( November 2018 , Limnology and Oceanography)

   Coastal upwelling of nutrients and metals along eastern boundary currents fuels some of the most biologically productive marine ecosystems. Although iron is a main driver of productivity in many of these regions, iron cycling and acquisition by microbes remain poorly constrained, in part due to the unknown composition of organic ligands that keep bioavailable iron in solution. In this study, we investigated organic ligand composition in discrete water samples collected across the highly productive California Coastal upwelling system. Siderophores were observed in distinct nutrient regimes at concentrations ranging from 1 pM to 18 pM. Near the shallow continental shelf, ferrioxamine B was observed in recently upwelled, high chlorophyll surface waters while synechobactins were identified within nepheloid layers at 60–90 m depth. In offshore waters characterized by intermediate chlorophyll, iron, and nitrate concentrations, we found amphibactins and an unknown siderophore with a molecular formula of C₃₃H₅₈O₈N₅Fe. Highest concentrations were measured in the photic zone, however, amphibactins were also found in waters as deep as 1500 m. The distribution of siderophores provides evidence for microbial iron deficiency across a range of nutrient regimes and indicates siderophore production and acquisition is an important strategy for biological iron uptake in iron limited coastal systems. Polydisperse humic ligands were also detected throughout the water column and were particularly abundant near the benthic boundary. Our results highlight the fine‐scale spatial heterogeneity of metal ligand composition in an upwelling environment and elucidate distinct sources that include biological production and the degradation of organic matter in suboxic waters.

    

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