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1. Surface macro- and micro-nutrients within the Copper River plume region respond to along-shore winds

   https://doi.org/10.1016/j.marchem.2025.104508  

   Ortega, ELS; Reister, I; Danielson, SL; Aguilar-Islas, AM (May 2025, Marine Chemistry)

   T he Copper River is a major source of freshwater to the Northern Gulf of Alaska (NGA) shelf with a seasonal cycle t hat reaches peak discharge in summer. This glacially-fed river also provides a large input of dissolved chemicals t o the NGA, and because of its large particle load, it impacts the distribution of particle-reactive elements. Summertime sampling of shelf water properties was carried out within the Copper River plume region during two y ears: first during a period of upwelling-favorable winds and higher river discharge (4–7 July 2019) and later during lower river discharge and more typical downwelling conditions (11–13 July 2020). Although these wind conditions were observed in separate years, both can occur over the course of a single summer. We found that the e xport of most nutrients to surface shelf waters was enhanced under upwelling-favorable winds accompanied by higher river discharge compared to downwelling conditions and lower discharge. For example, greater cross- shelf plume transport in 2019 resulted in higher mid-shelf surface inventories for nitrate +nitrite (N +N), silicic acid (H4 SiO 4 ), phosphate (PO4 3 − ), dissolved Fe (dFe), and dissolved Cu (dCu) compared to 2020. Entrainment of relatively macronutrient-rich subsurface waters under upwelling conditions may also have contributed t o the enhancement of these mid-shelf nutrient inventories. The observed high N:P ratios in plume waters were likely driven by the scavenging of P within particle-laden plume waters. Similarly, we observed lower than expected [dFe] (1.58 to 6.12 nM) in particle-laden plume waters, likely a result of enhanced scavenging combined with low concentrations of dissolved Fe-binding ligands. Although dNi and dZn have a river source, we observed lower concentrations in surface shelf waters under upwelling conditions, suggesting enhanced dilution b y relatively micronutrient-poor subsurface waters. Results highlight the influence of sub-seasonal variations in atmospheric forcing on nutrient distributions and suggest that this forcing also impacts the location and timing of primary production hotspots during summer, adding to the ecological mosaic of the NGA across a range of temporal and spatial scales. 

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2. More Than Deoxygenation: Linking Iodate Reduction to Nitrogen, Iron, and Sulfur Chemistry in Reducing Regimes

   https://doi.org/10.1029/2024JC021013  

   Evans, Natalya; Johnson, Emma; Taing, Amanda; Schnur, Alexi A; Chace, Peter J; Richards, Samantha; Hardisty, Dalton S; Moffett, James W (November 2024, Journal of Geophysical Research: Oceans)

   Abstract A striking feature of Oxygen Deficient Zones (ODZs) on the eastern boundary of the Pacific Ocean are large subsurface plumes of iodide. Throughout the oceans, iodate is the predominant and thermodynamically favored species of dissolved iodine, but iodate is depleted within these plumes. The origin of iodide plumes and mechanism of reduction of iodate to iodide remains unclear but is thought to arise from a combination of in situ reduction and inputs from reducing shelf sediments. To distinguish between these sources, we investigated iodine redox speciation along the Oregon continental shelf. This upwelling system resembles ODZs but exhibits episodic hypoxia, rather than a persistently denitrifying water column. We observed elevated iodide in the benthic boundary layer overlying shelf sediments, but to a much smaller extent than within ODZs. There was no evidence of offshore plumes of iodide or increases in total dissolved iodine. Results suggest that an anaerobic water column dominated by denitrification, such as in ODZs, is required for iodate reduction. However, re‐analysis of iodine redox data from previous ODZ work suggests that most iodate reduction occurs in sediments, not the water column, and is also decoupled from denitrification. The underlying differences between these regimes have yet to be resolved, but could indicate a role for reduced sulfur in iodate reduction if the sulfate reduction zone is closer to the sediment‐water interface in ODZ shelf sediments than in Oregon sediments. Iodate reduction is not a simple function of oxygen depletion, which has important implications for its application as a paleoredox tracer. 

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3. Acclimation of Phytoplankton Fe:C Ratios Dampens the Biogeochemical Response to Varying Atmospheric Deposition of Soluble Iron

   https://doi.org/10.1029/2022GB007491  

   Wiseman, N A; Moore, J K; Twining, B S; Hamilton, D S; Mahowald, N M (April 2023, Global Biogeochemical Cycles)

   Abstract Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition. 

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4. Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

   https://doi.org/10.1029/2022GB007330  

   Rahman, Shaily; Shiller, Alan M; Anderson, Robert F; Charette, Matthew A; Hayes, Christopher T; Gilbert, Melissa; Grissom, Karen R; Lam, Phoebe J; Ohnemus, Daniel C; Pavia, Frank J; et al (June 2022, Global Biogeochemical Cycles)

   Abstract Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg⁻¹) and increase to 70–80 and 140–150 nmol kg⁻¹in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa_xs) formation and dissolution rates were tracked by normalizing particulate excess²³⁰Th activities. Th‐normalized pBa_xsfluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m⁻² year⁻¹average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa_xsburial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year⁻¹. Estuarine mixing processes may add another 3–13 Gmol year⁻¹. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year⁻¹), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance. 

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5. Controls on Dissolved Barium and Radium‐226 Distributions in the Pacific Ocean Along GEOTRACES GP15

   https://doi.org/10.1029/2023GB008005  

   Le_Roy, Emilie; Charette, Matthew A; Henderson, Paul B; Shiller, Alan M; Moore, Willard S; Kemnitz, Nathaniel; Hammond, Douglas E; Horner, Tristan J (June 2024, Global Biogeochemical Cycles)

   Abstract Radium‐226(²²⁶Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of²²⁶Ra activities (T_1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of²²⁶Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher²²⁶Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with²²⁶Ra inputs primarily resulting from the decay of²³⁰Th in sediments. Further, dissolved²²⁶Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of²²⁶Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions. 

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