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1. The Distribution and Redox Speciation of Iodine in the Eastern Tropical North Pacific Ocean

   https://doi.org/10.1029/2019GB006302  

   Moriyasu, Rintaro ; Evans, Natalya ; Bolster, Kenneth M. ; Hardisty, Dalton S. ; Moffett, James W. ( February 2020 , Global Biogeochemical Cycles)

   The distributions of iodate (IO₃⁻), iodide (I⁻), nitrite (NO₂⁻), and oxygen (O₂) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centered within a potential density of 26.2–26.6 kg/m³, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP.

    

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2. A coastal N2 fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

   https://doi.org/10.1002/lno.11727  

   Selden, Corday R. ; Chappell, P. Dreux ; Clayton, Sophie ; Macías‐Tapia, Alfonso ; Bernhardt, Peter W. ; Mulholland, Margaret R. ( April 2021 , Limnology and Oceanography)

   In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N₂fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the¹⁵N₂bubble release method and used to build empirical models of regional N₂fixation via a random forest machine learning approach. N₂fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂fixation rates (> 100 nmol N L⁻¹d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

    

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3. 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)

   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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4. Biological, Physical, and Atmospheric Controls on the Distribution of Cadmium and Its Isotopes in the Pacific Ocean

   https://doi.org/10.1029/2022GB007441  

   Sieber, Matthias ; Lanning, Nathan T. ; Bunnell, Zachary B. ; Bian, Xiaopeng ; Yang, Shun‐Chung ; Marsay, Chris M. ; Landing, William M. ; Buck, Clifton S. ; Fitzsimmons, Jessica N. ; John, Seth G. ; et al ( January 2023 , Global Biogeochemical Cycles)

   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.

    

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5. Detection of Transient Denitrification During a High Organic Matter Event in the Black Sea

   https://doi.org/10.1029/2018GB006032  

   Fuchsman, Clara A. ; Paul, Barbara ; Staley, James T. ; Yakushev, Evgeniy V. ; Murray, James W. ( February 2019 , Global Biogeochemical Cycles)

   N₂production by denitrification can occur in anoxic water or potentially inside organic particles. Here we compare data from the Black Sea, a permanently anoxic basin, during two organic matter regimes: suspended particulate organic matter concentrations were high in the oxycline after the spring bloom in March 2005 compared to lower organic matter concentrations in June 2005, May and October 2007, July 2008, and May 2001. For all cruises, N₂gas had a maximum in the suboxic zone (O₂ < 10 μmol/L). During the high organic matter event (March 2005), an additional shallower N₂gas andδ¹⁵N‐N₂maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30–50 μmol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. Theδ¹⁵N of biologically produced N₂in the oxycline in March 2005 was significantly enriched (+7‰ to +38‰), not depleted, as would be expected from water column fractionation. A simple diffusion calculation indicated that ammonium produced from remineralization inside particles could be oxidized to nitrate and then completely consumed by denitrification inside the particle. In this calculation, half of denitrified N atoms originated from organic N [δ¹⁵N = 11‰] and half of N atoms originated from ambient nitrate [δ¹⁵N = 5‰–7‰], producing enrichedδ¹⁵N‐N₂values. We suggest that denitrifiers were active in microzones inside particulates in hypoxic waters above the suboxic zone of the Black Sea. Denitrification in particles may also explain previous data from the oxycline above ocean oxygen deficient zones.

    

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