Description: R/V Meteor cruise M131 carried out a physical oceanography research program with a biogeochemical sampling component in the South Equatorial Atlantic Ocean and eastern boundary upwelling region off Angola and Namibia. The program was part of the EU collaborative project PREFACE (“Enhancing prediction of tropical Atlantic climate and its impacts”) and the sampling for and analysis of nutrient data were linked to BMBF collaborative project GENUS (“Geochemistry and Ecology of the Namibian Upwelling System”). CTD data of the expedition M131 are archived under doi:10.1594/PANGAEA.910994. The bottle files corresponding to water samples analysed for nutrient concentrations and nitrate isotopic composition were provided by Gerd Krahmann (GEOMAR). Depths and data for temperature, salinity, oxygen concentrations labeled “CTD“ in the table are values from calibrated CTD sensors at closure of the bottles, numbers for fluorescence are uncalibrated. Water samples were taken by Maria-Elena Vorrath during M131 and were analyzed after shipment in the laboratory of Kirstin Dähnke at Helmholtz-Zentrum Geesthacht. Nutrient concentrations were measured with an AutoAnalyzer 3 system (Seal Analytics) using standard colorimetric methods by Markus Ankele. Nitrate was determined after reduction to nitrite, followed by a reaction with sulfanilamide to form a red azo dye (Grasshoff and Anderson 1999). Phosphate was measured after formation of a blue antimony-phosphorous colour complex, according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). The relative errors of duplicate sample measurements were below 1.5% for NOx and phosphate concentrations, and below 0.3% for ammonium and silicate. The detection limit was 〈0.5 µmol kg−1 for NOx, 〉 0.1 µmol kg−1 for phosphate, 〉0.013 µmol kg−1 for ammonium and 〉0.016 µmol kg−1 for silicate. Delta15N_NO3 and Delta18O_NO3 were determined in the laboratory at Helmholtz-Zentrum Geesthacht (Kirstin Dähnke) with the denitrifier method (Sigman et al. 2001; Casciotti et al. 2002). The isotopic composition was determined with a GasBench II coupled to a Delta Plus XP mass spectrometer (ThermoFinnigan). Replicate measurements were performed, and two international standards (IAEA-N3, Delta15N = 4.7‰, Delta18O=25.6‰ and USGS 34 (Delta15N=-1.8‰, Delta18O=-27.9‰; Böhlke et al. 2003), were measured with each batch of samples. To correct for exchange with oxygen atoms from water, a bracketing correction was applied (Sigman et al. 2009). The standard deviation for standards and samples was 0.2‰ for Delta15N and 0.4‰ for Delta18O.

Description: The coastal upwelling system offshore Namibia is well-known for its high primary productivity in surface waters and denitrification in OMZ. Both processes gradually enrich the residual nitrate pool in 15N that is assimilated by phytoplankton to synthesize Chl-a and particulate organic nitrogen. To unveil the connection between the upwelled nitrate pool, molecule-specific nitrogen and bulk sediment nitrogen, we analyzed the δ15N of nitrate in seawater, and of Chl-a and bulk nitrogen in 27 surface sediments samples of northern BUS (17°S – 25°S). An offshore increase in both δ15Nsed and δ15NChl-a in the northern (17°-20°S,11°–14°E) sector is consistent with 15N enrichment during progressive nitrate assimilation by phytoplankton. In the southern sector (20°-23°S, 11°–14°E) where the OMZ is prominent, an offshore decrease of δ15Nsed and δ15NChl-a reflect assimilation of nitrate enriched in 15N by water column denitrification. The δ15Nsed and δ15NChl-a values also increase along the inner shelf from 17° to 23°S, interpreted to reflect a time-averaged imprint of denitrification and concomitant increase in δ15N of upwelled nitrate. Despite of the overall similarity in spatial patterns, paired δ15Nsed and δ15NChl-a unexpectedly lack a significant correlation (R2 = 0.22). The deviation between δ15Nsed and δ15NChl-a (△) suggests a varying post-depositional bias of diagenetic processes on the δ15Nsed whereas the original δ15NChl-a is retained. We conclude that intense diagenetic alteration of bulk N in temporally anoxic sediments shifts the δ15Nsed, whereas the sedimentary δ15NChl-a captures the integrated N-loss in the water column, and is an unbiased isotope proxy for denitrification in the OMZ.

Description: The University of Hamburg is part of environmental studies in the INDEX Program, which was establishes by the BGR (Federal Institute of Geosciences and Natural Resources) in Hanover to explore Massive Sulphides with regard to a potential future deep sea mining. The INDEX license area is located in the oligotrophic subtropical gyre of the South Indian Ocean. The water samples were collected with a CTD water rosette during two ship cruises with R/V Merian in 2016 (MSM 59/2 "INDEX 2016-2"; November−December 2016) and R/V Sonne in 2017 (SO 259 "INDEX 2017"; August−October 2017) and were analysed for nutrients and stable isotopes of nitrate.

Description: Temperature, salinity and fluorescence data of CTD (Seabird 911+) bottle files (reference to data set doi:10.1594/PANGAEA.784365). Concentrations of nutrients in discrete samples from rosette after filtration of seawater by auto-analyser. Nutrients of samples were measured with a Bran & Luebbe autoanalyzer in the home laboratory in Hamburg by applying standard methods. Nitrate isotope composition (delta15N-nitrate) referenced against air-N2 determined by mass spectrometry after conversion of nitrate to N2O by the denitrifyer method. Methods are detailed in Nagel et al. (2013).

Description: In situ fluxes of solutes were measured in benthic chambers along an E-W transect at 18.1 oN using autonomous Biogeochemical Observatories (BIGO). A total of nine BIGO deployments were made at nine stations along a latitudinal transect across the shelf/slope covering a horizontal distance of 50 km from ~50 to ~1110 m water depth. The design and implementation of the BIGO landers has been discussed in detail previously (Krahmann et al. 2021). Each BIGO (I and II) contained two circular flux chambers (K1 and K2) with an internal diameter of 28.8 cm where the sediment was incubated for ~30 h. Discrete samples were taken using glass syringes (eight per chamber) at pre-programmed time intervals for chemical analysis. After BIGO recovery, the syringes were immediately transferred to the onboard cool room (4°C) for filtering (0.2 µm) and sub-sampling. Benthic lander samples were analyzed for the isotopic composition (in per mil units, ‰) of nitrate (δ15N-NO3- and δ18O-NO3-) and ammonium (δ15N-NH4+). Most of the samples were analyzed for δ15N-NO3- and δ18O-NO3-. Nitrate dual isotopes were analyzed using the denitrifier method Samples for δ15N-NH4+ were analyzed using the hypobromite/azide-method where NH4+ concentrations were above the detection limit for a reliable isotope analysis (1 µM). In the denitrifier method, NO3- and NO2- are quantitatively converted to N2O by Pseudomonas aureofaciens (ATTC 13985). The hypobromite/azide-method is based on the chemical conversion of NH4+ to N2O by a subsequent addition of a hypobromite and azide solution. For both methods, the sample volume was adjusted to a sample size of 10 nmol of N2O. N2O was extracted from the sample vials by purging with helium and measured with a GasBench II, coupled to an isotope ratio mass spectrometer (Delta Plus, Thermo Fisher Scientific, Germany). All samples were measured with international standards. NO2- comprised on average 2 % of the combined NO2- + NO3- pool and the contribution of NO2- interference to the reported δ15N and δ18O nitrate values was considered negligible. N and O isotope ratios are reported in per mil (‰), relative to the analytical standards (N2 in air for δ15N, and Vienna Standard Mean Ocean Water (VSMOW) for O).

Description: Within the BMBF funded project GENUS (Geochemistry and Ecology of the Namibian Upwelling System) the mole fraction of CO2 (xCO2) was measured in surface waters by using an underway pCO2 system (SUNDANS) duirng seven cruises. SUNDANS was developed by "marine analytics and data" (MARIANDA, Germany, www.marianda.com) according to the recommendations of the 2002 underway pCO2 system workshop in Miami. It was equipped with a shower type equilibrator, an open pre-equilibrator and a non-dispersive dual cell infrared gas analyzer (LI-7000). The LI-7000 was calibrated by using nitrogen gas (zero CO2) and a two additional standard gas for CO2. The standard gases were obtained from the company Deuste Steininger GmbH, Germany and revealed CO2 concentrations of 350 to 480 ppm (Std1) and around 800 ppm (Std2). The CO2 standard gases were calibrated against the standard gases provided by NOAA at the Institute for Baltic Sea Research in Warnemünde, Germany (Ref. No. CA07600 and CC311968) and the Centre for Tropical Marine Research in Bremen, Germany (Ref. No. CB08923 and CA06265). The xCO2 data were recorded each 6 seconds and subsequently averaged minute by minute. Minute by minute data on atmospheric pressure, wind speed, seawater temperature and salinity were measured by underway systems mounted on board the research vessels. xCO2 was converted into pCO2 by using the CO2 sys program. The difference between the equilibrator and the sea water temperature was taken into account as suggested by Dickson et al. (2007, SOP5, page 8). During the RV Metoer cruise M67/2 between May 15 and June 05 2008 the pCO was measured by using PSI CO2-ProTM underwater carbon dioxide sensor designs by Pro-Oceanus Systems In., USA.

Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marshall, T., Granger, J., Casciotti, K. L., Dahnke, K., Emeis, K.-C., Marconi, D., McIlvin, M. R., Noble, A. E., Saito, M. A., Sigman, D. M., & Fawcett, S. E. The Angola Gyre is a hotspot of dinitrogen fixation in the South Atlantic Ocean. Communications Earth & Environment, 3(1), (2022): 151, https://doi.org/10.1038/s43247-022-00474-x.

Description: Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate estimates vary five-fold, with models attributing most dinitrogen fixation to the western basin. From hydrographic properties and nitrate isotope ratios, we show that the Angola Gyre in the eastern tropical South Atlantic supports the fixation of 1.4–5.4 Tg N.a−1, 28-108% of the existing (highly uncertain) estimates for the basin. Our observations contradict model diagnoses, revealing a substantial input of newly-fixed nitrogen to the tropical eastern basin and no dinitrogen fixation west of 7.5˚W. We propose that dinitrogen fixation in the South Atlantic occurs in hotspots controlled by the overlapping biogeography of excess phosphorus relative to nitrogen and bioavailable iron from margin sediments. Similar conditions may promote dinitrogen fixation in analogous ocean regions. Our analysis suggests that local iron availability causes the phosphorus-driven coupling of oceanic dinitrogen fixation to nitrogen loss to vary on a regional basis.

Description: This work was supported by the South African National Research Foundation (114673 and 130826 to T.M., 115335, 116142 and 129320 to S.E.F.); the US National Science Foundation (CAREER award, OCE-1554474 to J.G., OCE-1736652 to D.M.S. and K.L.C., OCE-05-26277 to K.L.C.); the German Federal Agency for Education and Research (DAAD-SPACES 57371082 to T.M.); the Royal Society (FLAIR fellowship to S.E.F.); and the University of Cape Town (T.M., J.G., S.E.F.). The authors also recognize the support of the South African Department of Science and Innovation’s Biogeochemistry Research Infrastructure Platform (BIOGRIP).