Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E–150°W)

Iron, phosphate and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls oceanic primary production. Here, we present a GEOTRACES zonal section (GP13; 30-33oS, 153oE-150oW) extending eastwards from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L-1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L-1 (170oW-150oW). The supply of dissolved iron into the upper ocean (<100m) from the atmosphere and vertical diffusivity averaged 11 ±10 nmol m-2 d-1. In the remote South Pacific Ocean (170oW-150oW) atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface-water nitrate concentrations averaged 5 ±4 nmol L-1 between 170oW and 150oW whilst surface-water phosphate concentrations averaged 58 ±30 nmol L-1. The supply of nitrogen into the upper ocean is primarily from deeper waters (24-1647 μmol m-2 d-1) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux, in remote South Pacific waters. The deep water N:P ratio averaged 16 ±3 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production

The Marine Biogeochemisty of Cadmium: Studies of Cadmium Isotopic Variations in the Southern Ocean

Cadmium (Cd) in the oceans closely mimics the behavior of the macronutrient phosphate (PO43-) and can be used in the enzyme carbonic anhydrase (CA), suggesting a biological uptake of Cd. This relationship between Cd and PO43- has been used extensively as a paleoproxy for historic nutrient cycling. However, the validity of this proxy is questionable due to the complexity of the Cd /PO43- relationship. To this end, Cd isotopic studies can provide critical insight into the mechanism controlling Cd uptake and may, in itself, be a useful paleoproxy for historic primary production. In this study, multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS), combined with double spiking techniques, was used to determine Cd isotopic compositions in surface waters from the subtropical convergence and Southern Ocean phytoplankton cultures. These analytical techniques were improved to increase the precision on low (<5 ng) Cd samples resolving small differences in natural Cd isotopic variations. These improvements included an investigation into the effects of organic resin-derived matrix on analytical accuracy, as percent-level isotopic shifts were observed in Cd standards analyzed after seawater samples. This study shows that resin-derived organics from Eichrom TRU resin are responsible for the observed anomalous shifts in Cd isotopic standards and that the effects behave in a non-mass dependent way. Oxidation of the samples before analysis eliminates this matrix effect on subsequent standard analyses. With these improved methods, surface samples collected seasonally from the subtropical convergence and subantarctic were analyzed, showing limited Cd isotopic variability despite the observed 50-fold decrease in dissolved Cd concentration from winter to summer. This decrease was coincident with the drawdown of zinc (Zn) and macronutrients (NO3-, PO43-, Si(OH)4), strongly suggesting that Cd concentrations are dependent on biological activity. Cd was preferentially taken up relative to PO43- resulting in significant seasonal shifts in Cd/PO43- ratios. These results prove that neither Cd/PO43- ratios nor Cd isotopic compositions are, or will be, valid paleoproxies for nutrient cycling for subantarctic surface waters. The lack of Cd isotopic shifts contrasts the limited Cd isotopic data from North Pacific seawater where Cd isotopic compositions follow a kinetic biological uptake model. This indicates that multiple factors control the biological uptake of Cd, which are likely to influence the global Cd/PO43- relationship. Importantly, in regions where Zn and Cd concentrations are both low (<0.2 nmol kg-1) year round, diffusion limitation of both Cd and Zn may be governing biological uptake. To provide further supporting evidence for this hypothesis, the Cd isotopic composition was also determined, for the first time, in a cultured marine phytoplankton (Proboscia inermis). The effects of Zn and/or Fe limitation on Cd uptake were investigated, and Cd isotopic compositions were used to calculate isotopic fractionation factors. Results showed that marine phytoplankton preferentially remove light isotopes from the medium, and the most fractionated Cd isotopic compositions were observed in Zn-replete samples despite higher cellular Cd concentrations in Zn-limited cultures. This suggests that phytoplankton have at least two different uptake mechanisms with one more highly selective for light isotopes than the other and that these mechanisms are dependent on Zn concentrations

Seabird guano enhances phytoplankton production in the Southern Ocean

Highlights • Empirical approach was used to study effects of seabird guano on marine phytoplankton. • Seabird guano enhances phytoplankton productivity in different water masses. • Nutrient run-off from seabird colonies is a significant nutrient source. Abstract Six incubation experiments were carried out to investigate the phytoplankton biomass response to seabird guano-enrichment under different nutrient regimes. Study locations included Antarctic waters of the Ross Sea and sub-Antarctic waters offshore of the Otago Peninsula, both being characterized by iron limitation of phytoplankton productivity in summer, the Sub-Tropical Frontal Zone offshore of the Snares Islands, which is generally micronutrient-replete, and the island wake waters of the Snares Islands, which have a high nutrient supply from land. In all of the experiments the increase of phytoplankton biomass was higher in the treatments with guano addition compared to the controls. Guano additions were compared to Fe and macronutrients treatments (both added in quantities similar to those in the guano treatment) to shed light on which constituent(s) of guano are responsible for the observed increases in phytoplankton biomass. Macronutrients increased the phytoplankton biomass in the Sub-Tropical Frontal Zone, however, the response was less prominent than in the Guano treatment, suggesting synergetic effects of nutrients in seabird guano on phytoplankton production. It was also found that the pattern of response varied between the water masses with 6–10 days lag phase in the Sub-Antarctic water mass and no lag phase in Sub-Tropical Frontal Zone. The calculations presented here suggest that micro- and macronutrients delivered from seabird colonies on some of the sub-Antarctic islands may provide a significant amount of limiting nutrients to the nutrient budget of the surrounding coastal waters, potentially sufficient to sustain a local phytoplankton bloom. Findings of the present study indicate that biological recycling of nutrients by seabirds likely supports marine primary production and enhances productivity of associated food webs in the vicinity of islands

Constraining the Contribution of Hydrothermal Iron to Southern Ocean Export Production Using Deep Ocean Iron Observations

Hydrothermal iron supply contributes to the Southern Ocean carbon cycle via the regulation of regional export production. However, as hydrothermal iron input estimates are coupled to helium, which are uncertain depending on whether helium inputs are based on ridge spreading rates or inverse modelling, questions remain regarding the magnitude of the export production impacts. A particular challenge is the limited observations of dissolved iron (dFe) supply from the abyssal Southern Ocean ridge system to directly assess different hydrothermal iron supply scenarios. We combine ocean biogeochemical modelling with new observations of dFe from the abyssal Southern Ocean to assess the impact of hydrothermal iron supply estimated from either ridge spreading rate or inverse helium modelling on Southern Ocean export production. The hydrothermal contribution to dFe in the upper 250 m reduces 4–5 fold when supply is based on inverse modelling, relative to those based on spreading rate, translating into a 36–73% reduction in the impact of hydrothermal iron on export production. However, only the spreading rate input scheme reproduces observed dFe anomalies >1 nM around the circum-Antarctic ridge. The model correlation with observations drops 3 fold under the inverse modelling input scheme. The best dFe scenario has a residence time for hydrothermal iron that is between 21 and 34 years, highlighting the importance of rapid physical mixing to surface waters. Overall, because of its short residence time, hydrothermal Fe supplied locally by circum-Antarctic ridges is most important to the Southern Ocean carbon cycle and our results highlight decoupling between hydrothermal iron and helium supply

Critical evaluation of a seaFAST system for the analysis of trace metals in marine samples

WOS:000460710200086International audienceA seawater preconcentration system (sPAFAST) with offline sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) detection was critically evaluated for ultra-low trace elemental analysis of Southern Ocean samples over a four-year period (2015-2018). The commercially available system employs two Nobias PAl resin columns for buffer cleaning and sample preconcentration, allowing salt matrix removal with simultaneous extraction of a range of trace elements. With a primary focus on method simplicity and practicality, a range of experimental parameters relevant to oceanographic analysis were considered, including reduction of blank levels (over weeks and years), instrument conditioning, extraction efficiencies over different pH ranges (5.8-6.4), and preconcentration factors (similar to 10-70 times). Conditions were optimised for the analysis of ten important trace elements (Cd, Co, Cu, Fe, Ga, Mn, Ni, Pb, Ti and Zn) in open ocean seawater samples, and included initial pre-cleaning and conditioning of the seaFAST unit for one week before each separate analytical sequence; a controlled narrow buffer pH of 6.20 +/- 0.02 used for extraction; and a sample preconcentration factor of 10 for (relatively) concentrated rainwater or sea ice, 40 for typical seawater samples, and up to 67 times for seawater samples collected in the remote open ocean such as the Southern Ocean. Method accuracy (both short - days to weeks - and long term - months to years) were evaluated through extensive analysis of a range of oceanographic standard reference samples including SAFe D1 (n = 20), D2 (n = 3), S (n = 15), GEOTRACES GD (n = 6), GSC (n = 42) and GSP (n = 42), as well as NASS-6 (n = 6). Measured values for oceanographic samples were found to agree with consensus values to within +/- 6% for Cd, Cu, Fe, Ni, Pb and Zn. Offsets were noted for Co (labile fraction only; no UV oxidation), Mn (difference also noted in other recent studies) and Ti (limited reference values). No consensus values currently exist for Ga. Iron and Mn in Southern Ocean samples were also independently verified via flow injection analysis methods (R-2 = 0.95, n = 244 (Fe) and 0.92, n = 85 (Mn), paired t-test, p \textless\textless 0.05). Precisions over four years were evaluated through analysis of community seawater samples as well as a range of bulk in-house seawaters (3 sources, each n-100) and acid blanks (n = 250), and were typically found to be within 5-8%, depending on analyte and concentration. Values presented here represent one of the largest independent data sets for these reference samples, as well as the most documented comprehensive suite of GSP and GSC values currently available (consensus values have not yet been released). Samples covering a range of salinities (0-60) were investigated to demonstrate method versatility, with excellent recoveries noted using the seaFAST Nobias PAl column (\textgreater 98% for most elements, with 70-80% for Ga and Ti). By way of example, data is presented showing the application of the method to samples collected on the Kerguelen plateau in the Indian sector of the Southern Ocean (HEOBI voyage, January February 2016) and in land-fast ice and brine collected near Davis station, Antarctica, in austral summer 2015 (with a salinity range from 0 to 73 g kg(-1)). Finally, a range of recommendations for successful implementation of a seaFAST system are provided, along with considerations for future investigation

Atmospheric trace metal deposition near the Great Barrier Reef, Australia

Aerosols deposited into the Great Barrier Reef (GBR) contain iron (Fe) and other trace metals, which may act as micronutrients or as toxins to this sensitive marine ecosystem. In this paper, we quantified the atmospheric deposition of Fe and investigated aerosol sources in Mission Beach (Queensland) next to the GBR. Leaching experiments were applied to distinguish pools of Fe with regard to its solubility. The labile Fe concentration in aerosols was 2.3–10.6 ng m−3, which is equivalent to 4.9%–11.4% of total Fe and was linked to combustion and biomass burning processes, while total Fe was dominated by crustal sources. A one-day precipitation event provided more soluble iron than the average dry deposition flux, 0.165 and 0.143 μmol m−2 day−1, respectively. Scanning Electron Microscopy indicated that alumina-silicates were the main carriers of total Fe and samples affected by combustion emissions were accompanied by regular round-shaped carbonaceous particulates. Collected aerosols contained significant amounts of Cd, Co, Cu, Mo, Mn, Pb, V, and Zn, which were mostly (47.5%–96.7%) in the labile form. In this study, we provide the first field data on the atmospheric delivery of Fe and other trace metals to the GBR and propose that this is an important delivery mechanism to this region