Iron (Fe) is an essential micronutrient for marine organisms, and it is now well established that low Fe availability controls phytoplankton productivity, community structure, and ecosystem functioning in vast regions of the global ocean. The biogeochemical cycle of Fe involves complex interactions between lithogenic inputs (atmospheric, continental, or hydrothermal), dissolution, precipitation, scavenging, biological uptake, remineralization, and sedimentation processes. Each of these aspects of Fe biogeochemical cycling is likely influenced by organic Fe-binding ligands, which complex more than 99% of dissolved Fe. In this review we consider recent advances in our knowledge of Fe complexation in the marine environment and their implications for the biogeochemistry of Fe in the ocean. We also highlight the importance of constraining the dissolved Fe concentration value used in interpreting voltammetric titration data for the determination of Fe speciation. Within the published Fe speciation data, there appear to be important temporal and spatial variations in Fe-binding ligand concentrations and their conditional stability constants in the marine environment. Excess ligand concentrations, particularly in the truly soluble size fraction, seem to be consistently higher in the upper water column, and especially in Fe-limited, but productive, waters. Evidence is accumulating for an association of Fe with both small, well-defined ligands, such as siderophores, as well as with larger, macromolecular complexes like humic substances, exopolymeric substances, and transparent exopolymers. The diverse size spectrum and chemical nature of Fe ligand complexes corresponds to a change in kinetic inertness which will have a consequent impact on biological availability. However, much work is still to be done in coupling voltammetry, mass spectrometry techniques, and process studies to better characterize the nature and cycling of Fe-binding ligands in the marine environment.

https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=1611&context=msc_facpub

The Organic Complexation of Iron in the Marine Environment: A Review

The concentration and the major phase composition (particulate organic matter, CaCO3, opal, lithogenic matter, and iron and manganese oxyhydroxides) of marine particles is thought to determine the scavenging removal of particle-reactive TEIs. Particles are also the vector for transferring carbon from the atmosphere to the deep ocean via the biological carbon pump, and their composition may determine the efficiency and strength of this transfer. Here, we present the first full ocean depth section of size-fractionated (1–51 µm, >51 µm) suspended particulate matter (SPM) concentration and major phase composition from the US GEOTRACES North Atlantic Zonal Transect between Woods Hole, MA and Lisbon, Portugal conducted in 2010 and 2011. Several major particle features are notable in the section: intense benthic nepheloid layers were observed in the western North American margin with concentrations of SPM of up to 1648 µg/L, two to three orders of magnitude higher than surrounding waters, that were dominated by lithogenic material. A more moderate benthic nepheloid layer was also observed in the eastern Mauritanian margin (44 µg/L) that had a lower lithogenic content and, notably, significant concentrations of iron and manganese oxyhydroxides (2.5% each). An intermediate nepheloid layer reaching 102 µg/L, an order of magnitude above surrounding waters, was observed associated with the Mediterranean Outflow. Finally, there was a factor of two enhancement in SPM at the TAG hydrothermal plume due almost entirely to the addition of iron oxyhydroxides from the hydrothermal vent. We observe correlations between POC and CaCO3 in large (>51 µm) particles in the upper 2000 m, but not deeper than 2000 m, and no correlations between POC and CaCO3 at any depth in small (<51 µm) particles. There were also no correlations between POC and lithogenic material in large particles. Overall, there were very large uncertainties associated with all regression coefficients for mineral ballast (“carrying coefficients”), suggesting that mineral ballast was not a strong predictor for POC in this section.

Size-fractionated major particle composition and concentrations from the US GEOTRACES North Atlantic Zonal Transect

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires1–3, arguably contributing to the severe 2019–2020 Australian wildfires4. The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols5–7. Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively8–10. It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity11,12, but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019–2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions1–3. A greater appreciation of the links between wildfires, pyrogenic aerosols13, nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacial–interglacial cycling of atmospheric CO2 and the global climate system. Oceanic deposition of wildfire aerosols can enhance marine productivity, as supported here by satellite and in situ profiling floats data showing that emissions from the 2019–2020 Australian wildfires fuelled phytoplankton blooms in the Southern Ocean.

Widespread phytoplankton blooms triggered by 2019–2020 Australian wildfires

In this paper, we present, describe, and model the first size-fractionated (0.8–51 µm; >51 µm) water-column particulate trace metal results from the US GEOTRACES North Atlantic Zonal Transect in situ pumping survey, with a focus on the lithogenic tracer elements Al, Fe and Ti. This examination of basin-wide, full-depth distributions of particulate elements elucidates many inputs and processes—some for bulk lithogenic material, others element-specific—which are presented via concentration distributions, elemental ratios, size-fractionation dynamics, and steady-state inventories. Key lithogenic inputs from African dust, North American boundary interactions, the Mediterranean outflow, hydrothermal systems, and benthic nepheloid layers are described. Using the refractory lithogenic tracer Ti, we develop a 1-D model for lithogenic particle distributions and test the sensitivities of size-fractionated open-ocean particulate Ti profiles to biotically driven aggregation, disaggregation rates, vertical sinking speeds, and dust input rates. We discuss applications of this lithogenic model to particle cycling in general, and to POC cycling specifically.

Cycling of lithogenic marine particles in the US GEOTRACES North Atlantic transect

Human exposure to toxic mercury (Hg) is dominated by the consumption of seafood1,2. Earth system models suggest that Hg in marine ecosystems is supplied by atmospheric wet and dry Hg(ii) deposition, with a three times smaller contribution from gaseous Hg(0) uptake3,4. Observations of marine Hg(ii) deposition and Hg(0) gas exchange are sparse, however5, leaving the suggested importance of Hg(ii) deposition6 ill-constrained. Here we present the first Hg stable isotope measurements of total Hg (tHg) in surface and deep Atlantic and Mediterranean seawater and use them to quantify atmospheric Hg deposition pathways. We observe overall similar tHg isotope compositions, with median Δ200Hg signatures of 0.02‰, lying in between atmospheric Hg(0) and Hg(ii) deposition end-members. We use a Δ200Hg isotope mass balance to estimate that seawater tHg can be explained by the mixing of 42% (median; interquartile range, 24–50%) atmospheric Hg(ii) gross deposition and 58% (50–76%) Hg(0) gross uptake. We measure and compile additional, global marine Hg isotope data including particulate Hg, sediments and biota and observe a latitudinal Δ200Hg gradient that indicates larger ocean Hg(0) uptake at high latitudes. Our findings suggest that global atmospheric Hg(0) uptake by the oceans is equal to Hg(ii) deposition, which has implications for our understanding of atmospheric Hg dispersal and marine ecosystem recovery. Mercury deposition pathways from the atmosphere to the ocean remain uncertain, but mercury stable isotope measurements from the Atlantic and Mediterranean show that ocean uptake of gaseous elemental mercury is more important than previously thought.

https://eartharxiv.org/repository/object/88/download/5181/

Mercury stable isotopes constrain atmospheric sources to the ocean.

The GEOTRACES Standards and Intercalibration (S&I) Committee is charged with ensuring that the data generated during GEOTRACES are as precise and accurate as possible, which includes all the steps from sampling to analysis. Thus, sampling methods for dissolved and particulate constituents must take a representative (of the water depth/water mass) and uncontaminated sample, the samples must be stored (or immediately analyzed) in a fashion that preserves the concentrations (activities) and chemical speciation, and the analyses of these samples must yield accurate data (concentration, activity, isotopic composition, chemical speciation). To this end, experiences from the 2008-2010 GEOTRACES Intercalibration Program, and other related intercalibration efforts, helped to create the protocols in this document. However, methods continually evolve and the GEOTRACES S&I Committee will monitor these advances as validated by intercalibrations and modify the methods as warranted. The protocols here are divided into trace element and isotope groups: Hydrography and Ancillary Parameters, Radioactive Isotopes, Radiogenic Isotopes, Trace Elements, and Nutrient Isotopes. Those who contributed to preparing these protocols are listed in Appendix 1 and are sincerely thanked for their efforts in helping GEOTRACES and the worldwide TEI community.

/pdf/sampling-and-sample-handling-protocols-for-geotraces-cruises-4ve3n310jg.pdf

Sampling and Sample-handling Protocols for GEOTRACES Cruises

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Sampling and Sample-handling Protocols for GEOTRACES Cruises