Iron Biogeochemistry in the High Latitude North Atlantic Ocean

The role of seabird guano in maintaining North Atlantic summertime productivity

Nutrients supplied via seabird guano increase primary production in some coastal ecosystems. A similar process may occur in the open ocean. To investigate this directly, we first measured bulk and leachable nutrient concentrations in guano sampled in the North Atlantic. We found that guano was strongly enriched in phosphorus, which was released as phosphate in solution. Nitrogen release was dominated by reduced forms (ammonium and urea) whilst release of nitrate was relatively low. A range of trace elements, including the micronutrient iron, were released. Using in-situ bioassays, we then showed that supply of fresh guano to ambient seawater increases phytoplankton biomass and photochemical efficiencies. Based on these results, modelled seabird distributions, and known defecation rates, we estimate that on annual scales guano is a minor source of nutrients for the surface North Atlantic. However, on shorter timescales in late spring/summer it could be much more important: Estimates of upper-level depositions of phosphorus by seabirds were three orders of magnitude higher than modelled aerosol deposition and comparable to diffusion from deeper waters.

Metals profile in deep-sea sediment from an active tectonic region around Simeulue Island, Aceh, Indonesia

Simeulue waters are adjacent to the northern part of Sumatra Island, which is undergoing massive land-use transformations; moreover, the waters are located in an active tectonic region. Land changes and tectonic activity might affect the metal pollution profile in this deep sea area. Thus, this study aimed to investigate the vertical profile and assess the sediment quality from the deep-sea marine sediment around Simeulue Island based on metal concentration. Seventy-six bottom sediment samples were collected from eight cores at a water depth of up to 2800 m in the Simeulue waters, Indonesia, in November 2017. Metals Cd, Cu, Fe, Ni, Pb, and Zn were quantified from the cores and multivariate analyses were carried out to understand the process. Metals distributions are analogous to the grain size parameters and LOI₅₅₀ distribution pattern, while Sumatra and Simeulue islands influenced grain size and LOI₅₅₀ spatial distribution. The vertical grain size profile exhibited no extreme oscillation in the investigated cores. Thus, sediment transport from the Island was the main suspect for these metals' profiles in the deep water, and the tectonic activity had a minor impact. Cu, Ni, Pb, and Zn tend to rise in the collected cores, suggesting that the accumulation of the metals is growing. While Fe tended to be stable and Cd oscillated in the cores. Indices were computed to assess the metal contamination profile. The cores were dominated by EF class 1 (none to slight enrichment) status and I_geo class 1 (unpolluted). Cd was the metal of concern in the study since a high Cd was observed in some layers (maximum EF = 26.45 and maximum I_geo = 3.81). Thus, this study can be used as a database to improve the regulation formulation for improved environmental managerial efforts in the region.

Variability of trace metals in coastal and estuary: Distribution, profile, and drivers

Ongoing global changes such as increasing sea-surface temperatures, decreasing acidity levels, and expanding oxygen-minimum zone may impact on the biogeochemical cycles of trace metals in ocean systems. Each trace metal has unique characteristics and a distinctive distribution pattern controlled by chemical, biological, and physical processes that occur in ocean systems. The correlations of variability drivers in trace metals are interesting topics for investigation. Following up on ocean research in the coastal and estuary area, we specifically review the distribution of trace metals in seawater and suspended and surface sediment. The marginal seas usually feature significant terrestrial inputs accompanied by several active water-mass currents. The purpose of this review is to provide an overview of variability related to trace-metal distribution in coastal and estuary systems and to specifically describe the distribution, profile and drivers that affect trace metals variability.

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

Organic soils in the Arctic-boreal region produce small aquatic humic ligands (SAHLs), a category of naturally occurring complexing agents for iron. Every year, large amounts of SAHLs-loaded with iron mobilized in river basins-reach the oceans via river runoff. Recent studies have shown that a fraction of SAHLs belong to the group of strong iron-binding ligands in the ocean. That means, their Fe(III) complexes withstand dissociation even under the conditions of extremely high dilution in the open ocean. Fe(III)-loaded SAHLs are prone to UV-photoinduced ligand-to-metal charge-transfer which leads to disintegration of the complex and, as a consequence, to enhanced concentrations of bioavailable dissolved Fe(II) in sunlit upper water layers. On the other hand, in water depths below the penetration depth of UV, the Fe(III)-loaded SAHLs are fairly resistant to degradation which makes them ideally suited as long-lived molecular transport vehicles for river-derived iron in ocean currents. At locations where SAHLs are present in excess, they can bind to iron originating from various sources. For example, SAHLs were proposed to contribute substantially to the stabilization of hydrothermal iron in deep North Atlantic waters. Recent discoveries have shown that SAHLs, supplied by the Arctic Great Rivers, greatly improve dissolved iron concentrations in the Arctic Ocean and the North Atlantic Ocean. In these regions, SAHLs play a critical role in relieving iron limitation of phytoplankton, thereby supporting the oceanic sink for anthropogenic CO₂. The present Critical Review describes the most recent findings and highlights future research directions.

The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

Climate change has led to a ~ 40% reduction in summer Arctic sea-ice cover extent since the 1970s. Resultant increases in light availability may enhance phytoplankton production. Direct evidence for factors currently constraining summertime phytoplankton growth in the Arctic region is however lacking. GEOTRACES cruise GN05 conducted a Fram Strait transect from Svalbard to the NE Greenland Shelf in summer 2016, sampling for bioessential trace metals (Fe, Co, Zn, Mn) and macronutrients (N, Si, P) at ~ 79°N. Five bioassay experiments were conducted to establish phytoplankton responses to additions of Fe, N, Fe + N and volcanic dust. Ambient nutrient concentrations suggested N and Fe were deficient in surface seawater relative to typical phytoplankton requirements. A west-to-east trend in the relative deficiency of N and Fe was apparent, with N becoming more deficient towards Greenland and Fe more deficient towards Svalbard. This aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in + N treatment near Greenland and + N + Fe near Svalbard. Collectively these results suggest primary N limitation of phytoplankton growth throughout the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. We suggest that the supply of Atlantic-derived N and Arctic-derived Fe exerts a strong control on summertime nutrient stoichiometry and resultant limitation patterns across the Fram Strait region.

First Quantification of the Controlling Role of Humic Substances in the Transport of Iron Across the Surface of the Arctic Ocean

One of the main reasons behind our current lack of understanding of iron cycling in the oceans is our inability to characterize the ligands that control iron solubility, photosensitivity, reactivity, and bioavailability. We currently lack consensus about the nature and origin of these ligands. Here, we present the first field application of a new methodological development that allows the selective quantification of the fraction of Fe complexed to humic substances (HS). In the HS-rich surface Arctic waters, including the Fe-rich Transpolar Drift (TPD), we found that HS iron binding groups were largely occupied by iron (49%). The overall contribution of Fe-HS complexes to DFe concentrations was substantial at 80% without significant differences between TPD and non-TPD waters. Stabilization and transport of large concentrations of DFe across the surface of the Arctic Ocean are due to the formation of high concentrations of Fe-HS complexes. Competition of Arctic Fe-HS complexes with desferrioxamine and EDTA indicated that their stability constants are considerably higher than the stability constants previously found for riverine HS in temperate estuaries and HS standard material. This is the first case of identification of the ligand-dominating iron speciation over a specific region of the global ocean.

Regulation of the Phytoplankton Heme b Iron Pool During the North Atlantic Spring Bloom

Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L^-1) in iron limited phytoplankton in cultures and in the field. Here, we determined heme b in marine particulate material (>0.7 μm) from the North Atlantic Ocean (GEOVIDE cruise - GEOTRACES section GA01), which spanned several biogeochemical regimes. We examined the relationship between heme b abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme b concentrations ranged from 0.16 to 5.1 pmol L^-1 (median = 2.0 pmol L^-1, n = 62) in the surface mixed layer (SML) along the cruise track, driven mainly by variability in biomass. However, in the Irminger Basin, the lowest heme b levels (SML: median = 0.53 pmol L^-1, n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L^-1, n = 25; chlorophyll a: median = 2.0 nmol L^-1, n = 23) pointing to regulatory mechanisms of the heme b pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L^-1, n = 11) in the Irminger Basin, but large diatoms (Rhizosolenia sp.) dominated. Hence, heme b depletion and regulation is likely to occur during bloom progression when phytoplankton class-dependent absolute iron requirements exceed the available ambient concentration of DFe. Furthermore, high heme b concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L^-1, n = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme b provides a snapshot of the cellular activity in situ and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.

Pyrogenic iron: The missing link to high iron solubility in aerosols

Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.

The eastern extent of seasonal iron limitation in the high latitude North Atlantic Ocean

The availability of iron (Fe) can seasonally limit phytoplankton growth in the High Latitude North Atlantic (HLNA), greatly reducing the efficiency of the biological carbon pump. However, the spatial extent of seasonal iron limitation is not yet known. We present autumn nutrient and dissolved Fe measurements, combined with microphytoplankton distribution, of waters overlying the Hebridean (Scottish) shelf break. A distinct biogeochemical divide was observed, with Fe deficient surface waters present beyond the shelf break, much further eastwards than previously recognised. Due to along and on-shelf circulation, the Hebridean shelf represents a much-localised source of Fe, which does not fertilise the wider HLNA. Shelf sediments are generally thought to supply large quantities of Fe to overlying waters. However, for this Fe to influence upper-ocean biogeochemical cycling, efficient off-shelf transport mechanisms are required. This work challenges the view that the oceanic surface waters in close proximity to continental margins are iron replete with respect to marine primary production demands.

Non-linear response of summertime marine productivity to increased meltwater discharge around Greenland

Runoff from the Greenland Ice Sheet (GrIS) is thought to enhance marine productivity by adding bioessential iron and silicic acid to coastal waters. However, experimental data suggest nitrate is the main summertime growth-limiting resource in regions affected by meltwater around Greenland. While meltwater contains low nitrate concentrations, subglacial discharge plumes from marine-terminating glaciers entrain large quantities of nitrate from deep seawater. Here, we characterize the nitrate fluxes that arise from entrainment of seawater within these plumes using a subglacial discharge plume model. The upwelled flux from 12 marine-terminating glaciers is estimated to be >1000% of the total nitrate flux from GrIS discharge. This plume upwelling effect is highly sensitive to the glacier grounding line depth. For a majority of Greenland's marine-terminating glaciers nitrate fluxes will diminish as they retreat. This decline occurs even if discharge volume increases, resulting in a negative impact on nitrate availability and thus summertime marine productivity.