Ocean Iron Research Articles

Weak overturning circulation and high Southern Ocean nutrient utilization maximized glacial ocean carbon

Circulation changes have been suggested to play an important role in the sequestration of atmospheric CO2 in the glacial ocean. However, previous studies have resulted in contradictory results regarding the strength of the Atlantic Meridional Overturning Circulation (AMOC) and three-dimensional, quantitative reconstructions of the glacial ocean constrained by multiple proxies remain scarce. Here we simulate the modern and glacial ocean using a coupled physical-biogeochemical, global, three-dimensional model constrained simultaneously by δ13C, radiocarbon, and δ15N to explore the effects of AMOC differences and Southern Ocean iron fertilization on the distributions of these isotopes and ocean carbon storage. We show that δ13C and radiocarbon data sparsely sampled at the locations of existing glacial sediment cores can be used to reconstruct the modern AMOC accurately. Applying this method to the glacial ocean we find that a surprisingly weak (6–9 Sv or about half of today's) and shallow AMOC maximizes carbon storage and best reproduces the sediment isotope data. Increasing the atmospheric soluble iron flux in the model's Southern Ocean intensifies export production, carbon storage, and further improves agreement with δ13C and δ15N reconstructions. Our best fitting simulation is a significant improvement compared with previous studies, and suggests that both circulation and export production changes were necessary to maximize carbon storage in the glacial ocean.

Experimental determination of stability constant of ferrous iron borate complex [FeB(OH)4+] at 25 °C from solubility measurements: Implications for transport of iron in reducing environments

In this study, the formation constant (log10β10) for FeB(OH)4+ as expressed in the following reaction,Fe2++BOH4−⇌FeBOH4+is determined as 3.70 ± 0.10 (2σ) at 25 °C based on our long-term solubility experiments up to 2121 days on Fe(OH)2(cr) in the presence of borate ranging from 0.01 to 0.03 mol·kg−1. In our solubility measurements, the equilibrium was attained from the direction of supersaturation. In the experimental design, we used Fe2(OH)3Cl (pure iron end member of hibbingite) as the starting material. When Fe2(OH)3Cl was in contact with solutions without background concentrations of chloride, it was completely converted to Fe(OH)2(cr).The stability constant of FeB(OH)4+ is expected to find applications in many areas of study. For instance, FeB(OH)4+ may have played an important role in transport of ferrous iron in primitive oceans at the surface of the Earth. In the near-field of geological repositories, the formation of FeB(OH)4+ can sequestrate soluble borate, lowering borate concentrations available to the formation of the Am(III)-borate aqueous complex.

Geoengineering Research Under U.S. Law

Geoengineering — the deliberate modification of the earth’s climate to offset carbon emissions from human activity — was once the stuff of science fiction. Ideas of warding off the sun’s rays, or pulling greenhouse gases out of the atmosphere, are now being discussed not only in academic settings and media, but also in the halls of government. Some think these techniques might save the planet or lend humanity a vital window of opportunity; others consider them a dangerous distraction from a decarbonized energy future. How will we ever find out? Through research, of course. But how geoengineering research can be conducted consistent with existing laws is highly complex. The authors first divide the techniques under consideration between solar radiation management (aerosols in the stratosphere, or greater white surfaces below) and carbon dioxide removal (ocean iron fertilization (OIF) and other forms of carbon capture and sequestration). They detail the analysis required for an environmental impact statement (EIS) under the National Environmental Policy Act (NEPA), the California Environmental Quality Act (CEQA), and a variety of other U.S. statutes and common-law doctrines. Research-specific compliance is urged rather than awaiting the full body of evidence best suited for a programmatic EIS for full deployment. Only by encouraging research will it be possible to make appropriate decisions on whether to deploy geoengineering at the scale and for the duration needed to help combat climate change.

Chemoattraction to dimethyl sulfide links the sulfur, iron, and carbon cycles in high-latitude oceans

Large marine regions, including the exceptionally productive Southern Ocean, are iron-limited. As a result, there has been substantial interest in iron-fertilizing high nutrient low chlorophyll (HNLC) areas in an effort to sequester atmospheric carbon dioxide. More recently, research has shifted to quantifying the beneficial effects of iron recycling by marine biota. Marine top predators such as whales and seabirds have been examined specifically in this regard as they have high biomass, form dense aggregations, and excrete bioavailable iron in concentrations seven orders of magnitude higher than ambient seawater. Despite it being well established that marine fauna link the iron and carbon cycles, the connection of this process to the sulfur cycle has rarely been considered. The chemoattraction of specific marine fauna to algal-derived dimethyl sulfide (DMS) is key in triggering dense, multi-species foraging aggregations that induce iron recycling, augmenting carbon assimilation. The goal of this paper is twofold; first, to highlight DMS chemoattraction as a behavior that catalyzes carbon sequestration via natural iron fertilization, and second, to identify knowledge gaps that recent biogeochemical advances can address. Fostering this interdisciplinary research will enhance our understanding of global climate regulation, ecosystem services provided by marine top predators, and the biogeochemical cycles of carbon, iron, and sulfur in HNLC waters.

Iron isotope fractionation during pyrite formation in a sulfidic Precambrian ocean analogue

The chemical response of the Precambrian oceans to rising atmospheric O2 levels remains controversial. The iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean, yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process, difficulties in translating the iron isotope systematics of experimental studies to natural settings, and insufficient iron isotope datasets for natural euxinic (i.e. anoxic and sulfidic) marine basins where pyrite formation occurs. Herein we demonstrate, that a large, permil-level shift in the isotope composition of dissolved iron occurs in the Black Sea euxinic water column during syngenetic pyrite formation. Specifically, iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between Fe(II) and FeS2 of 2.75 permil (‰), the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry. These iron isotope systematics offer the potential to generate permil-level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory. The implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shifts between pyrite formation under sulfur-limited conditions and pyrite formation under iron-limited conditions. To this end, the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean, rather than its oxygenation state. On this basis, the reinterpreted sedimentary pyrite Fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the ‘Great Oxidation Event’ around 2.3 billion years ago. Importantly, this does not require the chemical state of the ocean to shift from mainly de-oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen, contrary to other interpretations based on iron isotope systematics.

Combined Effects of Atmospheric and Seafloor Iron Fluxes to the Glacial Ocean

AbstractChanges in the ocean iron cycle could help explain the low atmospheric CO2 during the Last Glacial Maximum (LGM). Previous modeling studies have mostly considered changes in aeolian iron fluxes, although it is known that sedimentary and hydrothermal fluxes are important iron sources for today's ocean. Here we explore effects of preindustrial‐to‐LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean's iron and carbon cycles in a global coupled biogeochemical‐circulation model. Considering variable atmospheric iron solubility decreases LGM surface soluble iron fluxes compared with assuming constant solubility. This limits potential increases in productivity and export production due to surface iron fertilization, lowering atmospheric CO2 by only 4 ppm. The effect is countered by a decrease in sedimentary flux due to lower sea level, which increases CO2 by 15 ppm. Assuming a 10 times higher iron dust solubility in the Southern Ocean, combined with changes in sedimentary flux, we obtain an atmospheric CO2 reduction of 13 ppm. The high uncertainty in the iron fluxes does not allow us to determine the net direction and magnitude of variations in atmospheric CO2 due to changes in the iron cycle. Our model does not account for changes to iron‐binding ligand concentrations that could modify the results. We conclude that when evaluating glacial‐interglacial changes in the ocean iron cycle, not only surface but also seafloor fluxes must be taken into account.

In situ estimates of iron-oxidation and accretion rates for iron-oxidizing bacterial mats at Lō’ihi Seamount

It is increasingly recognized that diffuse, hydrothermal venting is an important source of iron to the deep-sea that can influence oceanic iron dynamics and abundance. Lithotrophic Fe-oxidizing bacteria (FeOB) are dominant at diffuse hydrothermal vent sites, producing microbial iron mats that are often centimeters or more thick. At present, little is known about in situ growth rates of FeOB, specific Fe-oxidation rates, or accretion rates for iron mats. An in situ productivity chamber was developed that took advantage of the unique mineral morphotypes produced by FeOB to estimate rates of Fe-oxidation and accretion. Chambers were placed at two diffuse vents (1179 and 1300mbsl) at Lō’ihi Seamount where they were colonized by FeOB for different amounts of time. From this analysis, it was estimated that Fe-oxidation rates could range from 8.2 to 51.9 × 10−6molh−1, and that iron mats could accrete at around 2.2cm. yr−1. Molecular analysis indicated that the relative abundance of Zetaproteobacteria, a group of known FeOB, accounted for 80–90% of the bacteria colonizing the chambers. There was a distinct difference between populations at the 1179m site (Pohaku), and the 1300m site (North Hiolo Ridge). Microscope slides placed within the productivity chambers were colonized by different morphotypes of FeOB. The cells responsible for one common morphotype that produces a Y-shaped filament were identified as Zetaproteobacteria by use of a small subunit rRNA probe. This work confirms the importance of FeOB in the formation of chemosynthetic iron mats, and provides the first estimates for in situ Fe-oxidation rates and mat accretion rates.

Consensus, Certainty, and Catastrophe: Discourse, Governance, and Ocean Iron Fertilization

States, transnational networks of scientists, corporate actors, and institutions in the climate change regime have known for decades that iron ore, when dumped in the ocean, can stimulate the growth of plankton. Over the past twenty years, normative disagreements about appropriate behavior have shaped international governance of the phenomenon. Prior to 2007, firms lobbied governments to treat the oceans as a carbon sink and to allow corporations that dumped iron to sell carbon credits on the international market. However, after 2007 a transnational coalition of oceanographers and advocates opposed this agenda by linking it to an emergent antigeoengineering discourse. Crucial to their efforts was their interpretation of uncertainty: for opponents, scientific uncertainty implied possibly devastating consequences of iron dumping, which was thus best addressed with extreme caution. This normative approach ultimately shaped governance, since advocates successfully used it to lobby institutions in ocean governance to prevent carbon credits from being issued for ocean fertilization. Since these subjective understandings of certainty influenced global ocean governance, this article explains international behavior as a consequence of changing norms.

The integral role of iron in ocean biogeochemistry.

The micronutrient iron is now recognized to be important in regulating the magnitude and dynamics of ocean primary productivity, making it an integral component of the ocean's biogeochemical cycles. In this Review, we discuss how a recent increase in observational data for this trace metal has challenged the prevailing view of the ocean iron cycle. Instead of focusing on dust as the major iron source and emphasizing iron's tight biogeochemical coupling to major nutrients, a more complex and diverse picture of the sources of iron, its cycling processes and intricate linkages with the ocean carbon and nitrogen cycles has emerged.

Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean

As an essential micronutrient, iron plays a key role in oceanic biogeochemistry. It is therefore linked to the global carbon cycle and climate. Here, we report a dissolved iron (DFe) isotope section in the South Atlantic and Southern Ocean. Throughout the section, a striking DFe isotope minimum (light iron) is observed at intermediate depths (200-1,300 m), contrasting with heavier isotopic composition in deep waters. This unambiguously demonstrates distinct DFe sources and processes dominating the iron cycle in the intermediate and deep layers, a feature impossible to see with only iron concentration data largely used thus far in chemical oceanography. At intermediate depths, the data suggest that the dominant DFe sources are linked to organic matter remineralization, either in the water column or at continental margins. In deeper layers, however, abiotic non-reductive release of Fe (desorption, dissolution) from particulate iron-notably lithogenic-likely dominates. These results go against the common but oversimplified view that remineralization of organic matter is the major pathway releasing DFe throughout the water column in the open ocean. They suggest that the oceanic iron cycle, and therefore oceanic primary production and climate, could be more sensitive than previously thought to continental erosion (providing lithogenic particles to the ocean), particle transport within the ocean, dissolved/particle interactions, and deep water upwelling. These processes could also impact the cycles of other elements, including nutrients.

Detection, dispersal and biogeochemical contribution of hydrothermal iron in the ocean

This review aims to bring into focus the current understanding of hydrothermal systems and plume dynamics, tracers of hydrothermalism and the contribution of iron from hydrothermal vents to the global oceanic iron budget. The review then explores hydrothermal effect on surface ocean productivity. It is now well documented that scarcity of iron limits the production of chlorophyll-producing organisms in many regions of the ocean that are high in macronutrients. However, it is only recently that hydrothermal inputs have gained recognition as a source of Fe to the deep oceans that may potentially affect surface ocean productivity in some regions. A compilation of iron measurements from hydrothermal vents reveals that although hydrothermal studies measuring iron have increased significantly in recent years, there is still a dearth of data below 40°S. New analytical approaches for tracing iron sources, coupled with increasing sampling coverage of the oceans, is quickly improving knowledge of the effect of hydrothermal sources on biogeochemical cycles, a vital component in predicting future climate scenarios.

Impact of hydrothermalism on the ocean iron cycle

As the iron supplied from hydrothermalism is ultimately ventilated in the iron-limited Southern Ocean, it plays an important role in the ocean biological carbon pump. We deploy a set of focused sensitivity experiments with a state of the art global model of the ocean to examine the processes that regulate the lifetime of hydrothermal iron and the role of different ridge systems in governing the hydrothermal impact on the Southern Ocean biological carbon pump. Using GEOTRACES section data, we find that stabilization of hydrothermal iron is important in some, but not all regions. The impact on the Southern Ocean biological carbon pump is dominated by poorly explored southern ridge systems, highlighting the need for future exploration in this region. We find inter-basin differences in the isopycnal layer onto which hydrothermal Fe is supplied between the Atlantic and Pacific basins, which when combined with the inter-basin contrasts in oxidation kinetics suggests a muted influence of Atlantic ridges on the Southern Ocean biological carbon pump. Ultimately, we present a range of processes, operating at distinct scales, that must be better constrained to improve our understanding of how hydrothermalism affects the ocean cycling of iron and carbon.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Developing a test-bed for robust research governance of geoengineering: the contribution of ocean iron biogeochemistry.

Geoengineering to mitigate climate change has long been proposed, but remains nebulous. Exploration of the feasibility of geoengineering first requires the development of research governance to move beyond the conceptual towards scientifically designed pilot studies. Fortuitously, 12 mesoscale (approx. 1000 km2) iron enrichments, funded to investigate how ocean iron biogeochemistry altered Earth's carbon cycle in the geological past, provide proxies to better understand the benefits and drawbacks of geoengineering. The utility of these iron enrichments in the geoengineering debate is enhanced by the GEOTRACES global survey. Here, we outline how GEOTRACES surveys and process studies can provide invaluable insights into geoengineering. Surveys inform key unknowns including the regional influence and magnitude of modes of iron supply, and stimulate iron biogeochemical modelling. These advances will enable quantification of interannual variability of iron supply to assess whether any future purposeful multi-year iron-fertilization meets the principle of 'additionality' (sensu Kyoto protocol). Process studies address issues including upscaling of geoengineering, and how differing iron-enrichment strategies could stimulate wide-ranging biogeochemical outcomes. In summary, the availability of databases on both mesoscale iron-enrichment studies and the GEOTRACES survey, along with modelling, policy initiatives and legislation have positioned the iron-enrichment approach as a robust multifaceted test-bed to assess proposed research into climate intervention.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Marine phytoplankton and the changing ocean iron cycle

Changes in the ocean affect the biogeochemical cycle of iron, which in turn impacts phytoplankton growth. This Perspective discusses what research is needed to predict the future marine iron cycle. The availability of the micronutrient iron governs phytoplankton growth across much of the ocean, but the global iron cycle is changing rapidly due to accelerating acidification, stratification, warming and deoxygenation. These mechanisms of global change will cumulatively affect the aqueous chemistry, sources and sinks, recycling, particle dynamics and bioavailability of iron. Biological iron demand will vary as acclimation to environmental change modifies cellular requirements for photosynthesis and nitrogen acquisition and as adaptive evolution or community shifts occur. Warming, acidification and nutrient co-limitation interactions with iron biogeochemistry will all strongly influence phytoplankton dynamics. Predicting the shape of the future iron cycle will require understanding the responses of each component of the unique biogeochemistry of this trace element to many concurrent and interacting environmental changes.

Going rogue? Scenarios for unilateral geoengineering

The unilateral use of technologies for counteracting global warming is regarded as a threat by decision-makers, academics and the wider public, yet existing studies do not address the scenario in a systematic manner. I develop a conceptual framework for the study of unilateral geoengineering which distinguishes different types of actors, their capacities and decision-making contexts. Focusing on Ocean Iron Fertilization and Stratospheric Aerosol Injections, I develop several short- and long-term scenarios in which actors are conceivably both willing and able to deploy those technologies on a scale sufficient for impacting the climate system. Beyond being the first paper to develop a systematic approach to unilateral geoengineering, I show that some scenarios which stir public concern are implausible.

Ocean Iron Fertilization and Indigenous Peoples' Right to Food: Leveraging International and Domestic Law Protections to Enhance Access to Salmon in the Pacific Northwest

Ocean iron fertilization (OIF) is a new and controversial climate change mitigation strategy that seeks to increase the carbon-absorbing capacity of ocean waters by depositing significant quantities of iron dust into the marine environment to stimulate the growth of phytoplankton blooms. The photosynthetic processes of these blooms absorb carbon from the atmosphere and sequester it to the ocean floor. OIF has been criticized on several grounds, including the foreseeable and unforeseeable adverse consequences it may cause to the marine environment, as well as the daunting challenge of reconciling several potentially overlapping sources of international and domestic environmental law, which may lead to difficulties in regulating OIF effectively. Notwithstanding these challenges, OIF recently has produced a valuable benefit unrelated to its carbon sequestration purpose. In 2012, the Haida indigenous community in Canada conducted an OIF experiment that sought to restore its decimated supply of Pacific Northwest salmon stocks, upon which the Haida community relies for subsistence and self-determination. The experiment significantly increased salmon stocks within the span of one year.This Article addresses whether indigenous communities like the Haida in the U.S. Pacific Northwest region could assert a legal right to employ such a strategy in the future to help restore and maintain a cultural food source that has been depleted in part due to climate change impacts. The Article confirms that international environmental law, international human rights law, and federal Indian Law in the United States provide a firm foundation for enshrining a legal right to food for federally recognized U.S. tribes in this region. It proposes a potential exception to a future international environmental law treaty framework governing OIF experiments that would protect indigenous communities’ rights to enhanced access to salmon as a subsistence and cultural food resource that is essential to self-determination.

Understanding the variability in the iron concentration of Antarctic krill

Antarctic krill may play a significant role in the Southern Ocean iron cycle. However, understanding the control on iron budgets by Antarctic krill is hampered by the large range in the reported iron concentration of krill. The aim of this study was to investigate the causes of the large range of iron concentrations in krill reported in the literature (6–190 mg kg−1). Antarctic krill samples were collected from three research voyages to Pyrdz Bay, Antarctica, and analysed individually. Iron concentrations were measured using sector field inductively coupled plasma mass spectrometry in whole krill specimens and in the isolated stomach, digestive gland, muscle, body (whole krill excluding stomach and digestive gland), exoskeleton and faecal pellets. Iron concentrations in stomach (6–98 mg/kg), digestive gland (14–82 mg kg−1), and faecal pellet (683–1039 mg kg−1) were higher compared to muscle (4–7 mg kg−1), exoskeleton (6–15 mg kg−1), and body (4–18 mg kg−1) indicating that krill may ingest more iron than they require for physiological processes. Iron concentrations in whole krill from March 2012 (10 ± 3 mg kg−1) were significantly lower compared to February 2003 (19 ± 7 mg kg−1) and February 2015 (18 ± 12 mg kg−1). Overall, the iron concentrations in krill from this study were consistently at the lower end of the published range. We propose that the large range in reported whole iron concentrations of krill can be accounted for by a combination of seasonal and regional differences in sampling, reflecting differences in the quantity and quality of their diet.

Photoreduction of Terrigenous Fe-Humic Substances Leads to Bioavailable Iron in Oceans.

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near‐coastal waters and shelf seas. River‐derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river‐derived Fe‐HS samples were probed in a combined X‐ray absorption spectroscopy (XAS) and valence‐to‐core X‐ray emission spectroscopy (VtC‐XES) study at the Fe K‐edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen‐containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of FeIII‐HS in oceanic conditions into bioavailable aquatic FeII forms, highlights the importance of river‐derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper‐ocean iron biogeochemistry cycle.

Photoreduction of Terrigenous Fe-Humic Substances Leads to Bioavailable Iron in Oceans.

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near‐coastal waters and shelf seas. River‐derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river‐derived Fe‐HS samples were probed in a combined X‐ray absorption spectroscopy (XAS) and valence‐to‐core X‐ray emission spectroscopy (VtC‐XES) study at the Fe K‐edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen‐containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of FeIII‐HS in oceanic conditions into bioavailable aquatic FeII forms, highlights the importance of river‐derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper‐ocean iron biogeochemistry cycle.

Volcanic ash as an oceanic iron source and sink

AbstractVolcanic ash deposition to the ocean forms a natural source of iron (Fe) to surface water microbial communities. Inputs of lithogenic material may also facilitate Fe removal through scavenging. Combining dissolved Fe (dFe) and thorium‐234 observations alongside modeling, we investigate scavenging of Fe in the North Atlantic following the Eyjafjallajökull volcanic eruption. Under typical conditions biogenic particles dominate scavenging, whereas ash particles dominate during the eruption. The size of particles is important as smaller scavenging particles can become saturated with surface‐associated ions. Model simulations indicate that ash deposition associated with Eyjafjallajökull likely led to net Fe removal. Our model suggests a threefold greater stimulation of biological activity if ash deposition had occurred later in the growing season when the region was Fe limited. The implications of ash particle scavenging, eruption timing, and particle saturation need to be considered when assessing the impact of ash deposition on the ocean Fe cycle and productivity.