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Antarctic ice core records suggest that atmospheric CO2 increased by 15–20 ppm during Heinrich stadials (HS). These periods of abrupt CO2 increase are associated with a significant weakening of the Atlantic meridional overturning circulation (AMOC), and a warming at high southern latitudes. As such, modeling studies have explored the link between changes in AMOC, high southern latitude climate and atmospheric CO2. While proxy records suggest that the aeolian iron input to the Southern Ocean decreased significantly during HS, the potential impact on CO2 of reduced iron input combined with oceanic circulation changes has not been studied in detail. Here, we quantify the respective and combined impacts of reduced iron fertilization and AMOC weakening on CO2 by performing numerical experiments with an Earth system model under boundary conditions representing 40,000 years before present (ka). Our study indicates that reduced iron input can contribute up to 6 ppm increase in CO2 during an idealized Heinrich stadial. This is caused by a 5% reduction in nutrient utilization in the Southern Ocean, leading to reduced export production and increased carbon outgassing from the Southern Ocean. An AMOC weakening under 40ka conditions and without changes in surface winds leads to a ∼0.5 ppm CO2 increase. The combined impact of AMOC shutdown and weakened iron fertilization is almost linear, leading to a total CO2 increase of 7 ppm. Therefore, this study highlights the need of including changes in aeolian iron input when studying the processes leading to changes in atmospheric CO2 concentration during HS. Key Points: Simulated marine ecosystems and CO2 respond linearly to the combined impact of freshwater addition and reduced iron flux into the oceanA maximum of 7 ppm increase in CO2 is simulated due to the AMOC shutdown and weakened iron fertilization under MIS 3 boundary conditionsReduced aeolian iron flux in the Southern Ocean during Heinrich Stadials can lead up to 6 ppm rise in CO2 [ABSTRACT FROM AUTHOR]

Water and iron are crucial elements for soybean growth and development, particularly in calcareous soils and arid climatic conditions. The aim of this study was to improve iron and water use efficiency and enhance soybean resilience to water scarcity and iron deficiency. So, the effect of 16 treatments; 4 deficit irrigation water regimes (25, 50, 75 and 100 % crop water requirements (CWR)) combined to 4 foliar iron gradual concentrations (F0=0, F1=1, F2=3 and F3=5 g/L of FeSO4) applied at 3–4 leaves, at the beginning of flowering and at the end of flowering; was investigated in this split plot experiment with 4 replicates. Our results showed that supplying iron and water to plants can improve chlorophyll florescence a, chlorophyll content, stomatal conductance, yield, iron uptake, and protein content. Determining the optimal combination of deficit irrigation treatment and gradual iron sulfate concentrations for soybean is an alternative to save water and improve growth parameters. In our manuscript, we can conclude that 75 % CWR × F2 is the best combination of the two factors that led to the same biological yield as 100 % CWR. Consequently, we can say that applying F2 as a foliar iron concentration led to an economy of 25 % of the soybean crop water requirement by ensuring an adequate supply of soluble iron, facilitating root uptake, promoting protein synthesis, enhancing chlorophyll formation, and supporting overall nutrient uptake and metabolism.

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The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production.

Increasing Asian dust fluxes, associated with late Cenozoic cooling and intensified glaciations, are conventionally thought to drive iron fertilization of phytoplankton productivity in the North Pacific, contributing to ocean carbon storage and drawdown of atmospheric CO2. During the early Pleistocene glaciations, however, productivity remained low despite higher Asian dust fluxes, only displaying glacial stage increases after the mid-Pleistocene climate transition (~800 ka B.P.). We solve this paradox by analyzing an Asian dust sequence, spanning the last 3.6 My, from the Tarim Basin, identifying a major switch in the iron composition of the dust at ~800 ka, associated with expansion of Tibetan glaciers and enhanced production of freshly ground rock minerals. This compositional shift in the Asian dust was recorded synchronously in the downwind, deep sea sediments of the central North Pacific. The switch from desert dust, containing stable, highly oxidized iron, to glacial dust, richer in reactive reduced iron, coincided with increased populations of silica-producing phytoplankton in the equatorial North Pacific and increased primary productivity in more northerly locations, such as the South China Sea. We calculate that potentially bioavailable Fe2+ flux to the North Pacific was more than doubled after the switch to glacially-sourced dust. These findings indicate a positive feedback between Tibetan glaciations, glaciogenic production of dust with enhanced iron bioavailability, and changes in North Pacific iron fertilization. Notably, this strengthened link between climate and eolian dust coincided with the mid-Pleistocene transition to increased storage of C in the glacial North Pacific and more intense northern hemisphere glaciations. [ABSTRACT FROM AUTHOR]

Wind systems are known as nutrient sources playing significant roles in the oceanic realm and global climate oscillations. This study explores, for the first time, the effect of winds on the winter blooms of the mixotrophic dinoflagellate, the green variant of Noctiluca scintillans (NSG) in the northern Arabian Sea. When the NSG abundance was lower (i.e., ∼10000 cells l−1), H4SiO4 and H4SiO4:DIN (dissolved inorganic nitrogen) ratio fell. The NSG was then intensely green and chlorophyll-a richer, attributed to a change in the mode of NSG's nutrition from heterotrophy to autotrophy-dominance. The back-trajectory model revealed that the winds were mostly northeasterly (NE) initially (during February) and were north-westerly (NW) towards the end of winter (March). Separately for the NE and NW winds, the NSG abundance was 10655±18628 and 28896±46225 cells l−1, respectively. The H4SiO4:DIN ratio correspondingly reached

Aerobic rice is projected as a sustainable rice production technology for the immediate future to address water scarcity and environmental safety. Micronutrient deficiency, particularly iron is one of the main factors responsible for low productivity in aerobic rice. With this perspective, a field experiment was conducted at farmer's field in 2022 at Kuttalam, Mayladuthurai district, Tamilnadu, in sandy clay loam (Padugai Series -- Typic Ustifluvents) to predict the response of aerobic rice (Oryza sativa) to iron nutrition. The experiment was laid out in randomised block design with eleven treatments (Recommended dose of fertilizers (RDF) NPK only (control)(T1), (RDF) + FeSO4 @ 25 kg ha-1 (SA) (T2), RDF + FeSO4 37.5 kg ha-1 (SA)(T3),RDF+ FeSO4 @ 50 kg ha-1 (SA)(T4), RDF+ FeSO4 @ 25 kg ha-1 (SA) + FeSO4 @ 1% (FS)(T5), RDF+ FeSO4 37.5 kg ha-1 (SA) + FeSO4 @ 1% (FS)(T6), RDF+ FeSO4 @ 50 kg ha-1 (SA) + FeSO4 @ 1% (FS)(T7), RDF+ FeSO4 @ 25 kg ha-1(SA) + Fe-EDTA @ 0.5% (FS) (T8), RDF+ FeSO4 37.5 kg ha-1 (SA) + Fe-EDTA @ 0.5% (FS)(T9), RDF+ FeSO4 @ 50 kg ha-1 (SA) + Fe-EDTA @ 0.5% (FS) (T10) and RDF + Seed priming 0.05M Fe-EDTA(T11)) and replicated thrice. The results revealed that application of FeSO4 @ 50 kg ha-1 (SA) + Fe-EDTA @ 0.5% (FS) with RDF recorded the highest growth and yield parameters and the highest grain (3438 kg ha-1) and straw yield (5078 kg ha-1) compared to other treatments including control. This study concluded that iron fertilization through the soil and foliar application could enhance aerobic rice productivity. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this work was to test the hypothesis that Fe mining sub-products and thiols can alleviate a moderate Fe-deficiency in the Strategy I species soybean (Glycine max) grown in a calcareous soil in greenhouse conditions. Methods: Combinations of three Fe sources [Fe(III)-EDDHA and two Fe mining sub-products, one containing Fe oxides and FeS2 and the other Fe oxides], and three thiols (glutathione, dithiothreitol and thiophenol) were applied in solution to the soil, three times in a 55 day period, and different parameters related to Fe deficiency were measured. The thiol-mediated solubilization of Fe from the Fe mining sub-products was assessed by measuring in the solution total Fe and the reducible Fe pool using an Fe(II) chelator. Results: Application of Fe-EDDHA, the two Fe mining sub-products and the three thiols relieved the Fe deficiency symptoms to different extents, increased the Fe concentrations and contents throughout the plant and changed the redox state of leaves and roots, as judged from the changes in reduced and oxidized glutathione, ascorbate and antioxidant enzymes. When using Fe(III)-EDDHA, the addition of thiols led to a better leaf regreening. However, the addition of thiols did not cause further regreening in the case of the Fe mining sub-products, in spite of being able to solubilize Fe from them. Conclusion: Application of Fe-mining sub-products, thiols and the combination of Fe(III)-EDDHA and thiols could be used to alleviate moderate Fe deficiency in G. max grown in a calcareous soil. [ABSTRACT FROM AUTHOR]

In the last decade, annona crop was introduced to Lebanon. This experiment was conducted to enhance the adaptation of annona to local conditions. Three scion/rootstock combinations were obtained from self– and cross– cleft grafting of Annona squamosa (Sq) and Annona cherimola (Ch), planted in calcareous “white soil” and clay–loamy “red soil”, and fertilized or not by iron (Fe) through fertigation. In the third year, the treatment red soil–iron fertilization–Sq/Ch had significantly highest plant height, trunk diameter, leaf number, number of primary roots, fruit number and yield. In the third year, despite iron fertilization, plants of the three graft combinations cultivated in red soil had higher flower number, fruit number, individual fruits weight and yield, than those planted in white soil. In the same year, iron fertilization in white soil had only affected total dry mass, leaf mass fraction and leaf iron content. However, iron fertilization of Sq/Ch planted in red soil has improved leaf number, primary root number, primary root length, leaf chlorophyll index, and leaf iron content compared to non–fertilized plants. Conclusively, iron fertilization in red soil could be a useful method improving the performance and yielding capacity of annona crop mainly in Sq/Ch combination.

Iron limitation in vast water bodies has been linked to decreased algal productivity, despite different iron-acquiring mechanisms, and the presence of ferritin in many algal species that act as an iron internal reservoir. Therefore, iron fertilization has been proposed to increase algal biomass and photosynthesis. This, in turn, will reduce carbon dioxide in the atmosphere and increase oxygen, thereby decreasing global warming, and achieving ecological balance. In addition, algal proliferation will hopefully lead to enhancement in biodiversity, Biological pump, fish productivity and, subsequently marine food industry. Many climate geoengineering experiments in the form of ocean iron fertilization have been conducted globally in order to achieve such a purpose. However, reservations remain as the outcomes are not as promising as were previously expected. As the temporal and spatial scales of iron fertilization experiments are limited, the effects on fish productivity remain speculative. On the other hand, side effects were also recorded. The main purpose of iron fertilization, for carbon dioxide sequestration and global warming mitigation, still remains to be fully realized and verified. Several improvements and future modifications are suggested, and legal issues are discussed in this review. [ABSTRACT FROM AUTHOR]

Large ash plumes emitted by the 2019–2020 Australian wildfires were associated with a widespread phytoplankton bloom in the iron‐limited Pacific sector of the Southern Ocean. In this study, we used satellite observations and aerosol reanalysis products to study the regional phytoplankton community response to wildfire emissions. The bloom was stimulated by pyrogenic iron fertilization and coincided with elevated cellular pigment concentrations, increased photochemical efficiency, and apparent community structural shifts. Physiological anomalies were consistent with previously observed phytoplankton responses to iron stress relief and persisted for up to 9 months. Supported by a regional iron budget, we conclude that the bloom was sustained by iron recycling and episodic inputs of pyrogenic and dust‐borne mineral iron. The continuous regeneration of iron was likely facilitated by the bloom's large size, mitigating edge dilution effects, as well as enhanced bioavailability of pyrogenic and mineral iron due to atmospheric and chemical processing during long‐range transport. Plain Language Summary: Phytoplankton are the "plant" plankton at the base of the ocean food chain. Phytoplankton growth in the Southern Ocean is limited by the availability of iron. Deserts and wildfires are two important sources of iron‐bearing particles that can be transported over long distances and deposited on the ocean surface, causing increased phytoplankton growth and species changes. In this study we used observations from satellites to study the physiological response of a large phytoplankton bloom stimulated by emissions from the 2019–2020 Australian wildfires. We found the cells became richer in pigments, and more efficient in their photosynthesis. The changes are directly attributable to wildfire emissions and correspond with previously observed phytoplankton responses to iron fertilization experiments and natural fertilization by dust, volcanic ash, and upwelled iron from the deep ocean. The phytoplankton response lasted almost half a year after wildfire particles stimulated the bloom. We attribute the prolonged response in part to iron recycling, re‐supplying dissolved iron inside the bloom, and to additional deposition of wildfire emissions and dust. Key Points: 2019–2020 Australian wildfire emissions stimulated phytoplankton community responses consistent with iron fertilizationPhysiological anomalies lasted 9 months and were sustained by iron recycling and atmospheric iron supply by dust and wildfire emissionsAtmospheric processing of wildfire emissions likely increased the solubility of pyrogenic iron, amplifying its effect on the phytoplankton [ABSTRACT FROM AUTHOR]

In the last decade, annona crop was introduced to Lebanon. This experiment was conducted to enhance the adaptation of annona to local conditions. Three scion/rootstock combinations were obtained from self– and cross– cleft grafting of Annona squamosa (Sq) and Annona cherimola (Ch), planted in calcareous “white soil” and clay–loamy “red soil”, and fertilized or not by iron (Fe) through fertigation. In the third year, the treatment red soil–iron fertilization–Sq/Ch had significantly highest plant height, trunk diameter, leaf number, number of primary roots, fruit number and yield. In the third year, despite iron fertilization, plants of the three graft combinations cultivated in red soil had higher flower number, fruit number, individual fruits weight and yield, than those planted in white soil. In the same year, iron fertilization in white soil had only affected total dry mass, leaf mass fraction and leaf iron content. However, iron fertilization of Sq/Ch planted in red soil has improved leaf number, primary root number, primary root length, leaf chlorophyll index, and leaf iron content compared to non–fertilized plants. Conclusively, iron fertilization in red soil could be a useful method improving the performance and yielding capacity of annona crop mainly in Sq/Ch combination. [ABSTRACT FROM AUTHOR]

Water and iron are crucial elements for soybean growth and development, particularly in calcareous soils and arid climatic conditions. The aim of this study was to improve iron and water use efficiency and enhance soybean resilience to water scarcity and iron deficiency. So, the effect of 16 treatments; 4 deficit irrigation water regimes (25, 50, 75 and 100 % crop water requirements (CWR)) combined to 4 foliar iron gradual concentrations (F0=0, F1=1, F2=3 and F3=5 g/L of FeSO 4) applied at 3–4 leaves, at the beginning of flowering and at the end of flowering; was investigated in this split plot experiment with 4 replicates. Our results showed that supplying iron and water to plants can improve chlorophyll florescence a, chlorophyll content, stomatal conductance, yield, iron uptake, and protein content. Determining the optimal combination of deficit irrigation treatment and gradual iron sulfate concentrations for soybean is an alternative to save water and improve growth parameters. In our manuscript, we can conclude that 75 % CWR × F2 is the best combination of the two factors that led to the same biological yield as 100 % CWR. Consequently, we can say that applying F2 as a foliar iron concentration led to an economy of 25 % of the soybean crop water requirement by ensuring an adequate supply of soluble iron, facilitating root uptake, promoting protein synthesis, enhancing chlorophyll formation, and supporting overall nutrient uptake and metabolism. • Supplying water and iron to soybeans can improve chlorophyll florescence a and chlorophyll content. • Deficit irrigation and gradual iron sulfate treatments in soybeans saves water and boosts growth. • Applying F2=3 g/L as foliar iron concentration saves 25 % of soybean crop water requirements (CWR). • 75 % CWR*F2 is the best combination that led to the same biological yield as 100 % CWR. • Adequate supply of water and iron facilitates root nutrient uptake and promotes protein synthesis. [ABSTRACT FROM AUTHOR]

Mineral dust can indirectly affect the climate by supplying bioavailable iron (Fe) to the ocean. Here, we present the records of dissolved Fe (DFe) and total Fe (TDFe) in North Greenland Eemian Ice Drilling (NEEM) ice core over the past 110 kyr BP. The Fe records are significantly negatively correlated with the carbon-dioxide (CO2) concentrations during cold periods. The results suggest that the changes in Fe fluxes over the past 110 kyr BP in the NEEM ice core are consistent with those in Chinese loess records because the mineral-dust distribution is controlled by the East Asian deserts. Furthermore, the variations in the dust input on a global scale are most likely driven by changes in solar radiation during the last glacial–interglacial cycle in response to Earth's orbital cycles. In the last glacial–interglacial cycle, the DFe/TDFe ratios were higher during the warm periods (following the post-Industrial Revolution and during the Holocene and last interglacial period) than during the main cold period (i.e. the last glacial maximum (LGM)), indicating that the aeolian input of iron and the iron fertilization effect on the oceans have a non-linear relationship during different periods. Although the burning of biomass aerosols has released large amounts of DFe since the Industrial Revolution, no significant responses are observed in the DFe and TDFe variations during this period, indicating that severe anthropogenic contamination has no significant effect on the DFe (TDFe) release in the NEEM ice core. [ABSTRACT FROM AUTHOR]

Abstract The objective of this research was to study the effects of phosphorous and foliar iron fertilizations rates on the of protein, fiber, and water contents in wheat grains. Treatments of fertilizers included a combination of three foliar iron rates (0, 1.2, and 2 L/ha, Fusion) and four phosphorus rates (0, 75, 150, and 225 kg/ha, P2O5) at three replications. The quality parameters of harvested seeds were measured by the NIR technique. Protein content increased but and fiber and water contents decreased significantly with an increase in the iron and phosphorus fertility rates. As the rates of foliar iron increased from 0 to 2 L/ha the mean values of protein content increased significantly from 13.47% to 15.48%. With increasing the rates of phosphorus from 0 to150 kg/ha, the mean values of protein content increased significantly from 13.56% to 15.53%, however, further increase in phosphorus, causing a decreased trend in the amount of protein. Fiber and water contents of wheat decreased significantly from 13.76% to 11.86% and from 7.36% to 7.28%, respectively, as the phosphorous rate increased from 0 to 225 kg/ha. Those changes were from 13.40% to 12.03% and from 7.57% to 7.21%, respectively, for increasing foliar iron from 0, 1.2, and 2 L/ha. Mathematical relationships composed of phosphorus and iron fertilizers were developed for accurately describing the wheat protein, fiber, and water contents under different fertilization rates.

The subarctic Pacific Ocean and Bering Sea comprise the second‐largest high nitrate, low chlorophyll region in the world, where primary production is limited by the availability of iron (Fe). To estimate the potential impact of different terrestrial aerosol Fe sources on marine ecosystems, we performed a suite of laboratory assessments following established protocols, including: (1) leaching with Milli‐Q water, (2) sequential extractions, (3) complete acid digestions, (4) X‐ray diffraction, and (5) grain size analysis. Measurements were performed on 20 fine‐grained (<5 μm) glacier‐derived sediments from Alaska and the Yukon, two fresh, never‐wetted volcanic ashes (Redoubt 2009 and Pavlof 2016), and six weathered ashes (Redoubt and Augustine) which span the past ∼8,700 years. We compared results to published data on Asian desert‐derived sediments, finding that the glacier‐derived sediments have five times higher easily reducible Fe (median 2.3 ± 0.6 wt.%) than desert‐derived samples (0.49 ± 0.1 wt.%) and fourteen times higher easily reducible Fe than fresh ash (0.16 ± 0.1 wt.%). In addition, fractional Fe solubility was higher in glacial sediment (median cumulative 0.31 ± 0.11% FeS) than volcanic ash (0.04 ± 0.02% FeS). Glacial sediments contained higher concentrations of other bioactive metals including Co, Ni, Cu, Zn, Mo, Cd, and Pb. Results suggest that glacier‐derived dust may provide the subarctic Pacific with more bioavailable iron per unit mass than either volcanic ash or Asian desert‐derived dust. Plain Language Summary: The subarctic Pacific Ocean is a region where the growth of algae at the base of the ocean's food web is limited by a lack of the nutrient iron. In offshore regions far from river discharge and the edges of continents, iron inputs from the atmosphere, such as from volcanic ash or long‐range‐transported dust (i.e., dust storms), can help fuel algae growth, supporting the ocean ecosystem and fisheries that depend on it. In order to learn how different terrestrial nutrient sources might impact algae growth, we compared the compositions of volcanic ash, dust from Asian deserts, and dust from glacier‐fed riverbeds in Alaska and Yukon Territory. We also looked at ash deposits of different ages, to see how compositions changed through time. We found that glacier‐derived dust had the most iron in a form usable by algae, nearly five times higher than desert dust and fourteen times higher than volcanic ash. Volcanic ash had the lowest amount of iron in a form usable by algae. We also found that the form of iron in volcanic ash changes with increasing time and environmental exposure in southcentral Alaska's climate, making it more biologically available to algae as it ages. Key Points: Alaska glacial sediments contain easily reducible Fe that is 5x higher than Asian desert sediments and 14x higher than volcanic ashIron in volcanic ash is relatively insoluble and non‐bioavailable, but it changes through time and environmental exposure in the climate of southcentral AlaskaAlaska glacier‐derived sediments contain five to thirty times higher Cu, Ni, and Pb, and ∼twice as high Co, Zn, Cd, and Mo as volcanic ash [ABSTRACT FROM AUTHOR]

Iron (Fe) fertilizer can reduce cadmium (Cd) uptake and toxicity in rice, but the underlying mechanisms of Cd mitigation by different fertilizers are poorly understood. Here, pot experiments in rice were conducted to characterize the effects of four types of foliar-applied Fe fertilizer (chelated ferrous Fe, ferric Fe, ionic ferrous Fe, and ferric Fe) at three doses (20, 50, and 100 mg L−1) on photosynthetic capacity, antioxidant ability, yield, and Cd accumulation in Cd-contaminated soil. The results showed that foliar Fe application increased the net photosynthesis rate by 19.3%, peroxidase (POD) by 18.2%, superoxide dismutase (SOD) by 26.9%, and catalase (CAT) by 19.6%, and led to a 7.2% increase in grain yield compared with the control. Moreover, foliar Fe application significantly reduced Cd accumulation by 15.9% in brown rice and decreased the translocation of Cd from roots to other plant tissues. Overall, application of moderate doses (50 mg L−1) of chelated ferrous Fe was the most effective method for reducing Cd uptake (decreasing the Cd concentration in brown rice by 29.0%) and toxicity in rice (decreasing malondialdehyde by 23.2% and increasing POD, SOD, and CAT by 54.4%, 51.6%, and 45.7%, respectively), which may stem from the fact that chelated ferrous Fe was a more stable and bioavailable source of Fe for rice. The Cd concentration in rice had negative relationship with Fe concentration, and the translocation of Cd from root to the other tissues was reduced by the higher Fe nutrition status in leaf, suggesting that a high Fe supply may decrease Cd content by inhibiting the expression of the Fe transport system. These results indicate that foliar application of chelated ferrous Fe provides a promising alternative approach for enhancing growth and controlling Cd accumulation in rice plants. Furthermore, these results advance our understanding of the associations between plant Fe nutrition status and Cd accumulation. [ABSTRACT FROM AUTHOR]

Biogeochemical cycling in the Southern Ocean (SO) plays a key role in the global sea-air CO2balance and in the ocean anthropogenic carbon inventory (Ito et al., 2010; Khatiwala et al., 2009; Sarmiento et al., 2004). Some previous studies suggest a decreasing trend in the Southern Ocean carbon sink (Le Quéré et al., 2007; Lovenduski et al., 2007; Wetzel et al., 2005). We investigate the interannual and decadal variations in sea-air CO2flux and phytoplankton production in the SO with hindcast simulations by an ocean biogeochemical model. Decreasing trends in sinking POC and primary production are found from 1979 to 2003, concurrent with a decreasing trend in carbon uptake from the atmosphere. Simulations show substantial interannual and decadal variability in productivity. The sea-air CO2flux is significantly correlated with sinking POC, especially in high productivity regions of the Southern Ocean. Both mixed layer depths and iron concentrations are important to the long-term trends in production and phytoplankton community structure. Sea ice cover also plays an important role at high latitudes. Variability in dust deposition in recent decades has little influence on total SO productivity and carbon uptake, however, there are regional impacts near dust source regions. Accurately representing mixed layer depths and their impacts on phytoplankton light stress are critical for understanding how climate change impacts SO ecosystems and biogeochemistry.

We explore how the iron dependence of the Si:P uptake ratio RSi:P of diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron fertilization. We use a data‐constrained model of the coupled Si‐P‐Fe cycles that features a mechanistic representation of nutrient colimitations for three phytoplankton classes and that is embedded in a data‐assimilated global ocean circulation model. We consider three parameterizations of the iron dependence of RSi:P, all of which are consistent with the available field data and allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply RSi:P decreases with increasing iron concentration, iron fertilization can either cause enhanced silicic acid leakage from the Southern Ocean or strengthened Southern Ocean silicon trapping. Enhanced silicic acid leakage occurs if decreases in RSi:P win over increases in diatom growth, while the converse causes strengthened Southern Ocean silicon trapping. Silicic acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low‐latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron‐replete conditions reduces the global opal export. Regardless of RSi:P parameterization, the global response of the biological phosphorus and silicon pumps is dominated by the Southern Ocean. The Si isotope signature of opal flux becomes systematically lighter with increasing iron‐induced silicic acid leakage, consistent with sediment records from iron‐rich glacial periods. Key Points: Iron fertilization leads to silicic acid leakage if the Si:P uptake ratio decreases sharply enough to overwhelm increased diatom growthSilicic acid leakage increases low‐latitude opal export, while the Southern Ocean dominates increases in diatom organic matter exportSilicic acid leakage in response to iron fertilization produces isotopically lighter biogenic opal flux [ABSTRACT FROM AUTHOR]

To better understand the role of iron in driving marine ecosystems, the Southern Ocean Iron Experiment (SOFeX) fertilized two surface water patches with iron north and south of the Antarctic Polar Front Zone (APFZ). Using 1-D coupled biological-physical simulations, we examine the biogeochemical dynamics that occurred both inside and outside of the fertilized patches during and shortly after the SOFeX field campaign. We focus, in particular, on three main issues governing the biological response to deliberate iron fertilization: the interaction among phytoplankton, light, macronutrient and iron limitation; dilution and lateral mixing between the fertilized patch and external, unfertilized waters; and the effect of varying mixed layer depth on the light field. At the patch south of the APFZ, sensitivity simulations with no dilution results in the maximum bloom magnitude, whereas dilution with external water extends the development of the north patch bloom by relieving silicon limitation. In model sensitivity studies for both sites, maximum chlorophyll concentration and dissolved inorganic carbon depletion inside the fertilized patches are inversely related to mixed layer depth, similar to the patterns observed across a number of iron fertilization field experiments. Our results suggest that Southern Ocean phytoplankton blooms resulting from natural or deliberate iron fertilization will tend to become iron-light co-limited unless the mixed layer depth is quite shallow. © 2007 Elsevier B.V. All rights reserved.

Aeolian dust can provide nutrients for the ocean and affect the growth of phytoplankton. However, the impacts of dust deposition on autotrophic and heterotrophic microorganisms have rarely been studied. In this study, we conducted two microcosm experiments in the low-nutrient and low-chlorophyll environment of the South China Sea and found that dust did not stimulate the abundance of autotrophic and heterotrophic microorganisms. Our results show that dust contains most of the unreacted iron-bearing minerals, and thus provides limited bioavailable iron and nitrogen for bacterioplankton and phytoplankton growth. These results elucidate the overlooked impacts of the properties of the iron-bearing minerals in aeolian dust on microbial communities, which may play an important role in marine ecosystems and climate change. • Two experiments were carried out to investigate the effect of dust input on marine microorganisms. • Dust input did not affect the abundance and composition of marine microorganisms. • Coarse particles of iron-bearing minerals in dust cannot provide bioavailable iron for organisms. [ABSTRACT FROM AUTHOR]

A global ocean biogeochemical model is used to quantify the sensitivity of marine biogeochemistry and air–sea CO2 exchange to variations in dust deposition over decadal timescales. Estimates of dust deposition generated under four climate states provide a large range in total deposition with spatially realistic patterns; transient ocean model experiments are conducted by applying a step-function change in deposition from a current climate control. Relative to current conditions, higher dust deposition increases diatom and export production, nitrogen fixation and oceanic net CO2 uptake from the atmosphere, while reduced dust deposition has the opposite effects. Over timescales less than a decade, dust modulation of marine productivity and export is dominated by direct effects in high-nutrient, low-chlorophyll regions, where iron is the primary limiting nutrient. On longer timescales, an indirect nitrogen fixation pathway has increased importance, significantly amplifying the ocean biogeochemical response. Because dust iron input decouples carbon cycling from subsurface macronutrient supply, the ratio of the change in net ocean CO2 uptake to change in export flux is large, 0.45–0.6. Decreasing dust deposition and reduced oceanic CO2 uptake over the next century could provide a positive feedback to global warming, distinct from feedbacks associated with changes in stratification and circulation.

The Kerguelen Plateau is one of the regions in the Southern Ocean where spatially large algal blooms occur annually due to natural iron fertilization. The analysis of ocean color data as well as in situ samples collected during the Heard Earth-Ocean-Biosphere Interactions (HEOBI) voyage in January and February 2016, surprisingly revealed that chlorophyll a concentrations in waters located close to Heard and McDonald islands were much lower than those on the central Kerguelen Plateau. This occurs despite high levels of both glacial and volcanic iron supply from these islands. The analysis of pigment and optical data also indicated a shift in the phytoplankton size structure in this region, from a microphytoplankton to nanophytoplankton dominated community. Possible explanations for this high nutrient, high iron (Fe), low chlorophyll (HNHFeLC) phenomenon were explored. Low light availability due to deep mixing and shading by re-suspended sediment particles and augmented by dilution with surrounding low chlorophyll waters in the Antarctic Circumpolar Current was shown to be an important mechanism shaping phytoplankton communities. The competing dynamics between stimulation and limitation illustrate the complexity of short-term responses to our changing climate and cryosphere.

This is a proposal for ocean iron fertilization as a means to reduce atmospheric carbon dioxide levels. The idea is to take advantage of nanoparticulate, poorly crystalline Fe-oxides produced by chemosynthetic iron-oxidizing bacteria as an iron source to the ocean. Upon drying these oxides produce a fine powder that could be dispersed at altitude by aircraft to augment wind-driven Aeolian dust that is a primary iron source to the open ocean. Based on Fe-oxidation rates for natural populations of iron-oxidizing bacteria it is estimated 1,500 hectares of production ponds (1 × 100 × 100 m) would be required to produce sufficient iron dust to supply the 30% of the global ocean that is iron-limited. In theory, iron replete conditions will stimulate the biological pump to quantitatively remove enough carbon dioxide from the atmosphere to partially mitigate anthropogenic inputs. Modifications to existing technologies would satisfy most of the technological challenges. Addition of biogenic iron to meso-scale eddies could provide an effective means of testing this process. Nonetheless, there are many unknowns, thus any such effort will require research and development integrated across oceanographic and Earth science disciplines to determine its long term efficacy.

Volcanic eruptions in the Aleutian Archipelago during the summer of 2008 deposited large quantities of iron‐laden ash to the subarctic North Pacific Ocean. The surface ocean ecosystem response has been previously characterized using limited in situ and autonomous field measurements and numerical modeling, but basin‐scale satellite remote sensing has been limited to a simple description of chlorophyll variability. Here, we show that the ecosystem response is a complex combination of phytoplankton biomass and physiology that can be described with satellite ocean color diagnostics such as the chlorophyll‐to‐carbon biomass ratio (Chl:Cphyto) and chlorophyll fluorescence yield. Together, these quantities outline a more complete picture of ecological responses spanning unique signals of iron stress (and relief from), photoacclimation, changes in phytoplankton growth rate, increases in biomass, and timescales of decay for these processes. Plain Language Summary: Phytoplankton growth in nearly one third of the global ocean is limited by the micronutrient iron. Outside the influence of continental margins, the only mechanisms to introduce new iron to the surface ocean are mixing up from the deep ocean (such as during winter storms) and through airborne deposition of desert dust. However, every once in a while large volcanic eruptions can provide a temporary source of iron to the ocean through deposition of volcanic ash. The effects of this "fertilization" are nearly impossible to measure in the field because of the unpredictability of volcanic eruptions. Satellite platforms provide the necessary coverage for this scale of event but have been woefully underutilized to date. We revisit a pair of well‐documented volcanic eruptions from the Aleutian Archipelago and describe their impact on the surface ocean ecosystem of the subarctic North Pacific Ocean using novel satellite–derived products supplemented by model output. We are able to characterize the ecosystem response in terms of both increased phytoplankton growth and adjustments in their physiology. The latter is often neglected yet can be of equal or greater magnitude than changes in growth rate. Key Points: The ash deposition from Aleutian volcanic eruptions in 2008 provides a case study for remote sensing analyses of ocean ecosystem responseClear signals are seen in bulk satellite properties and phytoplankton physiological indices (Chl:C and chlorophyll fluorescence yield)Ocean ecosystem response to iron fertilization is complex and can only be characterized with multiple satellite remote sensing products [ABSTRACT FROM AUTHOR]

The objective of this research was to study the effects of phosphorous and foliar iron fertilizations rates on the of protein, fiber, and water contents in wheat grains. Treatments of fertilizers included a combination of three foliar iron rates (0, 1.2, and 2 L/ha, Fusion) and four phosphorus rates (0, 75, 150, and 225 kg/ha, P2O5) at three replications. The quality parameters of harvested seeds were measured by the NIR technique. Protein content increased but and fiber and water contents decreased significantly with an increase in the iron and phosphorus fertility rates. As the rates of foliar iron increased from 0 to 2 L/ha the mean values of protein content increased significantly from 13.47% to 15.48%. With increasing the rates of phosphorus from 0 to150 kg/ha, the mean values of protein content increased significantly from 13.56% to 15.53%, however, further increase in phosphorus, causing a decreased trend in the amount of protein. Fiber and water contents of wheat decreased significantly from 13.76% to 11.86% and from 7.36% to 7.28%, respectively, as the phosphorous rate increased from 0 to 225 kg/ha. Those changes were from 13.40% to 12.03% and from 7.57% to 7.21%, respectively, for increasing foliar iron from 0, 1.2, and 2 L/ha. Mathematical relationships composed of phosphorus and iron fertilizers were developed for accurately describing the wheat protein, fiber, and water contents under different fertilization rates. In this study, the effects of phosphorus and foliar iron fertilization rates on the protein, fiber, and water contents in whole wheat grain were determined by the NIR technique. [ABSTRACT FROM AUTHOR]

Ice crystals form in supercooled seawater beneath several Antarctic ice shelves; as they rise to the ice‐shelf base they scavenge particles from the water and incorporate them into the growing basal ice. The resulting marine ice can be ~100 m thick; it differs from sea ice in that it is clear, desalinated, and bubble‐free. Icebergs of marine ice vary in color from blue to green, depending on the nature and abundance of foreign constituents in the seawater that became trapped in the ice as it grew. A red or yellow material (i.e., one that absorbs blue light), in combination with the blue of ice, can shift the wavelength of minimum absorption to green. Previously, dissolved organic carbon (DOC) had been proposed to be responsible for the green color. Subsequent measurements of low DOC values in green icebergs, together with the recent finding of large concentrations of iron in marine ice from the Amery Ice Shelf, suggest that the color of green icebergs is caused more by iron‐oxide minerals than by DOC. These icebergs travel great distances from their origin; when they melt they can deliver iron as a nutrient to the Southern Ocean. Plain Language Summary: Glacier ice, originating from snowfall, flows off the Antarctic Ice Sheet to float on the ocean as ice shelves. At the front of the ice‐shelf, icebergs break off. They appear bluish‐white, intermediate between the blue of pure ice, and the white of snow, because glacier ice contains numerous bubbles that scatter light. Seawater freezes to the base of some ice shelves, forming marine ice, which incorporates organic and inorganic particles from the water. On the resulting composite icebergs, the marine ice can be exposed to view if the iceberg capsizes. The marine‐ice part of such icebergs is clear, dark, and often green in color, because red or yellow particles from the seawater, in combination with the blue of ice, can shift the color to green. Previously, dissolved organic carbon had been proposed to be responsible for the green color. The recent finding of large concentrations of iron in marine ice from the Amery Ice Shelf suggests that the color of green icebergs is caused more by iron‐oxide minerals. These icebergs travel great distances from their origin; when they melt they can deliver iron as a nutrient to the Southern Ocean. Key Points: Icebergs of marine ice vary in color from blue to greenThe color is probably caused more by iron oxides than by dissolved organic carbonThe color may indicate the ability of icebergs to deliver iron as a nutrient [ABSTRACT FROM AUTHOR]

Abstract Availability of dissolved iron and light are both regulating factors for primary productivity in high (macro)nutrient, low chlorophyll regions of the Southern Ocean. Here, using on-board iron/light incubation experiments conducted in 2015 in the Atlantic sector of the Southern Ocean, we show that irradiance limited significant phytoplankton growth (in chlorophyll-a and particulate organic carbon) north of the Polar Front (46 °S 08 °E), while iron addition resulted in growth stimulation even at low light levels in the Antarctic zone (65 °S 0 °E). The phytoplankton community in the Polar Frontal Zone showed a greater functional diversity than the one in the Antarctic Zone. The community structure changed over the course of the incubations in response to increased iron and light. The observed increase in chlorophyll-a under high light in the Polar Frontal Zone was driven predominantly by an increase in pico-(0.2–2 µm) and large (> 5 µm) nanophytoplankton. Pigment fingerprinting indicated an increase in the contribution of diatoms and Phaeocystis over the course of the incubation. In contrast, in the Antarctic Zone, the increase in chlorophyll-a after iron enrichment was predominantly due to an increase in the contribution of diatoms and large nanophytoplankton. The photochemical efficiency (Fv/Fm) was low at both sites at the beginning of the incubations, but increased upon iron fertilization in both water masses, indicating stress relief. However, the acclimation strategies fundamentally differed between the two communities. The ratio of photoprotective versus light-harvesting pigments increased under high light in the Polar Frontal Zone independent of iron enrichment, whereas this ratio declined upon iron enrichment in the Antarctic Zone even under high light. At the same time, the functional cross section of photosystem II (σ PSII) decreased upon iron enrichment in the Antarctic Zone, but not in the Polar Frontal Zone. Our experiments support the need to take biogeographical differences between Southern Ocean water masses into account when interpreting ecosystem dynamics. Graphical abstract fx1 Highlights • Phytoplankton communities respond differently to addition of iron and light. • Communities north of Polar Front appeared to be rather light limited. • Communities south of Polar Front appeared to be rather iron limited. • Acclimation strategies differed between the communities. • Community composition changed in response to iron and light addition. [ABSTRACT FROM AUTHOR]

The objective of this study is to analyze the daily variation of aeolian dust in Puerto Madryn, Patagonia, Argentina, based on a series of data of more than 11 years (from December 2004 to April 2016). In order to accomplish it, atmospheric dust was collected by means of a high volume sampler placed at the El Tehuelche airport located at 10 km westward from Puerto Madryn. A correlation between aeolian dust and wind intensity was studied. An elemental and mineral composition and granulometry analyses of dust particles were performed. Monthly aeolian dust concentration ranged between 1ug*m-3 and 2700ug*m-3. Volcanic eruptions ocurred throughout the study. The effects of volcanoes activities were evident and signifincatly increased the concentration of dust by the massive emission of ashes into the atmosphere. Thus, the mean monthly variation of dust concentration was analyzed considering the full data set and a sub-set of those years without volcanic eruptions. In the first case, the variation did not presents a pattern of variation, whereas, in the second case, a clear seassonal pattern was observed. Minimum dust concentration was recorded in Winter (20μg*m-3) and maximum in Summer (60μg*m-3). The correlation between dust concentration and wind intensity was lower than 0.5, suggesting that other factors (e.g. soil moisture and vegetation coverage) may play an important role on the suspension of mineral particles in the study region. The elemental composition was characterized by a high proportion of Si and O, whereas Fe, S, K, Mg, and P were present in a lower proportion. Clasts were mostly silicates such as quartz, horblende, and plagioclase. The granulometry was studied using a subset of 10 filters. The mean size of dust particles was 11 μm and ranged form 0.5 μm to 120 μm. These results suggest that dust transported from Patagonia to the Southern Atlantic Ocean is permanent and highly variable and may represent and important input of Fe into the sea. [ABSTRACT FROM AUTHOR]

Calcified coccolithophores, a diverse and widely distributed group of marine microalgae, produce biogenic calcite in the form of coccoliths located on the cell surface. Using batch incubations of the coccolithophorid Pleurochrysis carterae, we investigated the responses of this calcification process to iron concentrations by changing the iron supply in the initial culture media from a normal concentration to 1 ppm (parts per million), 5 ppm, and 10 ppm. Time-dependent measurements of cell population, production of inorganic carbon (coccoliths), and organic carbon (organic cellular components) showed that elevated iron supply in the growth medium of P. carterae stimulates carbon sequestration by increasing growth along enhanced photosynthetic activity and calcification. In addition, the acquired time-dependent UV-Vis and FT-IR spectra revealed that iron fertilization-enhanced coccolith calcification is accompanied by a crystalline phase transition from calcite to aragonite or amorphous phase. Our results suggest that iron concentration has a significant influence on the marine carbon cycle of coccolithophores. [ABSTRACT FROM AUTHOR]

The LOHAFEX iron fertilization experiment was conducted for 39 days in the closed core of a cyclonic mesoscale eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~ 20 μM), low dissolved iron (DFe ~ 0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. DFe concentrations in the upper 200 m were not significantly affected by the two fertilizations but after day 14 showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~ 5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe 48h ) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe 48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean. [ABSTRACT FROM AUTHOR]