Iron Cycle in Oceans

By Stéphane Blain and Alessandro Tagliabue

This book presents an up to date view of iron biogeochemistry in the ocean. It encompasses the description of iron speciation, the analytical methods used to measure the different iron forms in seawater and the different iron biogeochemical models.

• Language: English
• Publisher: Wiley
• Release date: Nov 22, 2016
• ISBN: 9781119136873

Book preview

Table of Contents

Cover

Title

Copyright

Preface

1 Iron Speciation in Seawater

1.1. The chemical element

1.2. Iron speciation

1.3. Applying speciation

2 Analytical Methods

2.1. Trace-metal clean sampling techniques

2.2. Processing of the sample before measurement of concentrations

2.3. Particle collection

2.4. Iron determination

3 Modeling Methods

3.1. Overview

3.2. Modeling frameworks

3.3. Modeling iron cycle processes

3.4. Synthesis

4 Iron Sources

4.1. Overview

4.2. Dust deposition

4.3. River supply

4.4. Continental margins

4.5. Hydrothermalism

4.6. Glaciers, icebergs and sea ice

4.7. Submarine groundwater discharge

4.8. Synthesis

5 Iron Cycling in the Ocean

5.1. The biological iron demand

5.2. Iron cycling in the surface ocean

5.3. Iron export and its cycling below the mixed layer

6 Dissolved Iron Distributions in the Ocean

6.1. Overview

6.2. Temporal evolution in the number of observations

6.3. The contemporary view of the distribution of iron in the ocean

6.4. The vertical profile of iron

6.5. Synthesis

7 The Iron Hypothesis

7.1. Introduction

7.2. From bottle incubations to mesoscale experiments

7.3. Natural iron fertilization

7.4. Paleo iron hypothesis

7.5. Large-scale iron fertilization: climate engineering

Bibliography

Index

End User License Agreement

List of Tables

1 Iron Speciation in Seawater

Table 1.1. Examples of iron minerals found in seawater [LAM 12]

List of Illustrations

1 Iron Speciation in Seawater

Figure 1.1. Examples of different categories of ligands in seawater. a) Important enzyme in photosynthetic or respiration chains that contain a porphyrin binding Fe(II). b) Example of different functional groups with iron binding capacity found in humic acid. c, d, e) Siderophores. In marinobactin (e) the complexing site is bound to an aliphatic chain of variable composition R

Figure 1.2. Redox potential of the couple Fe(II)/Fe(III) as a function of the nature of the ligand and implication for their reactivity

Figure 1.3. Speciation of iron in anoxic environments (data from [LAN 88])

Figure 1.4. Operational definitions of the different fractions commonly used in iron biogeochemistry. For comparison, operational definitions of high molecular weight (HMW)/low molecular weight (LMW) of dissolved organic matter [BEN 92] and for colloids are also indicated. A few examples of entities containing iron belonging to the different operational fractions are also indicated

Figure 1.5. a) Solubility of iron in natural seawater as a function of pH (data from [KUM 96]). b) Solubility of iron as a function of temperature (note that the samples used were collected at different in situ temperatures but the measurements were made at 20 °C, a correction from this bias has been proposed by the authors (see [SCH 12])

Figure 1.6. Reaction scheme for the photolysis of two Fe(III)--siderophore complexes: a) vibrioferrin [AMI 09], b) aquachelin [BAR 01]

Figure 1.7. Major chemical species and reaction schemes for oxido-reduction reactions with Fe(II)/Fe(III). The upper panel shows the production of the superoxide (see text for definition of O2–*) and hydrogen peroxide that are then used as an oxidizing or reducing agent in Fe(II)/Fe(III) redox chemistry. The solid phase containing Fe(II) or Fe(III) is represented by the hexagons. For clarity, the redox reactions of Fe(II)L and Fe(III)L are not mentioned but they should not be ignored (see text and Figures 1.6 and 1.8)

Figure 1.8. Bioavailability of iron for eucaryotic phytoplankton and models for iron uptake: a) the Fe' model [HUD 90], b) Fes model [SHA 05]

Figure 1.9. Geological perspective of iron speciation in the surface ocean. This figure is based on chronology presented by [SAI 03] for iron and sulfide and [HOL 06] for oxygen and banded iron formation. Because some of these issues are still in debate, and some others (e.g. organic ligand) are still completely unknown, this plot should be considered cautiously; however, broadly speaking, there is no doubt that iron was at the heart of the biogeochemical evolution of the planet and that iron speciation has played in the past a similar central role as it plays in the modern ocean

2 Analytical Methods

Figure 2.1. Historical perspective of the change in the range and average concentrations of dissolved iron in the ocean

Figure 2.2. Example of an FIA manifold [OBA 93] used for the determination of Fe(III) concentration in seawater

Figure 2.3. Iron complexation and speciation determination by the electrochemical method. a) Typical signals are obtained when iron is added at different concentrations to the seawater sample, b) titration curve, c) and d) illustration of two different methods used to estimate concentrations and stability constants of organic ligands in seawater (redrawn from [WU 09])

Figure 2.4. Calibration curve (sigmoid) for a biosensor [BOY 07a]

3 Modeling Methods

Figure 3.1. An example of the most complex treatment of iron speciation used in global, regional and process-based models. It represents Fe(II), Fe(III), solid Fe(III) and Fe(III) complexed by strong (Fe(III)La) and weak (Fe(III)Lb) ligands. Reproduced from Tagliabue et al. [TAG 09a]

Figure 3.2. Schematic representation of a) a threshold model for iron speciation, where a fixed ligand concentration controls the partitioning of iron between organically complexed and free components, and b) a dynamic model that represents colloidal dynamics and a variable concentration of organic iron-binding ligands

Figure 3.3. Illustration of the differences between a Monod (left) and Quota (right) treatment of nutrient limitation. Dissolved inorganic carbon (DIC), dissolved inorganic phosphorus (DIP), dissolved inorganic nitrogen (DIN) and dissolved iron (DFe) are represented as resources. In the Monod approach, only the external concentrations of resources are important. In a Quota model, uptake of a resource produces a cell quota (Qi for resource i), whose relative abundance then controls growth. Illustration modified from Follows and Dutkiewicz [FOL 11]. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

4 Iron Sources

Figure 4.1. Schematic representation of the atmospheric iron source. Dust iron enters the atmosphere from uplift of desert dust, volcanoes, fire and anthropogenic activities. Dust is then transported away from the source with a gradual reduction in particle size and chemical processing in the atmosphere. Finally, dust is deposited by both dry and wet deposition. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 4.2. Schematic representation of the river iron source. Iron enters the river upstream from weathering and is transported primarily as suspended sediments downstream along with dissolved organic material (DOM). While large particles and colloids may flocculate in the estuary, the iron associated with DOM may be transported into the open ocean. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 4.3. Schematic representation of the continental margin iron source. Diagenesis of PFe sinking into sediments can promote resupply of iron into the overlying water. This occurs by diffusion and resuspension, and the associated supply of DFe and PFe can be transported offshore. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 4.4. Schematic representation of the hydrothermal iron source. This emphasizes the generation of a hydrothermal iron plume due to the transformation of fluid within the hydrothermal vent. Although large amounts of iron are lost as PFe, pyrite (Fe-S) and ligands entrained into the plume can aid the propagation of a DFe plume away from the vent site. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 4.5. Schematic representation of the ice-associated iron source. Fe is supplied from three main ice-related sources: (1) iron is exchanged with sea ice during sea ice formation and melting; (2) glaciers that may accumulate atmospheric iron or iron released at the glacier–sediment interface; (3) icebergs that break from glaciers and are transported away into the open ocean. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

5 Iron Cycling in the Ocean

Figure 5.1. Internal iron cycle in the ocean. a) The tentative diagram of the iron cycle in the sea proposed by Cooper in 1935. b) The current view of the iron cycle. Gray-shaded forms represent the different iron pools. Iron fluxes between different pools are represented by arrows and the names associated with the fluxes are indicated by white circles

Figure 5.2. Interrelations between uptake rate, quota and growth rate. Log–log graphs reduced to asymptotes for iron uptake rate (ρ), iron quota (Q) and growth rate (µ) as a function of iron concentration. The hyperbolic functions for ρ and µ are characterized by the half-saturation constants Kρ and Kµ that define the range of concentrations of Fe where the cell can acclimate by changing Q and ρ. This figure is redrawn from Morel [MOR 87], where detailed definitions and equations are provided

Figure 5.3. Iron quota of phytoplankton [TWI 13]: (a) iron quota normalized to phosphorus determined by different approaches are indicated below the horizontal axis. (*) denotes the mean value provided by Ho et al. [HO 03]; (b) iron quota determined by SXRF for phytoplankton collected in different regions are indicated below the horizontal axis

Figure 5.4. Iron stocks and fluxes determined during FeCycle [BOY 05a, STR 05]

6 Dissolved Iron Distributions in the Ocean

Figure 6.1. The year-by-year evolution of the cumulative number of dissolved iron observations in the ocean

Figure 6.2. Dissolved iron concentrations, gridded onto a 5 × 5 degree horizontal grid and averaged over different depth ranges. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 6.3. The number of unique calender months for which there are dissolved iron measurements, gridded onto a 5 × 5 degree horizontal grid and quantified over different depth ranges. For a color version of this figure, see www.iste.co.uk/blain/iron.zip

Figure 6.4. Generic vertical iron profiles, illustrating different types of general behaviour

7 The Iron Hypothesis

Figure 7.1. Effect of iron addition (0–10 nM final concentrations) on a) chlorophyll concentrations and b) nitrate consumption during bottle incubation experiments in the subarctic Pacific (redrawn from Martin and Fitzwater [MAR 88b])

Figure 7.2. Iron concentrations and air CO2 concentrations in Vostok ice core as presented in Martin [MAR 90b] (data from Angelis et al. [ANG 87] and [BAR 87])

Figure 7.3. Systems used during IronEx I (redrawn from Coale et al. [COA 98]) to disperse iron and SF6. A dual tank system (2,400 l) was used to mix seawater with FeSO4 and HCl. The iron solution at the final concentration of 0.5 M was then delivered to the mixer at 12 l/min. The saturated solution of SF6 was prepared in another tank and delivered at 1.2 l/min. A bladder located within the tank was continuously filled with freshwater as the tank emptied. This avoided degassing of SF6 and thus allowed the injection of seawater with a constant iron/SF6 ratio

Figure 7.4. Locations of artificial iron fertilization experiments (white stars) superimposed on the annual average concentrations of nitrate in the surface water (data from World Ocean Atlas 2013 processed by Ocean Data View). Two major studies in naturally iron-fertilized regions are also shown (white