Iceberg-hosted nanoparticulate Fe in the Southern Ocean: Mineralogy, origin, dissolution kinetics and source of bioavailable Fe

doi:10.1016/j.dsr2.2010.11.011

Deep Sea Research Part II: Topical Studies in Oceanography

Volume 58, Issues 11–12, June 2011, Pages 1364-1375

https://doi.org/10.1016/j.dsr2.2010.11.011 Get rights and content

Abstract

Sediments from icebergs and glaciers contain nanopartculate Fe(III) oxyhydroxides (including ferrihydrite) which form in aqueous, oxic (micro)environments where Fe(II)-bearing rock minerals oxidise and high degrees of supersaturation are promoted by freezing and thawing. An ascorbic acid extraction dissolves only labile Fe present in fresh (loosely aggregated) ferrihydrite that is directly or indirectly bioavailable. Glacial and iceberg sediments contain ferrihydrite aggregates that provide 0.04 to 0.17% Fe soluble in ascorbic acid, rather larger than the concentrations in a limited suite of atmospheric dusts. The dissolution behaviour of labile Fe from glacial and iceberg sediments by ascorbic acid is controlled by the access of reactant, or removal of solute, through micropores to or from active sites in the interior of ferrihydrite aggregates. A first-order kinetic model is presented to examine the rates at which bioavailable Fe can be supplied by melting icebergs in the Weddell Sea. The model utilizes rate constants from the literature for the processes which solubilise Fe from nanoparticulate ferrihydrite (dissolution, photochemical reduction and grazing) and the processes that remove Fe nanoparticulates (sinking, scavenging and incorporation in faecal material), and render them less reactive (transformation, aging). Model results demonstrate that icebergs can supply bioavailable Fe to the Weddell Sea by the dissolution of nanoparticulate ferrihydrite (despite loss/removal of nanoparticles by sinking, aging, transformation, scavenging and incorporation into faecal pellets) at rates that are comparable to atmospheric dust. Dissolution enhanced by photochemical reduction and grazing provides the most rapid rates of bioavailable Fe production.

Introduction

Bioavailable Fe is supplied to surface waters in the Southern Ocean from a variety of sources, including atmospheric dust, continental shelf sediments and porewaters, upwelling and vertical diffusion from deep waters, sea ice and sediment released by melting icebergs (see Cassar et al., 2007). For many years atmospheric dust has been considered to be the main source of bioavailable Fe external to the marine reservoir although the links between dust and productivity may be weaker than previously believed (Boyd et al., 2010). However recent work has shown that icebergs may be a significant source of bioavailable Fe to the Southern Ocean. Ice rafting is clearly a major source of sediment to the Southern Ocean but this sediment has usually been considered as essentially inert. Recent data suggest otherwise. First, Smith et al. (2007) observed that melting icebergs in the Weddell Sea were associated with hot spots of biological activity and suggested that enhanced productivity was caused by the release of terrigenous debris that supplied bioavailable Fe. Furthermore iceberg-hosted sediment was demonstrated to stimulate productivity in experiments carried out with diatoms under Fe-limited conditions, such as those existing in the Southern Ocean. Second, Raiswell et al., 2006, Raiswell et al., 2008a showed that glacial and iceberg-hosted sediment contained nanoparticles of Fe oxyhydroxides that were potentially bioavailable and were present in sufficiently large concentrations to influence the Fe biogeochemical cycle in the Southern Ocean. Hence iceberg-hosted sediment clearly has the potential to stimulate significant primary productivity in the Southern Ocean.

Quantifying this potential presents considerable difficulties. First, it is necessary to identify the reactive iron-bearing minerals that exist in iceberg sediment. Second, it is necessary to determine whether Fe is present in these minerals in a form that can be directly or indirectly rendered as bioavailable. Third, it is necessary that the kinetics of Fe release from such minerals are rapid relative to the processes which remove particulates from the water column. This contribution begins by briefly describing the characteristics and origin of the main iron oxyhydroxides found in glacial and iceberg sediments. The paper continues by evaluating the use of an ascorbic acid extraction to estimate bioavailability and the kinetics of iron release during ascorbic acid extraction of glacial and iceberg sediments. Finally, a simple kinetic model of iron release from iceberg-hosted sediment is presented that assesses whether bioavailable iron can be released at rates that exert a significant regional influence on the Fe biogeochemical cycle.

Section snippets

Potentially bioavailable forms of Fe in glacial sediments

Iron, although the fourth most abundant element in the crust, is poorly soluble and may be biologically scarce in seawater. Iron (III) is the thermodynamically stable form of iron in surface environments but generally occurs in minerals that are stable and poorly-soluble in oxic environments. Common iron (II) minerals (e.g. pyrite and iron-bearing carbonates and aluminosilicates) are formed in sub-surface environments where oxygen is depleted but slowly weather to form Fe(III) oxyhydroxides

Determination of labile, potentially bioavailable iron.

Unfortunately it is difficult to quantify the bioavailability of particulate Fe oxyhydroxides because of variations in mineralogy, grain-sizes, crystallinity and surface characteristics, and because extraction of bioavailable Fe may occur by a variety of mechanisms, including inorganic dissolution, photochemical reduction and protozoan grazing (see 4.1.3). There is no simple way to replicate the diversity of these processes but Raiswell et al. (2008a) have utilised an extraction scheme based on

A simple kinetic model for the iceberg supply of bioavailable, nanoparticulate iron

This section presents an idealised, semi-quantitative model which evaluates the relative importance of different mechanisms by which bioavailable Fe may be supplied and removed during iceberg melting in the absence of turbulent mixing (see later). The model assumes;

(a)
Icebergs containing sediment concentrations of S kg km⁻³ melt at a constant rate V (km³ day⁻¹) supplying sediment containing f kg/kg of iron present as nanoparticles that are extractable by ascorbic acid. Thus nanoparticulate Fe is

Conclusions

The contribution of bioavailable Fe from terrigenous debris to the Southern Ocean has been examined by a range of techniques appropriate for the recognition and quantification of trace amounts of reactive iron minerals. High resolution microscopy can be used to identify the presence of small concentrations of nanoparticulate Fe oxyhydroxides that are the most reactive (and potentially the most bioavailable) Fe minerals in sediments frozen into icebergs. Iceberg-hosted sediments from the Weddell

Acknowledgment

Rob Raiswell is pleased to acknowledge the award of a Leverhulme Emeritus Fellowship which funded this work. Martyn Tranter, one anonymous reviewer and Ken Smith are thanked for their input.

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Iceberg-hosted nanoparticulate Fe in the Southern Ocean: Mineralogy, origin, dissolution kinetics and source of bioavailable Fe

Abstract

Introduction

Section snippets

Potentially bioavailable forms of Fe in glacial sediments

Determination of labile, potentially bioavailable iron.

A simple kinetic model for the iceberg supply of bioavailable, nanoparticulate iron

Conclusions

Acknowledgment

Atmospheric Environment

Colloids Surfaces

Geochimica Cosmochimica Acta

Geochimica et Cosmochimica Acta

Marine Chemistry

Journal Colloid Interface Science

Applied Geochemistry

Marine Chemistry

Marine Chemistry

Geochimica Cosmochimica Acta

Marine Chemistry

Geochimica et Cosmochimica Acta

Marine Chemistry

Atmospheric Environment

Atmospheric Environment

Deep-Sea Research II

Applied Geochemistry

Deep Sea Research II

Chemical Geology

Chemical Geology

Geochimica et Cosmochimica Acta

Chemical Geology

Deep-Sea Research II

Deep-Sea Research II

Deep-Sea Research II

Journal Colloidal Interface Science

Marine Chemistry

Deep-Sea Research

Marine Chemistry

Journal Geochemical Exploration

Chemical Geology

Geochemistry, Groundwater and Pollution

Globalizing results from ocean in situ fertilization studies

Global Biogeochemical Cycles

Role of protozoan grazing in relieving iron limitations of plankton

Nature

Laboratory and field studies of colloidal iron dissolution asmediated by phagotrophy and photolysis

Limnology Oceanography

Photochemistry of organic Fe (III) complexing ligands in oceanic systems

Photochemistry Photobiology

A mesoscale phytoplankton bloom in the polar southern Ocean stimulated by iron fertilization

Nature

The Southern Ocean biological response to aeolian iron deposition

Science

The Iron Oxides: Structure, Properties, reactions, occurrences and uses