Iron Transport from the
Continents to the Open Ocean:
The Aging–Rejuvenation Cycle
Robert Raiswell*
1811-5209/11/0007-00101$2.50 DOI: 10.2113/gselements.7.2.101
T
he biogeochemical cycle of iron plays a key role in the ocean by delivering bioavailable iron that controls plankton productivity. Transport
through the iron cycle occurs mainly as nanoparticulate (oxyhydr)oxides, which are physically and chemically intermediate between aqueous
and particulate forms and can be directly or indirectly bioavailable. Iron
nanoparticles transform with time to more stable forms by increased crystallinity, aggregation and growth, and they also alter to other nanominerals.
These age transformations can be inhibited or reversed. The resulting aging–
rejuvenation cycle first produces stability during long-distance transport and
then reverses the process such that bioavailable and labile iron can be
produced and delivered to the open ocean.
ments in rivers and glacial
meltwaters (Poulton and Raiswell
2005) and glacial and iceberghosted sediments (Fig. 1; Raiswell
et al. 2006, 2008). These studies
have recorded nanoparticles of
ferrihydrite, goethite, lepidocrocite, hematite and schwertmannite (Fig. 2), which display a large
range in stability, surface area,
adsorption capacity and chemical
reac t iv it y
(Cor nell
and
Schwertmann 2003). Ferrihydrite
[Fe3+ 4–5 (OH,O)12 ], the least stable
iron (oxyhydr)oxide, is formed
Keywords : iron (oxyhydr)oxide, nanoparticle, estuaries, shelf sources,
directly by weathering and is
wind-blown dust, icebergs, iron cycle
always nanoparticulate, but with
time it coalesces to form microporous nanoparticulate aggregates
INTRODUCTION
and/or
alters
to
goethite–hematite
mixtures. Lepidocrocite
Turning on a domestic water supply sometimes produces
reddish-coloured water due to the presence of particles of (γ–FeOOH) is more stable than ferrihydrite but is only
formed in fluctuating redox environments. Goethite (α–
iron (oxyhydr)oxides. This occurrence shows that the
transport of iron as particulate (oxyhydr)oxides is possible FeOOH) is more stable than lepidocrocite and is formed
by the oxidation of Fe(II) dissolved from minerals in rocks
and that separation of iron-bearing particulates from water
or by the transformation of ferrihydrite or lepidocrocite.
is far from easy. Geoscientists conventionally separate
Hematite (Fe2O3) is the most stable (oxyhydr)oxide and is
dissolved and particulate iron by filtration through 0.45 µm
filters. These remove visible particulates and some colloidal mainly formed in soils by the transformation of ferrihymaterial (defined as <1 µm in diameter) but pass aqueous drite. Schwertmannite is not a simple (oxyhydr)oxide, but
species, colloids (<0.45 µm in diameter) and nanoparticles a ferric (oxy)hydroxyl-sulfate that forms by the oxidative
weathering of pyrite at low pH and is readily transformed
(defined as <100 nm in diameter). The use of finer filters
(0.2–0.02 µm in pore diameter) is now more common, but to ferrihydrite.
models of the iron biogeochemical cycle commonly utilize
dissolved-iron data based on <0.45 µm filtrates, which
include dispersed nanoparticles and small nanoparticulate
aggregates but exclude larger aggregates and sediment–
nanoparticulate composites. The nanoparticles measured
as dissolved iron and those excluded by filtration, however,
behave differently from aqueous iron species in that their
reactivity and bioavailability may decrease during storage
and transport. The challenge that emerges is thus to determine how iron nanoparticles behave during transport and
what processes control iron bioavailability during transport. Mineralogy is an essential starting point.
IRON (OXYHYDR)OXIDE
NANOPARTICULATES: MINERALOGY
AND ORIGIN
Iron-bearing nanoparticles are present in most near-surface
sediments, including soils (Theng and Yuan 2008), windblown dust (Buseck and Adachi 2008), suspended sedi* School of Earth and Environment, Leeds University
Leeds LS2 9JT, UK
E-mail: [email protected]
E lements , V ol . 7,
pp.
101–106
Figure 1
101
Sediments containing nanoparticulate iron (oxyhydr)oxides in an iceberg in the Southern Ocean
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The occurrence of nanoparticulate iron (oxyhydr)oxides
in a wide range of weathering environments, from tropical
through temperate to polar, and linked to widely different
rock types provides compelling evidence for an origin from
common minerals via widespread surface-weathering
processes. Iron (oxyhydr)oxides are poorly soluble and
precipitate at high degrees of supersaturation as nanoparticles. The rapid oxidation of dissolved Fe2+ initially forms
octahedral molecular clusters of Fe(O,OH,OH2) 6 that slowly
aggregate to form nanoparticles and then colloids
(Waychunas et al. 2005).
A
Nanoparticles of iron (oxyhydr)oxides can form in soils
and sediments via three main mechanisms:
ƒƒ By the oxidation of Fe2+ -bearing rock minerals such as
carbonates, sulfides and aluminosilicates. The presence
of schwertmannite is clear evidence that iron (oxyhydr)
oxide nanoparticles originate, at least partly, from pyrite
oxidation, which is the only known mechanism capable
of simultaneously generating sulfate, iron and low-pH
solutions.
B
ƒƒ When Fe 2+ -bearing freshwaters or porewaters in
reducing sediments are brought into contact with
oxygen and the dissolved iron is rapidly oxidized.
ƒƒ By the transformation of pre-existing nanoparticles, for
example, schwertmannite to ferrihydrite and ferrihydrite to goethite and hematite (Bigham et al. 1996;
Schwertmann et al. 2004).
PROPERTIES OF NANOPARTICULATE IRON
(OXYHYDR)OXIDES
Nanoparticles represent only the smallest 10% of the
colloid size range, but the nanoparticulate fraction lies
between two extremes where chemical behavior is determined by either aqueous or particulate reactivity. Between
these two extremes, changes in chemical reactivity arise
from mineralogical transformations and phenomena
related to surface area, which become progressively more
important with decreasing grain size. The mineralogical
transformation of ferrihydrite to goethite and/or hematite
exerts the main influence on reactivity in the iron cycle
because this transformation is accompanied by large
changes in crystallinity, surface area and stability. This
transformation is particularly important because the
absence of iron in a bioavailable form limits plankton
growth in parts of the ocean (de Baar and de Jong 2001).
Plankton require iron for biological functions that utilize
redox reactions (including interactions with O2 ) and they
are able to use aqueous iron species and ferrihydrite (either
directly or indirectly), but iron in other iron minerals,
including hematite, is significantly less bioavailable.
C
The rate at which ferrihydrite transforms to goethite and/
or hematite plays a vital role in the delivery of bioavailable
iron. This transformation occurs rapidly (<500 days; Fig. 3)
at typical surface temperatures and is essentially independent of pH. However, at polar temperatures, the transformation is much slower and is pH dependent (~700 days at
pH 8, ~2400 days at pH 6). The transformation of ferrihydrite can also be slowed down by adsorbed natural organic
matter (NOM), silica and trace elements, but the formation
of hematite and goethite is increasingly likely the longer
the time interval is between the formation of ferrihydrite
and its delivery to ocean waters.
(A) High-resolution scanning electron microscope
(SEM) image of a nanoparticulate ferrihydrite aggregate in iceberg sediment from Antarctica. (B) High-resolution SEM
image of nanoparticulate goethite laths in iceberg sediment from
Antarctica. (C) High-resolution transmission electron microscope
(TEM) image of nanoparticulate schwertmannite as “pin-cushion”
spheroidal aggregates in subglacial sediment, Antarctica
Figure 2
E lements
The large surface areas of nanoparticles increase the influence of surface charge. For iron (oxyhydr)oxide nanoparticles, surface charge mainly arises from the ionization of
surface oxide and hydroxyl groups, which can undergo
protonation (producing a positive surface charge) or depro102
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of “dissolved iron” content against salinity lie on a curve
below the linear mixing line. This dissolved iron is present
mainly as iron (oxyhydr)oxides, which are stabilized in a
colloidal suspension by humic organic material until more
saline waters provide counterions that shield the surface
charge and allow aggregation (Chester 2003).
tonation (producing a negative surface charge). However
the surface charge of most nanoparticles (irrespective of
their composition) in natural systems is controlled by the
adsorption of films of NOM and, specifically, the negative
charges conveyed by the associated acidic functional
groups. This surface charge attracts a layer of positively
charged ions (the counterions) from the surrounding solution, and together, the negative and positive ions comprise
a double layer. At low ionic strength the influence of the
double layer extends over distances many times greater
than the nanoparticle diameter. This produces a strong
repulsion that prevents close approach and aggregation
and thus stabilizes the suspension. The layer of counterions
is compressed at high ionic strength and shrinks to within
the layer of adsorbed NOM. The distance of closest approach
is then small enough to allow bonding between the
adsorbed NOM on colliding nanoparticles (Sander et al.
2004) or between nanoparticles and sediment grains.
Large surface areas also affect solubility because smaller
particles have more surface energy per unit volume and
are, thus, less stable. Cornell and Schwertmann (2003)
estimated that size effects increase the solubilities of
goethite and hematite by more than an order of magnitude
once particle size decreases below ~10 nm. Aggregation
also affects reactivity and bioavailability of iron because
aggregates are microporous and access of reactants to iron
in the interior of an aggregate is difficult. Suspensions of
ferrihydrite in water reorganize in less than 100 days into
aggregates that are an order of magnitude less soluble
(Fig. 4). Sampling the same suspension at different times
by filtration and air-drying produces compacted aggregates
that are more than three orders of magnitude less soluble
(Fig. 4). A sample filtered from a freshly prepared suspension, air-dried and stored in water also produces poorly
soluble compact aggregates (Fig. 4). Aggregation caused by
just one drying event produces a significant decrease in
reactivity, which cannot be reversed by subsequent contact
with, or storage in, water. Aggregation, adsorption and
growth all decrease the reactivity and bioavailability of
ferrihydrite (Fig. 5).
IRON TRANSPORT THROUGH ESTUARIES
TO THE CONTINENTAL SHELF
The so-called “dissolved iron” (filtered to <0.45 µm) does
not follow a simple dilution trend with salinity during
passage through an estuarine salinity gradient, and plots
E lements
How do composite NOM–ferrihydrite nanoparticles behave
during transport? In turbulent, sediment-rich estuaries,
nanoparticles readily aggregate and collide with sediment
grains, becoming attached to them. High grain concentrations optimize deposition of the nanoparticles in the sediment and minimize transport of nanoparticles through
estuaries; conversely low grain concentrations and dilute
suspensions may facilitate transport. Typically 70–90% of
the nanoparticulate iron is removed in estuaries, and most
of the remaining nanoparticles that are attached to sediment grains will be deposited on the continental shelf.
Only a small proportion of nanoparticulate iron may
escape deposition and undergo long-range transport into
the open ocean. These far-travelled aggregated and/or
attached nanoparticles will age during transport and
become increasingly hematitic and/or goethitic and non-
Log Dissolution Rate
Constant (sec -1)
Variation in the half-life of the transformation of
f­errihydrite to mixtures of goethite and hematite as a
function of temperature and pH. Data from Schwertmann et al. (2004)
Figure 3
This pattern of iron colloid aggregation has been established for many of the world’s major rivers, but more detail
has emerged from the use of techniques that allow
“dissolved iron” to be separated into nanoparticles and
aqueous species (Dai and Martin 1995). The aqueous iron
(<3 nm in diameter) content then shows a linear, conservative trend with salinity, but the nanoparticulate fraction
(approximately 3 nm to 0.4 µm in diameter) produces
curved trends with increasing salinity that are similar to
those originally found for “dissolved iron” (to which it is
the main contributor). Stolpe and Hassellöv (2007) found
that the dissolved fraction (0.5 to 3 nm in diameter) in
river water has a strong UV absorbance typical of natural
organic matter and fluoresces under ultraviolet light like
fulvic acids, whereas the nanoparticulate component
(>3 nm in diameter) is associated with humic material, is
iron-rich and has a chemical composition and morphology
consistent with ferrihydrite nanoparticles.
DISSOLUTION RATE CONSTANT CHANGES WITH
TIME AND TREATMENT
-3
suspension
-4
Samples removed
from suspension by
filtering
-5
-6
Suspension filtered and stored in water
-7
0
25
50
75
100
125
Days
Variation over time in the rate constant for the dissolution of ferrihydrite in ascorbic acid with or without
storage and filtering (plus drying). The red curve shows rate
constant changes for a ferrihydrite suspension with time. The
arrows to the brown curve show the decrease in the rate constant
for samples removed from the suspension by filtration and air-dried,
before dissolution in ascorbic acid. The brown curve shows rate
constant changes for a sample removed from the suspension by
filtration shortly after preparation, then air-dried and stored in
water. Data from R aiswell et al. (2010)
103
Figure 4
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bioavailable (Fig. 5). By contrast, nanoparticles deposited
on the shelf can be rejuvenated by physical and biological
reworking (see below).
Do geomorphological factors control the scale of iron
removal in estuaries? It seems likely that high relief, steep
gradients and narrow shelves optimize delivery through
an estuary to the ocean. Another intriguing possibility is
that suspended-sediment concentrations could exceed the
local availability of NOM and, with less aggregation as a
result, delivery of nanoparticulate iron might be enhanced.
We can speculate that this effect might have been important in Precambrian estuaries, when terrestrial vegetation
was lacking.
IRON DELIVERY TO THE OPEN OCEAN
FROM SHELF SEDIMENTS:
THE IRON SHUTTLE
Many global models of the iron cycle acknowledge that
significant amounts of iron are supplied from the continental shelf to the open ocean (de Baar and de Jong 2001;
Poulton and Raiswell 2002; Aumont and Bopp 2006;
Raiswell et al. 2006; Moore and Braucher 2008: Lancelot
et al. 2009; Tagliabue et al. 2009). Thus, the role of
nanoparticles in such delivery demands consideration.
Much of this iron originates from the resuspension of shelf
sediments, which forms nanoparticles (see below) that can
be transported large distances in a process known as iron
shuttling. However, quantifying this source requires that
“dissolved iron” is recognized as comprising nanoparticles
and aqueous species, which behave differently. Remote
areas of the ocean with low clastic sedimentation rates
contain high proportions of a sediment component
enriched in goethite and/or hematite. Estimates of the rate
of accumulation of these minerals are low (<3 µg cm-2 y-1;
Chester 2003), and accumulation is usually overwhelmed
by the clastic sediment flux unless sedimentation rates are
extremely low (<1 cm/1000 y). However, significant iron
enrichments do occur in sediments deposited beneath a
euxinic (presence of free sulfide) water column, as in the
Black Sea (Canfield et al. 1996; Wijsman et al. 2001; Lyons
and Severmann 2006). In these settings iron is mobilized
from shelf sediments and transported to the deeper parts
of the basin, where it is precipitated as pyrite in the water
column. Euxinic iron enrichments can be recognized
throughout the Phanerozoic record (Poulton and Raiswell
2002), and thus source, transport and sink mechanisms of
iron are persistent through geological time.
Organic carbon–bearing shelf sediments produce Fe 2+
through the microbial reduction and dissolution of iron
(oxyhydr)oxides following sediment deposition. In most
Aging decreases reactivity and bioavailability
through physical and chemical transformations
Growth and
Aggregation
Ferrihydrite
Figure 5
Mineralogical
Transformation
Attachment
Clay
Goethite
Hematite
Aging–rejuvenation transformations of (oxyhydr)oxide
nanoparticulates in the biogeochemical cycle of iron
E lements
continental shelf sediments, iron reduction is closely
followed by sulfate reduction, and the iron dissolved in
the porewater is mainly precipitated as iron sulfides.
However, in circumstances where sulfate reduction is
suppressed (see Taylor and Macquaker 2011 this issue), sediment porewaters may contain high dissolved-iron concentrations, producing sediments that have the potential to
recycle dissolved porewater-iron back to the water column.
Recycling can occur by diffusion or by physical or ­biological
reworking of the sediment. Measurements of diffusive
fluxes of iron from shelf sediments are typically
<20 µg cm-2 y-1 (Elrod et al. 2004), but numerical models
suggest that diffusive fluxes up to an order of magnitude
higher are possible (Raiswell and Anderson 2005). Recycling
occurs in a particularly efficient form in the sediments of
the Amazon inner shelf. There, a classic study by Aller et
al. (1986) demonstrated how rapid rates of physical
reworking produce sediments in which sulfate reduction
is suppressed (and much of the pyrite formed is reoxidized);
as a consequence, high concentrations of dissolved iron
occur over depths of several metres. Physical reworking
mixes dissolved porewater iron into the water column
where nanoparticulate iron (oxyhydr)oxides are formed,
and these may be exported from the shelf. Transient physical and/or biological reworking events may transfer significant amounts of dissolved iron into overlying waters, where
oxidation produces nanoparticulate (oxyhydr)oxides. A
significant proportion of these nanoparticles will be scavenged by suspended sediments and redeposited, but
repeated cycles of reduction, resuspension and re-oxidation
allow some nanoparticles to escape the shelf (Lyons and
Severmann 2006). Repetitive, episodic shelf-sediment
reworking is the engine that drives the “iron shuttle”.
This recycling produces nanoparticulate iron, which is
initially labile and bioavailable, from more crystalline iron
oxides in a process that can be termed rejuvenation.
However, the concentration and reactivity of these
nanoparticulate (oxyhydr)oxides decrease rapidly through
settling, attachment, aggregation and aging until a quasistable suspension results that is sufficiently fine-grained
and dilute to permit long-range transport (Fig. 5). In the
Black Sea, the basin-wide continuity of millimetre-scale
clastic laminae testifies to the sustained efficiency of longrange transport. The time elapsed during transport will
favour transformation to hematite and/or goethite, both
of which are non-bioavailable but react with dissolved
hydrogen sulfide; transformation of the iron to pyrite thus
provides an efficient chemical trap for the iron in the deep
basin. In other words, transport times are long enough for
the aging transformations to produce nanominerals that
are stable during transport but still sufficiently reactive to
precipitate as sulfides.
IRON DELIVERY TO THE OPEN OCEAN
BY WIND-BLOWN DUST
Wind-blown mineral dust derived from the continents has
long been recognized as an important source of soluble,
bioavailable iron to the oceans (Jickells et al. 2005). The
rates at which dust is delivered by winds into the ocean
are relatively low compared to the rates of delivery by
rivers, but winds are able to reach parts of the ocean that
are inaccessible to rivers. In fact mineral dust is only one
of a wide range of particle types in the atmosphere with
very different physical and chemical properties (Buseck
and Adachi 2008), but mineral dust is the only significant
source in the pre-anthropogenic iron cycle.
Wind-blown dust particles generated by continental
erosion are removed from the atmosphere by dry deposition or washed out by rain. Very large particles are quickly
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lost, but particles less than 10 µm in diameter can be transported large distances; the average diameter of dust particles several hundred kilometres from source is 3 µm. The
number of particles increases by many orders of magnitude
as the grain size decreases, but most particle surface area
is associated with the 10–1000 nm size range (Buseck and
Adachi 2008).
Wind-blown mineral dust is derived mainly from arid and
semi-arid regions, mostly in North Africa and China, and
contains iron in a wide range of minerals that reflect the
geology and weathering processes in the source area
(Engelbrecht and Derbyshire 2010). However dusts from
widely separated geographical regions are actually rather
similar in bulk iron mineralogy, and the major ironcontaining minerals are clays (illite, smectite and chlorite)
accompanied by minor amounts of iron (oxyhydr)oxides
(Lafon et al. 2004; Engelbrecht and Derbyshire 2010). Both
clays and Fe (oxyhydr)oxides may be eroded directly from
bedrock and transported essentially unaltered, although,
even in arid regions, intermittent exposure to moisture
weathers iron(II) -bearing minerals to produce new
nanoparticulate iron (oxyhydr)oxides.
To most workers, dust-bound iron is the principal source
of bioavailable iron to the oceans. In this light, could these
nanoparticles represent a source of soluble, bioavailable
iron without further processing? Probably not, because
repeated cycles of wetting and drying occur even in arid
environments and these decrease the solubility of nanoparticles through aggregation and transformation (see Figure 4).
Furthermore, ferrihydrite, the most reactive iron (oxyhydr)
oxide, ages rapidly to form mixtures of goethite and hematite (Fig. 3), and the delivery of nanoparticulate ferrihydrite
in wind-blown dust would require erosion from the source
area and transport within very short timescales. The iron
(oxyhydr)oxide minerals in wind-blown dust must surely
require rejuvenation to become soluble and bioavailable.
Numerous researchers have adopted the view that the
minerals in wind-blown dust are altered or rejuvenated in
clouds by cycling with moisture-containing aerosols that
have a low pH from interactions with acidic gases (e.g.
HNO3 and H2SO4). Dust particles may be subject to periods
of evaporation (intensifying acid exposure) and drying,
interspersed with periods of exposure to moist, nearneutral pH conditions. Treatment of atmospheric dust with
dilute nitric or sulfuric acid seems an obvious approach
for quantifying the amount of iron that could be solubilized, but the method has yielded variable results. Does
this variability result from different dust mineralogies?
Some important clues can be found in the study by Shi et
al. (2009), who found nanoparticulate goethite in wetdeposited Saharan dust. The goethite contained trace
concentrations of Al, Cr, Si and Ca, indicating formation
by processing clay minerals in acidic clouds. Experimental
simulation of repetitive cloud processing also produced
similar goethite nanoparticles. It is unclear whether these
newly formed goethite nanoparticles are soluble in
seawater; that work remains to be done. However ferrihydrite, which is more soluble and bioavailable than goethite,
is found enveloped by, and preserved within, a carbonate
coating in mineral dust derived from paleo-lake deposits
(Shi et al. 2011). This carbonate coating dissolves during
cloud processing, releasing and rejuvenating ferrihydrite,
which is potentially bioavailable. These observations
demonstrate a need for detailed characterization of the
mineralogy, morphology and aging transformations of
potentially soluble iron-bearing phases, such as clays and
(oxyhydr)oxide nanoparticles, before and after acid dissolution. Dissolution of nanoparticulate iron minerals in
E lements
seawater must also discriminate between iron physically
released as nanoparticles and iron chemically dissolved as
aqueous species. Given that repetitive cloud cycling creates
an aging–rejuvenation cycle, the crucial questions focus
on the influence of rejuvenation on dust mineralogy, solubility and bioavailability.
IRON DELIVERY TO THE OPEN OCEAN
BY ICEBERGS
Since the advocates of iron supply by wind dust have
detailed issues of rejuvenation to consider, it is reasonable
to look elsewhere for sources that may not need rejuvenation; enter icebergs as the new player (Fig. 1). Powerful
agents of physical erosion, glaciers incorporate frozen
subglacial sediments derived from grinding bedrock. The
occurrence of freshly ground rock enables chemical weathering to be significant in proglacial and supraglacial environments, where air and meltwater are abundant and
iron(II)-bearing rock minerals oxidize to produce nanoparticulate iron (oxyhydr)oxides (F ig. 2). Weathering is
possible even in subglacial environments because ice melts
under pressure to produce water at 0 oC at the bedrock
interface; even in colder glaciers, like those in Antarctica
(where temperatures may be below 0 oC), water may still
be present in micro-environments. A critical feature of
iceberg delivery to the oceans is that freezing into ice
preserves the most reactive nanoparticles by limiting access
to water and slowing down the rate at which transformation to less reactive iron (oxyhydr)oxides occurs. Freezing
halts the progression of aging in icebergs, and rejuvenation
into a bioavailable form simply happens by melting.
Iron has long been recognized to limit plankton productivity in the Southern Ocean, and it has been widely
assumed that wind-blown dust is the main source of iron
in this region. However, recent evidence indicates that freefloating icebergs represent another, until recently overlooked, iron source. New global estimates of the flux of
glacial sediments to the Southern Ocean suggest that
icebergs contribute at least as much, or perhaps even more,
bioavailable iron than wind-blown dust (Raiswell et al.
2008). Potential bioavailable iron inputs from glacial ice
have been overlooked, primarily because the iron in glacial
sediments was assumed to be too inert for use by plankton.
However glacial sediments contain nanoparticulate ferrihydrite, the delivery rates of which to the Southern Ocean
are sufficiently large that the dissolution of only a small
proportion (aided by photochemical reactions or ingestion
by organisms) produces significant amounts of iron
bioavailable to plankton, thus enabling productivity
(Raiswell 2011).
Observations of floating icebergs reinforce the role of iron
in sediment released by melting. Smith et al. (2007)
observed that melting icebergs in the Weddell Sea are associated with hotspots of biological activity. In addition to
significant enrichments of glacial sediment grains around
two icebergs, they also found high concentrations of chlorophyll, krill and seabirds. Extrapolating their results to
the Weddell Sea as a whole, Smith et al. (2007) estimated
that similar-sized icebergs already influence 39% of the
surface ocean in this area. This extent of influence is
expected to increase, as atmospheric warming continues
the trend of ice sheet disintegration and iceberg production
throughout the Southern Ocean.
Iceberg delivery of nanoparticulate iron differs from other
parts of the iron cycle in that nanoparticulate ferrihydrite
can be preserved by freezing. This allows transport of iron
off the continental shelf to the open ocean, where melting
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delivers fresh nanoparticles that are little altered by aging
and retain their reactivity and bioavailability (Fig. 5).
CONCLUDING REMARKS
Nanoparticles play a crucial, but as yet poorly understood,
role in the transport and delivery of iron to the oceans.
Nanoparticulate (oxyhydr)oxides are the shape-shifters of
the iron biogeochemical cycle. Subtle changes in
morphology, grain size and mineralogy with age produce
shifts in stability and reactivity that can allow long-range
transport and allow rejuvenation to labile and bioavailable
forms. Transformations with age produce aggregation and
more stable nanominerals, and these changes allow trans-
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ACKNOWLEDGMENTS
I thank the Leverhulme Trust for the award of an Emeritus
Fellowship. Tim Lyons and Karen Hudson-Edwards are
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