Iceberg-hosted sediments and atmospheric dust transport potentially
bioavailable iron to the Arctic and Southern oceans as ferrihydrite.
Ferrihydrite is nanoparticulate and more soluble, as well as potentially more
bioavailable, than other iron (oxyhydr)oxide minerals (lepidocrocite,
goethite, and hematite). A suite of more than 50 iceberg-hosted sediments
contain a mean content of 0.076 wt % Fe as ferrihydrite, which produces
iceberg-hosted Fe fluxes ranging from 0.7 to 5.5 and 3.2 to 25 Gmoles yr
Iron (Fe) is an essential limiting nutrient for phytoplankton. Its supply
exerts a significant impact on marine productivity with important
implications for the carbon cycle and climate change (Mackenzie and
Andersson, 2013). Quantifying Fe sources to the oceans, especially those that
may be influenced by climate change, is therefore critical. Global Fe cycles
commonly recognise important supplies of dissolved Fe (dFe, < 0.2 or
0.45
The Southern Ocean (SO) is the largest high nutrient–low chlorophyll area where productivity is limited by the delivery of Fe (e.g. Moore et al., 2013). Recent modelling studies in the SO have focussed on understanding the factors which control spatial variations in productivity but reach different conclusions due to different representations of the Fe cycle and different assumptions regarding Fe solubility and scavenging. For example, Tagliabue et al. (2009) modelled measurements of dFe derived from atmospheric dust and shelf sediments. Atmospheric dust entering seawater was assumed to have a fractional solubility (soluble Fe expressed as a percentage of total Fe) of 0.5 % with continued slower dissolution during sinking occurring at a rate of 0.0002 % per day. Overall sediments were more important than atmospheric dust, although dust supplies dominated in some regions depending on the model assumptions used. Lancelot et al. (2009) modelled dFe supplies from atmospheric dust, iceberg melt, and shelf sediments. Sediments were the major source, iceberg melt was of lesser significance, and atmospheric dust (assumed to have fractional solubility of 2 %) had little influence. The models gave good agreement with patterns of phytoplankton growth but large uncertainties were acknowledged in the magnitude of these sources. Boyd et al. (2012) compared biological utilisation patterns using four mechanisms of Fe supply (vertical diffusivity in areas free of sea ice, iceberg melt, atmospheric dust, and shelf sediments) that were found to have substantial areal extent. Phytoplankton Fe utilisation was highest in regions supplied by Patagonian dust (using fractional solubilities varying from 1 to 10 %) and, to a lesser extent, shelf sediments. Wadley et al. (2014) compared the relative magnitudes and variations in supply of dFe from melting icebergs, shelf sediments and atmospheric dust. Sediments were again shown to be the most important source but considerable uncertainty was noted over the flux of Fe from iceberg-hosted sediments. Death et al. (2014) considered a range of sources that included iceberg-hosted sediments and atmospheric dust and found that modelled productivity was significantly enhanced in areas receiving iceberg-hosted sediments and subglacial melt compared to the productivity arising from atmospheric dust (assumed fractional solubility of 2 %). However, the contribution from iceberg-hosted sediments was based on a suite of only six samples (Raiswell et al., 2008) that contained 0.15 wt % Fe as ferrihydrite.
These studies show that SO models produce significant differences in the
relative magnitudes of the different Fe sources which complicate attempts to
isolate overlapping contributions. For example, Tagliabue et al. (2016) show
that global dust fluxes of dFe range from 1 to 30 Gmoles yr
Comparison of the FeA content of different size fractions of iceberg sediment.
Increases in iceberg-hosted sediment delivery are also likely in the Arctic Ocean (AO). A relatively high proportion of primary production occurs on the AO shelves (Pabi et al., 2008) where ice-free areas experience intense phytoplankton blooms due to favourable light and nutrient conditions. Nitrate appears to be the primary limiting nutrient otherwise Fe and/or light become limiting (Popova et al., 2010). Hawkings et al. (2014) have estimated Fe delivery by meltwaters from the Greenland Ice Sheet but no data are available for Fe delivery from iceberg-hosted sediments, although marine-terminating glaciers in the AO are likely to respond to climate change, as in the SO, by producing more icebergs (Bamber et al., 2012) and thus increasing sediment Fe delivery.
Modelling the polar Fe cycles and assessing the impact of climate change
requires an improved estimate of the Fe currently released from the
particulates present as iceberg-hosted sediments and atmospheric dust. There
is a substantial disagreement as to the strength of different sources and
reducing their uncertainty is important (Tagliabue et al., 2016). This
contribution presents new data for potentially bioavailable Fe from
iceberg-hosted sediments and atmospheric dust and also shows how ice
transport and storage may influence Fe delivery to the polar regions. The AO
and the SO differ in several important respects. The AO receives a
substantial riverine flux (
The Fe budgets for the AO use the area > 60
Over 60 sediment samples have been collected from icebergs and glaciers at 15 different Arctic and Antarctic locations (Table S1 in Supplement). Data have previously been reported for only 15 of these samples (from 7 localities; see Table S1) and thus the new samples provide a significant expansion of the existing data that now represent a substantial database for Fe in ice-hosted sediments. A set of 41 new iceberg samples were collected from floating icebergs with sediment-bearing layers present in dense, clear blue ice, indicating compressed glacier ice rather than accreted frozen seawater. An additional suite of nine new glacial ice samples was collected from sediment-rich bands in the main body of glaciers (i.e. land-based ice, not icebergs). These samples represent basal ice which has been in contact with the ice–rock interface.
Samples were collected with a clean ice axe, geological hammer, or chisel. The
outer layers of ice that might be contaminated were allowed to melt and drain
away before the remaining ice was transferred into a new polyethylene bag and
allowed to melt. Some loss of dissolved Fe by adsorption or the precipitation
of (oxyhydr)oxides during melting is possible (Conway et al., 2015), but the
presence of organic complexes (see later) may stabilise dissolved Fe. In any
event, melt dFe concentrations are too low (Hawkings et al., 2014) to produce
any significant increase in sediment Fe contents. Sediment samples were
collected as soon as melting was complete by filtration through a Whatman 542
(2.7
A suite of 15 atmospheric dust samples (Table S2) has been analysed by the
same extraction techniques used for the iceberg and glacial samples to ensure
data comparability. Seven new samples were collected during a cruise through
the eastern tropical Atlantic and into the Sea of Marmara (Baker et al.,
2006). Aerosol samples (
Each sample of air-dried sediment was treated for 24 h by an ascorbic
acid solution buffered at pH 7.5. Air drying at room temperature does not
achieve complete water loss but < 10 wt % more water is removed
by oven drying. The extractant was a solution of 0.17 M sodium citrate and
0.6 M sodium bicarbonate to which ascorbic acid was added to produce a
concentration of 0.057 M. This solution was deoxygenated (by bubbling with
nitrogen; see Reyes and Torrent, 1997). Approximately 10–40 mg of sample
were mixed with 10 mL of the ascorbate solution, shaken for 24 h at room
temperature and then filtered through a 0.45
Ferrihydrite only exists as a fine-grained and highly defective nanomaterial. The more disordered form (Hiemstra, 2013) contains two diffraction lines (two-line ferrihydrite, often called hydrous ferric oxide) and exists as smaller crystallites than the form with six diffraction lines (six-line ferrihydrite). The measurement of ferrihydrite is important because this mineral phase is directly or indirectly bioavailable (Wells et al., 1983; Rich and Morel, 1990; Kuma and Matsunga, 1995; Nodwell and Price, 2001). The delivery of fresh ferrihydrite to the open ocean thus has the potential to stimulate productivity in Fe-limited areas (Raiswell et al., 2008; Raiswell, 2011).
The residual sediment was treated for 2 h with a solution of 0.29 M sodium dithionite in 0.35 M acetic acid and 0.2 M sodium citrate, buffered at pH 4.8 (Raiswell et al., 1994). Following the ascorbic acid extraction step, the dithionite extracts the remaining (oxyhydr)oxide Fe (aged ferrihydrite, goethite, lepidocrocite, and hematite; Raiswell et al., 1994). Dithionite-soluble Fe is hereafter termed FeD and is reported as dry wt %. Both the FeA and FeD extractant solutions were analysed for Fe either by an atomic absorption spectrometer with an air–acetylene flame or by spectrophotometry using ferrozine (Stookey, 1970). Replicate analysis of a river sediment internal laboratory standard gave analytical precisions of 3 % for FeA and 10 % for FeD using this sequential extraction. Errors associated with sampling glacial sediments are examined below. Blank corrections were negligible.
Estimates of the solubility of Fe in atmospheric dust have utilised a variety
of extraction techniques which have produced estimates of fractional
solubility ranging from 0.2 to 80 % (Jickells and Spokes, 2001),
depending on time, pH, and the extractant (Baker and Croot, 2010). Recent
studies have attempted to recognise a soluble Fe fraction (extracted with
ultra-pure distilled water or seawater) and/or a labile or leachable fraction
(using a low pH chemical extraction). Distilled water leaches (Sedwick et
al., 2007; Berger et al., 2008; Conway et al., 2015) provide a consistent and
reproducible result but losses of Fe can occur due to precipitation of
Fe(OH)
Few of the extractions used to determine labile or leachable Fe have been
fully calibrated against different Fe minerals. Baker et al. (2006) extracted
Fe using ammonium acetate at pH 4.7, which dissolves negligible
concentrations of Fe (oxyhydr)oxides but significant concentrations of Fe as
carbonate (Poulton and Canfield, 2005). Chen and Siefert (2003) extracted Fe
with a 0.5 mM formate–acetate buffer at pH 4.5, which was stated to dissolve
Fe (oxyhydr)oxides (mineralogy unspecified). Berger et al. (2008) use a pH 2
leach with acetic acid and hydroxylamine hydrochloride followed by a 10 min
heating step at 90
We recognise two particulate fractions (Raiswell and Canfield, 2012) that
contain Fe (oxyhydr)oxide minerals (ferrihydrite, lepidocrocite, goethite,
and hematite), as described below.
FeA reported as wt % Fe that is extractable by ascorbic acid and
consists mainly of fresh ferrihydrite (Raiswell et al., 2011). FeD reported as wt % Fe that is extractable by dithionite.
Extraction of FeD following removal of FeA mainly dissolves residual, aged
ferrihydrite plus lepidocrocite, goethite, and hematite (Raiswell et al.,
1994).
An important issue concerns the bioavailability of FeA and FeD. Experimental work suggests that some part of sediment Fe can support plankton growth (Smith et al., 2007; Sugie et al., 2013). Sediment Fe present as fresh ferrihydrite (the most soluble Fe (oxyhydr)oxide) is directly or indirectly bioavailable (see above) and is extracted as FeA. FeA mainly comprises nanoparticulate ferrihydrite that probably encompasses a range in bioavailabilities (Shaked and Lis, 2012) due to variations in the extent of aggregation and associations with organic matter (which may partially or wholly envelope Fe (oxyhydr)oxide minerals; Raiswell and Canfield, 2012). We are concerned with Fe mineral reactivity at the point of delivery to seawater where ferrihydrite measured as FeA is more labile than FeD (the dithionite-soluble (oxyhydr)oxides which are relatively stable and poorly bioavailable). However, Fe present as FeD may become partially bioavailable after delivery to seawater (for example by dissolution and grazing; Barbeau et al., 1996; Shaked and Lis, 2012), but these complex interactions are outside the scope of the present contribution.
The collection of small samples from heterogeneous sediment with a range of
grain sizes (clay up to sand size and beyond) is difficult to do
reproducibly. Our approach has been to examine the variability both within
and between different size fractions. Our previous practice (Raiswell et al.,
2008) has been to remove only coarse material > 1 mm diameter,
which might severely affect our ability to analyse sub-samples of 10–40 mg
reproducibly. Table 1 compares the composition of different size fractions
produced by sieving iceberg sediment (from Wallenbergfjorden, Svalbard)
first to < 1 mm and then by taking two further replicate subsamples:
one sieved to < 250
A Student's
Reproducibility of the < 1 mm fraction of iceberg sediments.
No consistent pattern emerged from the data presented in Table 2. Samples
with low wt % FeA values (K2 and K3) tended to show the most variation.
However, a
Table 3 summarises the wt % FeA and FeD contents of the iceberg and glacier sediments and the mean and standard deviations of FeA and FeD. Wide variations mainly result from source area geology but there are no significant differences between the compositions of the Arctic and Antarctic icebergs (when the outlying data for Weddell Sea IRD4 are ignored; see Table S3) and hence we are justified in presenting all the iceberg samples as a single group (Table 3).
Composition of iceberg, glacial ice, and atmospheric dust samples (number of samples in brackets).
Low and high values each represent 1 logarithmic standard
deviation from the logarithmic mean, except for (FeA
The wt % FeA and FeD data approach a log-normal distribution and hence
logarithmic means are used to calculate the mean values and the logarithmic
standard deviations are used to derive the low and high values in Table 3.
This approach produces a logarithmic mean FeA content of 0.076 wt % for
the iceberg sediments and a range of 0.030 to 0.194 %. These new values
are based on more than 50 iceberg samples; thus this mean is more reliable
than the earlier mean value of 0.15 wt % FeA (based on only six samples
from Raiswell et al., 2008) and the large number of samples also permit an
estimate of the variation. A Student's
The wt % FeA and FeD contents of the iceberg sediments are significantly higher than the glacier-hosted sediments. The icebergs were not all derived from the land-based glaciers we sampled, and part of the differences in FeA and FeD may result from mineralogical/geochemical variations in the glacial bedrock. An alternative explanation for the high wt % FeA and FeD values is that iceberg sediments have undergone alteration during post-calving transport as temperature fluctuations induced melting/freezing cycles that caused dissolution and precipitation. The slightly acidic pH (5.5–6.0) of glacial ice melt (Meguro et al., 2004; Tranter and Jones, 2001) accompanied by the presence of extracellular polymeric substances (EPS) (Lannuzel et al., 2014; Lutz et al., 2014; Hassler et al., 2011, 2015) is able to accelerate the dissolution of Fe (oxyhydr)oxides.
Experimental work by Jeong et al. (2012) showed enhanced dissolution rates of
goethite and hematite trapped in ice compared to dissolution rates in water.
The degree of enhancement depended on the presence of organic ligands and the
surface area of the iron (oxyhydr)oxides; thus the high surface area of
ferrihydrite (compared to goethite and hematite) should produce large
enhancements. Jeong et al. (2012) found that dissolution was ligand-enhanced
and not reductive. However, Kim et al. (2010) have also observed that UV
radiation causes the photoreductive dissolution of Fe (oxyhydr)oxides
(goethite, hematite) encased in ice to ferrous Fe. Photoreductive dissolution
was significantly faster in ice than in aqueous solutions at pH 3.5 (and was
7–8 times faster than the dissolution rates observed by Jeong et al., 2012)
and was not influenced by the presence of electron donors. Acids are
concentrated by several orders of magnitude at the ice-grain boundary due to
freeze concentration effects, and the resulting low pH (
Figure 1 shows an idealised melting/freezing reaction scheme for any sediment
in which Fe (oxyhydr)oxides are initially absent and that only contains
silicate Fe. Dissolution is initiated in acidic snow melt where Fe is leached
slowly by silicate dissolution (Step 1). Subsequent freezing initially
concentrates the acids and accelerates dissolution until complete freezing
(or consumption of the acids) halts dissolution and induces the precipitation
and aggregation of Fe (oxyhydr)oxides as FeA and FeD (Step 2). The
transformation of ferrihydrite (FeA) to goethite/hematite (FeD) has a
half-life of several years at
Simplified reaction scheme for the behaviour of ice-hosted sediments during melting/freezing cycles.
The iceberg-hosted FeA flux (Table 4) is based on sediment encased in
icebergs and excludes sediments associated with seasonal ice (see later). The
solid ice discharge from Antarctica has been determined as
1321
Fluxes of FeA derived from iceberg-hosted sediment by melting.
The ice mass loss does not represent the mass of icebergs delivered into
coastal waters, as significant melting may occur for glaciers that calve into
long fjords (Hopwood et al., 2016). Such losses are relatively small in
Antarctica where most icebergs are calved from massive, marine-terminating
ice shelves and the remainder from outlet glaciers that calve directly into
the sea (Silva et al., 2006; Diemand, 2008). However, the characteristics of
Greenlandic glaciers vary. One endmember represents fast moving glaciers
where the ice mass loss is mostly by calving into the ocean, and the other
endmember represents glaciers entering long (up to 100 km) fjords where the
ice mass loss is mainly by melting in the fjord (Straneo and Cedenese, 2015;
Hopwood et al., 2016). For this latter endmember, fjord circulation patterns
largely prevent iceberg-hosted sediments from being delivered directly to
coastal waters (Hopwood et al., 2015, 2016). However, the five largest ice
mass losses from Greenlandic glaciers occur from the Jakobshavn, Køge Bugt,
Ikertivaq, Kangerdlugssuaq, and Helheim glaciers (together representing an ice
mass loss of
Raiswell et al. (2006) and Death et al. (2014) point out that the sediment
content of icebergs is poorly constrained but use a value of 0.5 g
litre
Mineralogy is a key factor in comparing particulate sources, and use of the
ascorbic acid extraction technique for the iceberg sediments and atmospheric
dust enables their ferrihydrite contents (as the most readily soluble and
potentially bioavailable Fe mineral) to be compared. The atmospheric dust
sample set is relatively small and mainly includes samples that are unlikely
to be delivered to the polar regions, although Patagonian dust is a possible
source to the SO (e.g. Schulz et al., 2012). Our Patagonian dust sample set
is small but a Student's
Our dust wt % FeA contents are low (mean 0.038 %; range 0.018 to
0.081 %) and are comparable to the wt % FeA contents of the sediments
present in glacial ice, but significantly lower (
Dust wt % FeD values (mean 0.87 %; range 0.43 to 1.76 %) are
significantly higher (
This FeA flux is based on dust transported through the atmosphere where there
is potential for processing (see above) and excludes soils. Localised areas
of the Ross Sea are subject to large dust inputs from local terrestrial sands
and silts but these appear to be only minor contributors to productivity
(Chewings et al., 2014; Winton et al., 2014). Here we proceed cautiously on
the basis that the FeA content of our atmospheric dust represents mineral
dust (with small to negligible contributions from combustion sources)
delivered to the polar regions. Dust deposition fluxes to the SO have been
variably estimated as 0.1 to 27 Tg yr
Atmospheric dust FeA fluxes.
Comparisons with other Fe flux estimates are difficult due to the different
methodologies used. Edwards and Sedwick (2001) measured Fe soluble at pH 2
from snow samples from East Antarctica, deriving a deposition flux of 0.3 to
2.0
However, the SO is more than 80 % covered by sea ice during winter
(declining to a minimum of
New dust Fe flux estimates to the AO (5.1 Tg yr
The new iceberg and atmospheric dust data presented here provide a valuable
insight into the iceberg and dust Fe sources to the polar oceans. They
substantiate the view that iceberg sediments have the potential to be a
significant source of bioavailable Fe as ferrihydrite (Table 6). We provide a
context for the iceberg sediment flux data by using the global shelf flux
value of Dale et al. (2015) to derive an order of magnitude estimate of shelf
sources (thought to be a dominant source in the SO, see earlier). The Arctic
and Antarctic shelf areas represent 11.5 and 7.3 % of the global shelf
area (< 200 m depth; Jahnke, 2010). Combining these area
percentages with the global shelf flux dFe value of 72 Gmol yr
Summary data for the main sources of iron to the Arctic and Southern oceans.
Sources of variation in Tables 4 and 5 relate both to the estimates of mass fluxes as well as the Fe analytical data but improved mass flux estimates may be difficult to achieve given their temporal and spatial variability. Table 6 and Fig. 2 summarise the flux ranges. At first sight there appear to be broad similarities in the magnitude of these Fe sources to the polar oceans but we list below three limitations to the current data set.
Ranges of FeA fluxes to the Arctic and Southern oceans. Dashed line shows rough estimates of shelf dFe based on Dale et al. (2015).
The iceberg FeA fluxes are based on data that are derived mainly from the Arctic. Iceberg melting losses during fjord transit are poorly known and, if underestimated here, might increase differences between the AO and the SO.
The atmospheric dust sample set is small and may not be representative of dust delivered to the polar regions.
FeA is present as ferrihydrite, which is potentially bioavailable to phytoplankton although acquisition rates are unknown and may vary substantially between organisms and with local environmental factors (Shaked and Lis, 2012).
Iceberg-derived FeA is a major source of Fe to both the AO and the SO that will likely increase as iceberg delivery increases with climate warming in the polar regions (Table 6 and Fig. 2). Our measurements of iceberg FeA contents are based on a substantial data set, although Antarctic data are still poorly represented. It is clear that iceberg FeA is a major source of potentially bioavailable Fe as ferrihydrite, unless the errors associated with the estimates of iceberg sediment contents exceed an order of magnitude (Raiswell et al., 2008; Death et al., 2014; Hawkings et al., 2014). Modelling the impact of iceberg FeA delivery on surface water dFe concentrations will be complex and will require kinetic models that incorporate scavenging, complexation, dissolution, and sinking (e.g. Tagliabue and Volker, 2011; Raiswell and Canfield, 2012). FeA attached to coarse material will settle out of surface waters quickly, but FeA present mainly as fine-grained material (or nanoparticles) may be held in suspension for long periods in the wake of icebergs. The basal and sidewall melt from icebergs creates complex patterns of upwelling and turbulence producing a persistent water column structure that may last for several weeks and whose influence extends for tens of kilometres and from the surface to 200–1500 m depth (Smith et al., 2013). Furthermore, giant icebergs (> 18 km in length) have a disproportionally large areal influence (compared to smaller bergs) which may last for longer than a month (Duprat et al., 2016). The proportion of the FeA found within this area of influence will clearly have a prolonged residence time that may be a key factor in its dispersion and utilisation away from iceberg trajectories into areas where other Fe supplies are limited.
Atmospheric dust fluxes are estimated to be a minor source of FeA to both the
AO and the SO, compared to iceberg-hosted sediment, although substantially
larger to the AO (Table 6). The dust database used here is small but appears
to be globally representative of mineral dust in that the range of wt %
FeD contents (2–5 %) overlaps that found in other studies (e.g. Lafon et
al., 2004, 2006). There are no comparable data for potential dust sources to
the polar regions although Patagonia atmospheric dust (Gaiero et al., 2007)
has wt % total Fe values ranging from 2.9 to 4.3 wt % (which overlaps
the 3.5 wt % total Fe value commonly used as a global average). Our
mineral dust flux estimates could be significantly increased by combustion
sources, estimates of which are very dependent on the flux model assumptions,
especially those for Fe solubility. Luo et al. (2008) show global maps of the
ratio (soluble Fe from combustion)/(total soluble Fe) which ranges from
10 to 40 % in the SO (> 60
The important features of the new FeA and FeD dust data presented here is
that they are closely tied to mineralogy, with FeA measuring the content of
fresh ferrihydrite, which is the most reactive and potentially bioavailable
Fe mineral. Thus these data enable direct comparison with iceberg sediment
FeA delivery. Furthermore we have estimated a potential role for ice
processing which appears to enhance FeA contents of dust delivered to sea
ice. Mean dust FeA concentrations of 0.095 wt % (if ice processed)
approximate to the mean concentration in icebergs (0.076 wt %), which
indicates that the former will dominate in areas where dust mass fluxes
exceed iceberg sediment delivery, assuming both types of particulates have
similar residence times in the ocean. Additional atmospheric dust samples
from the polar regions are needed to support these cautious conclusions and
to clarify the role of combustion sources. Wet deposition is thought to be
the main mechanism of deposition to the SO but fluxes are poorly known
(Mahowald et al., 2011). Very high soluble Fe contents (Heimburger et al.,
2013) have been found in wet deposition samples from the Kerguelen Islands
(at 48
All the data used in this manuscript are included in the Supplement.
Robert Raiswell thanks the School of Earth and Environment for Greenland fieldwork support and Michael D. Krom acknowledges support from the Leverhulme Foundation with grant RPG-406 and LGB from the UK Natural Environment Research Council grant number NE/J008745/1. Jon R. Hawkings, Martyn Tranter, and Jemma Wadham were funded by the NERC DELVE project (NERC grant NE/I008845/1 and the associated NERC PhD studentship). The authors are grateful to Lyndsay Hilton from the Thomas Hardye School, Dorset, who provided Antarctic glacial ice samples collected during participation on a Fuchs Foundation charity expedition. The Patagonian dust samples were supplied by S. Clerici. We are grateful to Mark Hopwood for drawing our attention to iceberg losses in Greenlandic fjords. Edited by: C. P. Slomp