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Global Iron Connections Between Desert Dust, Ocean Biogeochemistry, and Climate


Abstract and Figures

The environmental conditions of Earth, including the climate, are determined by physical, chemical, biological, and human interactions that transform and transport materials and energy. This is the "Earth system": a highly complex entity characterized by multiple nonlinear responses and thresholds, with linkages between disparate components. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans, affecting ocean biogeochemistry and hence having feedback effects on climate and dust production. Here we review the key components of this cycle, identifying critical uncertainties and priorities for future research.
Schematic view of global iron and dust connections. Highlighted are the four critical components (clockwise from top): the state of the land surface and dust availability, atmospheric aerosol loading, marine productivity, and some measure of climatic state (such as mean global surface temperature). The sign of the connections linking these varies; where the correlation is positive (for example, increased atmospheric aerosol loading Y increased marine productivity), the line is terminated with a solid arrowhead. Where the correlation is negative (for example, increased marine productivity Y lower CO 2 and a colder climate), the termination is an open circle. Connections with an uncertain sign are terminated with an open arrowhead. The mechanism by which the link acts (for example, the impact of a change in atmospheric CO is 2 via the radiative forcing of climate) is displayed in italics. Finally, the ‘‘water tap’’ symbols represent a secondary mechanism modulating the effect of a primary mechanism; for instance, a change in global precipitation strength and distribution will alter the efficiency with which entrained dust is transported to the open ocean. If a path of successive connections can be traced from any given component back to itself, a closed or feedback loop is formed. An even number (including zero) of negatively correlated connections counted around the loop gives a positive feedback, which will act to amplify a perturbation and tend to destabilize the system. Conversely, an odd number of negative correlations gives a negative feedback, dampening any perturbation and thus stabilizing the system. For instance, atmospheric aerosol loading Y marine productivity Y climatic state Y dust availability Y atmospheric aerosol loading contains two negative and two positive correlations and thus is positive overall. In contrast, marine productivity looping back onto itself contains a single negative correlation and thus represents a negative feedback.
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Global Iron Connections Between Desert
Dust, Ocean Biogeochemistry, and Climate
T. D. Jickells,
Z. S. An,
K. K. Andersen,
A. R. Baker,
G. Bergametti,
N. Brooks,
J. J. Cao,
P. W. Boyd,
R. A. Duce,
K. A. Hunter,
H. Kawahata,
N. Kubilay,
J. laRoche,
P. S. Liss,
N. Mahowald,
J. M. Prospero,
A. J. Ridgwell,
I. Tegen,
R. Torres
The environmental conditions of Earth, including the climate, are determined by
physical, chemical, biological, and human interactions that transform and transport
materials and energy. This is the ‘Earth system’’: a highly complex entity characterized
by multiple nonlinear responses and thresholds, with linkages between disparate
components. One important part of this system is the iron cycle, in which iron-
containing soil dust is transported from land through the atmosphere to the oceans,
affecting ocean biogeochemistry and hence having feedback effects on climate and dust
production. Here we review the key components of this cycle, identifying critical
uncertainties and priorities for future research.
ron is an essential nutrient for all organisms,
used in a variety of enzyme systems,
including those for photosynthesis, respira-
tion, and nitrogen fixation (1, 2). However, iron
is very insoluble under oxidizing conditions
above pH 4 (3). For marine phytoplankton,
separated from the iron-rich sediment of the
ocean floor by considerable water depths,
physiological iron requirements must be met
from within the water column. Iron supply is a
limiting factor on phytoplankton growth over
vast areas of the modern ocean, although this
may not have been so in the distant past, when
prokaryotes first evolved in a less oxic ocean (1).
Iron supply reaches the oceans mainly from
rivers as suspended sediment in a vast global
transport system (Table 1). However, fluvial and
glacial particulate iron is efficiently trapped in
near-coastal areas (4), except where rivers dis-
charge directly beyond the shelf. Hydrothermal
inputs are rapidly precipitated at depth in the
oceans. Hence, the dominant external input of
iron to the surface of the open ocean is aeolian
dust transport, mainly from the great deserts of
the world. Currently hyper-arid areas such as the
Sahara desert occupy 0.9 billion hectares and
drylands occupy 5.2 billion hectares, which is
one-third of global land area. These environ-
ments are particularly sensitive to global change
pressures (5, 6), and such changes could alter
ocean productivity and hence climate. There are
other possible contributors to atmospheric iron
supply, including volcanic, anthropogenic, and
extraterrestrial sources (7, 8), whose iron may be
more soluble than iron in soil aluminosilicates
(7), and these merit further study.
Dust produced in arid areas has important
and disparate effects throughout the Earth
system, as illustrated in Fig. 1 and discussed
below. These need to be incorporated into
climate models to correctly predict impacts of
global change pressures. We first consider
each component of the system before attempt-
ing a global synthesis.
Climate Effects on Dust/Iron Fluxes
Satellite imagery has greatly increased our
knowledge of large-scale dust source regions,
emphasizing the importance of localized
sources, which vary seasonally. There are sim-
ilar climatic and geomorphological controls on
many source regions (6), and dried-out lake
systems such as the Bodele Depression in
North Africa appear to be particularly impor-
tant. Dust production depends on the supply of
wind-erodible material, which ironically usu-
ally requires fluvial erosion, often from
adjacent highlands, followed by subsequent
drying out and the loss or absence of veg-
etative protection (6, 9–11). Dust production
arises from saltation or sandblasting, when
winds above a threshold velocity transport soil
grains horizontally, producing smaller par-
ticles, a small proportion of which get carried
up into the atmosphere for long-range trans-
port. These processes depend on rainfall,
wind, surface roughness, temperature, topog-
raphy, and vegetation cover, which are inter-
dependent factors linked to aridity and climate
in a highly nonlinear way. Wind tunnel studies
show dust production to be proportional to the
cube of wind speed (5).
Desert dust aerosol is dominated by par-
ticles of diameter 0.1 to 10 mm, with the mean
size being around 2 mm. Such aerosols have a
lifetime of hours to weeks, allowing long-
range transport over scales of thousands of
School of Environmental Sciences, University of East
Anglia, Norwich NR47TJ, UK.
State Key Lab of Loess
and Quaternary Geology, Institute of Earth Environ-
ment, Chinese Academy of Sciences, AS, 10 Fenghui
South Road, Post Office Box 17, China.
Niels Bohr
Institute, University of Copenhagen, Juliane Maries Vej
30, 2100 Copenhagen, Denmark.
Laboratoire Inter-
universitaire des Syste
mes Atmospherique, Universite
Paris 7 and Paris 12, UMR CNRS 7583, Paris, France.
National Institute of Water and Atmospheric Research
Centre for Chemical and Physical Oceanography, De-
partment of Chemistry;
Department of Chemistry, Post
Office Box 56; University of Otago, Dunedin, New
Departments of Oceanography and Atmo-
spheric Sciences, Texas A&M University, TAMU 3146,
College Station, TX 77843–3146, USA.
Survey of Japan, National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba-
higashi 1-1-1, Ibaraki 305–8567, Japan.
Institute of
Marine Sciences, Middle East Technical University,
Post Office Box 28, Erdemli-Mersin 33731, Turkey.
Leibniz-Institute fu¨r Meereswissenchaften IFM-GEO-
MAR, Marine Biogeochemistry, Geba
ude Westufer,
Du¨sternbrooker Weg 20, 24105 Kiel, Germany.
tional Center for Atmospheric Research. Post Office
Box 3000, Boulder, CO 80307, USA.
School of Marine and Atmospheric Sciences, University
of Miami, 4600 Rickenbacker Causeway, Miami, FL
33149–1089, USA.
Department of Earth and Ocean
Sciences, University of British Columbia, 6339 Stores
Road, Vancouver, British Columbia V6T 1Z4 Canada.
Max-Planck-Institute for Biogeochemistry. Post Of-
fice Box 10, 01 64 07701 Jena, Germany.
dad de Conception, Departamento de Oceanografia,
Casilla 160C, Chile.
*Present address: Institute for Tropospheric Research,
Permoserstrasse 15 04318, Leipzig, Germany.
Table 1. Global iron fluxes to the ocean (in Tg
of Fe year
). From Poulton and Raiswell (4),
with modified atmospheric inputs from Fig.
2. ‘Authigenic fluxes’ refer to releases from
deep-sea sediments during diagenesis. We
distinguish only separately dissolved and
particulate for fluvial inputs, because it is
clear that fluvial particulate iron, along with
iron from coastal erosion and glacial sedi-
ment sources, does not reach the oceans,
whereas authigenic, atmospheric, and hy-
drothermal iron all reach the oceans regard-
less of their phase.
Source Flux
Fluvial particulate total iron 625 to 962
Fluvial dissolved iron 1.5
Glacial sediments 34 to 211
Atmospheric 16
Coastal erosion 8
Hydrothermal 14
Authigenic 5 SCIENCE VOL 308 1 APRIL 2005
kilometers (5, 11) but producing strong gra-
dients of dust deposition and concentrations
that vary substantially on time scales of È1
day. Dust production, transport, and deposition
to the oceans again depend on climatic factors,
particularly atmospheric structure, which regu-
lates uplift, and wind speed and precipitation,
which influence removal. Much of the transport
of dust occurs at altitudes of several kilo-
meters, with subsequent removal by wet
deposition. Hence, satellite images of dust may
not reflect dust inputs to the oceans (7).
Dust removal occurs by wet and dry
deposition, processes whose efficiency varies
with aerosol particle size (7). Measurements at
a limited number of sites and modeling studies
suggest that 30 to 95% of total removal is by
wet deposition (7, 12). Wet deposition is
spatially variable, reflecting several climatical-
ly sensitive factors, including aerosol size
distribution, rainfall patterns, and transport
altitude. This results in considerable uncertain-
ties in estimating the contribution of wet
deposition to total deposition.
Dust fluxes can be estimated from direct
measurements and subsequent extrapolation
(13), models (11, 14, 15), and satellite obser-
vations (16). The various approaches all yield
similar dust deposition estimates of 1000 to
2000 Tg year
(1 Tg 0 10
g), varying
substantially from year to year. However,
models are usually tuned to match observations,
and hence the agreement is not truly an
independent validation.
Existing models of global dust transport
(14, 15) include only first-order physical rep-
resentations of the key dust production
processes, largely because of the lack of
suitable data sets of global surface character-
istics. Despite this, global models seem able to
simulate dust deposition fluxes reasonably
well. We estimate production at 1700 Tg
, with almost two-thirds from North
Africa and 26% of the dust reaching the
oceans (Fig. 2). Changes in the hydrological
cycle and/or vegetative cover affect global dust
production (10) as recorded over glacial/
interglacial cycles in loess, ice core, and
marine sediment records. Dust fluxes were 2
to 20 times higher during the last glaciation
(17–19) because of stronger winds, aridity,
changes in vegetation cover, lowered sea level,
and reduced precipitation. It has been sug-
gested that changing land use practices over
recent decades have altered dust fluxes by up
to 50% (14, 20), although recent work suggests
lower values (15). Although the global impor-
tance of land use change as a dust source is
currently uncertain, effects at a regional scale
are clear, such as around the Aral Sea and the
in the U.S. 1930s Dust Bowl storms (6). Dust
storm frequency over the Sahel appears to
have increased since 1950. This may be related
to multidecadal-scale climate variability or
landusechange(21). Dust transport over
China, the United States, and North Africa
has been related to large-scale climatic cycles
(22–26), and the variability in dust transport
can be influenced by climatic cycles such as El
Nin˜o–Southern Oscillation and North Atlantic
Oscillation (23, 24). Some climate models
suggest that enhanced greenhouse warming
could ‘green’ the Sahel and southern Sahara
(14, 27), drastically altering global dust pro-
duction. Different models of global dust flux
Fig. 1. Schematic view of global iron and dust connections. Highlighted are the four critical
components (clockwise from top): the state of the land surface and dust availability, atmospheric
aerosol loading, marine productivity, and some measure of climatic state (such as mean global
surface temperature). The sign of the connections linking these varies; where the correlation is
positive (for example, increased atmospheric aerosol loading Y increased marine productivity),
the line is terminated with a solid arrowhead. Where the correlation is negative (for example,
increased marine productivity Y lower CO
and a colder climate), the termination is an open
circle. Connections with an uncertain sign are terminated with an open arrowhead. The
mechanism by which the link acts (for example, the impact of a change in atmospheric CO
via the radiative forcing of climate) is displayed in italics. Finally, the ‘water tap’ symbols
represent a secondary mechanism modulating the effect of a primary mechanism; for instance, a
change in global precipitation strength and distribution will alter the efficiency with which
entrained dust is transported to the open ocean. If a path of successive connections can be traced
from any given component back to itself, a closed or feedback loop is formed. An even number
(including zero) of negatively correlated connections counted around the loop gives a positive
feedback, which will act to amplify a perturbation and tend to destabilize the system. Conversely,
an odd number of negative correlations gives a negative feedback, dampening any perturbation
and thus stabilizing the system. For instance, atmospheric aerosol loading Y marine productivity Y
climatic state Y dust availability Y atmospheric aerosol loading contains two negative and two
positive correlations and thus is positive overall. In contrast, marine productivity looping back onto
itself contains a single negative correlation and thus represents a negative feedback.
(14, 15) predict similar present-day dust fluxes
(Fig. 2), but their predictions for the next 100
years range from a modest increase (þ12%) to
a significant decrease (–60%), with different
regional deposition patterns. These differences
reflect variations in the relative importance of
land use changes and CO
fertilization, as well
as differences in climate predictions.
Dust has important but uncertain direct
impacts on climate and radiative budgets
(20, 28) and possibly rainfall patterns (29).
We note the importance of these physical
effects but focus here on the biogeochemical
effects. In a biogeochemical context, the key
flux to the oceans is not dust, but soluble or
bioavailable iron. Although the iron content of
soil dust (average, 3.5%) is variable globally,
the uncertainty introduced by this variability is
small compared with other uncertainties in the
iron cycle (7). Iron solubility from soil dust is
low [G1to2%(7)]. Higher solubilities of
aerosol iron have been reported (12). The
controls on aerosol iron solubility include
photochemistry (photoreduction of Fe III to
Fe II) and acidity, particularly during aerosol
cloud processing (7). Emissions of acid
precursors (SO
and NO
) have more than
doubled from the preanthropogenic state, and
emissions are expected to continue to
increase (20). Organic complexation may play
a role in regulating atmospheric iron solubility.
Consequently, emissions of organic matter
from natural sources (such as soil humic acids
and plant terpenes) and anthropogenic sources
(such as biomass burning and industrial/urban
emissions) may influence atmospheric iron
cycling (7). We know very little about the
organic chemistry of aerosols or of the active
microbial community identified in aerosols,
which may influence iron solubility (7, 30). All
these factors (acidity, organic complexation,
and photochemistry) will alter with global
change pressures.
Dust/Iron Impacts on the Ocean
The physicochemical environment of atmo-
spheric iron changes dramatically on entering
the oceans. At a seawater pH of 8, soluble ferric
iron rapidly reprecipitates, setting up a competi-
tion between adsorption to water column partic-
ulates, active biological uptake, and organic
complexation, which evolves over the surface
water residence time of dust [tens of days (31)].
Experimental measurements of the solubility of
aerosol iron have generally been conducted
over shorter time scales and hence may not
adequately predict the solubility of aerosol iron.
Measuring total and speciated iron concen-
trations in the ocean is difficult, but global data
are now emerging (32). Total dissolved iron
shows nutrient-like oceanic profiles, with low
surface water concentrations (0.03 to 1 nmol
where photochemically produced Fe II
may be significant) increasing to deep water
concentrations of 0.4 to 2 nmol liter
(32, 33).
Significant colloidal iron is present in the
water column and is potentially biogeochemi-
cally labile (32). The impact of atmospheric
deposition on surface water iron concentrations
has been demonstrated (34), as has recycling
from sediments and coastal regions (35, 36).
Within the oceans, dissolved iron is predomi-
nantly organically complexed, stabilizing it
against rapid scavenging (37), although its
residence time is still probably only decades
(32, 33). The source, biological function, and
structure of these organic iron-complexing
ligands are essentially unknown. Electrochemical
titrations suggest that some have similar binding
strength to that of true siderophores: strong iron-
specific ligands (3, 33, 37). Siderophores have
been found in marine bacteria and coastal sea-
water (38). Although many species may be able
to use siderophore-bound iron, siderophore syn-
thesis systems are not readily identifiable in the
genomes of important picophytoplankton species
such as Synecochoccus and Prochlorococcus
(39, 40), though they may be present in
Trichodesmium and Crocosphaera (two marine
diazotrophs) and in uncultured heterotrophic
bacterial genomes from the Sargasso Sea (41).
Iron limitation reflects deep-water Fe/N
concentration ratios that are inadequate to
meet phytoplankton iron requirements (36)
because of scavenging of iron regenerated
from sinking organic matter in the deep ocean
at faster rates than N. Thus, sustaining open
ocean phytoplankton primary production
requires an additional input of iron to that
produced from upwelling, which is usually
atmospheric. The relative importance of atmo-
spheric and upwelling sources varies throughout
the oceans (36). Iron limitation of phytoplank-
ton primary production in as much as 30% of
the oceans has now been suggested (1, 36, 42).
In some areas, such as the Southern Ocean,
this results in incomplete use of macronutrients
(N, P, and Si) and relatively low algal abun-
dance, hence the term ‘high-nutrient low-
chlorophyll’ (HNLC) regions. Recent studies
emphasize more complex interactions within the
ocean than simple iron limitation or sufficiency,
with evidence in some areas of simultaneous
limitation of primary production by iron, light,
Table 2. Effects of dust/iron (Fe) on ocean biogeochemistry. (In addition, there are dust effects on the climate system via albedo and the hydrological
cycle; see text.)
Interaction Mechanism Area* Reference
Primary productivity Reduction in Fe limitation allows more efficient use of macro-
nutrients and hence CO
HNLC and other Fe-limited areas (36, 42)
fixation Reduction in Fe limitation on nitrogen fixation increases primary
production and hence CO
Subtropical gyres (1, 43)
Changes in species
Species-selective relief of iron stress. Global (42)
Ballast effect Increases sinking rate of organic matter, reducing organic matter
regeneration within seasonal mixed layer; promotes CO
Probably only significant in areas
of high dust deposition
DMS Increased productivity leads to increased DMS emissions and
increased aerosol formation.
HNLC and other Fe-limited areas (54)
O and NO
Increased fluxes of organic matter to deep waters lower oxygen
concentrations and promote denitrification, release N
O, and
lower oceanic nitrate inventory.
Upwelling systems (53)
O and CH
Increased productivity leads to changes in euphotic zone methane
and N
O concentrations.
HNLC and other Fe-limited areas (54, 57)
S Increased fluxes of organic matter to deep waters lower oxygen
concentrations and promote sulfate reduction; sulphide produc-
tion lowers iron inventory.
Upwelling systems
Halocarbons and
alkyl nitrates
Biogenic gases linked to primary productivity. These are greenhouse
gases, linked to aerosol formation and to the ozone cycle.
As for DMS (54)
Isoprene and CO Biogenic trace gases linked to primary productivity. These gases
influence atmospheric oxidizing capacity.
As for DMS (54, 57)
*Most impacts have effects throughout the oceans, but where appropriate, we identify here areas that are most sensitive to changes in dust/iron flux.
macronutrients (42, 43), and trace nutrients
(suchasCoandZn)(2, 44). Furthermore
atmospheric inputs supply not only iron but also
other nutrients and carbonate, which may
influence ocean biogeochemistry (45, 46).
Luxury iron uptake has been demonstrated
for some phytoplankton, allowing them to better
adapt to episodic atmospheric supply (47). Iron
availability influences algal community struc-
ture as well as overall productivity. Open ocean
phytoplankton generally need less iron than
coastal species, which have evolved in a more
iron-rich environment, although iron-limited
coastal systems are known (36). A reduced
iron requirement can be achieved by reducing
cell size or minimizing the number of iron-
containing enzymes (39). The success of
Prochlorococus in HNLC areas depends on
both strategies. Relief of iron stress results in
the growth of phytoplankton taxa character-
ized by larger cells, particularly diatoms with
less dense opal skeletons (36). A similar
process may arise for coccolithophores as a
result of Fe/Zn co-limitation (44). Changes in
skeleton density should influence sinking
rates and hence carbon export to depth,
although this has not been seen in field
experiments (48). Changes in coccolithophore
abundance directly affect atmospheric partial
pressure of CO
), because their cal-
cification produces CO
(36, 42). In addition
to direct limitation of primary production in
the HNLC regions, iron may limit (or co-limit
with P) nitrogen fixation by photosynthetic
diazotrophs in tropical oceans, where stratifi-
cation creates high temperature and irradiance
and low nitrate concentrations in surface
waters, which favor this process (1, 43). The
best characterized photosynthetic diazotroph,
Trichodesmium, requires 5 to 10 times more
iron for growth based on nitrogen fixation, as
compared to ammonium (47).
The supply of dust to the oceans is very
important in maintaining oceanic primary pro-
duction and CO
uptake but is sensitive to
climate change, although the overall effect
will vary between ocean biogeochemical prov-
inces (Table 2). In HNLC regions, changes in
iron supply will directly affect primary pro-
duction and species composition, whereas in
subtropical/tropical oligotrophic regions, the
impact will be mainly via changes in nitrogen
fixation. The dust supply from the great North
African and Asian deserts directly affects the
tropical North Atlantic and temperate North
Pacific, respectively, and effects in the two
regions can be expected to be different. The
largest HNLC region, the Southern Ocean
(36), has the biggest potential to influence
atmospheric CO
. Here atmospheric dust
supply is low (Fig. 2), originating from small
dust sources in Argentina, Australia, and South
Africa (6). Changes in these small and little-
studied desert regions may have a dis-
proportionately large global impact.
Because the solubility of iron from dust is
low, it follows that there is a large flux of
particulate iron through the deep ocean,
particularly beneath the major dust plumes. If
some of this dust dissolves at depth, it will
increase abyssal dissolved iron concentrations
and, over the long term, productivity in
upwelling regions such as the Southern Ocean.
Deep-water dust dissolution will depend on
organic ligand concentrations and possibly
sediment redox (33).
Martin (49) proposed that increased dust
transport during the last glaciation reduced
iron limitation in HNLC regions, increasing
primary production and CO
uptake. The
complexity of iron biogeochemistry and
nutrient co-limitation means that higher gla-
cial dust loadings need not necessarily cause
increased productivity. Current models and
ice core data yield very different results,
predicting that glacial/interglacial changes in
dust fluxes will change atmospheric pCO
5 to 45 parts per million (ppm) as a con-
tribution to the total change of 80 to 100 ppm
(19, 50). Bopp et al.(50) reviewed much of
the existing marine sediment core data on
glacial/interglacial ocean productivity changes
and found no simple global pattern of change.
However, there are regional patterns (51)with
increases in productivity in the northwest
Pacific, South Atlantic, and Indian Oceans
north of the polar front, with decreases south
of it. South Pacific productivity appears to be
little changed. Some of these patterns can be
reproduced in ocean models (50).
Effect on Climate of Iron Inputs
to the Oceans
The oceans clearly exert a major influence on
climate via heat transport and related physical
processes (20). Large-scale reorganization of
oceanic circulation will also affect the trans-
port of iron, effects driven predominantly from
within the ocean. Climate change will induce a
variety of physicochemical changes in the
open ocean, particularly by changing stratifi-
cation and nutrient supply ratios (42), with
unpredictable effects. We acknowledge these
important issues but focus on the dust cycle,
considering now ways in which this can affect
the oceans and climate, beside the direct iron
limitation of primary production and nitrogen
fixation discussed above (Table 2).
Changes in iron fluxes can result in species
shifts and changes in phytoplankton size
distribution, changing oceanic CO
uptake by
altering the efficiency of organic carbon export
to deep water. Dust may also play a direct role
in regulating export via the ballast effect (52).
In most areas, dust is a minor ballast compo-
nent compared to opal and calcite, but their
production is also influenced by dust/iron
supply. Changes in ocean productivity and
organic carbon export to deep water will
influence subsurface oxygen levels and thereby
denitrification in oxygen minima zones, oce-
anic nitrate inventories and productivity, and
nitrous oxide emissions (53). Changes in
sediment H
S in such areas could affect deep-
ocean iron concentrations and productivity.
Up to eightfold changes in dimethyl sulfide
(DMS) concentrations are seen in iron addition
experiments (54). DMS oxidizes in the atmo-
sphere to form acidic sulfate aerosol, a highly
effective scatterer of solar radiation. Modeling
suggests that a twofold global rise in DMS
fluxes produces a global temperature decrease
of 1-C, proving a climate feedback and linking
Fig. 2. Dust fluxes to the world oceans based on a composite of three published modeling studies
that match satellite optical depth, in situ concentration, and deposition observations (11, 14, 15).
The models have been extensively compared to observations, and although individual models
show strengths and weaknesses, this composite appears to match observations well. Total
atmospheric dust inputs to the oceans 0 450 Tg year
. Percentage inputs to ocean basins based
on this figure are as follows: North Atlantic, 43%; South Atlantic, 4%; North Pacific, 15%; South
Pacific, 6%; Indian, 25%; and Southern Ocean, 6%.
the C, Fe, and S cycles (55). DMS is only one
of a group of trace gases that can influence
climate and whose emissions are sensitive to
iron concentrations (54, 56, 57). These include
gases that directly affect greenhouse gas
forcing (nitrous oxide and methane), ozone
cycling (halocarbons and alkylnitrates), and
atmospheric oxidizing capacity (isoprene and
carbon monoxide). Impacts on ozone are
important in radiative forcing (20) and via
ultraviolet-B impacts on phytoplankton com-
munity structure (42).
Global Iron Connections
Our analysis demonstrates the complexity of the
global iron cycle (Fig. 1). Low iron solubility
leads to limitation of marine productivity, with
potentially large-scale feedbacks within the
global climate system. These could act to either
amplify future global climate change via a
positive (destabilizing) feedback or diminish it
via a negative (stabilizing) feedback. There are
considerable uncertainties in our understanding
of these interactions, requiring research that
integrates across the whole Earth system. We
suggest the following research priorities: (i) dust
deposition processes, (ii) aerosol iron bioavail-
ability, and (iii) the impact of iron on marine
nitrogen fixation and trace gas emissions. These
should lead to improvements in global models,
allowing realistic predictive capability that can
be tested against improved results from the
paleo record of the biogeochemical response to
changing dust fluxes.
There are discussions about changing
terrestrial land uses to create carbon sinks to
help mitigate global change. Such changes
may reduce dust fluxes to the ocean and
thereby reduce primary productivity, offsetting
gains in terrestrial carbon storage (58). There is
also discussion about fertilizing the ocean with
iron to increase CO
uptake (59). Our analysis
demonstrates that such a scheme could pro-
duce many changes in marine biogeochemical
systems. Clearly, we need a comprehensive
understanding of the current and future dust/
iron cycle before we can contemplate such
engineering of the Earth system.
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... Mineral dust, an important component of global tropospheric aerosol, has been widely studied due to its significant impacts on air quality (Giannadaki et al., 2014;Lachatre et al., 2020;Li et al., 2018), human health (de Longueville et al., 2013;Giannadaki et al., 2014), climate change (Huang et al., 2014;Kok et al., 2018), and marine ecosystem (Bishop et al., 2002;Jickells et al., 2005). Mineral dust is blown into the atmosphere mainly from arid and semi-arid regions, and the major dust source areas in the world include North African, Arabian Peninsula, and East Asia (Choobari et al., 2014;Middleton and Kang, 2017). ...
... Through deposition, dust could exhibit impacts on marine primary productivity and global climate change, as it can promote the growth of marine phytoplankton by introducing nutrients, i.e., nitrogen, phosphorus, and iron (Jickells et al., 2005;Mahowald et al., 2017;Shi et al., 2012). Therefore, the amount of dust deposition, especially the amount of nutrients in Fig. 4. Multi-sensor AAI (a-c) and meteorological field (500 hPa geopotential height, 850 hPa wind vector, and sea level pressure, a′-c′) during Dust-2. ...
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In March 2021, China experienced three dust events (Dust-1, 2, 3), especially the first of which was reported as the strongest one in recent ten years. Their environmental impacts have received great attention, demanding comprehensive study to assess such impacts quantitatively. Multiple advanced measurement methods, including satellite, ground-based lidar, online aerosol speciation instrument, and biogeochemical Argo float, were applied to examine and compare the transport paths, optical and chemical properties, and impacts of these three dust events on urban air quality and marine ecosystem. The results showed that Dust-1 exhibited the largest impacts on urban area, increasing PM10 concentration in Beijing, Shuozhou, and Shijiazhuang up to 7525, 3819, and 2992 μg m⁻³, respectively. However, due to fast movement of the Mongolian low-pressure cyclone, the duration of northwest wind over the land was quite short (e.g., only 10 h in Beijing), which prevented the transport of dust plume to the northwestern Pacific, resulting in limited impact on the ocean. Dust-2 and Dust-3, though weaker in intensity, were transported directly to the sea, and led to a substantial increase in chlorophyll-a concentration (up to near 3 times) in the northwestern Pacific, comparing to climatological value. This indicates that the impacts of dust events on ocean was not necessarily and positively correlated to their impacts on land. Based on the analyses of land-ocean-space integrated observational data and synoptic systems, this study examined how marine ecosystem responded to three significant Asian dust events in spring 2021 and quantitatively assessed the overall impacts of mega dust storms both on land and ocean, which could also provide an interdisciplinary research methodology for future research on strong aerosol emission events such as wildfire and volcanic eruption.
... Рис. 5 -Распределение растворенного кремния в поверхностном слое вод в районе Канарского апвеллинга на разрезах: первом (а); втором (б); третьем (в); четвертом (г) В области между 10° и 5° с. ш. на всех разрезах наблюдалось повышение растворенного кремния, обусловленное выносом аэрозолей из пустынных и полупустынных районов Африки северо-восточными пассатами (Клювиткин и др., 2004;Jickells et al., 2005). На первом, третьем и четвертом разрезах в этом районе величина растворенного кремния увеличивается с минимальных значений до 1-1.6 µМ. ...
The work is devoted to the modern distribution of hydrochemical parameters in the surface layer of waters of tropical latitudes of the Atlantic Ocean. The materials were collected based on the results of two expeditions onboard the R/V “Academik Mstislav Keldysh” – AMK 79 (2019–2020) and AMK 87 (2021–2022). Four longitudinal transatlantic sections were considered. The hydrochemical characteristics of the Canary upwelling areas, the zones of aerosol transport from the desert regions of Africa, the areas of influence of the river discharge of the Rio de la Plata are described in detail.
... However, a recent study shows that dust-rainfall interactions are still of high uncertainty and not clearly understood (Alpert et al., 2021). Over the continents, dust storms can degrade soil fertility, destroy crop fields, pollute water (Thiagarajan and Aeolus Lee, 2004) and cause poor visibility (Furman, 2003), while once dust settles into the ocean, phytoplankton can bloom because of the presence of iron in the Aeolian dust (Jickells et al., 2005). ...
High dust concentrations in the Eastern Mediterranean - Middle East (EMME) region have serious effects on air quality, human health and climate. This study used long-term aerosol datasets during the main dusty season (April–July: AMJJ) over the EMME from 2000 to 2020, based on Moderate-Resolution Imaging Spectroradiometer (MODIS)/Terra-C6.1, Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2), and Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) retrievals and analyzed the spatio-temporal variations and trends of dust, as well as the influencing factors. The dust aerosol optical depth (DAOD) experienced a significant upward trend during 2000–2010, followed by a significant decrease during 2010–2017. After 2017 and till 2020, the DAOD presented rather a stable trend. Aerosol Robotic Network (AERONET) data in the EMME region display trends compatible to those of both MERRA-2 and CAMSRA DAOD. The DAOD trends were linked to changes in regional meteorological parameters in the EMME. A significant downward trend in AMJJ sea-level pressure (SLP) during the early period (2000−2010) induced hot and dry winds from desert regions towards the EMME, which reduced relative humidity (RH) and raised temperature, thus favored soil drying and dust outbreaks through enhancing evaporation. In contrast, a significant increase in winter SLP during the late period (2010–2017), accompanying an increase in North Atlantic Oscillation index, induced cold, wet winds from northwest regions, which increased RH and lowered temperature, thus reducing dust loading in EMME. Positive anomalies in winter soil moisture persisted in the following AMJJ, and consequently suppressed dust activity. DAOD variability over the dust-prone regions was linked to various meteorological parameters via a multiple linear regression (MLR) model. The results show that climatic variability strongly affects the dust trends and contribute to better understanding of meteorological – dust dynamics in the EMME region.
... Atmospheric nitrogen deposition is widely regarded as one of the important sources of nutrients in the marine system (Duce et al., 2008;Galloway et al., 2008). Aerosol has direct or indirect effects on climate, environment, and human health (Jickells et al., 2005;Pöschl, 2005;Das and Jayaraman, 2012;Xu and Penner, 2012;Seinfeld and Pandis, 2016). The rapid population growth has been egregiously negatively impacting on the coastal landscape and atmospheric environment in many parts of the world (Arteaga et al., 2019;Aswini and Hegde, 2021;Mohamed et al., 2021;Refulio-Coronado et al., 2021;Wang et al., 2021). ...
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The coastal atmospheric environment is one of the most complex environments on earth. It is shaped by terrestrial, marine, and atmospheric processes and acts as an external nutrient source for coastal waters. At present, there are few observations of inorganic nitrogen isotopes of China coastal aerosols, let alone the Yellow Sea. In this study, a weekly collection of total suspended particulate aerosols was conducted on the Qianliyan Island in 2018 for the measurements of inorganic nitrogen species (NO 3 ⁻ and NH 4 ⁺ ) and their isotopic ratios (δ ¹⁵ N-NO 3 ⁻ , δ ¹⁸ O-NO 3 ⁻ , and δ ¹⁵ N-NH 4 ⁺ ). At the Qianliyan Island, the average NO 3 ⁻ and NH 4 ⁺ concentrations were 2.49 ± 2.12 and 3.33 ± 2.68 μg·m ⁻³ , respectively; the average δ ¹⁵ N-NO 3 ⁻ , δ ¹⁸ O-NO 3 ⁻ , and δ ¹⁵ N-NH 4 ⁺ were 2.4‰ ± 5.7‰, 78.7‰ ± 8.0‰, and −2.6‰ ± 6.3‰, respectively. The major nitrate formation pathways were •OH oxidation and N 2 O 5 hydrolysis paths, and the dominant sources of inorganic nitrogen aerosols were coal combustion (29% ± 7%), marine (19% ± 15%), and fertilizer (16% ± 13%). Aerosol δ ¹⁵ N-NO 3 ⁻ and δ ¹⁸ O-NO 3 ⁻ were obviously higher in winter and lower in summer; conversely, aerosol δ ¹⁵ N-NH 4 ⁺ was slightly higher in summer and slightly lower in winter. The difference in nitrogen sources was considered to be the best explanation for the aerosol δ ¹⁵ N-NO 3 ⁻ and δ ¹⁵ N-NH 4 ⁺ differences between summer and winter, of which coal combustion contributed the most. The seasonal difference in nitrate formation paths was considered to be the best explanation for the difference of Qianliyan aerosol nitrate δ ¹⁸ O-NO 3 ⁻ between summer and winter. Aerosol inorganic nitrogen deposition flux was estimated to be 3.4 nmol N·m ⁻² ·s ⁻¹ , which induced less than 1% to marine primary production, and aerosol inorganic nitrogen deposition, compared with N 2 fixation, contributed some 80% of δ ¹⁵ N-NO 3 ⁻ depression of the summer Yellow Sea thermocline.
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The Taklamakan Desert and the Gobi Desert in East Asia constitute the second-largest sources of dust in the world. In particular, dust originating from the Gobi Desert is more susceptible to long-range transport, with consequent impacts in downwind Asian countries and the Northwest Pacific region. Two intensive dust events (the 3·15 dust event and the 3·28 dust event) were experienced in North China in March 2021. The 3·15 dust process was rated as the most intensive dust process in China in the past 10 years. In this study, by using a combination of spaceborne remote sensing datasets from geostationary and polar-orbiting satellites, ground-based columnar observations of aerosol optical parameters, meteorological reanalysis data, and backward trajectory simulations of air masses, the transport pathways and the three-dimensional structure characteristics of dust aerosols during the transport of the two dust events in March 2021 were cross-validated. The results of the study indicated that the two dust events were induced by the Mongolian cyclone. Due to the different configurations of the ground meteorological system conditions, a backflow process occurred during the 3·15 dust event transmission process. After passing over North China and the Bohai Sea, the direction of transport of the dust plume was reversed. The wind deflected from northwest to northeast, and the dust reached the eastern coastal areas of China and was finally deposited on land. The 3·28 dust event exhibited aerosol stratification in the transport path, the higher pure dust layer reached up to 9 km height, and the lower layer underwent aerosol mixing and became a polluted dust aerosol. This study implies that the investigation of dust aerosol transport and the deposition processes, the impact on the ocean, and the impact of marine aerosols on land also needs to be taken into consideration; the integration of advanced satellites and ground-based remote sensing data, the meteorological reanalysis data and the backward trajectories simulation, which complemented and verified each other, can enhance the ability to delineate the transport pathways and the three-dimensional structural characteristics of dust events.
Flow velocity, potentially linked to dry deposition velocity and wind speed parameters, show non-marginal significance in the heterogeneous oxidation of SO2 on α-Fe2O3 particles. Our uptake measurement results produce lab-based evidence that an increase in flow velocity can potentially strengthen SO2 uptake capability over α-Fe2O3 by nearly one order of magnitude within the range of considered reaction conditions. Additionally, specimen analysis of surface S-containing products indicates a distinct SO2 oxidation regime, shifting from the heterogeneous reaction mediated by active sites under low flow velocity and low RH to an aqueous-like multiphase-dominated pathway under high flow velocity and high RH. An attempt was further made to link this experimental parameter to dry deposition velocity regulated by wind force in the atmosphere. This work highlights the significance of flow velocity in triggering fast sulfate production concerning dust chemistry.
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Paleo-loess and silty eolian-marine strata are well recognized across the Carboniferous-Permian of equatorial Pangaea. Eolian-transported dust and loess appear in the late Devonian in the west, are common by the Late Carboniferous, and predominate across equatorial Pangaea by the Permian. The thickest loess deposits in Earth history –>1000 m− date from this time, and archive unusually dusty equatorial conditions, especially compared to the dearth of equatorial dust in the Cenozoic. Loess archives a confluence of silt generation, eolian emission and transport, and ultimate accumulation in dust traps that included ephemerally wet surfaces and epeiric seas. Orogenic belts sourced the silt, and mountain glaciation may have exacerbated voluminous silt production, but remains controversial. In western Pangaea, large rivers transported silt westward, and floodplain deflation supplied silt for loess and dust. Expansion of dust deposition in Late Pennsylvanian time records aridification that progressed across Pangaea, from west to east. Contemporaneous volcanism may have created acidic atmospheric conditions to enhance nutrient reactivity of dusts, affecting Earth’s carbon cycle. The late Paleozoic was Earth’s largest and most long-lived dust bowl, and this dust represents both an archive and agent of climate and climate change. Supplementary material at
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The availability of iron (Fe) and phosphorus (P) has been shown to be a key factor regulating rates of nitrogen fixation in the western subtropical Pacific. However, the relative importance of Fe and P at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by (i) carrying out measurements of finer-scale spatial variabilities in N2 fixation rates and diazotroph nifH gene abundances throughout these regions and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the nifH gene abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates and nifH gene abundances were lower in the NSCS than around the Luzon Strait and the western North Pacific. The nutrient addition bioassay experiments demonstrated that N2 fixation rates in the central NSCS were co-limited by Fe and P, whereas at the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in N2 fixation rates, and in six out of eight experiments the largest responses in nifH gene abundances were dominated by either Trichodesmium or UCYN-B (unicellular diazotrophic cyanobacteria group B). In general, nutrient addition had a relatively restricted impact on the composition of the six phylotypes that we surveyed apart from on UCYN-B. This unicellular cyanobacterium group showed increased contribution to the total nifH gene abundance following P addition at sites where N2 fixation rates were P-limited. Our study provides comprehensive evidence of nutrient controls on N2 fixation biogeography in the margin of the western North Pacific. Future research that more accurately constrains nutrient supply rates to this region would be beneficial for resolving what controls diazotroph community structure.
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Here, we used a modified single-particle soot photometer (SP2) coupled with a concentric pneumatic nebulizer to measure the size-resolved number and mass concentrations of light-absorbing iron oxide aerosols (FeOx) in liquid water (CNFeOx and CMFeOx, respectively). The SP2 could selectively detect individual FeOx particles in mixed wüstite–fullerene soot laboratory samples and melted Arctic snow samples. The nebulizer efficiency for FeOx particles was about 50% within the 70–650 nm diameter range, as derived from the ratio of the volume of ammonium sulfate before and after extraction by the nebulizer and the size-resolved transmission efficiency in the nebulizer–SP2 sampling line. Uncertainty from the boundary lines empirically drawn to discriminate the scatterplots of FeOx and black carbon in the mixed wüstite–fullerene soot suspensions and snow samples was approximately 3.0% and 10%, respectively. Overall uncertainty in total CNFeOx and CMFeOx (220–1400 nm) was approximately 19% and 18%, respectively. After storage at 4 °C for 16 months, the FeOx particle size distributions in melted Arctic snow had remained stable, and CNFeOx and CMFeOx had changed by less than 19% and 1.0%, on average, respectively. Most of the FeOx on dust particles measured by this system was estimated to be in the diameter range smaller than 1000 nm, considering the nebulizer efficiency for dust particles. The high accuracy of the CNFeOx and CMFeOx measurements will help to improve our quantitative understanding of the wet deposition of FeOx and provide more accurate estimates of the effects of FeOx on snow surface albedo.
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The production, transport, and deposition of mineral dust exert major influences on climate change and Earth’s biogeochemical cycles. Furthermore, their imprint, as recorded in pelagic sediments, provides an avenue for determining past changes in terrestrial aridity and atmospheric circulation patterns in response to global climate change. Here, by examining geochemical and magnetic data obtained from a ferromanganese crust in the western Pacific Ocean, we investigate the eolian dust source-region conditions and dust transport mechanisms from the Asian interior to the Pacific Ocean since the Pliocene. We identify a gradual provenance change in the dust source regions, from a dominant Gobi Desert source during the early Pliocene to a mixed Gobi-Taklimakan Desert source during the late Pliocene and Pleistocene, alongside increasing chemical weathering in those source areas. Climate model simulations suggest that these changes were related to an equatorward shift of the westerly jet and humidification of Central Asia during the gradual transition from a warm Pliocene climate to the cool Pleistocene.
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Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. -from Authors
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[1] Biological productivity in a number of ocean regions appears to be at least partly limited by the availability of iron. Any reduction in the present-day aeolian iron supply to the open ocean is therefore likely to result in further limitation of productivity. The stabilization of soils for the purpose of carbon sequestration could give rise to such an effect. With the aid of a global carbon cycle model, we show that the effectiveness of carbon removal from the atmosphere by sequestration on land will be diminished as a result of a reduction of up to 9% in the rate of anthropogenic CO2 uptake by the ocean. This interconnectedness, both within the `natural' system and in relation to human activities, highlights the importance of analyzing global change within an integrated ‘Earth system’ framework.
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Desert dust deposition to the ocean may be a significant source of biogeochemically important elements, specifically iron. The bioavailability of iron in the oceans requires it to be in a soluble form, and because atmospheric iron in desert dust is typically insoluble, understanding the atmospheric processes that convert insoluble iron to more soluble forms is essential. Understanding these relationships is especially important in remote ocean regions where iron may be the limiting nutrient. Observations of soluble iron from 2001 cruise-based aerosol measurements over the Atlantic and Pacific Oceans ranged from 0 to 45% (mean of 4 +/- 9%) in the fine mode (
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IRON is essential to the growth of organisms, and iron derived from the atmosphere may be the limiting nutrient for primary productivity in some oceanic regions1-6. Aeolian mineral dust is the chief source of marine iron in many areas1-3,5,7, but there is little information on the chemical form of the iron in this dust. Here we report that Fe(n) contributed 56+/-32% of the total iron in marine aerosol samples collected over the central North Pacific and 49 + 15% at Barbados. We suggest that the key reaction that produces Fe(n), and hence increases the solubility of marine aerosol iron in sea water, is [Fe(in)(OH)(H2O)5]2+ + H2O^ [Fe(n)(H2O)6]2+ + OH- (refs 8-10). The presence of Fe(n) in remote marine aerosols suggests that the OH radical has been produced in these heterogeneous reactions. From consideration of both the marine biological production of dimethylsulphide and the subsequent oxidation of reduced forms of sulphur in the atmosphere, we propose that the iron and sulphur cycles in both the atmosphere and the ocean may be closely coupled.
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Simulations of Asian dust emissions over the past 43 years are presented based on a size-dependent soil dust emission and transport model (NARCM) along with supporting data from a network of surface stations. The deserts in Mongolia and in western and northern China (mainly the Taklimakan and Badain Juran, respectively) contribute ∼70% of the total dust emissions; non-Chinese sources account for ∼40% of this. Several areas, especially the Onqin Daga sandy land, Horqin sandy land, and Mu Us Desert, have increased in dust emissions over the past 20 years, but efforts to reduce desertification in these areas may have little effect on Asian dust emission amount because these are not key sources. The model simulations indicate that meteorology and climate have had a greater influence on the Asian dust emissions and associated Asian dust storm occurrences than desertification.
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The International Global Atmospheric Chemistry Program (IGAC) has conducted a series of Aerosol Characterization Experiments (ACE) that integrate in situ measurements, satellite observations, and models to reduce the uncertainty in calculations of the climate forcing due to aerosol particles. ACE-Asia, the fourth in this series of experiments, consisted of two focused components: (1) An intensive field study that sought to quantify the spatial and vertical distribution of aerosol concentrations and properties, the processes controlling their formation, evolution, and fate, and the column-integrated radiative effect of the aerosol (late March through May 2001). (2) A longer-term network of ground stations that used in situ and column-integrated measurements to quantify the chemical, physical, and optical properties of aerosols in the ACE-Asia study area and to assess their spatial and temporal (seasonal and interannual) variability (2000-2003). The approach of the ACE-Asia science team was to make simultaneous measurements of aerosol chemical, physical, and optical properties and their radiative impacts in a variety of air masses, often coordinated with satellite overpasses. Three aircraft, two research ships, a network of lidars, and many surface sites gathered data on Asian aerosols. Chemical transport models (CTMs) were integrated into the program from the start, being used in a forecast mode during the intensive observation period to identify promising areas for airborne and ship observations and then later as tools for integrating observations. The testing and improvement of a wide range of aerosol models (including microphysical, radiative transfer, CTM, and global climate models) was one important way in which we assessed our understanding of the properties and controlling processes of Asian aerosols. We describe here the scientific goals and objectives of the ACE-Asia experiment, its observational strategies, the types of observations made by the mobile platforms and stationary sites, the models that will integrate our understanding of the climatic effect of aerosol particles, and the types of data that have been generated. Eight scientific questions focus the discussion. The intensive observations took place during a season of unusually heavy dust, so we have a large suite of observations of dust and its interaction with air pollutants. Further information about ACE-Asia can be found on the project Web site at
Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of ∼45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.
Interplanetary dust particles accrete on the Earth at a rate of ~40 ktons yr-1. Some 90% of this material evaporates in the atmosphere, producing a bioavailable iron flux of 3x10-7 mol Fe m-2 yr-1. This extraterrestrial Fe flux is 30 - 300% of the eolian flux of bioavailable iron transported from terrestrial sources in remote marine regions and ~20% of the upwelled Fe flux in the Southern Ocean. Extraterrestrial Fe may play an important role in regulating the marine carbon cycle in these regions.
Particle fluxes measured with time series sediment traps deployed below 2000 m at 68 sites in the world ocean are combined with satellite-derived estimates of export production from the overlying water to assess the factors affecting the transfer of particulate organic matter from surface to deep water. Multiple linear regression is used to derive an algorithm suggesting that the transfer efficiency of organic carbon, defined as the settling flux of organic carbon normalized to export production, increases with the flux of carbonate and decreases with water depth and seasonality. The algorithm predicts >80% of the organic carbon transfer efficiency variability in diverse oceanic regions. The influence of the carbonate flux suggests that the ballasting effect of this biogenic mineral may be an important factor promoting export of organic carbon to the deep sea by increasing the density of settling particles. However, the lack of a similar effect for biogenic opal suggests that factors other than particle density also play a role. The adverse effect of increasing seasonality on the transfer efficiency of carbon to the deep sea is tentatively attributed to greater biodegradability of organic matter exported during bloom events. In high latitude opal-dominated regions with high f-ratios and seasonality, while a higher fraction of net production is exported, a higher fraction of the exported organic matter is remineralized before reaching bathypelagic depths. On the other hand, in warm, low latitude, carbonate-dominated regions with low f-ratios and seasonality, a higher fraction of the exported organic matter sinks to the deep sea.