Search

The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron. Greater atmospheric dust deposition could have fertilized the equatorial Pacific with iron during the last ice age--the Last Glacial Period (LGP)--but the effect of increased ice-age dust fluxes on primary productivity in the equatorial Pacific remains uncertain. Here we present meridional transects of dust (derived from the (232)Th proxy), phytoplankton productivity (using opal, (231)Pa/(230)Th and excess Ba), and the degree of nitrate consumption (using foraminifera-bound δ(15)N) from six cores in the central equatorial Pacific for the Holocene (0-10,000 years ago) and the LGP (17,000-27,000 years ago). We find that, although dust deposition in the central equatorial Pacific was two to three times greater in the LGP than in the Holocene, productivity was the same or lower, and the degree of nitrate consumption was the same. These biogeochemical findings suggest that the relatively greater ice-age dust fluxes were not large enough to provide substantial iron fertilization to the central equatorial Pacific. This may have been because the absolute rate of dust deposition in the LGP (although greater than the Holocene rate) was very low. The lower productivity coupled with unchanged nitrate consumption suggests that the subsurface major nutrient concentrations were lower in the central equatorial Pacific during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, we propose that the central equatorial Pacific data are consistent with more nutrient consumption in the Subantarctic Zone, possibly owing to iron fertilization as a result of higher absolute dust fluxes in this region. Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.

No single mechanism can account for the full amplitude of past atmospheric carbon dioxide (CO2) concentration variability over glacial-interglacial cycles. A build-up of carbon in the deep ocean has been shown to have occurred during the Last Glacial Maximum. However, the mechanisms responsible for the release of the deeply sequestered carbon to the atmosphere at deglaciation, and the relative importance of deep ocean sequestration in regulating millennial-timescale variations in atmospheric CO2 concentration before the Last Glacial Maximum, have remained unclear. Here we present sedimentary redox-sensitive trace-metal records from the Antarctic Zone of the Southern Ocean that provide a reconstruction of transient changes in deep ocean oxygenation and, by inference, respired carbon storage throughout the last glacial cycle. Our data suggest that respired carbon was removed from the abyssal Southern Ocean during the Northern Hemisphere cold phases of the deglaciation, when atmospheric CO2 concentration increased rapidly, reflecting--at least in part--a combination of dwindling iron fertilization by dust and enhanced deep ocean ventilation. Furthermore, our records show that the observed covariation between atmospheric CO2 concentration and abyssal Southern Ocean oxygenation was maintained throughout most of the past 80,000 years. This suggests that on millennial timescales deep ocean circulation and iron fertilization in the Southern Ocean played a consistent role in modifying atmospheric CO2 concentration.

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

The �iron fertilisation� hypothesis suggests controversially that atmospheric CO2 has been influenced by transport of iron-containing dust to the ocean surface1-3. Experiments in the Southern Ocean show that productivity, and subsequent CO2 drawdown, are enhanced by iron additions4. A carbon cycle model (forced by large values of Southern Ocean dust flux) indicates productivity changes during past glacial times could have reduced atmospheric CO2 by ~40ppm 5. However, Southern Ocean dust flux is very low at present and was increased, but still low, during past glaciations. Thus, as for the equatorial Pacific 1, 6, 7, distally-supplied, upwelled iron may be more significant than local dust-borne iron. Hence, Northern, not Southern, hemisphere dust may drive Southern Ocean productivity8,9. Here, we examine the flux and timing of N. Atlantic dust inputs in relation to the Vostok climate record. For the penultimate glaciation, two N. Atlantic dust peaks occurred. At 155 ka, the Atlantic dust flux was 2500x that at Vostok10, but declined well before the onset of the Vostok CO2 rise. The second dust peak, at 130ka, substantially post-dated the CO2 rise. Thus, low Southern Ocean dust fluxes, and this mismatch between N. hemisphere dust peaks and Southern Ocean climate change, appear not to support the suggested role of dust-mediated iron fertilisation in the Southern Ocean at Termination II.

The growth of populations is known to be influenced by dispersal, which has often been described as purely diffusive. In the open ocean, however, the tendrils and filaments of phytoplankton populations provide evidence for dispersal by stirring. Despite the apparent importance of horizontal stirring for plankton ecology, this process remains poorly characterized. Here we investigate the development of a discrete phytoplankton bloom, which was initiated by the iron fertilization of a patch of water (7 km in diameter) in the Southern Ocean. Satellite images show a striking, 150-km-long bloom near the experimental site, six weeks after the initial fertilization. We argue that the ribbon-like bloom was produced from the fertilized patch through stirring, growth and diffusion, and we derive an estimate of the stirring rate. In this case, stirring acts as an important control on bloom development, mixing phytoplankton and iron out of the patch, but also entraining silicate. This may have prevented the onset of silicate limitation, and so allowed the bloom to continue for as long as there was sufficient iron. Stirring in the ocean is likely to be variable, so blooms that are initially similar may develop very differently.

Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial approximately 40 p.p.m. of glacial-interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known.

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis'. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

There is compelling evidence that phytoplankton growth is limited by iron availability in the subarctic Pacific1, and equatorial Pacific2 and Southern oceans3. A lack of iron prevents the complete biological utilization of the ambient nitrate and influences phytoplankton species composition in these open-ocean ‘high-nitrate, low-chlorophyll’ (HNLC) regimes4. But the effects of iron availability on coastal primary productivity and nutrient biogeochemistry are unknown. Here we present the results of shipboard seawater incubation experiments which demonstrate that phytoplankton are iron-limited in parts of the California coastal upwelling region. As in offshore HNLC regimes, the addition of iron to these nearshore HNLC waters promotes blooms of large chain-forming diatoms. The silicic acid:nitrate (Si:N) uptake ratios in control incubations are two to three times higher than those in iron incubations. Diatoms stressed by a lack of iron should therefore deplete surface waters of silicic acid before nitrate, leading to a secondary silicic acid limitation of the phytoplankton community. Higher Si:cell, Si:C and Si:pigment ratios in diatoms in the control incubations suggest that iron limitation leads to more silicified, faster-sinking diatom biomass. These results raise fundamental questions about the nature of nutrient-limitation interactions in marine ecosystems, palaeoproductivity estimates based on the sedimentary accumulation of biogenic opal, and the controls on carbon export from some of the world's most productive surface waters.

THE concentrations of bioavailable iron in the surface waters of some ocean regions may indirectly modulate climate by controlling phytoplankton productivity and thus the amounts of carbon dioxide1 and dimethyl sulphide (DMS) that are exchanged with the atmosphere. Oxidation of DMS is involved in the formation of atmospheric sulphate particles, which can exert a climate cooling effect2 directly (by scattering and absorbing solar radiation), and indirectly (by affecting cloudiness and hence global albedo). But direct evidence supporting the hypothesis that DMS production in the ocean is affected by iron availability is lacking. Here we report changes in the concentrations of DMS in response to in situ iron-enrichment during two ecosystem-scale experiments designed to investigate the biological and chemical effects of iron fertilization of under-productive surface ocean waters3,4. The first such experiment revealed a limited overall biological response3 and no significant changes in DMS concentrations, although the concentrations of its biochemical precursor doubled5. The second experiment, designed to better mimic the natural process of iron enrichment, elicited a much stronger biological response4, and DMS concentrations increased by a factor of 3.5. This result provides direct support for an important link in the iron–DMS–climate hypothesis.

The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability.

A fundamental issue in marine science is the identification of the factors controlling biological uptake of CO2, in high-nitrate, low-chlorophyll regions. A recent in situ iron fertilization experiment demonstrated that iron limitation is responsible for low phytoplankton stocks in the equatorial Pacific4. Here we show that flavodoxin, a biochemical marker of iron limitation, can be used to map the degree of iron stress in natural populations. Flavodoxin assays along a 900-km east-west transect in the northeastern subarctic Pacific revealed a pronounced increase in iron stress in the region west of the 135° W meridian. Addition of dissolved iron alleviated this stress. Immunostaining of single cells from the most western station showed that flavodoxin is present specifically within the chloroplasts of diatoms. Our approach provides a rapid means of defining the extent of iron stress in the ocean5 and supports the hypothesis that diatoms are iron stressed in the northeast Pacific.

THE iron hypothesis1–3—the suggestion that iron is a limiting nutrient for plankton productivity and consequent CO2 drawdown— has been tested by small-scale experiments in incubation bottles in the subarctic Pacific2,4 and Southern5 –7 Oceans, and by a recent large-scale experiment in the equatorial Pacific Ocean8,9. Here we test the idea by looking at natural levels of productivity in regions of the Southern Ocean with differing iron abundance. In the southerly branch of the Antarctic circumpolar current (ACC), upwelling of deep waters supplies sufficient iron to the surface to sustain moderate primary production but not to permit blooms to develop. In contrast, within the fast-flowing, iron-rich jet of the polar front (PF), spring blooms produced phytoplankton biomass an order of magnitude greater than that in southern ACC waters, leading to CO2 undersaturation. The plankton-rich PF waters were sharply delineated from adjacent iron-poor waters, indicating that iron availability was the critical factor in allowing blooms to occur.

The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km2 in the open equatorial Pacific Ocean with iron. This resulted in a doubling of plant biomass, a threefold increase in chlorophyll and a fourfold increase in plant production. Similar increases were found in a chlorophyll-rich plume down-stream of the Galapagos Islands, which was naturally enriched in iron. These findings indicate that iron limitation can control rates of phytoplankton productivity and biomass in the ocean.

It has been suggested (1–3) that fertilizing the ocean with iron might offset the continuing increase in atmospheric CO2 by enhancing the biological uptake of carbon, thereby decreasing the surface-ocean partial pressure of CO2and drawing down CO2 from the atmosphere. Using a box model, we present estimates of the maximum possible effect of iron fertilization, assuming that iron is continuously added to the phosphate-rich waters of the Southern Ocean, which corresponds to 16% of the world ocean surface. We find that after 100 years of fertilization, the atmospheric CO2 concentration would be 59 p.p.m. below what it would have been with no fertilization, assuming no anthropogenic CO2 emissions, and 90–107 p.p.m. less when anthropogenic emissions are included in the calculation. Such a large uptake of CO2 is unlikely to be achieved in practice, owing to a variety of constraints that require further study; the effect of iron fertilization on the ecology of the Southern Ocean also remains to be evaluated. Thus, the most effective and reliable strategy for reducing future increases in atmospheric CO2 continues to be control of anthropogenic emissions.

THE equatorial Pacific Ocean is a 'high-nitrate, low-chlorophyll' region where nitrate and phosphate are abundant all year round. These nutrients cannot therefore be limiting to phytoplankton production. It has been suggested that the bioavailability of iron—a micronutrient—may be preventing full biological utilization of the major nutrients1–3. The results of a previous in situ iron fertilization experiment in this region provided support for this hypothesis4, but the observed biological response resulted in only a small decrease in surface-water CO2 fugacity5. Here we report a much larger, biologically induced uptake of surface-water CO2 that occurred during a second study6. The fugacity of CO2 in the centre of the (iron-fertilized) patch of surface ocean fell from a background value near 510 μatm to approximately 420 μatm, corresponding to a transient 60% decrease in the natural ocean-to-atmosphere CO2 flux. We conclude that iron supply to this ocean region can strongly modulate the local short-term source of CO2 to the atmosphere, but has little long-term influence on atmospheric CO2 partial pressure. However, if such a modulation also occurs in the Southern Ocean, then iron bioavailability at high southern latitudes could have a significant effect on atmospheric CO2 partial pressure7–11, for example over glacial–interglacial periods.

The discovery that marine algal blooms deposit organic carbon to the deep ocean answers some — but not all — of the questions about whether fertilizing such blooms is a viable strategy for mitigating climate change. See Article p.313 The iron hypothesis proposes that the supply of iron-bearing dust to certain oceanic regions stimulates phytoplankton blooms that, by sinking out to the deep ocean when the organisms die, sequester carbon absorbed from exchange with the atmosphere. Experiments carried out to test the idea have provided clear evidence that iron addition generates phytoplankton blooms in waters with high -nutrient but low -chlorophyll concentrations, as proposed. But the fate of phytoplankton blooms and hence the extent of any carbon sequestration to the deep ocean has remained uncertain. This paper presents multiple lines of evidence, collected by aboard RV Polarstern in 2004 during the European Iron Fertilization Experiment (EIFEX), that suggest carbon is exported to the deep ocean as a result of iron fertilization, with at least half of the bloom biomass sinking to below 1,000 metres.

IT has long been hypothesized that iron concentrations limit phyto-plankton productivity in some parts of the ocean1–3. As a result, iron may have played a role in modulating atmospheric CO2 levels between glacial and interglacial times4, and it has been proposed5 that large-scale deposition of iron in the ocean might be an effective way to combat the rise of anthropogenic CO2 in the atmosphere. As part of an experiment in the equatorial Pacific Ocean6, we observed the effect on dissolved CO2 of enriching a small (8x8 km) patch of water with iron. We saw significant depression of surface fugacities of CO2 within 48 hours of the iron release, which did not change systematically after that time. But the effect was only a small fraction (∼10%) of the CO2 drawdown that would have occurred had the enrichment resulted in the complete utilization of all the available nitrate and phosphate. Thus artificial fertilization of this ocean region did not cause a very large change in the surface CO2 concentration, in contrast to the effect observed in incubation experiments3, where addition of similar concentra-tions of iron usually results in complete depletion of nutrients. Although our experiment does not necessarily mimic all circum-stances under which iron deposition might occur naturally, our results do not support the idea that iron fertilization would signifi-cantly affect atmospheric CO2 concentrations.

MARTIN et al. have proposed an ingenious means by which the rise in atmospheric CO2 content generated by the burning of fossil fuels and deforestation might be partially compensated1. The idea is that plant production in the nutrient-rich surface waters of the Antarctic could be stimulated by the addition of dissolved iron, thereby reducing the CO2 partial pressure in these waters and allowing CO2 to flow from the atmosphere into the Antarctic Ocean. We have used a box model calibrated with transient tracer data to examine the dynamical aspects of this proposal, and conclude that after 100 years of totally successful fertilization the CO2 content of the atmosphere would be lowered by only 10±5% below what it would have been in the absence of fertilization. So if after 100 years the CO2 content of the atmosphere were 500 fjiatm without fertilization, it would be between 425 and 475 µ atm with full fertilization. In other words, if our model calibration is correct, even if iron fertilization worked perfectly it would not significantly reduce the atmospheric CO2 content.

WE are testing the hypothesis that Antarctic phytoplankton suffer from iron deficiency1–3 which prevents them from blooming and using up the luxuriant supplies of major nutrients found in vast areas of the southern ocean. Here we report that highly productive4 (∼3 g C m−;2 day−1), neritic Gerlache Strait waters have an abundance of Fe (7.4 nmol kg−1) which facilitates phytoplankton blooming and major nutrient removal, while in low-productivity4 (∼0.1 g C m−2 day−1), offshore Drake Passage waters, the dissolved Fe levels are so low (0.16 nmol kg−1) that the phytoplankton are able to use less than 10% of the major nutrients available to them. The verification of present-day Fe deficiency is of interest as iron-stimulated phytoplankton growth may have contributed to the drawing down of atmospheric CO2 during glacial maxima2,3; it is also important because oceanic iron fertilization aimed at the enhancement of phytoplankton production may turn out to be the most feasible method of stimulating the active removal of greenhouse gas CO2 from the atmosphere, if the need arises (J.H.M., manuscript in preparation).

An iron-enrichment experiment in the equatorial Pacific Ocean shows that increased availability of this trace nutrient induces dramatic biological and biogeochemical changes in the surface waters.

An interesting oceanographic problem concerns the excess major plant nutrients (PO4, NO3, SiO3) occurring in offshore surface waters of the Antarctic1–3 and north-east Pacific subarctic Oceans4. In a previous study5, we presented indirect evidence suggesting that inadequate Fe input was responsible for this limitation of growth; recently we had the opportunity to seek direct evidence for this hypothesis in the north-east Pacific subarctic. We report here that the addition of nmol amounts of dissolved iron resulted in the nearly complete utilization of excess NO3, whereas in the controls—without added Fe—only 25% of the available NO3 was used. We also observed that the amounts of chlorophyll in the phytoplankton increased in proportion to the Fe added. We conclude that Fe deficiency is limiting phytoplankton growth in these major-nutrient-rich waters.