Boyd, PW; Law, CS; Wong, CS; Nojiri, Y; Tsuda, A; Levasseur, M; Takeda, S; Rivkin, R; Harrison, PJ; Strzepek, R; Gower, J; Mckay, RM; Abraham, E; Arychuk, M; Barwell-Clarke, J; Crawford, W; Crawford, D; Hale, M; Harada, K; Johnson, K; Kiyosawa, H; Kudo, I; Marchetti, A; Miller, W; Needoba, J; Nishioka, J; Ogawa, H; Page, J; Robert, M; Saito, H; Sastri, A; Sherry, N; et al.

Nature [Nature]. Vol. 428, no. 6982, pp. 549-553. 1 Apr 2004.

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.

Bishop, JKB; Wood, TJ; Davis, RE; Sherman, JT

Science (Washington) [Science (Wash.)]. Vol. 304, no. 5669, pp. 417-420. 16 Apr 2004.

Autonomous floats profiling in high-nitrate low-silicate waters of the Southern Ocean observed carbon biomass variability and carbon exported to depths of 100 m during the 2002 Southern Ocean Iron Experiment (SOFeX) to detect the effects of iron fertilization of surface water there. Control and "in-patch" measurements documented a greater than fourfold enhancement of carbon biomass in the iron-amended waters. Carbon export through 100 m increased two- to sixfold as the patch subducted below a front. The molar ratio of iron added to carbon exported ranged between 10 super(4) and 10 super(5). The biomass buildup and export were much higher than expected for iron-amended low-silicate waters.

Buesseler, KO; Andrews, JE; Pike, SM; Charette, MA

Science (Washington) [Science (Wash.)]. Vol. 304, no. 5669, pp. 414-417. 16 Apr 2004.

An unresolved issue in ocean and climate sciences is whether changes to the surface ocean input of the micronutrient iron can alter the flux of carbon to the deep ocean. During the Southern Ocean Iron Experiment, we measured an increase in the flux of particulate carbon from the surface mixed layer, as well as changes in particle cycling below the iron-fertilized patch. The flux of carbon was similar in magnitude to that of natural blooms in the Southern Ocean and thus small relative to global carbon budgets and proposed geoengineering plans to sequester atmospheric carbon dioxide in the deep sea.

Buesseler, KO; Boyd, PW

Science (Washington) [Science (Wash.)]. Vol. 300, no. 5616, pp. 67-68. 5 Apr 2003.

Iron fertilization of the ocean--a potential strategy to remove CO sub(2) from the atmosphere--has generated much debate among ocean and climate scientists. It is viewed as particularly attractive by geoengineers because the addition of relatively small amounts of iron to certain ocean regions may lead to a large increase in carbon sequestration at a relatively low financial cost.

Boyd, PW; Law, CS

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)]. Vol. 48, no. 11-12, pp. 2425-2438. 2001.

This volume is dedicated to the Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron fertilisation experiment performed in the polar waters of the Southern Ocean. SOIREE was an interdisciplinary study involving participants from six countries, and took place in February 1999 south of the Polar Front in the Australasian-Pacific sector of the Southern Ocean. Approximately 3800kg of acidified FeSO sub(4).7H sub(2)O and 165g of the tracer sulphur hexafluoride (SF sub(6)) were added to a 65-m deep surface mixed layer over an area of similar to 50km super(2). Initially, mean dissolved iron concentrations were similar to 2.7nM, but decreased to ambient levels within days, requiring subsequent additions of 1550-1750kg of acidified FeSO sub(4).7H sub(2)O on days 3, 5 and 7 of the experiment. During the 13-day site occupation there were iron-mediated increases in phytoplankton growth rates, with marked increases in chlorophyll a (up to 2 mu gl super(-1)) and production rates (up to 1.3gCm super(-2)d super(-1)). These resulted in subsequent changes in the pelagic ecosystem structure, and in the cycling of carbon, silica and sulphur, such as a 10% drawdown of surface CO sub(2). The SOIREE bloom persisted for >40 days following our departure from the site, as observed via SeaWiFS remotely sensed observations of Ocean Colour. Papers in this volume report in detail on aspects of this study, from the oceanographic setting of the experiment to a modelling simulation of the SOIREE bloom. A CD-ROM accompanies this volume and contains the main SOIREE datasets and ancillary information including the pre-experiment `desktop' database study for site-selection, and satellite images of the SOIREE bloom.

Boyd, PW; Watson, AJ; Law, CS; Abraham, ER; Trull, T; Murdoch, R; Bakker, DCE; Bowie, AR; Buesseler, KO; Chang, Hoe; Charette, M; Croot, P; Downing, K; Zeldis, J; et al.

Nature [Nature]. Vol. 407, no. 6805, pp. 695-702. 12 Oct 2000.

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, remineralization and timescales of water mass subduction.

Abraham, ER; Law, CS; Boyd, PW; Lavender, SJ; Maldonado, MT; Bowie, AR

Nature [Nature]. Vol. 407, no. 6805, pp. 727-730. 12 Oct 2000.

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.

Cavender-Bares, KK; Mann, EL; Chisholm, SW*; Ondrusek, ME; Bidigare, RR

Limnology and Oceanography [Limnol. Oceanogr.]. Vol. 44, no. 2, pp. 237-246. Mar 1999.

Recent unenclosed iron-fertilization experiments in the equatorial Pacific Ocean have shown that phytoplankton biomass can be increased substantially by the addition of iron. Analyses of size-fractionated chlorophyll indicate that much of the increase during the most recent fertilization experiment, IronEx II, occurred in the >10- mu m size fraction. We used flow cytometry, combined with taxon-specific pigment measurements by high-performance liquid chromatography (HPLC), to analyze the responses of five different groups of phytoplankton: Prochlorococcus, Synechococcus, ultraplankton, nanoplankton, and pennate diatoms. These results are unique in the suite of measurements from the IronEx studies in that they simultaneously examine individual cell properties, which are grazer independent, and population dynamics, which reflect the net result of growth and grazing. Our results show that the overall increase of chlorophyll a (Chl a) in the patch was due in part to increases in chlorophyll content per cell and in part to increases in cell numbers of specific groups. Cellular fluorescence was stimulated by iron addition in all five groups to a qualitatively similar degree and was correlated with taxon-specific changes in cellular pigments. In terms of net cell growth, however, these groups responded very differently. The groups that dominated the community before the addition of iron increased at most twofold in cell number; Prochlorococcus actually decreased. In contrast, the initially rare pennate diatoms increased 15-fold in number by the peak of the iron-induced bloom. Within 1 week, this differential response led to a dramatic change in the phytoplankton community structure, from one dominated by picoplankton to one dominated by large diatoms. It is not known whether this shift would be sustained over extended periods of fertilization, a response that would ultimately change the structure of the food web.

Coale, KH; Johnson, KS; Fitzwater, SE; Gordon, RM; Tanner, S; Chavez, FP; Ferioli, L; Sakamoto, C; Rogers, P; Millero, F; Steinberg, P; Nightingale, P; Cooper, D; Cochlan, WP; Kudela, R

Nature, vol. 383, no. 6600, pp. 495-501, 1996

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.

Cooper, DJ; Watson, AJ; Nightingale, PD

Nature, vol. 383, no. 6600, pp. 511-513, 1996

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 nutrients. The results of a previous in situ iron fertilization experiment in this region provided support for this hypothesis, but the observed biological response resulted in only a small decrease in surface-water CO sub(2) fugacity. Here we report a much larger, biologically induced uptake of surface-water CO sub(2) that occurred during a second study. The fugacity of CO sub(2) in the centre of the (iron-fertilized) patch of surface ocean fell from a background value near 510 mu atm to approximately 420 mu atm, corresponding to a transient 60% decrease in the natural ocean-to-atmosphere CO sub(2) flux. We conclude that iron supply to this ocean region can strongly modulate the local short-term source of CO sub(2) to the atmosphere, but has little long-term influence on atmospheric CO sub(2) 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 CO sub(2) partial pressure, for example over glacial-interglacial periods.

Frost, BW

Nature, vol. 383, no. 6600, pp. 475-476, 1996

What regulates phytoplankton abundance and production in the equatorial upwelling zone of the eastern Pacific Ocean? The major nutrients required for phytoplankton growth, such as nitrate and phosphate, are available there in high concentrations, yet phytoplankton seem unable to use them efficiently and phytoplankton abundance remains low. The papers report an experimental test of the late John H. Martin's hypothesis that it is the availability of iron that limits phytoplankton in the HNLC regions. Iron is nearly insoluble in sea water, but is an essential trace nutrient required by phytoplankton for many biochemical processes (chlorophyll synthesis and nitrate reduction, for example). Measurements showing that surface-water concentrations of dissolved iron are extraordinarily low (sub-nanomolar) led Martin to first test the iron hypothesis using shipboard nutrient bioassays. Natural phytoplankton incubated in bottles usually increased in abundance when provided with extra iron. But bottles of sea water as microcosms of the ocean are always suspect. So an in situ iron fertilization experiment was designed and has been carried out twice, first in 1993 and then again last year. In the first experiment (IronEx I), reported without fanfare in this journal two years ago, a single dose of iron, raising the dissolved iron concentration to 4 nM in a 64-km super(2) patch, resulted in significant increases in phytoplankton abundance and production rate, but had little effect on nitrate concentration or partial pressure of CO sub(2). In contrast, during the second experiment (IronEx II, described in this issue), the fertilized patch remained at the surface and retained its integrity while drifting 1,500 km. The same amount of iron was added as in IronEx I, but in three sequential infusions over a week to effect a more sustained increase of dissolved iron in the surface layer of a 72-km super(2) patch. The fertilization had an immediate and dramatic effect. Within the enriched patch, phytoplankton photosynthetic capacity, growth rate and abundance increased, and nitrate decreased. As the phytoplankton bloomed, its species composition changed radically. Diatoms became dominant and accounted for most of the increased use of nitrate. Significantly, these events parallel those observed in previous shipboard nutrient bioassays in the equatorial Pacific.

Gao, Y; Fan, S-M; Sarmiento, JL

Journal of Geophysical Research. D. Atmospheres [J. Geophys. Res. (D Atmos.)]. Vol. 108, no. D7, [np]. Apr 2003.

Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for ~40% of the total deposition over the coastal oceans and ~60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 x 10 super(12) g yr super(-1), which is 4-30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6-2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles.

Gervais, F; Riebesell, U; Gorbunov, MY

Limnology and Oceanography [Limnol. Oceanogr.]. Vol. 47, no. 5, pp. 1324-1335. Sep 2002.

EisenEx--the second in situ iron enrichment experiment in the Southern Ocean--was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate ( super(14)C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m super(-2)), low photosynthetic efficiency (F sub(v)/F sub(m) approximately 0.3), and low daily primary productivity (130-220 mg C m super(-2) d super(-1)). Between 70 and 90% of Chl a was found in pico- and nanophytoplankton. During the induced bloom, F sub(v)/F sub(m) increased up to similar to 0.55, primary productivity and Chl a reached the maximum values of 790 mg C m super(-2) d super(-1) and 231 mg Chl a m super(-2), respectively. As a consequence, the euphotic depth decreased to similar to 41 m. Picophytoplankton biomass hardly changed. Nano- and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly <40 m, primary productivity was highly correlated with Chl a. In the third week, productivity was much lower than predicted from Chl a, probably because of a reduction in photosynthetic capacity as a consequence of increased physical variability in the upper water column. These results provide unequivocal evidence that iron supply is the central factor controlling phytoplankton primary productivity in the Southern Ocean, even if the mixing depth is >80 m.

Hannon, E; Boyd, PW; Silvoso, M; Lancelot, C

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)]. Vol. 48, no. 11-12, pp. 2745-2773. 2001.

The impact of a mesoscale in situ iron-enrichment experiment (SOIREE) on the planktonic ecosystem and biological pump in the Australasian-Pacific sector of the Southern Ocean was investigated through model simulations over a period of 60-d following an initial iron infusion. For this purpose we used a revised version of the biogeochemical SWAMCO model (Lancelot et al., 2000), which describes the cycling of C, N, P, Si, Fe through aggregated chemical and biological components of the planktonic ecosystem in the high nitrate low chlorophyll (HNLC) waters of the Southern Ocean. Model runs were conducted for both the iron-fertilized waters and the surrounding HNLC waters, using in situ meteorological forcing. Validation was performed by comparing model predictions with observations recorded during the 13-d site occupation of SOIREE. Considerable agreement was found for the magnitude and temporal trends in most chemical and biological variables (the microbial food web excepted). Comparison of simulations run for 13- and 60-d showed that the effects of iron fertilization on the biota were incomplete over the 13-d monitoring of the SOIREE bloom. The model results indicate that after the vessel departed the SOIREE site there were further iron-mediated increases in properties such as phytoplankton biomass, production, export production, and uptake of atmospheric CO sub(2), which peaked 20-30 days after the initial iron infusion. Based on model simulations, the increase in net carbon production at the scale of the fertilized patch (assuming an area of 150km super(2)) was estimated to 9725 t C by day 60. Much of this production accumulated in the upper ocean, so that the predicted downward export of particulate organic carbon (POC) only represented 22% of the accumulated C in the upper ocean. Further model runs that implemented improved parameterization of diatom sedimentation (i.e. including iron-mediated diatom sinking rate, diatom chain-forming and aggregation) suggested that the downward POC flux predicted by the standard run might have been underestimated by a factor of up to 3. Finally, a sensitivity analysis of the biological response to iron-enrichment at locales with different initial oceanographic conditions (such as mixed-layer depth) or using different iron fertilization strategies (single vs. pulsed additions) was conducted. The outcomes of this analysis offer insights in the design and location of future in situ iron-enrichments.

Piketh, SJ; Tyson, PD; Steffen, W

South African Journal of Science [S. Afr. J. Sci.]. Vol. 96, no. 5, p. 244. May 2000.

Aeolian transport of iron-bearing fine dust is shown to take place from southern Africa to the central South Indian Ocean area in a pathway leading towards Australasia. Deposition of dust to the ocean takes place by subsidence of transport trajectories to the surface in the region of the centre of the South Indian Anticyclone and over the area of the ocean where a large carbon sink has been shown to occur. It is proposed that the sink is the consequence of aeolian transport of aerosols and iron fertilization of the marine biota in the south Indian Ocean in a manner similar to that used in iron-supplementation ocean experiments to enhance phytoplankton chlorophyll and biological productivity.

Pollard, RT; Lucas, MI; Read, JF

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)]. Vol. 49, no. 16, pp. 3289-3305. 2002.

The primary control on the N-S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2 degree C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilization.

Trull, T; Rintoul, SR; Hadfield, M; Abraham, ER

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)]. Vol. 48, no. 11-12, pp. 2439-2466. 2001.

The Southern Ocean Iron Release Experiment (SOIREE) was carried out in late summer (February 1999) south of Australia (61 degree S, 140 degree E). This region of the southern Antarctic Zone (AZ-S), between the southern branch of the Polar Front (PF) and the southern front of the Antarctic Circumpolar Current (SAACF), is characterized by weak currents and is remote from the influence of sea-ice or coastal waters. The SOIREE site exhibits high nutrient concentrations year-round (phosphate, nitrate and silicate remain above 10 mu M), low chlorophyll accumulations (<0.5 mu g/l), and moderate summer mixed-layer depths (50-70m). The SOIREE iron fertilization led to a large increase in algal biomass, particularly large diatoms, and persisted into March well after normal seasonal production is complete. No increase in carbon export occurred during the SOIREE 13-day observation period. The seasonal cycles of mixed-layer development and low biomass accumulation at the SOIREE site are representative of most of the region between the PF and the SACCF, i.e. between similar to 54 and similar to 62 degree S, and to a lesser extent the Polar Frontal Zone. However, north of similar to 59 degree S surface waters are depleted in silica by mid-summer (as occurs year-round north of the Subantarctic Front). A different response to iron fertilization is likely under these conditions, possibly the promotion of lightly silicified diatoms and non-siliceous organisms, whose ability to export carbon is uncertain. The SOIREE fertilized waters are likely to have remained at the surface in the AZ-S throughout the winter. In general, carbon sequestration by subduction of iron-enhanced biomass accumulations is unlikely south of the SAF, except in very limited regions. Moreover, intermediate water masses formed in the Southern Ocean sink with little pre-formed silicate, so that the "silica pump" is already working at close to maximal capacity. Therefore, in the absence of significant changes in community structure or algal physiology, which increase the ratio of carbon export to silicate export, increased iron supply is unlikely to increase the magnitude of carbon sequestration.

Turner, SM; Nightingale, PD; Spokes, LJ; Liddicoat, MI; Liss, PS

Nature, vol. 383, no. 6600, pp. 513-517, 1996

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 dioxide 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 effect 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 waters. The first such experiment revealed a limited overall biological response and no significant changes in DMS concentrations, although the concentrations of its biochemical precursor doubled. The second experiment, designed to better mimic the natural process of iron enrichment, elicited a much stronger biological response, 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.

Watson, AJ; Bakker, DCE; Ridgwell, AJ; Boyd, PW; Law, CS

Nature [Nature]. Vol. 407, no. 6805, pp. 730-733. 12 Oct 2000.

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 CO sub(2). 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 CO sub(2) 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 CO sub(2) changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial similar to 40 p.p.m. of glacial-interglacial CO sub(2) change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO sub(2) 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.

Watson, AJ; Law, CS; Van Scoy, KA; Millero, FJ; Yao, W; Friedderich, GE; Liddicoat, MI; Wanninkhof, RH; Barber, RT; Coale, KH

Nature, vol. 371, no. 6493, pp. 143-145, 1994

It has long been hypothesized that iron concentrations limit phytoplankton productivity in some parts of the ocean. As a result, iron may have played a role in modulating atmospheric CO sub(2) levels between glacial and interglacial times, and it has been proposed that large-scale deposition of iron in the ocean might be an effective way to combat the rise of anthropogenic CO sub(2) in the atmosphere. As part of an experiment in the equatorial Pacific Ocean, we observed the effect on dissolved CO sub(2) of enriching a small (8 x 8 km) patch of water with iron. We saw significant depression of surface fugacities of CO sub(2) within 48 hours of the iron release, which did not change systematically after that time. But the effect was only a small fraction ( similar to 10%) of the CO sub(2) 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 CO sub(2) concentration, in contrast to the effect observed in incubation experiments, where addition of similar concentrations of iron usually results in complete depletion of nutrients. Although our experiment does not necessarily mimic all circumstances under which iron deposition might occur naturally, our results do not support the idea that iron fertilization would significantly affect atmospheric CO sub(2) concentrations.

Fuhrman, JA; Capone, DG

Limnology and Oceanography [LIMNOL. OCEANOGR.], vol. 36, no. 8, pp. 1951-1959, 1991

We consider biogeochemical secondary effects that could arise from an increase in ocean productivity, such as may occur via fertilization with Fe. These processes and feedback loops are infrequently discussed in this context, yet are likely to be highly relevant to the understanding of global change in general. In particular, we suggest that increased productivity may increase the production and efflux of greenhouse gases, such as nitrous oxide (N sub(2)O) and methane (CH sub(4)) and that shifts in phytoplankton species and productivity may cause changes in another climate-related gas, dimethylsulfide (DMS). N sub(2)O is also implicated in the destruction of stratospheric ozone. Factors contributing to amplified release include both increased nutrient cycling in general and possible development of low oxygen conditions from fertilization. It is also remotely possible that reduced oxygen from an initial fertilization could mobilize existing Fe pools, inducing uncontrolled self-fertilization. Although lack of relevant physiological and ecological data makes it difficult to provide quantitative limits on the extent of the undesired effects, rough calculations suggest that the enhanced release of N sub(2)O alone could totally negate any potential benefit from fertilization and likely worsen global warming and ozone depletion.

Peng, T-H; Broecker, WS

Nature, vol. 349, no. 6306, pp. 227-229, 1991

Martin et al. have proposed an ingenious means by which the rise in atmospheric CO sub(2) content generated by the burning of fossil fuels, and deforestation might be partially compensated. 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 CO sub(2) partial pressure in these waters and allowing CO sub(2) 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 CO sub(2) content of the atmosphere would be lowered by only 10 plus or minus 5% below what it would have been in the absence of fertilization. So if after 100 years the CO sub(2) content of the atmosphere were 500 mu atm without fertilization, it would be between 425 and 475 mu 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 CO sub(2) content.

Martin, JH; Gordon, RM; Fitzwater, SE

Nature, vol. 345, no. 6271, pp. 156-158, 1990

The authors test the hypothesis that Antarctic phytoplankton suffer from iron deficiency which prevents them from blooming and using up the luxuriant supplies of major nutrients found in vast areas of the Southern Ocean. Here it is reported that highly productive ( similar to 3 g C/m super(2)/day), neritic Gerlache Strait waters have an abundance of Fe (7.4 nmol (kg) which facilitates phytoplankton blooming and major nutrient removal, while in low-productivity ( similar to 0.1 g C/m super(2)/day), offshore Drake Passage waters, the dissolved Fe levels are so low (0.16 nmol/kg) that the phytoplankton are able to use 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 CO sub(2) during glacial maxima; 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 CO sub(2) from the atmosphere, if the need arises.

Coale, KH; Johnson, KS; Chavez, FP; Buesseler, KO; Barber, RT; Brzezinski, MA; Cochlan, WP; Millero, FJ; Falkowski, PG; Bauer, JE; Wanninkhof, RH; Kudela, RM; Altabet, MA; Hales, BE; Takahashi, T; Landry, MR; Bidigare, RR; Wang, X; Chase, Z; Strutton, PG; Friederich, GE; Gorbunov, MY; Lance, VP; Hilting, AK; Hiscock, MR; Demarest, M; Hiscock, WT; Sullivan, KF; Tanner, SJ; Gordon, RM; Hunter, CN; Elrod, VA; Fitzwater, SE; Jones, JL; Tozzi, S; Koblizek, M; Roberts, AE; Herndon, J; et al.

Science (Washington) [Science (Wash.)]. Vol. 304, no. 5669, pp. 408-414. 16 Apr 2004.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

Cao, Yong; Li, Daoji; Zhang, Jing

Marine science bulletin/Haiyang Tongbao [Mar. Sci. Bull./Haiyang Tongbao]. Vol. 21, no. 6, pp. 83-90. 2002.

Iron plays an important role in the growth of marine phytoplankton. In the HNLC areas, iron is the limiting factor to the growth of phytoplankton. Iron fertilization has a great future in decreasing the greenhouse effect. Iron is the trigger for the nearshore red tide. This paper introduces the progress in the research on marine iron limitation, and its influence on the global climate.

Coale, KH; Fitzwater, SE; Gordon, RM; Johnson, KS; Barber, RT

Nature, vol. 379, no. 6566, pp. 621-624, 1996

The "iron hypothesis" states that phytoplankton growth and biomass are limited by low concentrations of available iron in large regions of the world's oceans where other plant nutrients are abundant. Such limitation has been demonstrated by experiments in which iron has been added to both enclosed and in situ (un-enclosed) phytoplankton populations. A corollary of the iron hypothesis is that most "new" iron is supplied by atmospheric deposition, and it has been suggested that changes in the deposition rates of iron-bearing dust have led to changes in biological productivity and, consequently, global climate. Here we report surface-water measurements in the Equatorial Pacific Ocean which show that the main iron source to equatorial waters at 140 degree W is from upwelling waters. Shipboard in vitro experiments indicate that sub-nanomolar increases in iron concentrations can cause substantial increases in carbon export to deeper waters in this region. These findings demonstrate that equatorial biological production is controlled not solely by atmospheric iron deposition, but also by processes which influence the rate of upwelling and the iron concentration in upwelled water.

Coale, KH

Encyclopedia of Ocean Sciences - Vol. 3 (I-M). pp. 1385-1397. 2001.

The trace element iron has been shown to play a critical role in nutrient utilization and phytoplankton growth and therefore in the uptake of carbon dioxide from the surface waters of the global ocean. Carbon fixation in the surface waters, via phytoplankton growth, shifts the ocean-atmosphere exchange equilibrium for carbon dioxide. As a result, levels of atmospheric carbon dioxide (a greenhouse gas) and iron flux to the oceans have been linked to climate change (glacial to interglacial transitions).

Fitzwater, SE; Johnson, KS; Gordon, RM; Coale, KH; Smith, WO Jr

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)]. Vol. 47, no. 15-16, pp. 3159-3179. 1 Jan 2000.

Dissolved and particulate trace metal concentrations (dissolved Fe, Zn, Cd, Co, Cu and Ni; particulate Fe, Mn and Al) were measured along two transects in the Ross Sea during austral summer of 1990. Total Fe concentrations in southern Ross Sea and inshore waters were elevated >3.5 times that of northern waters. Dissolved Zn, Cd and Co concentrations were lower by factors of 4.5, 3.5 and 1.6 in southern surface waters relative to northern waters. Dissolved Cu and Ni concentrations were similar in both areas. Elevated Fe concentrations coincided with areas of increased productivity, phytoplankton biomass and nutrient drawdown, indicating that Fe is an important factor controlling the location of phytoplankton blooms in the Ross Sea. Particulate concentrations of Fe, Mn and Al indicate two possible sources of iron to the Ross Sea, resuspension of continental shelf sediments and iron incorporated in annual sea ice and released with meltwaters.

Fitzwater, SE; Coale, KH; Gordon, RM; Johnson, KS; Ondrusek, ME

Deep-Sea Research (Part II, Topical Studies in Oceanography) [Deep-Sea Res. (II Top. Stud. Oceanogr.)], vol. 43, no. 4-6, pp. 995-1015, 1996

Several experiments were conducted in the equatorial Pacific at 140 degree W during the Joint Global Ocean Flux Study, equatorial Pacific, 1992 Time-series I (TS-I, 23 March-9 April), Time-series II (TS-II, 2-20 October) and FeLINE II cruises (10 March-14 April), to investigate the effects of added Fe on phytoplankton communities. Seven series of deckboard iron-enrichment experiments were performed, with levels of added Fe ranging from 0.13 to 1000 nM. Time-course measurements included nutrients, chlorophyll a and HPLC pigments. Results of these experiments showed that subnanomolar (sub-nM) additions of Fe increased net community specific growth rates, with resultant chlorophyll a increases and nutrient decreases. Community growth rates followed Michaelis-Menten type kinetics resulting in maximum rates of 0.99 doublings per day and a half-saturation constant of 0.12 nM iron. The dominant group responding to iron enrichment was diatoms.

Joos, F; Sarmiento, JL; Siegenthaler, U

Nature, vol. 349, no. 6312, pp. 772-774, 1991

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. After 100 years, the atmospheric CO sub(2) concentration would be 59 p.p.m. below what it would have been with no fertilization, assuming no anthropogenic CO sub(2) emissions, and 90-107 p.p.m. less when anthropogenic emissions are included in the calculation. The most effective and reliable strategy for reducing future increases in atmospheric CO sub(2) continues to be control of anthropogenic emissions.

Kurz, KD; Maier-Reimer, E

MPIM-84; ETN-93-93678, , 1992, 20 pp

A proposal that an artificial iron fertilization could increase the productivity of the Antarctic Ocean and therefore reduce substantially the atmospheric carbon dioxide level is addressed. A set of experiments was performed with the three dimensional Hamburg global carbon cycle model to study the impact of such a fertilization. Under the assumption that only the magnitude but not the basic structure of the particulate downward flux is changed by the fertilization, the structure of the oceanic circulation is concluded to be able to support a higher remineralization as well as higher primary production. The reduction of the atmospheric content of carbon dioxide by the fertilization does not exceed 60 ppm.

Lam, PJ; Tortell, PD; Morel, FMM

Limnology and Oceanography [Limnol. Oceanogr.]. Vol. 46, no. 5, pp. 1199-1202. Jul 2001.

Bottle and mesoscale experiments have demonstrated that iron additions enhance phytoplankton growth and reduce surface pCO sub(2) in high-nutrient, low-chlorophyll (HNLC) regions of the world oceans. Here we show that iron additions specifically stimulate organic but not inorganic carbon production in the HNLC Subarctic Pacific. Five-hour super(14)C labeling experiments performed during incubation of surface water samples demonstrated a large increase in the rate of organic carbon produced but no change in the rate of inorganic carbon production. The same result was obtained on two different dates: one when coccolithophores formed a relatively large proportion of total autotrophic biomass; the other when coccolithophores were less abundant. Together with previous taxonomic observations, our results imply that iron fertilization may be particularly effective in drawing down CO sub(2) in surface waters by stimulating primary production but not calcium carbonate precipitation, which augments CO sub(2).

Landry, MR; Ondrusek, ME; Tanner, SJ; Brown, SL; Constantinou, J; Bidigare, RR; Coale, KH; Fitzwater, S

Mar. Ecol. Prog. Ser. Vol. 201, pp. 17-42. 2000.

During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flow cytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 and diatoms by >70-fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chl a ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia, median length 20 to 24 mu m) dominated the patch bloom, with smaller pennate species and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 /d) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cell size, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto- and heterotrophic populations <10 mu m in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and >3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may have played an active role in controlling the phytoplankton response to increased iron availability.

Martin, JH; Coale, KH*; Johnson, KS; Fitzwater, SE; Gordon, RM; Tanner, SJ; Hunter, CN; Elrod, VA; Nowicki, JL; Coley, TL; Barber, RT; Lindley, S; Watson, AJ; Van Scoy, K; Law, CS

Nature, vol. 371, no. 6493, pp. 123-129, 1994

The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km super(2) 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 downstream 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.

Peng, T-H; Broecker, WS

Limnology and Oceanography [LIMNOL. OCEANOGR.], vol. 36, no. 8, pp. 1919-1927, 1991

A limit on the reduction in atmospheric CO sub(2) partial pressure (pCO sub(2)) in the next century resulting from purposeful Fe fertilization of the Antarctic Ocean is estimated with an advection-diffusion model calibrated with transient tracer distributions. To evaluate the possible increase in atmospheric CO sub(2) with and without fertilization, we adopt a "business-as-usual" scenario of anthropogenic CO sub(2) emission. Such increase is computed from the atmospheric pCO sub(2) in the ocean-atmosphere total C system as it responds to this emission scenario. The length of the productive season influences the extent of pCO sub(2) reduction. The maximum O sub(2) consumption in our standard case is estimated to be 133 mu mol kg super(-1) at a depth of 600 m. However, O sub(2) consumption depends on the reoxidation function in the subsurface water. If the organic flux reoxidizes completely in the upper 2,000 m, the maximum consumption of O sub(2) at 500 m could reach 500 mu mol kg super(-1). Hence, depending on the reoxidation function, an anoxic Antarctic thermocline could result from Fe fertilization. Both calculations regarding the seasonality of production and those regarding oxygen reduction are highly sensitive to parameters over which we have little control. They are included only to emphasize their potential importance.

Pitchford, JW; Brindley, J

Journal of Plankton Research [J. Plankton Res.]. Vol. 21, no. 3, pp. 525-547. Mar 1999.

Deficiency in bioavailable iron and grazing pressure have been proposed as alternative explanations for the existence of oceanic high nutrient-low chlorophyll (HNLC) regions. We present a four-component population model, describing the dynamics of two classes each of phytoplankton and zooplankton, having different growth rates, grazing habits and response to iron availability. The model shows that the two explanations should be regarded as complementary, rather than alternatives. Results from the model, which displays excitable dynamics, are compared with results of recent large-scale iron fertilization experiments (IRONEX I and II).

Sedwick, P; DiTullio, G; Mackey, D

Antarctic Journal of the United States [Antarct. J. U.S.]. Vol. 30, no. 5, pp. 199-201. 1995.

The antarctic continental shelves are among the most productive areas of the southern oceans (Comiso et al. 1992), although the mechanisms that control phytoplankton biomass and productivity in these relatively shallow waters remain uncertain. Recent observations of low [less than 1 nanomolar (nM)] dissolved-iron concentrations in surface waters of the southern ocean (Martin, Fitzwater, and Gordon 1990; Martin, Gordon, and Fitzwater 1990; de Baar et al. 1995), together with results of "iron-fertilization" experiments (Martin et al. 1989; Martin, Fitzwater, and Gordon 1990; Martin, Gordon, and Fitzwater 1990; Martin et al. 1994), provide strong evidence that dissolved iron deficiency may be one factor limiting algal production in antarctic waters replete with major plant nutrients nitrate, phosphate, and silica. It has also been suggested that dissolved manganese might limit phytoplankton growth in such areas (Martin, Gordon, and Fitzwater 1990). In the Ross Sea, results of bottle-incubation experiments suggest that iron deficiency limits phytoplankton growth, at least during the summer (Martin, Fitzwater, and Gordon 1990). Here we present preliminary results from a survey of dissolved iron and manganese in surface waters of the Ross Sea during the spring-bloom period. The work was carried out aboard R/V Nathaniel B. Palmer in November and December 1994 as part of a multidisciplinary investigation of the spring phytoplankton bloom in the southern Ross Sea.

Martin, JH

Paleoceanography, vol. 5, no. 1, pp. 1-13, 1990

Several explanations for the 200 to 280 ppm glacial/interglacial change is atmospheric CO sub(2) concentrations deal with variations in southern ocean phytoplankton productivity and the related use nonuse of major plant nutrients. An hypothesis is presented herein in which arguments are made that new productivity in today's southern ocean (7.4 x 10 super(13) g/hr) is limited by iron deficiency, and hence the phytoplankton are unable to take advantage of the excess surface nitrate/phosphate that, if used, could result in total southern ocean new production of 2-3 x 10 super(15) g C yr super(-1). As a consequence of Fe-limited new productivity, Holocene interglacial CO sub(2) levels (preindustrial) are as high as they were during the last interglacial ( approximately equals 280 ppm). In contrast, atmospheric dust Fe supplies were 50 times higher during the last glacial maximum. Because of this Fe enrichment, phytoplankton growth may have been greatly enhanced, larger amounts of upwelled nutrients may have been used, and the resulting stimulation of new productivity may have contributed to the LGM drawdown of atmospheric CO sub(2) to levels of less than 200 ppm. Background information and arguments in support of this hypothesis are presented.

Watson, A; Liss, P; Duce, R

Limnology and Oceanography [LIMNOL. OCEANOGR.], vol. 36, no. 8, pp. 1960-1965, 1991

We consider the design of an in situ enrichment experiment to test the hypothesis that Fe deficiency limits primary productivity in some regions of the ocean. A small-scale (10-100 km super(2)) experiment would be preferable for logistical reasons, but the chief practical difficulty is that the patch of enriched water will fragment or streak severely. This problem is made tractable by adding a conservative marker such as sulfur hexafluoride (SF sub(6)) along with the Fe release, enabling rapid detection of enriched water despite fragmentation. The concentration of SF sub(6) would indicate the amount of dilution the original injection of Fe had undergone, and regression of parameters such as pCO sub(2) and Chl against SF sub(6) at several times postinjection would enable the degree of fertilization to be assessed. Due to the complexity of Fe chemistry in seawater, we cannot be certain that the added Fe will adequately mimic the input of natural aerosol Fe to the surface.

Mann, EL; Chisholm, SW*

Limnology and Oceanography [Limnol. Oceanogr.]. Vol. 45, no. 5, pp. 1067-1076. Jul 2000.

Prochlorococcus, a small unicellular cyanobacterium, is an important member of the phytoplankton community in the eastern equatorial Pacific. When these waters were enriched with iron during IronEx II, the chlorophyll per cell and cell size of Prochlorococcus increased, implying that they were iron limited. The extent of this limitation was unclear, however, and the number of Prochlorococcus remained constant. To examine whether cell division rates were stimulated significantly by iron, we used a cell cycle analysis approach to measure them in and out of the Fe-enriched patch and in Fe-enriched bottles. The cell division rate increased from 0.6 to 1.1 d super(-1) over 6 d of exposure to the elevated iron concentrations in the patch. Cells incubated in bottles with additional iron had rates of 1.4 d super(-1) or two doublings per day. Prochlorococcus mortality rates, measured independently, nearly doubled after the addition of iron. This matched the increase in the cell division rate and maintained a relatively constant population size. Thus the cell division rates of even the smallest phytoplankton in the equatorial Pacific are significantly iron limited, but biomass is constrained by both iron limitation and microzooplankton grazing. The differential response of individual phytoplankton groups to the addition of iron during IronEx II was at least partially a result of differential mortality rates over the time course of the experiment. How the community would respond to sustained fertilization, however, is not obvious.

Sigman, Daniel M; Boyle, Edward A

Nature, London, England. Vol. 407, no. 6806, pp. 859-869. 2000.

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean's `biological pump', the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.