Iron fertilization of the Southern Ocean: Regional simulation and analysis of C-sequestration in the Ross Sea

Kevin Arrigo, Stanford University

In the face of anticipated changes in the climate system due to increasing atmospheric CO₂, important questions that need to be addressed include "How will climate change likely alter marine ecosystem dynamics and C cycling in the SO?", "Can sequestration of anthropogenic CO₂ be increased by artificial trace metal fertilization of the ocean surface?", and "Will there be any feedback between trace-metal ocean fertilization, altered marine trophic structure, and the efficiency of the biological pump?" Answers to these questions are crucial to the ability to understand the capacity for increased C-sequestration by the ocean, but have eluded researchers thus far because of the current inability to include the necessary mechanistic biological detail in large-scale global climate models. Recent investigations at the regional scale are beginning to shed light on these issues, however, and early results suggest that the feedback between marine ecosystem structure and anticipated climate changes are important. To address these issues, an existing fully coupled 3-dimensional numerical model of the pelagic, ice/ocean ecosystem in the Ross Sea will be modified to include inorganic and organic C pools, O2, PO4, air-sea gas exchange, and a more sophisticated formulation for particle export. This ecological model will be used to investigate

1. Annual to decadal variations in the capacity of the Ross Sea to act as a sink for atmospheric CO₂,
2. Biotic and abiotic factors controlling variability in the efficiency of the biological pump in the Ross Sea,
3. The extent, rate, and locus of organic carbon remineralization following primary production, and
4. How 1-3 would change under a strategy of increased Fe availability.

Photosynthetic characteristics, carbon metabolism, and nutrient requirements of Phaeocystis antarctica and diatoms from the Ross Sea, Antarctica.

Kevin Arrigo, Stanford University and University and Dale Robinson, San Francisco State University

Despite the generally low phytoplankton abundance in the Southern Ocean, intense phytoplankton blooms occasionally develop, consistently dominated either by diatoms or Phaeocystis antarctica. CO₂ drawdown within these blooms is sufficient to maintain a positive gradient between the ocean and the atmosphere, facilitating the influx of CO₂. There is evidence that the potential for C-fixation can be increased if additional Fe is made available. Model simulations show that within the Ross Sea, the most productive area of the Southern Ocean, only about two-thirds of the available macronutrients are consumed by phytoplankton prior to the exhaustion of the available Fe. Were more Fe made available, annual production in these waters could be increased by about 50%.

However, critical questions remain to be addressed before scientists can begin to understand the consequences of Fe-fertilization in the Southern Ocean. First, how is Fe fertilization likely to affect phytoplankton community composition, which controls food web structure, and ultimately, the flux of carbon from the euphotic zone? This is important because P. antarctica takes up twice as much CO₂ per mole of PO₄ removed than do diatoms. Second, why does P. antarctica exhibit N/P and C/P uptake ratios that are twice that of diatoms? This issue is critical because if the reason is differing taxon-specific cellular C:N:P ratios, given Fe replete conditions, P. antarctica would be driven to NO3 limitation while diatoms would be limited by PO₄. Understanding the cause of these differences is essential to reliably predicting the outcome of a C-sequestration program based on Fe-fertilization.

Characterizing the production and retention of dissolved iron as Fe(II) across a natural gradient in chlorophyll concentrations in the Southern Drake Passage.

Kathy Barbeau, Scripps Institution of Oceanography/University of California, San Diego

Recent mesoscale iron fertilization experiments in the Southern Ocean have demonstrated the potential utility of iron addition as a means to increase the productivity and carbon sequestration potential of these low chlorophyll waters. Studies of the fate of added iron in these experiments have revealed that the residence time of iron as soluble Fe(II) in surface waters was unexpectedly long, which may have contributed to the development of extensive and prolonged phytoplankton blooms in response to iron additions. Retention of biologically available iron in surface waters is a key parameter in the design of effective iron fertilization protocols for carbon sequestration in oceanic waters. Given the potential significance of the Southern Ocean as an area for the development of future iron fertilization programs, it is critical that more is learned about the redox cycling of iron and the expected lifetime of soluble Fe(II) in this regime, particularly under conditions of sustained iron enrichment. This study will characterize Fe(II) systematics across a natural gradient of iron and chlorophyll concentrations in the Southern Drake Passage, focusing on several basic questions to increase the understanding of Fe(II) production, cycling, and preservation in Antarctic waters: Are there horizontal and vertical gradients in Fe(II) that coincide with chlorophyll gradients in our study area? Is there a diurnal cycle in Fe(II) concentrations in the surface waters due to photochemical redox cycling? Do some components of the dissolved organic carbon pool in Antarctic waters interact with Fe(II) to prolong its lifetime in the surface ocean?

Direct Experiments on the ocean disposal of fossil fuel CO₂

James Barry and Peter Brewer, Monterey Bay Aquarium Research Institute Moss Landing, CA

Concern over rising greenhouse gas concentrations in the atmosphere has led to consideration of novel approaches of "carbon management", including the idea of direct injection of waste CO₂ into the deep ocean, to mitigate the rapid rise in atmospheric CO₂. Using this approach, carbon dioxide would be stripped from the flue gases of fuel burning power plants, pressurized, and released at deep-sea depths (>2000 - 3000 m) where it would remain for centuries. While this would reduce the peak levels of atmospheric CO₂ expected in the next 200 years, concern over the efficiency of carbon storage and potential biological and ecological impacts require study.

The project continues previous DOE funding to investigate the physical, chemical and biological processes associated with direct ocean CO₂ injection. Technically advanced methods have been developed to perform in situ deep-sea experiments involving small-scale (ca. 20-70 l CO₂) releases of liquid CO₂. Results from field experiments have advanced the understanding of the dissolution kinetics of liquid CO₂ and CO₂ hydrate, and have determined the sensitivities of some deep-sea animals to changes in seawater chemistry expected under CO₂ injection scenarios. In this renewal project, studies will be continued of the physics and chemistry of liquid CO₂ and CO₂ hydrate, and will broaden studies of the potential biological impacts of CO₂ injection. Manipulative field experiments and laboratory studies of deep-sea animals will be used to measure changes in oxygen uptake and metabolism associated with exposure to the high-CO₂, low pH (acidic) seawater caused by CO₂ injection.

Autonomous assessment of ocean carbon flux

James K.B. Bishop, Lawrence Berkeley National Laboratory

A major gap exists in the understanding of the downward transport of biologically fixed carbon from surface waters and its fate in the deep sea. This gap exists because most information on carbon sedimentation in the upper kilometer of the ocean is from short-term ship-based studies. To close the gap, LBNL is developing an autonomous, optically-based, sediment trap system capable of recording the high frequency (hours to days) variations of organic and inorganic carbon flux within the upper kilometer of the ocean over seasonal time scales.

Once developed, the high-frequency records of flux variability and process mechanistic information achieved through image analysis of collected material will provide fundamentally new insight into particulate carbon sedimentation and remineralization. This will lead to significantly improved model simulations of the ocean's carbon cycle and to better predictions of how carbon sequestration in the ocean will change in the future.

LBNL's autonomous flux instrument prototype was first tested at sea in 2002. Herein, LBNL proposes to stepwise achieve a refined autonomous imaging carbon flux sensor and to integrate it with the Sounding Oceanographic Lagrangian Observer, a robotic profiling float developed by Scripps Institution of Oceanography. The carbon flux instrument and the integrated autonomous Carbon Flux Explorer will be deployed in the Pacific Ocean in 2004 (near Hawaii) and 2005 (near Japan), respectively. The work is part of the NSF/DOE supported VERtical Transport In the Global Ocean experiment (VERTIGO) experiment. See also http://www-ocean.lbl.govf.

Ocean carbon dynamics and transfer processes: SOFEX and beyond

James K.B. Bishop, Lawrence Berkeley National Laboratory.

Iron fertilization is an ocean carbon sequestration strategy that requires careful scientific assessment. The 2002 NSF/DOE funded Southern Ocean Iron Fertilization Experiment (SOFEX) added iron to waters at 55S and 66S, near 170W to investigate the hypothesis that silica limitation at 55S would quench the effects of the added iron. In 2001, DOE initiated our SOFEX work in two areas: (1) We deployed 4 autonomous robotic profiling floats -- called Carbon Explorers -- inside and outside of the iron treated waters to perform daily assessment of carbon biomass variability from the surface to 1000 meters depth and to gather information on carbon sedimentation rates near 100m. (2) We collected 80 size-fractionated particulate matter samples from the surface to 1000 meters using the ship-deployed Multiple Unit Large Volume in situ Filtration System (MULVFS).

Carbon Explorer results showed that iron stimulated phytoplankton growth at 55S more strongly than it did in silica-rich waters at 66S. MULVFS samples will be analysed in the laboratory for major, minor and trace elements to investigate why addition of iron at 55S caused the unexpectedly strong carbon response. Synchrotron XRAY microanalysis of MULVFS samples will investigate the effects of iron addition down the water column. We will analyze Carbon Explorer records from the first 2 months of 2002 when SOFEX effects were recorded and we will extend analysis of these unique -- now 6 month long -- records of Southern Ocean carbon biomass variability. MULVFS will be repaired to correct damage sustained in freezing Antarctic conditions.

Web link (http://www-ocean.lbl.gov/)

Comparison of Cartesian and isopycnal simulations of oceanic carbon sequestration via iron fertilization and deep Injection

Rainer Bleck, Los Alamos National Laboratory

Los Alamos supports two oceanic global circulation models that treat ocean circulation numerics in substantially different ways. By simulating iron fertilization of the surface ocean and injection of liquefied carbon dioxide into the deep sea in both the z-coordinate Parallel Ocean Program (POP) and the density coordinate-based Hybrid Coordinate Ocean Model (HYCOM), some of the key transport- and mixing- related uncertainties in oceanic carbon sequestration will be identified. A previously developed ecodynamics and geochemistry package will be inserted into both circulation codes. Side-by-side simulations will be performed of carbon drawdown and general environmental effects during likely stages in the evolution of a worldwide marine carbon engineering industry. Manifold patch iron enrichment, broad scale surface nutrient enhancement, injection of condensed CO₂ below the thermocline, and creation of abyssal CO₂ lakes will all be considered. Owing to differences in the handling of vertical mixing and transport associated with wind-driven subduction and thermohaline-forced deep overturning, results from the two models are expected to diverge in several ways. Emphasis in this project will be placed upon intercomparison of the results, with the ultimate goal of heightening confidence in numerical simulations of oceanic carbon sequestration.

Simulating fertilization of the ocean as a carbon sequestration strategy - Effectiveness and unintended consequences

Ken Caldeira , Lawrence Livermore National Laboratory

The primary objectives of this project are to assess, and improve our understanding of: 1) The effectiveness of various proposals to intentionally store carbon in the ocean through fertilization of the surface ocean with iron and/or macronutrients; and 2) Biologically relevant consequences of long-term and extensive ocean fertilization. The PISCES ocean biogeochemistry model, developed at the MPI in Hamburg, Germany, and IPSL in Saclay, France will be used in this study. This model considers Fe, N, P, O₂, Si, alkalinity, and carbon, in organic and inorganic, dissolved and particulate forms. The model represents diatoms, coccolithophorids, nitrogen fixers, and two classes of zooplankton. This model will be incorporated into the LLNL ocean GCM, which is already being applied to other problems in ocean carbon sequestration.

After coupling the ocean biogeochemistry and circulation models, the reliability of this model will be evaluated by comparison to observations. These include observations of natural ecological and biogeochemical variation and observations of small-scale iron fertilization experiments (e.g. SOFeX, IRONEx). This strategy will produce a tested model with predictive capability that we will use to address the following important questions: What is the long-term effectiveness of ocean carbon sequestration via different ocean fertilization strategies? What are the long-term environmental consequences of prolonged or widespread ocean fertilization? What processes need to be included in the models, to better reproduce effects observed in iron fertilization experiments? What should the next experiment measure to better aid the models?

The influence of deep-sea-bed CO₂ sequestration on small metazoan (meiofaunal) community structure and function

Kevin R. Carman and John W. Fleeger, Louisiana State University
David Thistle, Florida State University

The influence of deep-sea CO₂ sequestration on the meiofaunal component of the benthic community will be examined. Meiofauna (metazoans < 1.0 mm in body length) are the most abundant and diverse metazoan organisms in the benthos, and their relative importance increases with increasing depth in the ocean. Because of their small size, meiofauna have higher metabolic rates and shorter population turnover times than do macrobenthos, and thus are particularly useful for the study of environmental disturbances over relatively short time scales. The research will be integrated with the DOE project of Drs. James Barry and Peter Brewer, which involves the experimental application of liquid CO₂ to the seabed at depths ranging from 3200-3600 m in the Monterey Canyon. Samples will be collected from treatment (exposed to CO₂) and control sites with an ROV. Both structural (abundance, taxonomic and functional-group composition, and vertical distribution in sediment) and functional (body size/biomass, reproductive status, and animal health) effects of CO₂ exposure on meiobenthos will be examined. Emergence traps (which capture meiofaunal animals that move from sediment to overlying water) and the vertical distribution of meiofauna within sediment will be used to test for effects on surface-dwelling meiofauna. Effects on community composition will be determined from species-level (harpacticoid copepods) and functional-group (nematode feeding type and tail morphology) analyses. Lipid content of individual harpacticoid copepods will be used as an indicator of health. Fecundity will be determined by measuring brood size in female harpacticoid copepods and abundances of larval and juvenile harpacticoid developmental stages.

Effects of oceanic disposal of carbon dioxide on benthic microfauna: foraminifera as indicators of dissolution and ecosystem health

James Kennett, University of California-Santa Barbara and Joan Bernhard, University of South Carolina

Carbon dioxide (CO₂) sequestration is being considered as a mechanism for reducing the release of carbon dioxide to the atmosphere. A crucial part of investigating oceanic CO₂ sequestration is to determine the biological and geochemical effects on microorganisms in surface sediments.

Studies will be conducted on foraminifera, a well-known group of diverse protists (microfossils) that are typically abundant in marine sediments. Many foraminifera precipitate shells composed of calcium carbonate, making them an ideal indicator to assess the effects of dissolving CO₂ hydrate, which increases pCO₂ levels and also decreases pH in the surrounding waters.

The foraminiferal data will allow monitoring of changes in space and time prior to and following carbon dioxide release experiments in the deep sea. This project will provide data on foraminiferal ecology, survival, and carbonate dissolution as a result of CO₂ injection in the deep sea. Foraminiferal survival will be determined using two independent methods. While the research is to assess the impact of CO₂ sequestration on a local scale, this work will assist in evaluating the feasibility of such endeavors on larger scales. Studies of the effects of CO₂ sequestration on foraminifera may have considerable disparate implications, including significant impacts on the benthic food chain and the utilization of carbonate-secreting organisms as a natural buffer to changing deep-sea pH.

The study will be based upon cores taken during ongoing investigations by MBARI researchers employing an ROV, offshore Monterey, California. This project represents a collaborative effort between MBARI, UCSB and USC researchers. Integration between this and ongoing studies at MBARI should provide a comprehensive assessment of the ecological impacts of deep-ocean CO₂ sequestration.

Pathways and mechanisms of ocean tracer transport: Implications for carbon sequestration

John Marshall and Mick Follows, Massachusetts Institute of Technology

A numerical ocean circulation model and its adjoint will be used to study mechanisms and pathways of ocean tracer transport with a particular emphasis on impacts of, and optimal strategies for, direct ocean carbon sequestration. The study comprises two connected themes:

(i) Sensitivity studies using adjoint ocean circulation and biogeochemistry models to comprehensively characterize the carbon sequestration efficiency for any source location and its dependence on key, and sometimes uncertain, parameters of ocean transport and biogeochemistry. The adjoint model allows this to be done extremely efficiently and systematically compared to traditional Green's function methods.

(ii) Study of the dispersion of dissolved inorganic carbon from specific, near-shore sources in an eddy resolving model high resolution model of the global ocean which explicitly represents the eddy (mesoscale) transfer, topographic complexity and effects of time-varying meteorological forcing. The study will explore the importance of modeling near-field influences accurately and allow us to evaluate the validity of coarse resolution models for far field studies of ocean CO₂ disposal.

Microbiological impacts of direct carbon dioxide injection - Exploratory studies under deep-sea conditions

Douglas C. Nelson, University of California, Davis

Among strategies being considered for combating an increase in atmospheric CO₂, is injection of this greenhouse gas into the deep-sea. Based on the formation of solid hydrates at sufficient pressure, discharge at a depth of approximately 3000 m at sites near the sea floor seems most likely. The biological consequences of the resultant local increase in CO₂ concentration and decrease in pH (= increase in acidity) are difficult to predict. Nonetheless, the narrow range of natural pH variability found at this depth suggests that both microbes and animals evolved for optimum performance there might be sensitive to CO₂-induced acidification. If a pH decrease near the point of injection is as much as a whole unit, localized die-offs of deep-sea animals are possible, which -- based on the activities of diverse microbes -- would result in increased local production of organic carbon compounds, ammonia, hydrogen sulfide and nitrate. The injected CO₂ stream is likely to be contaminated with other industrial gasses such as H₂, CO, and SO₂, which adds to the microbial transformations that must be considered. Microorganisms in the deep-sea uniquely perform several of the fundamental transformations that allow global cycles of nitrogen, carbon and sulfur to be sustained. In its essence the research in this proposal begins to test whether deep-sea injection of CO₂ and resultant acidification might adversely impact the global cycle of certain major biological elements. The processes to be tested at in situ and more acidic conditions include:

1. Respiration of organic matter in the presence of oxygen. Typically, this would occur in the water column.
2. Fermentation or respiration of organic matter in the absence of oxygen. Typically this would occur in marine sediments and might also involve the reduction of nitrogen- or sulfur-compounds.
3. Oxidation of reduced inorganic compounds, e.g. ammonia, hydrogen sulfide, molecular hydrogen or carbon monoxide. These compounds will either be co-injected with CO₂ or arise from sediment microbial processes discussed in point (2).

Initial studies will be undertaken with samples of natural seawater and sediment collected from 3000 m water depth at hydrostatic pressures ranging from 1 to 300 atmospheres. Changes in, and composition of, microbial communities will be monitored using molecular fingerprinting techniques.

Organic matter composition, recycling susceptibility, and the effectiveness of the biological pump - An evaluation using NMR spectra of marine plankton

Adina Paytan, Stanford University

The long-term effectiveness of the "biological pump" depends on a net transfer of C from the upper ocean-atmosphere system to the deep ocean where the C is removed from contact with the atmosphere for an extended period of time. This C removal can be equated to the amount of C fixation by phytoplankton minus the C cycling and regeneration (biodegradation) in the euphotic zone.

The degree of organic matter biodegradation and recycling depends on the "reactivity" of compounds synthesized by the biota, which in turn, is controlled by the structural characteristic of these compounds. There is considerable evidence that different phytoplankton taxa differ substantially in their biogeochemical characteristics and it is likely that the relative abundance of different compounds synthesized by these distinct taxa, and even within each group at different growth conditions, will differ too. This variability in biosynthesis and thus abundance of a wide range of organic compounds in the water column would lend itself to different susceptibility for biodegradation and regeneration. Knowledge of the distribution of various organic matter structural groups synthesized by distinct taxa, the dependence of the organic matter compound classes on different growth conditions and the selective susceptibility of these compound to regeneration can be derived from 13C and 31P NMR spectroscopy. These data are crucial for estimating the potential for rapid regeneration in the euphotic zone, and thus the effectiveness of the "biological pump".

Sequestration of Dissolved Organic Carbon in the Deep Sea

Dan Repeta, Woods Hole Oceanographic Institution

Approximately 600GT of dissolved organic carbon (DOC) are sequestered in the deep sea. Isotopic measurements show deep sea DOC to be depleted in radiocarbon, with an apparent age between ~4000y (Atlantic) and ~6000y (Pacific). From the old radiocarbon age of deep sea DOC we can infer that this organic matter is inert and does not cycle on less than millennial time scales. However, high precision DOC measurements show small variations in deep sea DOC concentrations between ocean basins, and radiocarbon measurements on sugars extracted from deep sea DOC shows this fraction to be enriched in C-14 to post 1945 levels. Although most deep sea DOC may not cycle rapidly, a small portion, equivalent to 10-30GT C may be cycling on decadal time scales.

The goal of this project is to establish the inventory and radiocarbon age of reactive deep sea DOC. A large sample of high molecular weight dissolved organic matter will be collected and the reactive DOC extracted as polysaccharides and proteins. Reactive DOC components will be quantified and their radiocarbon values determined by accelerator mass spectrometry. Using this information, the flux and composition of dissolved organic matter needed to support the deep sea inventory can be determined

Carbon sequestration by patch fertilization: A comprehensive assessment using coupled physical- ecological-biogeochemical models

J.L. Sarmiento, Princeton University and Nicolas Gruber, University of California-Los Angeles

Fundamental research will be performed on ocean iron fertilization as a means to enhance the net oceanic uptake of CO₂ from the atmosphere. A suite of coupled physical- ecological-biogeochemical models will be applied to (i) determine to what extent enhanced carbon fixation from iron fertilization will lead to an increase in the oceanic uptake of atmospheric CO₂ and how long this carbon will remain sequestered (efficiency), and (ii) examine the changes in ocean ecology and biogeochemical cycles resulting from iron fertilization (consequences). In this round of funding, three factors will be modeled that control the efficiency and consequences of iron fertilization:

1. A suite of mechanistic scenarios relating to iron chemistry and its interaction with ocean biology will be tested. Prior results from this project suggested that differing behavior of the added iron could alter fertilization efficiency by a factor of 20.
2. The role of phytoplankton functional diversity will be examined. A significant body of research, some supported by this program, has found that environments that are dominated by different functional groups, e.g., diatoms versus CaCO3 secreting organisms, have large variations in the fraction of fixed carbon that gets exported from the surface ocean and in how deep the exported carbon is carried into the ocean.
3. High-resolution simulations will be run with a range of patch sizes and durations of fertilization to examine the influence of smaller scale processes on the efficiency and consequences of fertilization. Recent work, including some of from this project, suggests that model resolution may alter the simulated response.

Improved estimation of sediment trap sampling characteristics during VERTIGO

David Siegel, University of California-Santa Barbara

The gravitational sinking of particulate organic carbon from the euphotic zone to the ocean interior does not occur vertically. Physical oceanographic processes advect these sinking particles making the sampling of the "vertical" sinking fluxes problematic both operationally and conceptually. Here, the role that physical processes play in determining upper ocean sinking fluxes and how they affect the sampling characteristics of neutrally buoyant, surface tethered and deep moored sediment trap arrays will be assessed. The research group will participate in the up- coming VERTIGO experiment (www.whoi.edu/science/MCG/vertigo) whose aim is to assess the efficiency which exported particulate carbon is sequestered in the mesopelagic. Specific goals are to:

1. Make Lagrangian observations of near-surface velocities using satellite-tracked surface drifters,
2. Use these data to estimate surface water convergence and evaluate the role that submesoscale physical processes play on trap fluxes,
3. Model the probable trajectories for sinking and neutrally buoyant water parcels using the surface drifter and other data, and
4. Characterize the sampling of sinking particles by neutrally buoyant, bottom moored and surface tethered sediment trap arrays.

Little is known about the roles that physical processes play in the sampling of sinking particle fluxes. The proposed physical oceanographic observations will enable us to assess the importance of trap sampling characteristics and their impact on sampled vertical fluxes. This will help assess the quality of sediment trap fluxes (long a contentious issue) and enable new hypotheses linking physical processes and sinking particle fluxes to be tested.

Sinking CO₂ particle plumes

Costas Tsouris, Oak Ridge National Laboratory and E. Eric Adams, Massachusetts Institute of Technology

Direct ocean carbon sequestration has been proposed as a possible measure to counteract anthropogenic increases in atmospheric CO₂. Liquid CO₂ is less dense than seawater at depths <3 km and, if injected at intermediate ocean depths, would form a rising plume prone to rapid atmospheric repartitioning. Ways are being explored to improve sequestration efficiency and reduce environmental impacts by employing a sinking plume formed from dense CO₂ hydrates.

Rapid formation of CO₂ hydrate requires vigorous agitation. In previous laboratory and field work, a co-flow injector was developed that produces a negatively buoyant, paste-like mixture containing CO₂-hydrate, liquid-CO₂, and water under ambient seawater conditions at depths of 1-1.5 km. Goals of the current project are to improve and scale up the injection method, increase the hydrate conversion rate, and determine the effects of various injection contaminants on hydrate formation. The rate of dissolution for the CO₂- hydrate composite particles and the spatial distribution of pH near the injector will be obtained using the ORNL Seafloor Process Simulator (SPS-70). A multi-phase sinking plume model developed at MIT will be extended based on the ORNL data to better understand plume behavior, predict sinking efficiency, and simulate near- field environmental effects of pH change and metal concentrations.

The effects of iron complexing ligands on the long term ecosystem response to iron enrichment of HNLC waters

Mark Wells, University of Maine

Iron is now known to limit diatom production in all major High Nitrate Low Chlorophyll (HNLC) regimes of the ocean. But substantial mesoscale iron infusions to HNLC waters has not resulted in complete macronutrient utilization, and there is so far little evidence that this added phytoplankton production leads to significant carbon sequestration to the deep ocean. Detailed observations from two of these experiments (IronEx II - Equatorial Pacific; SOIREE - Southern Ocean) indicate that the diatoms began re-experiencing iron stress even though dissolved iron concentrations remained elevated in the patch. This surprising outcome likely is related to the observed increased concentrations of strong Fe(III)-complexing ligands in seawater; experiments suggest that eukaryotes cannot readily access iron in these complexes. The specific goals of the project are to: 1) determine how different natural and synthetic iron chelators affect iron availability to phytoplankton species representative of offshore HNLC waters, 2) elucidate how the changes in absolute concentrations of these chelators will affect the ecosystem response beyond the normal (1-4 week) period of observations, and 3) ascertain how changes in the ligand composition affect cell sinking and aggregation rates; measures of the efficiency of carbon sequestration to the deep. These experiments will provide a more detailed understanding of iron complexing ligand effects on long-term ecosystem structure and carbon cycling. This knowledge is essential for ascertaining not only the effect of iron enrichment on short-term carbon sequestration in the oceans, but also the potential effect of iron enrichment in modifying ecosystem structure.

The impact of enhanced N fixation on carbon sequestration: A reassessment of the inorganic carbon system in LNLC regions.

Patricia L. Yager, University of Georgia

Currently, human activities are indirectly enhancing the flux of iron to large regions of the tropical and subtropical oceans and it is thought that this iron fertilization is stimulating enhanced rates of nitrogen fixation. We intend to determine whether these enhanced rates of N₂ fixation yield enhanced carbon sequestration via the biological carbon pump. We also aim to determine how shifts in phytoplankton community structure (with fertilization) influence the export of carbon from these regions. Our project complements a major NSF-funded Biocomplexity research program (MANTRA/PIRANA, http://biology.usc.edu/bc/) investigating N₂ fixation in subtropical and tropical oceans. The Biocomplexity program will measure and model ecosystem processes involved with marine nitrogen fixation in the Western Equatorial Atlantic and Subtropical Pacific, focusing on the fertilization effects of wind-driven dust and riverine Fe inputs enhanced by land use changes. Our complementary measurements of the key CO₂ system parameters (total dissolved inorganic carbon, alkalinity, surface pCO₂, and isotopic composition of the inorganic carbon) along with deep remineralization rates will significantly enhance the value of the existing program and will allow us to determine the rates of net air-sea CO₂ exchange, total oceanic and anthropogenic CO₂ concentrations, and carbon export in regions where these reservoirs and fluxes are poorly known. The proposed research is intended to provide direct estimates of the impact of N₂ fixation and community structure on the uptake of atmospheric CO₂ and to yield a quantitative link between anthropogenic and climate-induced changes in marine N₂ fixation and the sequestration of anthropogenic CO₂ in the ocean.