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

**Grant DE-FG02-00ER63009**

**DOE Office of Biological and Environmental Research**

**FINAL REPORT**

PRINCETON UNIVERSITY

Jorge L. Sarmiento, PI

Anand Gnanadesikan, Co-I

UNIVERSITY OF CALIFORNIA, LOS ANGELES

Nicolas Gruber, PI

# I. Overview

This final report summarizes research undertaken collaboratively between
Princeton University, the NOAA Geophysical Fluid Dynamics Laboratory on
the Princeton University campus, the State University of New York at
Stony Brook, and the University of California, Los Angeles between
September 1, 2000, and November 30, 2006, to do *fundamental research on
ocean iron fertilization as a means to enhance the net oceanic uptake of
CO~2~ from the atmosphere*. The approach we proposed was to develop and
apply a suite of coupled physical-ecological-biogeochemical models in
order to

(i) determine to what extent enhanced carbon fixation from iron
    fertilization will lead to an increase in the oceanic uptake of
    atmospheric CO~2~ and how long this carbon will remain sequestered
    (*efficiency*), and

(ii) examine the changes in ocean ecology and natural biogeochemical
     cycles resulting from iron fertilization (*consequences*).

The award was funded in two separate three-year installments:

-   September 1, 2000 to November 30, 2003, for a project entitled
    "Ocean carbon sequestration by fertilization: An integrated
    biogeochemical assessment." A final report was submitted for this at
    the end of 2003 and is included here as Appendix 1.

-   December 1, 2003 to November 30, 2006, for a follow-on project under
    the same grant number entitled "Carbon sequestration by patch
    fertilization: A comprehensive assessment using coupled
    physical-ecological-biogeochemical models."

This report focuses primarily on the progress we made during the second
period of funding subsequent to the work reported on in Appendix 1.

When we began this project, we were thinking almost exclusively in terms
of long-term fertilization over large regions of the ocean such as the
Southern Ocean, with much of our focus being on how ocean circulation
and biogeochemical cycling would interact to control the response to a
given fertilization scenario. Our research on these types of scenarios,
which was carried out largely during the first three years of our
project, led to several major new insights on the interaction between
ocean biogeochemistry and circulation. This work, which is described in
the following Section II on "Large scale fertilization," has continued
to appear in the literature over the past few years, including two high
visibility papers in Nature.

Early on in the first three years of our project, it became clear that
small \"patch-scale\" fertilizations over limited regions of order 100
km diameter were much more likely than large scale fertilization, and we
carried out a series of idealized patch fertilization simulations
reported on in *Gnanadesikan et al. (2003)*. Based on this paper and
other results we had obtained by the end of our first three-year grant,
we identified a number of important issues that needed to be addressed
in the second three-year period of this grant. Section III on "patch
fertilization" discusses the major findings of this phase of our
research, which is described in three major manuscripts that will be
submitted for publication in the near future. This research makes use of
new more realistic ocean ecosystem and iron cycling models than our
first paper on this topic. We have several major new insights into what
controls the efficiency of iron fertilization in the ocean.

Section IV on "model development" summarizes a set of papers describing
the progress that we made on developing new ecosystem models during the
period of this grant.

# II. Large scale fertilization

## References to discuss

Gnanadesikan, A., J. P. Dunne, R. M. Key, K. Matsumoto, J. L. Sarmiento,
R. D. Slater, and P, S. Swathi, 2004. Oceanic ventilation and
biogeochemical cycling: Understanding the physical mechanisms that
produce realistic distributions of tracers and productivity. Global
Biogeochem. Cycles, 18, GB4010, doi:10.1029/2003GB002097.

Marinov, I., 2005. Controls on the air-sea balance of carbon dioxide,
PhD thesis, Atmospheric and Oceanic Sciences Program, Princeton
University.

Marinov, I., M. Follows, A. Gnanadesikan, R.D. Slater and J. L.
Sarmiento, in preparation. What controls the atmospheric CO~2~ response
to surface nutrient depletion ?.

Marinov, I. A. Gnanadesikan, J. R. Toggweiler, and J. L. Sarmiento,
2006. The Southern Ocean biogeochemical divide. Nature, 441: 964-967.

Marinov, I., A. Gnanadesikan, J. L. Sarmiento, J. R. Toggweiler, M.
Follows and B.K. Mignone, submitted. Impact of oceanic circulation on
biological carbon storage in the ocean and atmospheric pCO~2~. (Global
Biogeochem. Cycles)

Mignone, B. K., A. Gnanadesikan, J. L. Sarmiento, R. D. Slater, 2006.
Central role of southern hemisphere winds and eddies in modulating the
oceanic uptake of anthropogenic carbon. Geophys. Res. Lett., 33, L01604,
doi:10.1029/2005GL024464.

Najjar, R. G., X. Jin, F. Louanchi, O. Aumont, K. Caldeira, S. C. Doney,
J.-C. Dutay, M. Follows, G. M. Kay, E. Maier-Reimer, R. J. Matear, K.
Matsumoto, A. Mouchet, J. C. Orr, G. K. Plattner, J. L. Sarmiento, M. F.
Weirig, Y. Yamanaka, and A. Yool, in press. Impact of circulation on
export production, dissolved organic matter and dissolved oxygen in the
ocean: Results from OCMIP-2. (Global Biogeochem. Cycles).

Nevision, C.D., R. F. Keeling, R. F. Weiss, B. N. Popp, X. Jin, P. J.
Fraser, L. W. Porter and P. G. Hess, 2005. Southern Ocean ventilation
inferred from seasonal cycles of atmospheric N~2~O and O~2~/N~2~ at Cape
Grim, Tasmania. Tellus B, vol. 57, 218-119.

Rodgers, K.B., O. Aumont, C. Menkes. and T. Gorgues, submitted. Decadal
variations in equatorial Pacific ecosystems and ferrocline/pycnocline
decoupling. (Global Biogeochem. Cycles).

Sarmiento, J. L., N. Gruber, M. A. Brzezinski, and J. P. Dunne, 2004.
High latitude controls of the global nutricline and low latitude
biological productivity. Nature, 427: 56-60.

Sarmiento, J. L., J. Simeon, and A. Gnanadesikan, in preparation.
Sub-Antarctic Mode Water pathways and nutrients.

## Relevant reports

**Irina Marinov**

The Southern Ocean has been identified as the place where depleting
nutrients will results in much higher atmospheric CO2 drawdown than
depleting nutrients in the North Atlantic, North Pacific, or tropical
ocean (Sarmiento and Orr, 1991). We have performed further research on
the roles of the Southern Ocean on controlling the air-sea CO~2~
balance.

**1.)The Southern Ocean Biogeochemical Divide**

We probe the system to determine whether certain regions of the Southern
Ocean are more critical than others for air-sea CO~2~ balance and
biological export production (Marinov et al., 2006). The probe used is
to increase surface nutrient drawdown (i.e., perform nutrient depletion)
in different regions of the Southern Ocean. We demonstrate that
atmospheric CO~2~ is controlled primarily by the biological pump and
circulation in the Antarctic deep-water formation region, whereas global
biological productivity is controlled primarily by the biological pump
and circulation in the Subantarctic intermediate and mode water
formation regions. Our work also highlights the crucial role of the
convective regions such as the Weddell Sea in setting the air-sea CO~2~
balance.

**2.)Sub-Antarctic Mode Water pathways and nutrients**

The Southern Ocean is one of the few places in the global ocean that
permits interaction of the atmosphere and the abyssal ocean.
Understanding the physical circulation of the Southern Ocean can lead to
better systems for managing emissions via air-sea interactions.
Sarmiento et al. (2004) presented the prevalence of Sub-antarctic Mode
Water (SAMW) as a control to low-latitude biological productivity.
Simeon et al (in prep) extends this study by illustrating the pathways
of SAMW from the base of thermocline to the surface. A suite of
simulations conducted in a general circulation model (GCM) is utilized
to elucidate and quantify the influence of Sub-Antarctic Mode Water
(SAMW) over the global thermocline. Three GCMs are employed to represent
various scenarios of global circulation, where the realizations feature
alternative deep-ocean-to-surface return pathways in the thermohaline
circulation. The dye tracer and nutrient, phosphate, are released in
four source regions: (1) the SAMW outcrop, (2) Antarctic Intermediate
Water outcrop, (3) north of 40N and (4) a deep, low-latitude region. The
dye represents fractional contributions of the source regions in the
upper thermocline as dye moves away from the release region. Phosphate
is tagged to identify it\'s pre-formed and remineralized states
associated with each source region. Low-latitude new production is
separately calculated from each of the tagged phosphate tracers. Float
trajectory simulations reconstruct the SAMW\'s Lagrangian pathways
between the thermocline and the surface.

Results show the upper thermocline is predominantly comprised of water
masses originating from the SAMW (\~50%), even when strong low-latitude
upwelling is specified. Float trajectories indicate the existence of two
general pathways for northward SAMW transport. One pathway is
\`\`fast\", O(10 years), where SAMW follows eastern boundary currents.
Eastern boundary currents bring in up to 0.6 Sv of SAMW through the base
of the thermocline. This path is impacted by near surface buoyancy
fluxes resulting in relatively faster transformations from heavy to
light waters. The second pathway is slow, O(100 years), and follows the
geostrophic gyre transport. These parcels tend to follow isopycnals and
remain enclosed in the interior ocean. Subtropical gyres act to draw
SAMW out of the upper thermocline at a rate less than or equal to 0.15
Sv.

Results from the phosphate simulations indicate an overturning
circulation whose primary return pathway is in the low-latitudes yields
the largest new production (0.5 GtC/year) as opposed to the
high-latitude return pathways (0.2 GtC/yr). The tropics are the region
of highest new production, with the Southern Hemisphere sub-antarctic
region producing the second largest signal in new production. The source
of nutrients for the new production is the SAMW in the Southern
Hemisphere and at low-latitudes remineralized nutrients, largely
originating from the SAMW region, is the source of nutrients.

The water originating from the SAMW outcrop is transported equatorward
largely by geostrophy and to a smaller degree, by Ekman dynamics. The
wind-driven transport tends to be rapid, but shallow. In contrast, SAMW
outcrop water driven by geostrophic transport, tends to slowly circulate
in the subtropical gyres. Pre-formed nutrients associated with the SAMW
that become entrained in the gyres, may yield a more limited or delayed
contribution to global biological productivity. The nutrients
originating from a particular source region are mostly consumed locally,
while the low-latitude new production is actually supported by a small
fraction of un-utilized source region nutrients and a large fraction of
recycled nutrients.

**3.) Impact of oceanic circulation on biological carbon storage in the
ocean and atmospheric pCO~2~**

We use both theory and ocean biogeochemistry models to examine the role
of the soft tissue biological pump in controlling atmospheric CO~2~
(Marinov, submitted). We demonstrate that atmospheric CO~2~ is
determined in a simple way by the remineralized and preformed nutrient
inventories in the ocean (which are both linearly related to the total
carbon storage in the ocean due to the soft tissue pump).

Previous model simulations have explored how changes in the surface
concentration in deepwater formation regions can modify the oceanic
preformed nutrient inventory. In this paper we demonstrate that
preformed nutrients can also be modified simply by changing the strength
of deep water formation even without modifying the surface nutrient
concentration. If deepwater formation strengthens (as a result of
increasing Southern Ocean winds or vertical mixing), the preformed
nutrient inventory and thus atmospheric CO~2~ increase.

An important consequence of these new insights is that the relationship
between surface nutrient concentrations, biological export production,
and atmospheric CO~2~ is more complex than previously predicted.

Contrary to conventional wisdom, we show that the soft tissue ocean
carbon storage can increase (causing atmospheric CO~2~ to increase),
while surface nutrients show minimal change and export production
decreases.

**4.) What controls the atmospheric CO~2~ response to surface nutrient
depletion.**

We study extensively the sensitivity of atmospheric pCO~2~ to surface
nutrient depletion in a suite of ocean biogechemistry GCMs (Marinov et
al., in prep). A depletion of unutilized surface nutrients through
increased biological export production results in lowering of surface
CO~2~ and atmospheric pCO~2~ drawdown. Here we show how the oceanic
uptake of atmospheric CO~2~ following nutrient depletion depends on the
region depleted, gas exchange rate and oceanic circulation.

We confirm that the Southern Ocean (and in particular the Antarctic
region south of the Polar Front) is the most important area for CO~2~
uptake in all models studied.

We show that ocean physics and the details of the air-sea gas exchange
mechanism are crucial in determining the atmospheric CO~2~ response to
surface nutrient depletion.

An increase in vertical mixing or Southern Ocean winds decreases the
sensitivity of atmospheric CO~2~ to Southern Ocean nutrient depletion.
An increase in isopycnal mixing or a decrease in Southern Ocean winds
increases the sensitivity of atmospheric CO~2~ to Southern Ocean
nutrient depletion.

Atmospheric CO~2~ sensitivity to high latitude processes is driven by a
subtle interplay of biology, circulation, and gas exchange. Models with
fast gas exchange can show either more sensitivity or less Southern
Ocean sensitivity (than models with regular gas exchange), depending on
the model circulation.

This work confirms that the preformed nutrient inventory and the
remineralized nutrient inventory are excellent metrics for atmospheric
pCO~2~ both before and after nutrient depletion.

**5.) Decadal variations in equatorial Pacific ecosystems and
ferrocline/pycnocline decoupling**

The Equatorial Pacific is known for its large interannual to decadal
variability in circulation. In particular, the changes which occurred in
1976/77 have received considerable attention in the physical climate
literature. What is the impact of this variability on biogeochemistry.
Can decadal variability in ocean circulation lead to a decoupling of the
ferrocline, the pycnocline, and the nitricline in the Equatorial
Pacific?

We applied output from simulations conducted with the ORCA2-PISCES model

(Aumont and Bopp, 2006) to this question (Rodgers, submitted). The model
consisted of the ORCA2 configuration of the OPA ocean model, which has a
foodweb/biogeochemistry model (PISCES) embedded in it. The model was
forced with both NCEP-1 and ERA-40 reanalysis fluxes over multiple
decades.

Our main finding with the model is that a decrease in the amplitude of
the surface

windstress in the tropical Pacific in the mid-to-late-1970s leads to a
decrease in Fe and Chl concentrations in the upwelling regions of the
Eastern Equatorial Pacific after 1976/77. These changes find expression
predominantly during the upwelling season (the seasonal maximum for Fe
and Chl concentrations), when surface Fe and Chl concentrations tent to
be significantly higher pre-1976/77 than post-1976/77. The changes in
Chl concentrations need to be understood as modulations of the amplitude
of the seasonal cycle.

In contrast to what is found for Fe and Chl, for NO3 the decadal changes
in surface concentrations in the upwelling region about 1976/77 can be
described as a shift in the mean state.

It is shown that the response in surface Fe and Chl in the upwelling
region about

1976/77 is proportionally larger than the decadal change in surface wind
stress forcing, and it is also larger than the previously reported
change in the strength of the meridional overturning strength of the
Subtropical Cells (STCs). Importantly, this amplified response reflects
a decoupling of the ferrocline and pycnocline within the equatorial
Pacific. The nonlinearities responsible for the pycnocline/ferrocline
decoupling have potentially important implications for the response of
Equatorial Pacific ecosystems to climate change.

# III. Patch fertilization

A measure of the overall impact of the addition of iron on atmospheric
CO~2~ is the carbon-to-iron fertilization ratio, which reflects how much
additional CO~2~ is taken up from the atmosphere for a given amount of
iron added to the ocean. This fertilization ratio can be split into an
efficiency part that reflects how much CO~2~ is taken up from the
atmosphere per unit change in biological export (termed the atmospheric
uptake efficiency), and into a biological iron utilization ratio that
reflects how a given amount of added iron stimulates the biological
export of carbon (termed the iron utilization ratio). By definition, the
product of the atmospheric uptake efficiency and the biological iron
utilization ratio is the fertilization ratio.

As summarized in our last report (2003), our short-term
patch-fertilization scenarios (*Gnanadesikan et al., 2003*) suggest that
the efficiency of small-scale iron fertilization scenarios, i.e., the
fraction of increased export production that comes from the atmosphere,
is likely to be extremely low (2% after 100 years). An important caveat
is that the biogeochemistry model that was used for these simulations is
an extremely simple diagnostic model based on forcing surface nutrients
back towards observations, or to 0 in the case of the nutrient depletion
scenarios. Dutkiewicz et al. (2006) find much greater efficiencies of
atmospheric drawdown and the highest uptake of atmospheric CO~2~ when
iron is added in the equatorial Pacific, dramatically differing from the
previous macronutrient manipulation studies (e.g., Sarmiento and Orr,
1991; Gnanadesikan et al, 2003). The Iron Fertilization Model
Intercomparison Project (IFMIP) was designed to investigate the
potential of iron fertilization to remove atmospheric CO2 using models
that fully resolve the marine iron cycle and include much more complex
(and more realistic) representations of the marine biosphere than
previously employed (Chavez, 2007). This has permitted us to overcome an
important limitation of the prior studies in that it is possible now to
determine not only the atmospheric uptake efficiency, but also the
biological iron utilization ratio, and hence the carbon-to-iron
fertilization ratio. Two studies have been conducted to investigate the
factors that control the carbon-to-iron fertilization ratio.

One group carry out a series of sensitivity studies to examine the
processes that determine how the uptake of CO~2~ across the air-sea
interface is related to the export of organic matter across the base of
the euphotic zone (*Jin and al., in prep).* Atmospheric uptake
efficiency, i.e., the amount of atmospheric CO~2~ draw-down per unit of
biological export induced by iron fertilization, is a critical criterion
to assess the impact of changes of the biological pump on atmospheric
CO~2~, e.g., the feasibility, effectiveness with regard to lowering
atmospheric CO~2~ using iron fertilization. we use a Pacific Ocean setup
of ROMS (the Regional Oceanic Modeling System) at an eddy-permitting
resolution of 0.5 degrees, coupled with an ecosystem model, including
three phytoplankton functional groups and multiple nutrient limitation
(N, P, Si, Fe), and performed experiments in different patch sizes and
fertilization durations to investigate the factors that control the
uptake efficiecny (Jin et al., 2007). We find very high atmospheric
uptake efficiency for our standard patch iron fertilization, arriving
0.81 for ten years duration of iron fertilization. This high efficiency
is of little sensitivity to patch sizes in which iron is added. We find
the main factor that controls the atmospheric uptake efficiency is the
place where, within the euphotic zone, the stimulated export production
occurs. The closer to the surface the dissolved inorganic carbon is
taken from, the higher this inorganic carbon is replaced from the
atmosphere, resulting in high uptake efficiencies. In the equatorial
Pacific Ocean, we find that the uptake efficiency is proportional to the
fraction of stimulated net primary production that occurs at upmost 10
meters to total stimulated production. The results we find are relevant
for any changes in the ocean\'s biological pump. They can be used in
explaining atmospheric CO~2~ changes during glacial-interglacial and/or
future climate. Despite high atmospheric uptake efficiencies,
patch-scale iron fertilization of the ocean\'s biological pump tends to
remove little

CO~2~ from the atmosphere over the decadal timescale considered here.

Another group (Sarmiento et al., in prep) analyze the biogeochemical
model simulations in more detail and to carry out a series of
biogeochemical sensitivity studies in order to examine what processes
specifically account for the differences between models and between
regions. The sensitivity studies described here include how the response
to iron fertilization is affected by: (1) the inclusion of an
atmospheric reservoir (which the *Dutkiewicz, et al.* \[2006\] study did
not); and (2) by iron biogeochemistry including (a) how much iron is
added, (b) how effectively the ocean recycles iron, and (c) the time
scale of iron fertilization. The conclusions are:

1.  Contrary to *Dutkiewicz, et al.* \[2006\], we find the largest
    response in the Southern Ocean region with the Equatorial Pacific
    and other locations all being lower in both the biological export
    and atmospheric CO~2~ uptake. Our result is due to the high C:Fe
    uptake ratio. We believe that this difference in behavior between
    our analysis and that of *Dutkiewicz, et al.* \[2006\] is due
    primarily to the fact that their model is light limited in the
    Southern Ocean.

2.  The very high biological C:Fe uptake ratio of the Southern Ocean is
    driven in turn by the fact that this locations is unique in having
    both a high fraction of the added iron continuing to participate in
    biological production rather than being lost to deep waters where it
    is not available for reuse, and a high C:Fe ratio in the organic
    matter that is produced. No other station has both of these high.
    The differences between the models is not due to iron retention in
    the water column (as opposed to loss from the ocean), which is the
    same across all models.

3.  There is a very large loss of CO~2~ from the ocean when a realistic
    atmospheric reservoir is included (cf. *Gnanadesikan, et al.*
    \[2003\]). By the 100^th^ year of the IFMIP simulation, half of the
    CO~2~ that is absorbed by the model with no atmospheric reservoir is
    lost back to the atmosphere in the model with a realistic
    atmospheric reservoir. Thus the *Dutkiewicz, et al.* \[2006\]
    simulation gives twice as large a CO~2~ uptake as it should.

4.  As might be expected, there is a very large sensitivity of the
    biological response to an iron addition when the iron is retained in
    the model than when it is used once and lost. The simulations with
    iron lost have a much smaller biological response. Interestingly,
    the physical efficiency is insensitive to whether the iron is
    retained or lost. This is very different from the *Gnanadesikan, et
    al.* \[2003\] box model analysis, which shows much lower physical
    efficiency for their macronutrient depletion scenario (analogous to
    our iron added and lost scenario) than for their macronutrient
    addition scenario (analogous to our iron added and retained
    scenario).

5.  A final point we examined was whether additional iron could lead to
    depletion of nutrients during at least part of the year. Our results
    show that all the stations can be driven to 0 nitrate in the summer
    time, but only the Equatorial Pacific can be driven to 0 mean
    nitrate year around. In fact, in the Southern Ocean and Ross Sea,
    the nutrient concentration maximum recovers all the way to its
    wintertime maximum even in the 1000x case, testifying to the
    effectiveness of the circulation in returning nutrients to these two
    regions.

We also investigated the factors that limit the draw down in the
Southern Ocean (M. Hiscock, in preparation). Despite the completion of
ten iron enrichment experiments of various sizes and durations, the
degree to which iron addition will draw down nutrients and export carbon
below the mixed layer is poorly understood. This question will likely be
difficult to fully resolve in the field due to the exponential expansion
and dilution of an iron-enriched patch. For example, in the Southern
Ocean Iron Experiments (SOFeX) of 2002, the two iron enriched patches
increased in area by an order of magnitude in twenty days. Such a
ten-fold dilution increases the macronutrients available to the patch
while decreasing the concentration of iron and biomass within the patch
and is not reflective of the potential drawdown and bloom of a basin
scale Southern Ocean iron enrichment. With a combination of in situ
iron-enrichment measurements and climatological data we model an
iron-replete Southern Ocean in an attempt to answer these questions: Is
it possible to draw down the vast supplies of nutrients in the various
physical zones of the Southern Ocean? To what extent is an iron-replete
Southern Ocean light limited? Results from this work suggest that,
indeed, basin scale iron addition draws down more nitrate than iron
fertilization experiments. However, basin Fe-addition fails to deplete
nitrate concentrations entirely. It appears that in iron-replete
conditions the Southern Ocean is ultimately light limited. Further
experiments in which seasonal North Atlantic mixed layer depths are
simulated in the Southern Ocean showed that decreasing mixed layer depth
has a greater effect on nitrate utilization than adding iron does.

# IV. Model development

## References to discuss

Boyd, P.W., T. Jickells, C. S. Law, S. Blain, E. A. Boyle, K. O.
Buesseler, K. H. Coale, J. J. Cullen, H. J. W. de Baar, M. Follows, M.
Harvey, C. Lancelot, M. Levasseur, N. P. J. Owens, R. Pollard, R. B.
Rivkin, J. Sarmiento, V. Schoemann, V. Smetacek, S. Takeda, A. Tsuda, S.
Turner, A. J. Watson, 2007. Mesoscale iron enrichment experiments
1993-2005: Synthesis and future directions. Science, 315: 612-617.

Armstrong, R.A. 2006. Optimality-based modeling of nitrogen allocation
and photoacclimation in photosynthesis. Deep-Sea Research II 53:513-531.

Armstrong, R.A., submitted. Modeling size dependence of nutrient uptake:
a Michaelis-form alternative to Aksnes and Egge. (Deep-Sea Research I)

Deutsch, C., J. L. Sarmiento, D. M. Sigman, N. Gruber, and J. P. Dunne,
2007. Spatial coupling of nitrogen inputs and losses in the ocean.
Nature, 445: 163-167.

Dunne, J. P., R. A. Armstrong, A. Gnanadesikan, and J. L. Sarmiento,
2005. Empirical and mechanistic models for the particle export ratio.
Global Biogeochem. Cycles, 19, GB4026, doi:10.1029/2004GB002390.

Dunne, J. P., J. L. Sarmiento, and A. Gnanadesikan, in press. A
synthesis of global particle export from the surface ocean and cycling
through the ocean interior and on the sea floor. (Global Biogeochem.
Cycles)

Jin, X., N. Gruber, J. P. Dunne, J. L. Sarmiento, and R. A. Armstrong,
2006. Diagnosing the contribution of phytoplankton functional groups to
the production and export of particulate organic carbon, CaCO~3~, and
opal from global nutrient and alkalinity distributions. Global
Biogeochem. Cycles, 20, GB2015, doi10.1029/2005GB002532.

We have made considerable progress on developing our
biogeochemistry/ecology models in the following aspects.

1.  **Diagnosing the contribution of phytoplankton functional groups to
    the production and export of particulate organic carbon, CaCO~3~,
    and opal from global nutrient and alkalinity distributions**

We diagnose the contribution of four main phytoplankton functional
groups to the production and export of particulate organic carbon (POC),
CaCO~3~ and opal by combining in a restoring approach global oceanic
observations of nitrate, silicic acid and alkalinity with a simple size
dependent ecological/biogeochemical model. In order to determine the
robustness of our results, we employ three different variants of the
ocean general circulation model (OGCM) required to transport and mix the
nutrients and alkalinity into the upper ocean. In our standard model,
the global export of CaCO~3~ is diagnosed as 1.1 PgC yr^-1^ (range of
sensitivity cases 0.8 to 1.2 PgC yr^-1^) and that of opal as 180 Tmol Si
yr^-1^ (range 160 to 180 Tmol Si yr^-1^). CaCO~3~ export is found to
have three maxima at approximately 40^o^S, the equator, and around
40^o^N. In contrast, the opal export is dominated by the Southern Ocean
with a single maximum at around 60^o^S. The molar export ratio of
inorganic to organic carbon is diagnosed in our standard model to be
about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform
spatially. The molar export ratio of opal to organic nitrogen varies
substantially from values around 2 to 3 in the Southern Ocean south of
45^o^S to values below 0.5 throughout most of the rest of the ocean,
except for the North Pacific. Irrespective of which OGCM is used, large
phytoplankton dominate the export of POC, with diatoms alone accounting
for 40% of this export, while the contribution of coccolithophorids is
only about 10%. Small phytoplankton dominate net primary production
(NPP) with a fraction of \~70%. Diatoms and coccolithophorids account
for about 15% and less than 2% of NPP, respectively. These diagnosed
contributions of the main phytoplankton functional groups to NPP are
also robust across all OGCMs investigated. Correlation and regression
analyses reveal that the variations in the relative contributions of
diatoms and coccolithophorids to NPP can be predicted reasonably well on
the basis of a few key parameters.

**2) Optimality-based modeling of nitrogen allocation and
photoacclimation in photosynthesis.**

Only recently has it become widely recognized in the biogeochemistry
modeling community that the taxonomic composition of planktonic
ecosystems may play a key role in determining atmospheric CO${}_{2}$
(Moore et al. 2002a,b; Armstrong 2003a,b). This recognition implies that
the ability to represent competition and exclusion among phytoplankton
and zooplankton will be critical to predicting the response of the
oceanic carbon cycle to environmental change (Totterdell et al. 1993;
Armstrong 2003a,b). Models with such capabilities will of necessity be
complicated, and will require accurate but simple representations of
both growth and grazing. To achieve this predictive capability, models
must be able to represent the characteristic physiology of each
taxonomic group with reasonable fidelity; but in order to prevent a
catastrophic inflation of parameter space, constraints on parameter
values within taxa must also be developed.

While biochemistry and molecular biology provide the ultimate
mechanistic basis for modeling, models based directly on molecular
mechanisms are at present too detailed to be useful for biogeochemistry
modeling. Instead, modeling of physiological processes using tradeoffs
in resource allocation among competing physiological demands may produce
models compact enough to be used in biogeochemistry.

Essentially all ecosystem models in ocean biogeochemistry use one of two
strategies for modeling the interaction of light and nitrogen in
photosynthesis. The first approach is based on Liebig\'s "Law of the
Minimum;" in this model, photosynthetic rate is set by the "most
limiting" factor, in this case either nitrogen or light. A second
approach is to view limitation as a multiplicative process. A third,
recently popular state-of-the-art model of light/nitrogen interaction,
is that of Geider et al. (1998), which has a more complicated structure
in which nitrogen limitation and temperature affect maximum
photosynthetic rate through the dark reactions of photosynthesis, but do
not affect photosynthetic rate at low light levels.

As part of their model, Geider et al. (1996, 1998) posit that the
chlorophyll:carbon ratio $q^{C}$ is determined by a "chlorophyll *a*
synthesis regulation term." The mathematical form of this term is not
derived from considerations of nitrogen allocation between light
reactions and dark reactions, but rather by considering the redox state
of photosystem II.

In contrast, Armstrong (2006) produced a prediction of $q^{C}$ based on
the idea that nitrogen should be allocated to light and dark reactions
to maximize the rate of photosynthesis.

A full set of parameters was then estimated using data from Laws and
Bannister (1980, 2004); these were in turn used to predict
chlorophyll:carbon and nitrogen:carbon ratios as functions of growth
rate. The Armstrong (2006) model was entered into the NASA-sponsored
ppar3 competition among models of column productivity, where it
performed exceptionally well despite not having been "tuned" for this
purpose. Its performance suggests that the improved conceptualization of
nitrogen:carbon and chlorophyll:carbon relationships in Armstrong (2006)
outweighs any disadvantages from lack of tuning, and suggests that
Armstrong (2006) may therefore provide a robust theoretical underpinning
for a new generation of models of multiple-nutrient limitation.

**3) Modeling size dependence of nutrient uptake: a Michaelis-form
alternative to Aksnes and Egge.**

Munk and Riley (1952) were first to explore mathematically the idea that
nutrient supply to phytoplankton cells might be limited by diffusive or
advective transport. Later, Pasciak and Gavis (1974, 1975) recognized
that nutrient uptake is not limited solely by physical transport, but
also by active biological transport across the cell surface; they
therefore required that diffusive nutrient flux to the cell surface
match the active transport flux across the cell membrane. Much later,
Aksnes and Egge (1991) proposed a simplified model that included both
cell-surface transport by porter proteins and diffusive transport
through the surrounding medium, yet could be expressed in terms of
standard Michaelis-Menton parameters. This model has been widely used
for size-dependent modeling within the biogeochemistry modeling
community. Here I showed that the Aksnes/Egge model has severe problems
with representing size-dependence of nutrient uptake, especially at
small cell sizes; I provided an alternative Michaelis-form approximation
that may prove more useful to ecosystem and biogeochemistry modelers.
Finally, I compared alterative models by fitting them to iron uptake
data from Sunda and Huntsman (1997).

# V. Summary of main findings

Outreach (Sept. 1, 2000-Nov. 30, 2006)

During the period of this grant, we wrote a series of overview papers
related to the carbon cycle of the ocean that were informed by the
research that was carried out as part of this project. None of these
acknowledge a specific grant. These include:

-   *Edmonds et al. (2004)*

-   *Greenblatt & Sarmiento (2004)*

-   *Gruber et al, (2004)*

-   *Iglesias-Rodriguez (2002)*

-   *Marinov & Sarmiento (2004)*

-   *Sarmiento and Gruber (2002)*

In addition, we published a major textbook/research monograph on ocean
biogeochemistry that summarizes the state of our understanding in a
broad range of fields relevant to ocean carbon sequestration:

-   *Sarmiento and Gruber (2006)*

Graduate students and postdoctoral researchers\
trained (Sept. 1, 2000-Nov. 30, 2006)

## Graduate students:

-   Dr. Curtis Deutsch, PhD awarded by Princeton University in 2003,
    subsequently a Post-Doc at the University of Washington and now a
    professor at UCLA.

-   Dr. Irina Marinov, PhD awarded by Princeton University in 2005,
    subsequently a Post-Doc at MIT and now at Woods Hole Oceanographic
    Institution.

## Post-docs:

-   Dr. John Dunne, Princeton University, now at GFDL/NOAA

-   Dr. Michael Hiscock, Princeton University

-   Dr. Xin Jin, UCLA.

Publications:

## Book

Sarmiento, J. L., and N. Gruber, 2006. Ocean Biogeochemical Dynamics,
Princeton University Press, Princeton. 503 pp.

## Journal and book articles

### [Sept. 1, 2000-Nov. 30, 2003 (published only; see Appendix 1)]{.underline}

Deutsch, C., 2003. Biogeochemical constraints on the modern and glacial
oceanic nitrogen cycle. Ph.D. Thesis, Princeton University, Princeton,
NJ.

Gnanadesikan, A., J. L. Sarmiento, and R. D. Slater, 2003. Effects of
patchy ocean fertilization on atmospheric carbon dioxide and biological
production. Global Biogeochem. Cycles, 17 (2), doi:
10.1029/2002GB001940.

Iglesias-Rodriquez, M. D., R. Armstrong, R. Feely, R. Hood, J. Kleypas,
J. D. Milliman, C. Sabine, and J. Sarmiento, 2002. The marine calcium
carbonate budget in a changing ocean. EOS Transactions, 83 (34): 365,
374-375.

Sarmiento, J. L., J. Dunne, A. Gnanadesikan, R. M. Key, K. Matsumoto,
and R. Slater, 2002. A new estimate of the CaCO~3~ to organic carbon
export ratio. Global Biogeochem. Cycles, 16 (4), 1107,
doi:10.1029/2002GB001919.

Sarmiento, J.L., and N. Gruber, 2002. Sinks for anthropogenic carbon,
Physics Today, August 2002, pp 30-36.

### [Dec. 1, 2003-Nov. 30, 2006 (published, in press, and in preparation)]{.underline}

Armstrong, R.A. 2006. Optimality-based modeling of nitrogen allocation
and photoacclimation in photosynthesis. Deep-Sea Research II 53:513-531.

Armstrong, R.A., submitted. Modeling size dependence of nutrient uptake:
a Michaelis-form alternative to Aksnes and Egge. (Deep-Sea Research I)

Boyd, P.W., T. Jickells, C. S. Law, S. Blain, E. A. Boyle, K. O.
Buesseler, K. H. Coale, J. J. Cullen, H. J. W. de Baar, M. Follows, M.
Harvey, C. Lancelot, M. Levasseur, N. P. J. Owens, R. Pollard, R. B.
Rivkin, J. Sarmiento, V. Schoemann, V. Smetacek, S. Takeda, A. Tsuda, S.
Turner, A. J. Watson, 2007. Mesoscale iron enrichment experiments
1993-2005: Synthesis and future directions. Science, 315: 612-617.

Deutsch, C., J. L. Sarmiento, D. M. Sigman, N. Gruber, and J. P. Dunne,
2007. Spatial coupling of nitrogen inputs and losses in the ocean.
Nature, 445: 163-167.

Dunne, J. P., R. A. Armstrong, A. Gnanadesikan, and J. L. Sarmiento,
2005. Empirical and mechanistic models for the particle export ratio.
Global Biogeochem. Cycles, 19, GB4026, doi:10.1029/2004GB002390.

Dunne, J. P., J. L. Sarmiento, and A. Gnanadesikan, in press. A
synthesis of global particle export from the surface ocean and cycling
through the ocean interior and on the sea floor. (Global Biogeochem.
Cycles)

Edmonds, J., F. Joos, N. Nakicenovic, R. G. Richels, and J. L.
Sarmiento, 2004. Scenarios, targets, gaps, and costs. In: The Global
Carbon Cycle, ed. C. B. Field and M. R. Raupach, Island Press,
Washington, D.C., pp. 77-102.

Gnanadesikan, A., J. P. Dunne, R. M. Key, K. Matsumoto, J. L. Sarmiento,
R. D. Slater, and P, S. Swathi, 2004. Oceanic ventilation and
biogeochemical cycling: Understanding the physical mechanisms that
produce realistic distributions of tracers and productivity. Global
Biogeochem. Cycles, 18, GB4010, doi:10.1029/2003GB002097.

Greenblatt, J. B., and J. L. Sarmiento, 2004. Variability and climate
feedback mechanisms in ocean uptake of CO~2~. In: The Global Carbon
Cycle, ed. C. B. Field and M. R. Raupach, Island Press, Washington,
D.C., pp. 257-275.

Gruber, N., P. Friedlingstein, C. B. Field, R. Valentini, M. Heimann, J.
E. Richey, P. R. Lankao, E.-D. Schulze, and C.-T A. Chen, 2004. The
vulnerability of the carbon cycle in the 21st Century: An assessment of
carbon-climate-human interactions. In: The Global Carbon Cycle, ed. C.
B. Field and M. R. Raupach, Island Press, Washington, D.C., pp. 45-76.

Hiscock, M.R., Sarmiento, J.L., Sweeney, C., in preparation. Modeling a
basin scale iron addition in the Southern Ocean.

Jin, X., H. Frenzel and N. Gruber (in preparation), The impact of iron
fertilization induced changes of the biological pump on atmospheric
CO~2~.

Jin, X. and N. Gruber, 2003, Offsetting the radiative benefit of ocean
iron fertilization by enhancing N~2~O emissions. Geophysical Research
Letters, 30 (24) doi:10.1029/2003GL018458.

Jin, X., N. Gruber, J. P. Dunne, J. L. Sarmiento, and R. A. Armstrong,
2006. Diagnosing the contribution of phytoplankton functional groups to
the production and export of particulate organic carbon, CaCO~3~, and
opal from global nutrient and alkalinity distributions. Global
Biogeochem. Cycles, 20, GB2015, doi10.1029/2005GB002532.

Marinov, I., 2005. Controls on the air-sea balance of carbon dioxide,
PhD thesis, Atmospheric and Oceanic Sciences Program, Princeton
University.

Marinov, I., M. Follows, A. Gnanadesikan, R.D. Slater and J. L.
Sarmiento, in preparation. What controls the atmospheric CO~2~ response
to surface nutrient depletion ?.

Marinov, I. A. Gnanadesikan, J. R. Toggweiler, and J. L. Sarmiento,
2006. The Southern Ocean biogeochemical divide. Nature, 441: 964-967.

Marinov, I., A. Gnanadesikan, J. L. Sarmiento, J. R. Toggweiler, M.
Follows and B.K. Mignone, submitted. Impact of oceanic circulation on
biological carbon storage in the ocean and atmospheric pCO~2~. (Global
Biogeochem. Cycles)

Marinov, I., and J. L. Sarmiento, 2004. The role of the oceans in the
global carbon cycle: An overview. In: The Ocean Carbon Cycle and
Climate, ed. M. Follows and T. Oguz, NATO ASI, Ankara, Turkey, Kluwer
Academic Publishers, pp. 251-295.

Mignone, B. K., A. Gnanadesikan, J. L. Sarmiento, R. D. Slater, 2006.
Central role of southern hemisphere winds and eddies in modulating the
oceanic uptake of anthropogenic carbon. Geophys. Res. Lett., 33, L01604,
doi:10.1029/2005GL024464.

Najjar, R. G., X. Jin, F. Louanchi, O. Aumont, K. Caldeira, S. C. Doney,
J.-C. Dutay, M. Follows, G. M. Kay, E. Maier-Reimer, R. J. Matear, K.
Matsumoto, A. Mouchet, J. C. Orr, G. K. Plattner, J. L. Sarmiento, M. F.
Weirig, Y. Yamanaka, and A. Yool, in press. Impact of circulation on
export production, dissolved organic matter and dissolved oxygen in the
ocean: Results from OCMIP-2. (Global Biogeochem. Cycles).

Nevision, C.D., R. F. Keeling, R. F. Weiss, B. N. Popp, X. Jin, P. J.
Fraser, L. W. Porter and P. G. Hess, 2005. Southern Ocean ventilation
inferred from seasonal cycles of atmospheric N~2~O and O~2~/N~2~ at Cape
Grim, Tasmania. Tellus B, vol. 57, 218-119.

Rodgers, K.B., O. Aumont, C. Menkes. and T. Gorgues, submitted. Decadal
variations in equatorial Pacific ecosystems and ferrocline/pycnocline
decoupling. (Global Biogeochem. Cycles).

Sarmiento, J. L., N. Gruber, M. A. Brzezinski, and J. P. Dunne, 2004.
High latitude controls of the global nutricline and low latitude
biological productivity. Nature, 427: 56-60.

Sarmiento, J. L., R. D. Slater, M. E. Maltrud, A. Gnanadesikan, and J.
Dunne, in preparation. Iron fertilization models revisited.

Sarmiento, J. L., J. Simeon, and A. Gnanadesikan, in preparation.
Sub-Antarctic Mode Water pathways and nutrients.

Additional References Discussed in Report

**Other?**

Suntharalingam, P., and J. L. Sarmiento, 2000. Factors governing the
oceanic nitrous oxide distribution: Simulations with an ocean general
circulation model. Global Biogeochem. Cycles, 14: 429-454.

**Armstrong**

Aksnes, D.L., and J.K. Egge. 1991. A theoretical model for nutrient
uptake in phytoplankton. Marine Ecology Progress Series 70:65-72.

Armstrong, R.A. 2003a. A hybrid spectral representation of phytoplankton
growth and zooplankton response: The \"control rod\" model of plankton
interaction. Deep-Sea Research II 50:2895-2916.

Armstrong, R.A. 2003b. Representing biogeochemical diversity and size
spectra in ecosystem models of the ocean carbon cycle. *In*: Canham,
C.D., J.J. Cole, and W.K. Laurenroth (*Eds*.), Models in ecosystem
science. Princeton University Press, Princeton, NJ, pp. 254-271.

Geider, R.J., H.L. MacIntyre, and T.M. Kana. 1996. A dynamic model of
photoadaptation in phytoplankton. Limnology and Oceanography 41:1-15.

Geider, R.J., H.L. MacIntyre, and T.M. Kana. 1998. A dynamic regulatory
model of phytoplanktonic acclimation to light, nutrients, and
temperature. Limnology and Oceanography 43:679-694.

Laws, E.A., and T.T. Bannister. 1980. Nutrient- and light-limited growth
of *Thalassiosira* *fluviatilis* in continuous culture, with
implications for phytoplankton growth in the ocean. Limnology and
Oceanography 25:457-473.

Laws, E.A., and T.T. Bannister. 2004. Erratum: Nutrient- and
light-limited growth of *Thalassiosira fluviatilis* in continuous
culture, with implications for phytoplankton growth in the ocean.
Limnology and Oceanography 49:2316.

Moore, J.K., S.C. Doney, J.A. Kleypas, D.M. Glover, and I.Y. Fung.
2002a. An intermediate complexity marine ecosystem model for the global
domain. Deep-Sea Research II 49:403-462.

Moore, J.K., S.C. Doney, D.M. Glover, and I.Y. Fung. 2002b. Iron cycling
and nutient-limitation patterns in surface waters of the world ocean.
Deep-Sea Research II 49:463-507.

Munk, W.H., and G.A. Riley. 1952. Absorbtion of nutrients by aquatic
plants. Journal of Marine Research 11:215-240.

Pasciak, W.J., and J. Gavis. 1974. Transport limitation of nutrient
uptake in phytoplankton. Limnology and Oceanography 19:881-888.

Pasciak, W.J., and J. Gavis. 1975. Transport limited nutrient uptake
rates in *Ditylum brightwellii*. Limnology and Oceanography 20:604-617.

Sunda, W.G., and S.A. Huntsman. 1997. Interrelated influence of iron,
light, and cell size on marine phytoplankton growth. Nature 390:389-392.

Totterdell, I.J., R.A. Armstrong, H. Drange, J.S. Parslow, T.M. Powell,
and A.H. Taylor. 1993. Trophic resolution. *In*: Evans, G.T., and M.J.R.
Fasham (*Eds*.), Towards a model of ocean biogeochemical processes.
Springer-Verlag, New York, pp. 71-92.

**Rodgers**

**references for manuscripts submitted which are not related to iron**

Rodgers, K.B., J.L. Sarmiento, R. Key, A. Gnanadesikan, O. Aumont, N.
Metzl, L. Bopp,

A. Ishida, Y. Yamanaka, and M. Ishii (2007), Detection of Anthropogenic
Changes in Column Inventories of Oceanic DIC,submitted to Geophysical
Research Letters.

Rodgers, K.B., J.L. Sarmiento, C. Crevoisier, A. Gnanadesikan, C. de
Boyer Montegut,

N. Metzl, and O. Aumont (2007), A wintertime uptake window for
anthropogenic CO2 in

the North Pacific, submitted to GLobal Biogeochemical Cycles.

Gorgues, T., C. Menkes, O. Aumont, K.B. Rodgers, G. Madec, and Y.
Dandonneau (2007), Indonesian Throughflow Control of the eastern
equatorial Pacific biogeochemistry, in revision for Geophys. Res. Lett.

Iudicone, D., K.B. Rodgers, R. Schopp, and G. Madec (2007), An exchange
window for the injection of Antarctic Intermediate Water into the South
Pacific, in press, J. Phys. Oceanogr.

Laurian, A., A. Lazar, G. Reverdin, K.B. Rodgers, and P. Terray (2006),

Poleward propagation of spiciness anomalies in the North Atlantic Ocean,
Geophys. Res. Lett., 33, L13603, doi:10.1029/200GL026155.

Naegler, T., P. Ciais, K.B. Rodgers, and I. Levin (2006), Excess
radiocarbon constraints on air-sea gas exchange and the uptake of CO2 by
the oceans, Geophys. Res. Lett., 33, L11802, doi:10.1029/2005GL025408.

Raynaud, S., O. Aumont, K.B. Rodgers, P. Yiou, and J.C. Orr (2006),

Interannual-to-decadal variability of North Atlantic air-sea CO2 fluxes,
Ocean Sciences Discussions, 2, 437-472.

Appendix 1

Final Report for first grant period from September 1, 2007 to November
30, 2003

**Ocean carbon sequestration by fertilization:**

**An integrated biogeochemical assessment**

**Grant DE-FG02-00ER63009**

**DOE Office of Biological and Environmental Research**

**FINAL REPORT**

PRINCETON UNIVERSITY

Jorge L. Sarmiento, PI

Anand Gnandesikan, Co-PI

UNIVERSITY OF CALIFORNIA, LOS ANGLES

Nicholas Gruber, PI

# Overview

This final report summarizes research undertaken collaboratively between
Princeton University, the NOAA Geophysical Fluid Dynamics Laboratory on
the Princeton University campus, the State University of New York at
Stony Brook, and the University of California, Los Angeles from
09/01/2000 to 08/31/2003 sponsored by DOE's and entitled "Ocean carbon
sequestration by fertilization: An integrated biogeochemical
assessment".

Under this grant, we investigated a range of issues associated with the
proposal to fertilize the ocean with nutrients (such as iron) in order
to increase the export of organic matter from the ocean's near surface
waters and consequently increase the uptake of CO~2~ from the
atmosphere. There are several critical scientific questions that have
the potential to be make-or-break issues for this proposed carbon
sequestration mechanism:

(1) If iron is added to the ocean, will export of organic carbon from
    the surface actually occur? Clearly, if no export occurs, then there
    will be no sequestration.

(2) If iron fertilization does lead to export of organic carbon from the
    surface of the ocean, how much CO~2~ will actually be removed from
    the atmosphere? Even if carbon is removed from the surface of the
    ocean, this does not guarantee that there will be significant
    removal of CO~2~ from the atmosphere, since the CO~2~ may be
    supplied by a realignment of dissolved inorganic carbon within the
    ocean.

(3) What is the time scale of any sequestration that occurs? If
    sequestered CO~2~ returns to the atmosphere on a relatively short
    time scale, iron fertilization will not contribute significantly to
    slowing the growth of atmospheric CO~2~.

(4) Can the magnitude of sequestration be verified? If verification is
    extremely difficult or impossible, this option is likely to be
    viewed less favorably.

(5) What unintended consequences might there be from fertilizing the
    ocean with iron? If these are severe enough, they will have a
    significant impact on policy decisions.

Most research on carbon sequestration by fertilization has focused on
the first of these issues. Although a number of in situ fertilization
experiments have successfully demonstrated that the addition of iron
leads to a dramatic increase in ocean productivity, the question of
whether this results in enhanced export remains an open one. The primary
focus of our research was on the development of models to examine topics
(2) through (5), although some of the research we accomplished has
implications for the first of these topics as well. In the following we
present and discuss our main results and products.

We start with a discussion of our results from large-scale fertilization
experiments using a relatively simple ecosystem model. While these
experiments are very instructive in highlighting the mechanisms and
consequences, it is very unlikely that fertilization will ever be
carried out on such scales. We therefore conducted a detailed study to
investigate how patch-scale fertilization differs from that conducted at
basin scale. After presenting the results of this study, we then discuss
the results we obtained with regard to consequences of fertilization on
ocean biogeochemistry and radiative forcing. Since the existing
ecological/biogeochemical models at the beginning of our grant were not
adequate to investigate many important components of how ocean ecology
and biogeochemistry will respond to the addition of iron, a substantial
fraction of our efforts went also into the development of a model that
would allow us to quantitatively predict phytoplankton functional group
diversity. After reporting on our model development work, we close with
a summary of our outreach activities and publications.

Note: Publications supported by this grant are identified by italics.

# Large Scale Fertilization

When we began this project, we were thinking almost exclusively in terms
of long-term fertilization over large regions of the ocean such as the
Southern Ocean, with much of our concern being on how the ocean
circulation would control the response to a given fertilization
scenario. Early on in our project, it became clear that small
\"patch-scale\" fertilizations over limited regions of order 100 km
diameter were much more likely. We describe here the large-scale
fertilization simulations we carried out, and discuss patch scale
fertilization simulations in the following section.

We assumed that iron addition is able to cause macronutrient depletion
and applied such depletion over different basins in a set of model
simulations (*Marinov et al., in prep.-b*). Because of the circulation
characteristics and large concentration of unused surface nutrients in
the Southern Ocean, depleting nutrients in this area results in much
higher atmospheric CO~2~ drawdown than depleting nutrients in the North
Atlantic, North Pacific, or tropical ocean, confirming earlier work on
this problem (Sarmiento & Orr, 1991).

Based on four sets of simulations, in which isopycnal and diapycnal
diffusivity coefficients were varied in the tracer equations
(*Gnanadesikan et al., submitted*), resulting in very different
circulation scenarios, we found that depleting nutrients results in
similar sequestration of CO~2~ despite large variations in the
productivity and nutrient profiles (*Marinov et al., 2003, in prep.-a*).
This surprising result is due in part to how the rest of the ocean
responds to Southern Ocean fertilization. Complete removal of
macronutrients in this region results in the removal of the performed
nutrients that are usually transported into the low latitudes
(*Sarmiento et al., in press*). As a consequence the nutrient levels
throughout the upper ocean are reduced, causing low latitude
productivity to plummet (*Sarmiento et al., in press*). In the case of a
model that has low vertical mixing, low latitude productivity decreases
by about 75% (*Marinov et al.*, *in prep.-b; Sarmiento et al., in
press)*. This effect extends all the way to the North Atlantic, the
other major source of deep water production, reducing surface nutrients
in this region as well. Thus depleting the Southern Ocean enhances the
efficiency of nutrient removal by the biological pump throughout the
global ocean (though at the cost of remote reduction in productivity).
Models with elevated vertical mixing show a larger local response in the
Southern Ocean but a weaker remote response in the rest of the global
ocean so that the total effect is very similar. .

In order to understand what areas of the Southern ocean are responsible
for most of the CO~2~ uptake with depletion, we preferentially depleted
the Southern Ocean only in areas with densities larger than sigma theta
=27.1, 27.3, 27.5, and 27.6, respectively (*Marinov et al., in
prep.-b*). Depleting Southern Ocean areas with densities larger than
sigma theta=27.1 (roughly south of 55°S) results in a 65.5 ppm drawdown
in atmospheric CO~2~. This accounts for most of the 73 ppm resulting
from the depletion of the Southern ocean south of 30°S. Interestingly,
convection centers covering a relatively small fraction of this region
prove to be a major player in the response to depletion, accounting for
the majority of the Southern Ocean uptake of CO~2~ following
fertilization.

# Patch fertilization

We performed simulations to examine the impact of fertilizing small
patches of ocean (a few hundred kilometers on a side) to sequester
carbon dioxide (*Gnanadesikan et al., 2003*). We found that less than
10% of the increased export from a one-month fertilization event came
from the atmosphere after 100 years. This efficiency dropped to 2% if
the organic matter was remineralized in the water column rather than
sinking all the way to the sediment water interface. Verification of
this removal is likely to pose a major challenge as the resulting
air-sea fluxes are well below our ability to detect them in the ocean,
and the time and space scales involved are very long. A way to enhance
the response is to add macronutrients at the same time as iron. If the
added iron is retained in the water column with the micronutrients upon
remineralization, the efficiency of the fertilization increases to 40%.
However, biogeochemists do not have a very good understanding yet of
whether the added iron would in fact be retained or scavenged by sinking
particles and deposited into the sediments.

We found that patch fertilization without macronutrient addition will
reduce biological productivity because it removes nutrients from the
upper ocean. The impact of this can be quite large, with the worst-case
scenario giving a 30-ton reduction in productivity for every 1-ton of
carbon removed from the atmosphere. In the following section, we discuss
additional work that we did on the impact of patch as well as
large-scale fertilization on the nitrous oxide concentration in the
atmosphere.

We also found that there can be a strong dependence of efficiency on the
way that biology is parameterized in the model, including the
remineralization profile. The biological model that we used in our patch
fertilization scenarios is a simple \"diagnostic\" model such as that
described in a later section. During the period of this grant, we made
considerable progress on the background work required to develop a more
realistic model. With the renewal of this grant, we are now in the final
stages of implementing this model and will use it to re-examine the
patch fertilization scenarios.

# Biogeochemical Consequences of Fertilization

The expected increase in the export of organic material from the surface
ocean into the interior as a consequence of a successful fertilization
will result in enhanced rates of remineralization in the deeper layers
of the ocean. *Fuhrman and Capone* (1991) pointed out that such an
increase in remineralization will likely enhance production of nitrous
oxide, that, when emitted into the atmosphere, will result in some
offset of the radiative benefit of the atmospheric CO~2~ reduction
stemming from the fertilization. This potential feedback is particularly
worthwhile investigating since N~2~O is 200 times more radiatively
active than CO~2~.

We investigated the consequences of iron fertilization on oceanic N~2~O
emissions with our diagnostic biological model, to which we added a
simple description of the N~2~O cycle following *Suntharalingam* (1997)
(*Jin and Gruber, in press*). We separately considered the nitrification
and low oxygen mechanisms that are known to produce N~2~O. As the
relative role of these processes are not well known, we set up two
simulations, one considering the nitrification pathway only and the
second considering the low oxygen pathway only. As the tracer equation
is linear we can combine the results of these extreme cases to obtain
any intermediate case. Our \"best estimate\" case is that which assumes
that about half of the global oceanic production of N~2~O comes from the
nitrification pathway, and the other half from the low oxygen pathway.
This best estimate is based on the studies of *Suntharalingam and
Sarmiento* (2000) and our own extensive comparison of our model results
with in situ data (*Jin and Gruber*, *in prep*). We conducted
large-scale and patch fertilization experiments in the tropics, the
Southern Ocean, the Northern North Pacific Ocean and the Northern North
Atlantic Ocean. We further considered a fertilization of 10 year
duration, followed by 90 years without fertilization and a fertilization
of 100 year duration.

We found that the magnitude of the warming offset caused by nitrous
oxide release to the atmosphere is substantial, but is highly dependent
on the location and duration of fertilization (*Jin and Gruber, in
press*). The largest offset (of the order of 100%) is found when
fertilization is undertaken in the tropics, particularly when it is of
limited duration and size. Smaller, but still substantial effects are
found when fertilization is undertaken elsewhere and over longer
periods. These results suggest that any assessment of ocean
fertilization as a mitigating option is incomplete without consideration
of the N~2~O feedback.

The oceanic oxygen concentrations play a key role in controlling the
magnitude of the N~2~O feedback, due to the very high N~2~O yields
occurring at low oxygen concentrations (*Jin and Gruber, in prep*). The
low oxygen concentration found in the thermocline of the tropical ocean
makes this region the most sensitive one with regard to the magnitude of
the N~2~O feedback. The substantial reduction of the thermocline oxygen
concentration in the North Pacific as a result of the long-term
fertilization turns this region from one whose N~2~O formation mechanism
is dominated by the nitrification pathway to one dominated by the
denitrification pathway, highlighting the non-linear response of the
system.

As discussed above for the Southern Ocean, a second important
consequence of iron fertilization is the decrease of primary production
in regions outside those that are fertilized (*Gnanadesikan et al.,
2003; Sarmiento et al, in press; Marinov et al., in prep-b*).
Interestingly, this decrease is another reason why the magnitude of the
N~2~O feedback is smallest in the Southern Ocean, as this decrease in
the low latitude production actually reduces the N~2~O production in the
low latitudes, where yields tend to be high, leading to quite a
substantial offset in the increased N~2~O production in the Southern
Ocean (*Jin and Gruber, in press*).

# Model Development

The existing ocean biogeochemistry/ecology models at the onset of this
project were insufficient to address many of the detailed questions
associated with the role of upper ocean ecosystem structure in
determining the response of the ocean to nutrient fertilization. In
particular, we suggested that the oceanic response to iron fertilization
would depend very strongly on the specific response of the main
phytoplankton functional groups. For example, the impact of increased
productivity on the air-sea balance of CO~2~ would depend on the
relative ratio of organisms that produce CaCO~3~ shells (hypothesized to
be due mainly to coccolithophorids) and those that just produce organic
matter. The most important group of species in the latter group are
diatoms, which require silicic acid to grow. Since the formation of
CaCO~3~ increases the partial pressure of CO~2~, a replacement of
coccolithophorids by diatoms or some other phytoplankton while keeping
export production the same would lead to a sequestration of CO~2~ from
the atmosphere. We have proposed such a mechanism to explain in part the
observed change in atmospheric CO~2~ between the last glacial maximum
and today (Brzezinski et al, 2002; Matsumoto et al., 2002), but an
assessment of the impact of such changes in ecology on human-scale ocean
iron fertilization requires a detailed predictive model that includes
such phytoplankton functional group diversity explicitly. No such model
existed at the onset of our project. We proposed to develop such a model
in three stages.:

-   A diagnostic biogeochemical model, in which all the nutrients and
    alkalinity are forced towards the observed fields at the surface of
    the ocean.

-   A semi-prognostic biogeochemical model, in which only phosphate is
    forced towards observations at the surface of the ocean, and export
    is predicted by a set of ecological rules.

-   A fully prognostic model, which explicitly models phytoplankton
    dynamics through light and nutrient limitation.

We made considerable progress on all three of these modeling approaches
and discuss each of them in turn. With the renewal of this grant, we are
now in the final stages of implementing these models and will use them
for a new set of fertilization scenarios that we expect will settle many
of the issues that were raised by our earlier studies.

## 1.)Diagnostic biogeochemistry

In this model, we infer biological productivity from the physical supply
of nutrients through the mismatch between modeled and observed surface
nutrient fields. By separately modeling nitrate, phosphate, silicate,
alkalinity and iron, we infer the stoichiometry of elemental fluxes.
Previous runs done at Princeton have had a very simple relationship
between production and export. We have developed a new diagnostic model
of biogeochemical cycling which takes advantage of the large amount of
data collected during the past decade relating primary production to
export, as well as distributions of dissolved organic matter (cf., *Jin
et al., in prep*.). This model starts by restoring surface phosphate
towards observations and then uses a simple ecosystem (*Dunne et al,
submitted, GBC)* to allocate the diagnosed total production to small and
large phytoplankton). Phosphorus surplus is used to diagnose nitrogen
fixation in the surface while anoxic remineralization is used to
diagnose denitrification in the interior (*Deutsch 2003*). The
contribution of diatoms and coccolithophorids to the large phytoplankton
production is determined by converting the diagnostically inferred opal
and CaCO~3~ productions into organic nitrogen production. This model
allows us to infer the partitioning of organic productivity into sinking
material, regenerated nutrients and dissolved organic material and
compare our estimates of productivity with satellite observations of
ocean color (*Dunne et al., 2002, Eos Trans.*).

We have coupled the cycling of particulate organic material in the water
column to that of silicon and alkalinity through the inclusion of the
protection of sinking organic material by opal and calcium carbonate
ballast, allowing us to reproduce the global constancy in the carbon
content in sinking material collected in deep sediment traps (*Dunne et
al., in prep.-b*). Our preliminary results show a global organic carbon
production of 13.9 Pg C/yr, a CaCO~3~ production of 1.16 Pg C/yr and an
opal production of 188 T mol Si/yr. The implied export ratios of
inorganic carbon to organic carbon are about 0.08:1, consistent with the
vertical ratio analysis of *Sarmiento et al. (2002)* (see below)*.*
Small phytoplankton are found to dominate, with more than 75% of global
production associated with this functional group. Their dominance is
particularly strong in the low latitudes. Among the large phytoplankton,
the diatoms, coccolithophorids and other large phytoplankton contribute
roughly equally to NPP. As expected, diatoms tend to dominate at high
latitudes, where they make up more than 40% of the large phytoplankton
production. Equatorward of the diatom-dominated belt, coccolithophorids
are dominant. A paper on this topic is currently in preparation (*Jin et
al., in prep*.).

In our attempt to characterize the cycling of dissolved organic matter,
we found the inclusion of differential production or regeneration of
dissolved organic material (DOM) critical in reproducing observed
meridional trends. Specifically, production in the oligotrophic regions
of DOM was inferred to be either a much larger fraction of the total
production, or much longer-lived. We are using new support to continue
experimenting with various mechanistic formulations of the sinking
detrital pool to investigate controls on remineralization in the ocean
interior including such effects as the protection of organic material by
mineral ballast, temperature, lability, sinking velocity and zooplankton
grazing (*Dunne et al., in preb.-b*).

##  2.)Semi-prognostic biogeochemistry

We developed a first version of this model, but put this project on hold
until we could complete analysis of the diagnostic model to develop an
improved model for prediction of stoichiometric ratios. A specific
result from this study was the necessity for including variable Si:N
ratios in any prognostic model. In particular, unless we simulate the
unusually high Si:N drawdown ratios of Southern Ocean iron limited
production, we predict excessive silicic acid concentrations in low
latitudes and an unrealistic domination of diatoms at all latitudes
(cf., Brzezinski et al., 2002; Matsumoto et al. 2002; *Sarmiento et al.,
in press*).

## 3.)Prognostic model development

The prognostic version of the model, which we expect to complete by
early next year with the renewal of this grant, builds on the two
previous models by incorporating explicit phytoplankton growth with a
simple light penetration model. It includes three phytoplankton groups:
small (patterned after cyanobacteria with coccolithophorids also
included as a fixed fraction of the small phytoplankton), large
(patterned after diatoms) and diazotrophic (patterned after
trichodesmium) (*Dunne et al., submitted*). Phytoplankton growth rates
are determined as a function of temperature, light and nutrient
limitation as well the C:Chl ratio in phytoplankton which is allowed to
vary instantaneously after the Geider et al. (1996) model of equilibrium
carbon and nutrient uptake. Nutrient limitation is expressed using
Liebig\'s law of the minimum by nitrate, ammonia, phosphate, silicate
and iron. Nitrogen limitation includes ammonia inhibition and nitrogen
fixation by diazotrophs. The export and remineralization
parameterizations developed with the diagnostic model (cf., *Dunne et
al., submitted,* and *Dunne et al., in prep.-b*) are implemented to
predict the export and fate of organic matter.

# Analyses of Observations and Models

As part of our efforts to develop improved models of ocean ecology and
to test those models, we did several combined model-data analysis
studies of how export from the surface of the ocean is related to the
primary production, and what the CaCO~3~ and diatom opal to organic
carbon export ratios are around the world. The response of the ocean to
a fertilization event will depend greatly on the nature of the organisms
that carry out the enhanced production, and their ability to export
organic matter from the surface of the ocean. The information gained
from these studies provided major insights that were or are being
incorporated into our models.

## 1.)Particle export ratio

Iron fertilization experiments in waters with unutilized nutrient have
unfailingly shown an increase in primary production, yet have failed so
far to clearly demonstrate an increase in export (Nodder and Waite,
2001). Is this a result of incomplete observations, or are there reasons
to expect that the additionally fixed nutrients merely cycle more
rapidly within the mixed layer? This question can be cast in terms of
the pe-ratio, the ratio of particulate export to primary production.

We have synthesized observations of the particle export ratio and used
this synthesis to calibrate empirical algorithms of the particle export
ratio as a function of the log of biomass (or productivity) and
temperature (*Dunne et al., submitted, GBC*). We also developed a
predictive, ecosystem-based model of the pe-ratio as it varies in space
and time through temperature, and productivity through changes in
phytoplankton size structure and mineral ballast protection of sinking
detritus. The model is driven by the difference in the functional form
of the grazing loss term and export efficiencies between large
phytoplankton (where grazing is proportional to the four-thirds power of
efficiently-exported biomass) and small phytoplankton (where grazing is
proportional to the second power of inefficiently-exported biomass). We
demonstrate that both the empirical regression and the simple ecosystem
size-based model reproduce the observed variability in pe-ratios (Dunne
et al., submitted).

This simple ecosystem model has been implemented within the diagnostic
ocean biogeochemistry model described in the previous section. This
model is able to reproduce major patterns in primary production as
interpreted from satellite-based estimates of ocean color, surface
irradiance, temperature and euphotic zone depth. Important differences
exist, however, which point to deficiencies in the model\'s
representation of ocean circulation, our biological representation of
ocean ecosystems, and the satellite chlorophyll and primary productivity
algorithms.

## 2.)A new estimate of the CaCO~3~ to organic carbon export ratio

We used an ocean biogeochemical-transport box model of the top 200 m of
the water column to estimate the CaCO~3~ to organic carbon export ratio
from observations of the vertical gradients of potential alkalinity and
nitrate (*Sarmiento et al., 2002*). We found a global average molar
export ratio of 0.06 +/-0.03. This is substantially smaller than earlier
estimates of 0.25 on which a majority of ocean biogeochemical models had
based their parameterization of CaCO~3~ production. Contrary to the
pattern of coccolithophore blooms determined from satellite
observations, which show high latitude predominance, we found maximum
export ratios in the equatorial region and generally smaller ratios in
the subtropical and subpolar gyres. Our results suggest a dominant
contribution to global calcification by low-latitude non-bloom forming
coccolithophores or other organisms such as foraminifera and pteropods.

## 3.)A new estimate of the opal to organic carbon export ratio

We used the same box model employed for estimating the CaCO~3~ to
organic carbon export ratio (see above) to estimate the opal to organic
nitrogen (and thereby, carbon) export ratio from observations of the
vertical gradients of silicic acid and nitrate (*Sarmiento et al., in
press*). This study shows that the Southern Ocean has exceptionally high
export ratios that result in a complete depletion of silicic acid in
waters that are still rich in nitrate. The export of this water to the
north explains the low concentration of silicic acid in much of the rest
of the world. Including this in models is essential to correctly
simulating diatom production, as noted previously.

##  4.)A new estimate of global particle export and remineralization of the major elements

We have used our particle export ratio estimates in combination with
satellite based estimates of primary production to calculate export
production of organic carbon in the world ocean. These export production
maps have been used to test our model simulations (*Gnanadesikan, et
al., submitted*). The box model method that we employed to estimate the
CaCO~3~ and opal to organic carbon export ratios can also be used to
estimate the export ratios of the other macronutrients (nitrogen and
phosphorus). Combining the organic carbon export production maps with
our export ratio estimates makes it possible to provide a new set of
global estimates of the particle export of all the major nutrients. We
can also use the remineralization functions to estimate the cycling of
this exported organic matter in the ocean interior. We are presently
writing a paper describing the results of this analysis (*Dunne et al.,
in prep.-a*), and will continue to use the results of these types of
analyses to test and improve our ocean biogeochemistry models for
predicting the response to fertilization.

# Outreach

During the period of this grant, we wrote a series of overview papers
related to the carbon cycle of the ocean that were informed by the
research that was carried out as part of this project, although they do
not explicitly acknowledge any specific grant. These include: *Edmonds
et al., in press, Gruber et al, in press, Marinov et al. in press,* and
*Sarmiento and Gruber (2002).*

# Summary

In concluding, we return to the set of five questions we raised in the
introductory overview and use these as a template to summarize the major
findings of our research with relevance to the use of fertilization to
sequester CO~2~ from the atmosphere.

(1) If iron is added to the ocean, will export of organic carbon from
    the surface actually occur?

Ultimately, this question must be answered by *in situ* experiments.
However, our empirical work on the relationship between particle export
and primary production (*Dunne et al., submitted*) is certainly
consistent with the general expectation that the increased primary
production that has been observed in iron fertilization studies should
result in increased export.

(2) If iron fertilization does lead to export of organic carbon from the
    surface of the ocean, how much CO~2~ will actually be removed from
    the atmosphere?

Our short-term patch-fertilization scenarios (*Gnanadesikan et al.,
2003*) suggest that the efficiency of small-scale iron fertilization
scenarios, i.e., the fraction of increased export production that comes
from the atmosphere, is likely to be extremely low (2-10% after 100
years). This low efficiency can go as high as 40% after 100 years, but
only if macronutrients (nitrate and phosphate) are added with the iron
and if the iron is retained in the water column rather than scavenged.
Long-term basin-scale fertilization scenarios are more efficient in
preventing the carbon from escaping back to the atmosphere.

An important caveat is that the biogeochemistry model that was used for
these simulations is an extremely simple diagnostic model based on
forcing surface nutrients back towards observations, or to 0 in the case
of the nutrient depletion scenarios. We did not include functional
groups in this model, and our modeling of the iron distribution and
interior remineralization of organic matter was also rudimentary. A
critical issue that remains to be addressed is how our conclusions would
be affected if we used a more realistic biogeochemistry model such as we
developed as part of this grant (cf., *Deutsch, 2003, Dunne et al.,
submitted, Gnanadesikan et al., submitted,* and *Dunne et al., in
prep.-b*). Ongoing research with a renewal of this grant should enable
us to implement our new biogeochemistry model and address these issues
in the near future.

(3) What is the time scale of any sequestration that occurs?

The work described under (2) shows that long-term sequestration requires
long-term fertilization. A major reason for the low efficiency we found
in our patch-fertilization scenarios is because of the rapid return of
the sequestered carbon back to the atmosphere at locations far away from
the region of fertilization.

(4) Can the magnitude of sequestration be verified?

The short answer to this is very likely not, at least for the
patch-fertilization scenarios (*Gnanadesikan et al., 2003*). Direct
verification by measurement of the air-sea CO~2~ fluxes that result from
a given fertilization event presents an insurmountable challenge with
present technology. The fluxes are far smaller than the natural
variability, and their spatial scale, particularly the longer-term
escape of the sequestered CO~2~, is vast. Indirect verification by
monitoring of the actual export of organic carbon is challenging, and
inferring the impact of such an export event requires a far better
understanding of what determines the efficiency of sequestration than we
have at present. Verification is also extremely challenging with regard
to the possible offsetting effect from N~2~O emissions. *Jin and Gruber
(in press)* show that the majority of the fertilization induced
outgassing of N~2~O into the atmosphere occurs well outside the patch
and continues well into the future even after fertilization has stopped.

(5) What unintended consequences might there be from fertilizing the
    ocean with iron?

Our research suggests that there may be quite significant negative
impacts from fertilization. In the patch-fertilization scenarios, we
find a long-term reduction in biological production as nutrients are
removed from the surface and sequestered at depth, unless macronutrients
are added with the iron and the iron is retained in the water column
(*Gnanadesikan et al., 2003*). Such changes in oceanic biological
production are even more marked in the large-scale fertilization
scenarios, where there may be as much as a 75% reduction in global
export production resulting from a Southern Ocean fertilization
(*Sarmiento et al., in press,* and *Marinov et al., in prep.-b*). There
is also a potentially large production of nitrous oxide, a potent
greenhouse gas that may counteract much of the effect of any CO~2~
removal that is achieved by a given fertilization event (*Jin and
Gruber, in press*).

# Students and postdoctoral researchers trained:

-   Dr. Xin Jin, Post-Doc, UCLA.

-   Irina Marinov, Graduate Student, Princeton University

-   Dr. Curtis Deutsch, Graduate Student, Princeton University,
    presently a Post-Doc at the University of Washington

-   Dr. John Dunne, Post-Doc, Princeton University, presently at
    GFDL/NOAA

# Publications:

## Journal and Book Articles (published, in press, and in preparation)

Deutsch, C., 2003. Biogeochemical constraints on the modern and glacial
oceanic nitrogen cycle. Ph.D. Thesis, Princeton University, Princeton,
NJ.

Dunne, J. P., R. A. Armstrong, A. Gnanadesikan, J. L. Sarmiento, and R.
D. Slater (in preparation-a) New estimates of global particle export of
the major elements and their cycling in the ocean interior. (Global
Biogeochem. Cycles)

Dunne, J. P., R. A. Armstrong, A. Gnanadesikan, J. L. Sarmiento, and R.
D. Slater, submitted. Empirical and mechanistic models for particle
export ratio. (Global Biogeochem. Cycles)

Dunne, J. P., A. Gnanadesikan, P. Swathi, J. L. Sarmiento and R.
Armstrong (in preparation-b) Controls on global particle export and
remineralization in a global ocean general circulation model, (Global
Biogeochemical Cycles)

Edmonds, J., F. Joos, N. Nakicenovic, R. Richels, and J. L. Sarmiento,
in press. Scenarios, targets, gaps, and costs. In: Toward CO2
Stabilization: Issues, Strategies, and Consequences, A SCOPE/GCP Rapid
Assessment Project, ed. C. Field and M. Raupach, Island Press,
Washington, D.C.

Gnanadesikan, A., J. P. Dunne, R. M. Key, K. Matsumoto, J. L. Sarmiento,
R. D. Slater, and P,. S. Swathi, submitted. Oceanic vertical exchange
and biogeochemical cycling: Comparisons between models and new
observational estimates. (Global Biogeochem. Cycles)

Gnanadesikan, A., J. L. Sarmiento, and R. D. Slater, 2003. Effects of
patchy ocean fertilization on atmospheric carbon dioxide and biological
production. Global Biogeochem. Cycles, 17 (2), doi:
10.1029/2002GB001940.

Gruber, N., P. Friedlingstein, C.B. Field, R. Valentini, M. Heimann,
J.E. Richey, P. Romero-Lankao, D. Schulze, and C.-T.A. Chen, in press.
The vulnerability of the carbon cycle in the 21st century: An assessment
of carbon-climate-human interactions, In: *Toward CO~2~ Stabilization:
Issues, Strategies, and Consequences* ,Eds: C. B. Field and M. R.
Raupach, Island Press.

Jin, X., J. Dunne, C. Deutsch and N. Gruber, in prep., Diagnosing
phytoplankton functional groups from global nutrient and alkalinity
distributions. (Global Biogeochemical Cycles)

Jin, X. and N. Gruber, in prep. Constraining oceanic N~2~O production
mechanisms by an inversion of large-scale oceanic N~2~O data. (Global
Biogeochemical Cycles)

Jin, X. and N. Gruber, in press. Offsetting the radiative benefit of
ocean iron fertilization by enhancing N2O emissions (Geophys. Res.
Lett.)

Marinov, I., A. Gnanadesikan, J.L. Sarmiento and R.D. Slater, in
preparation-a, What controls the air-sea flux of carbon dioxide? (Global
Biogeochem. Cycles.)

Marinov, I., and J. L. Sarmiento, in press. The role of the oceans in
the global carbon cycle: An overview. In: Ocean Carbon Cycle and
Climate, ed. M. Follows and T. Oguz, NATO ASI, Ankara, Turkey.

Marinov, I., J.R. Toggweiler, A. Gnanadesikan, K. Matsumoto, J.L.
Sarmiento and R.D. Slater, in preparation-b, Impacts of nutrient
depletion in general circulation models and box models. (Global
Biogeochem. Cycles)

Sarmiento, J. L., J. Dunne, A. Gnanadesikan, R. M. Key, K. Matsumoto,
and R. Slater, 2002. A new estimate of the CaCO~3~ to organic carbon
export ratio. Global Biogeochem. Cycles, 16 (4), 1107,
doi:10.1029/2002GB001919.

Sarmiento, J.L., and N. Gruber, 2002. Sinks for anthropogenic carbon,
Physics Today, August 2002, pp 30-36.

Sarmiento, J. L., N. Gruber, M. A. Brzezinski, and J. P. Dunne, in
press. High latitude controls of the global nutricline and low latitude
biological productivity. (Nature).

## Conference Abstracts:

Dunne, J.P., R. A. Armstrong, C. Deutsch, A. Gnanadesikan, J.L.
Sarmiento, 2002. A multi-element ecosystem model for global
biogeochemical cycles, Eos Trans. AGU, 83(4), Ocean Sci. Meet. Suppl.,
Abstract OS32L-12.

Gnanadesikan, A., J.L. Sarmiento, and R.D. Slater, 2001. Ocean
fertilization and biological productivity, Proceedings, 1st National
Conference on Carbon Sequestration, National Energy Technology
Laboratory, Paper 6D-1.

Gnanadesikan, A., J.L. Sarmiento and R.D. Slater, 2002. Changes in
atmospheric carbon dioxide and biological productivity induced by patchy
fertilization, Eos Trans. AGU, 83(4), Ocean Sci. Meet. Suppl., Abstract
OS51F-08.

Gnanadesikan, A., J.L. Sarmiento and R.D. Slater, 2002. Efficiency and
effects of carbon sequestration through ocean fertilization,
Proceedings, Sixth International Conference on Greenhouse Gas Control
Technologies, Paper I3-5.

Jin, X., C. Deutsch, N. Gruber, and K. Keller, 2002. Assessing the
consequences of iron fertilization on oceanic N2O emissions and
radiative forcing, Eos Trans. AGU, 83(4), Ocean Sci. Meet. Suppl.,
Abstract OS51F-10.

Jin, X. and N. Gruber, 2002. Offsetting the radiative benefit of ocean
iron fertilization by enhancing ocean N2O emissions, Eos Trans. AGU,
83(47), Fall Meet. Suppl., Abstract U21A-0006.

Jin, X, N. Gruber, J.P. Dunne, and C. A. Deutsch. Diagnosing net
production by the major phytoplankton functional groups from ocean
nutrient and alkalinity data. Eos Trans. AGU, Ocean Sci. Meet. Suppl, in
press.

Marinov, I., A. Gnanadesikan, K. Matsumoto, J.L. Sarmiento, R. D.
Slater, 2002. An analysis of large-scale ocean fertilization under
different circulation scenarios, Eos Trans. AGU, 83(4), Ocean Sci. Meet.
Suppl., Abstract OS51F-12.

# Additional References Discussed in Report

Brzezinski MA, Pride CJ, Franck VM, Sigman DM, Sarmiento JL, Matsumoto
K, Gruber N, Rau GH, Coale KH, 2002. A switch from Si(OH)(4) to NO3-
depletion in the glacial Southern Ocean, Geophys. Res. Lett., 29,
10.1029/2001GL014349.

Codispoti, L.A. and J.P. Christensen, 1985. Nitrification,
denitrification and nitrous oxide cycling in the Eastern Tropical South
Pacific Ocean. Marine Chemistry, 16, 277-300.

Fuhrman, J.A., and D.G. Capone, 1991. Possible biogeochemical
consequences of ocean fertilization. *Limnol. Oceanogr.* 36: 1951-1959.

Geider, R., H. MacIntyre, T. Kana, 1996. Dynamic model of phytoplankton
growth and acclimiation: responses of the balanced growth rate and the
chlorophyll a:carbon ratio to light, nutrient-limitation and
temperature. Marine Ecological Progress Series, 148, 187-200.

Masumoto, K., J. L. Sarmiento, and M. A. Brzezinski, 2002. Silicic acid
leakage from the Southern Ocean as a possible mechanism for explaining
glacial atmospheric pCO~2~. Global Biogeochem. Cycles, 16:
10.1029/2001GB001442.

Nodder, S. D., and A. M. Waite, 2001. Is Southern Ocean organic carbon
and biogenic silica export enhanced by iron-stimulated increases in
biological production? Sediment trap results from SOIREE, Deep-Sea Res.,
II, 48, 2681-2701

Suntharalingam, P., 1997. Modeling the global oceanic nitrous oxide
distribution. Ph. D. thesis, Princeton University, Princeton, NJ.

Suntharalingam, P. and J.L. Sarmiento, 2000. Factors governing the
oceanic nitrous oxide distribution: Simulations with an ocean general
circulation model. *Global Biogeochemical Cycles*, 14(1), 429-454.