# Presentation: 716405

## Carbon sequestration through iron fertilization: A review of the major issues

- Anand Gnanadesikan
- NOAA/Geophysical Fluid Dynamics Laboratory, Princeton NJ
- NOAA Central Library Brown Bag Seminar
- September 10, 2008

## Outline

- Why does the ocean hold so much carbon?
- Why the speculation that iron fertilization could increase this amount?
- What are some of the practical issues surrounding iron fertilization as a carbon sequestration strategy?
- Verification
- Consequences

## How do you know when water on the stove is getting hot?

- Bubbles appear in the fluid.
- Dissolved gas comes out of solution.
- ~20 M of CO2 dissolves in water

- http://www.chem.purdue.edu/gchelp/liquids/boil.html

## How much carbon does this mean the ocean should hold?

- 1M= 1 millimole/m3
- =1 megamole/km3 = 12 tons C/km3

- Ocean area is about 360,000,000 square km.
- Depth is about  4km.

- 1 millmole/m3~ 17 billion t C or 17 Gt C => 20 M =340 GtC

- Volume = 1.44 billion cubic km

## How much carbon does the ocean actually hold?

- Ocean actually holds about 34000 GtC!

- Atmosphere only holds 800 GtC.
- What form is this carbon in?

- Average concentration
-  ~2000 mmol/m3!

## Why so much carbon?

- Ocean can hold a lot of carbon- because carbon dioxide is buffered by carbonate ion.
- Increasing carbon dioxide will result in reducing the amount of carbonate ion (ocean acidification).

*Orr et al., Nature, 2005*

## Role of biology

- Nutrients are released, oxygen consumed when reaction  runs backwards.

## How much phosphate is associated with carbon?- Look at deep oxygen

- Extending this back we get nonzero
- “preformed” phosphate!

## High preformed nutrients= high surface nutrients

- Points to dominant role of Southern Ocean, potential role
- for other regions.

## So ocean could hold more carbon...

- 0.1 mmol/m3 phosphate = 11.7 mmol/m3 C
- ~200 Gt C ~90 ppmv (!)= 900 ppmv/M
- But... remember the buffering equation
- So over a long time, much of the response to changing preformed nutrients will be compensated.

## What can models add to this?

- Role of *different regions*
*Time scale* of change compared with buffering.
*Relationship* between reduction in CO2 and increase in sinking organic matter.

## Diagnostic ocean models

- Restore ocean surface to observed values of T,S, nutrients.
- Apply “observed” fluxes of momentum, heat, freshwater
- Predict internal structure, flows using dynamics.
- Run depletion scenarios setting nutrients to zero

## Preformed nutrient changes and carbon drawdown

- Larger preformed nutrient drawdown results in larger carbon drawdown.
- Southern Ocean dominates.
- Long time scales required to get large impact on atmospheric CO2.
- Significant compensation over century time scales.

- Entire tropics

- Entire globe

- Gnanadesikan and Marinov, MEPS, 2008

## Runs with different models show no necessary relationship with global export!

- Gnanadesikan and Marinov, 2008

- Equilibrium response is well characterized in terms of **preformed nutrients**, poorly characterized by looking at export alone.

## Summary: Ocean carbon cycle

- Ocean holds a lot of carbon because of carbonate buffering.
- Biology adds additional carbon to the system...
- But it is inefficient, because not all the nutrients in the system get used.
- If we could associate some of these nutrients, the result would be to reduce atmospheric CO2.
- Over time, response is buffered by ocean.

## What’s the connection with iron?

- Metalloenzymes involved in
- Photosynthesis
- Nitrate reduction
- Lab culture studies show
- increased growth rates
- with increased levels of iron.
- Fe:P ratios~ 1:1000
- Fe:C ratios ~ 1:120,000

- Sunda and Huntsman, Nature, 1997

## Why is iron rare in the ocean?

- Blain et al., DSRII, 2007.

- Enough iron is found
- in surface waters to take up
- 0.1 mmol/m3 phosphate.
- Higher values at depth are associated with topography- not higher nutrients.
- Iron is preferentially stripped out by sinking particles.

## Implications

- Big cells (with low surface area to volume ratio) need a lot of dissolved iron.
- These cells tend to be more efficient at exporting carbon

- Small cells can survive at lower levels of iron, but tend to be tightly coupled to grazing.

- Iron could explain
- High Nutrient Low Chlorophyll Regions!

- 10 micron scale bar

- 0.1 micron scale bar

## SoFEX experiment

- Led by Ken Coale (Moss Landing), Supported by NSF and  DOE, with participation from NOAA/AOML.

- Photos: K. Buesseler

## Results: Increased chlorophyll

- Coale et al., Science, 2004.

## Less evidence for changes in species composition

- Hiscock et al., PNAS, 2007

- Bars show fraction of species by size and pigment. Small changes seen inside and outside the patch.
- Circles show absolute concentrations of Chl and particulate carbon. Large changes inside and outside the patch.

## Clear evidence for changes in nutrient uptake

- Coale et al., Science, 2004

- Inside the patch relative to outside...

- Nutrients go down.
- Total CO2 is lower
- Particulate carbon is higher.

## Some increased export

- Bishop et al. (Science, 2004) shows results from two profiling floats, one inside the patch, the other outside.
- Color contours show increased particulate matter inside path and increased export...
- BUT.... export doesn’t go very deep.

## Counterexample- EiFeX

- Assmy et al., 2006

- In this experiment a substantial amount of material appears to have gone right to the bottom!

## Summary: Iron fertilization

- All experiments to date have seen a bloom.
- Nutrients are drawn down in the bloom region.
- Very few experiments have stayed around for long enough to see increased export.
- Unclear whether export changes are significant- and therefore how long term changes in surface nutrients are likely to be.

## Additionaly potential issues regarding fertilization?

- Can it be verified?
- Will it change ocean chemistry?
- Oxygen
- Other greenhouse gasses
- Will there be downstream consequences on ecosystems?
- Reduced/increased production downstream
- Toxic algal blooms

## Problems with verification- patch fertilization simulations

- Simulation in which restoring value for nutrient is set to zero for one month, then returned to climatology (added iron quickly lost).
- Simulation in which we add “supernutrient” to system (added iron always retained).
- Result of fertilization depends on behavior of iron.
- Local fluxes insufficient to get total flux.

- Gnanadesikan, Sarmiento and Slater, Global Biogeochemical Cycles, 2003.

## Distribution of gas exchange resulting from patch drawdown

- Gnanadesikan, Sarmiento and Slater, Global Biogeochemical Cycles, 2003.

## Problems with ocean chemistry

- Desired result of iron fertilization is to associate more phosphate with carbon.
- But this means less oxygen!
- Increase in anoxic regions in deep ocean, increasing denitrification.

## Other greenhouse gasses

- Nitrous oxide production enhanced during SOIREE experiment. May offset significant fraction of carbon uptake.

## Potential problems- are we borrowing trouble?

- Marinov, Gnanadesikan, Toggweiler and Sarmiento,Nature,  2006

- Nutrient depletion of the Subantarctic has remote consequences over many centuries.

## Results from a full ecosystem model

- Aumont and Bopp, Global Biogeochemical Cycles, 2006

- Aumont and Bopp show regions at edge of upwelling zones losing productivity.
- Atmospheric impact is 33 ppmv
- When fertilization is stopped, production is slightly lower.

## Harmful algal blooms?

- Some species of phytoplankton shown to be responsive to fertilization (Pseudonitzchia) are associated with marine toxin domoic acid.
- Domoic acid appears to bind iron.

- A. Marchetti et al., J. Phycology, 2008

## How much does iron fertilization actually cost?

- Boyd (MEPS, 2008) shows that as one looks further and further down the chain more iron is lost, and cost appears to go up.

## Alternative versions of iron fertilization

- LNLC fertilization
- Seeks to enhance nitrogen fixation

- Organic matter produced by nitrogen fixation has a higher N:P, and thus C:P ratio (White et al., Limn. Oceanogr. 2006)

## Iron fertilization could decrease C:P ratios

- Arrigo et al., GRL,
- 2002
- If iron fertilization leads to the replacement of diatoms in the Southern Ocean, carbon sequestration could *drop*.

## Slide 37

| Scenario | Detectability | Impact on production |
| --- | --- | --- |
| Massive increase in deep export (EIFEX) | Good as regards export. Impact on preformed nutrients unclear. | Productivity drops |
| Iron enhances surface cycling (Aumont/Bopp) | Biggest challenge to detect marginal change. | Positive in most regions, negative at edges of productive regions |
| Change in C:P ratio | Easily detectable in particle traps. | Depends on interaction with low-oxygen regions, ecosystem structure |

## Summary 

- Ocean fertilization is not a panacea- best estimates put potential atmospheric drawdown at ~33 ppmv.
- Verification is hard- requires estimating preformed carbon/nutrients, not just looking at export of organic matter.
- Ocean is three-dimensional- changing the nutrient cycle at one location changes biogeochemistry at others.

## Where can I learn more?

- Marine Ecology Progress Series, v 364 open-access theme section on iron fertilization.
- Oceanus magazine, report on the Woods Hole workshop on iron fertilization.

## Ethics issues and iron fertilization

- Ocean as “wilderness”
- Problematic- there are no pristine ecosystems
- Legitimate- in that we don’t know what’s there, and that known consequences (increasing anoxia) are problematic
- “Gardening” vs. “Grazing”
- Gardens are productive...
- But tend to have low biodiversity.

## Legal issues and ocean fertilization

- Law of the Sea- London Dumping Convention
- Is iron fertilization “dumping” or “emplacement”
- Is CO2 added by iron fertilization “dumping”?
- Clean development mechanism
- Difficult to see how iron fertilization could comply with verifiability requirements for natural sinks
- Would require renegotiation

## Ice ages and iron fertilization

- From Petit et al., 1999 reproduced in Boyd, MEPS, 2008

- Are iron and CO2 both responding to the same climatic forcing?
- Or does iron drive CO2?

## NOAA and iron fertilization

- Chemical oceanographers eager to see scale-up to larger experiment- knowledge about iron’s role in the ocean.
- Fisheries oceanographers are worried about impacts.