Biological pump

Biological pump• Low latitude versus high latitudes

Low-latitude ecosystem• Productivity limited by nutrient

supply to the mixed layer

Mixed layer

Mixed layer nutrient and Chl-a

• Chlorophyll is maximum at about 100m near Hawaii

• What causes this deep chlorophyll maximum?

High latitude ecosystem• Stronger seasonality in solar radiation,

nutrients and productivity

Seasonal cycle of mixed layer depth

Y. Takano

• Shallow mixed layer = More light

Low vs high latitude ecosystem• Low-latitude, low-nutrient

condition• Small cell size• Efficient recycling of nutrient

• High-latitude, high-nutrient condition

• Large cell size• Efficient export of nutrient

Surface nutrient vs chlorophyll

Chlorophyll-a

Nitrate

Sarmiento and Gruber (2006)

NO3-Chl relationship

• HNLC (High-Nutrient Low-Chlorophyll)

• Southern Ocean• Equatorial Pacific• Subarctic North Pacific

HNLC region and iron limitation• Southern Ocean nutrient problem

– Siegenthaler and Wenk (1984); Sarmiento and Toggweiler (1984); Knox and McEloy (1984)

– Utilization of excess nutrient in the Southern Ocean

The iron hypothesis

• Phytoplankton needs trace amount of iron as a micro-nutrient

• Due to the remoteness of the Southern Ocean from the continents, phytoplankton growth is limited by the availability of iron (Martin, 1990)

• Macro-nutrient such as NO3 are not fully utilized in the Southern Ocean

Atmospheric dust deposition in present climate

Southern Ocean Iron Release Experiment (Boyd et al., 2000)

• Monitor two similar “patches” of surface waters in the Southern Ocean

• One patch is seeded with high-level of iron• The other patch is not seeded• Measure photosynthesis after the iron

addition and compare the two patches

Satellite Images from

Results from SOIREE

• Photosynthesis responded to the artificial addition of iron– Increased chlorophyll and primary production

• The seeded patch is mixed with the environment after a few weeks– Long-term effect is difficult to determine

• Carbon export to the deep ocean was not confirmed

Implications

• Can we increase ocean CO2 uptake by adding iron to the Southern Ocean?

• Is there any geologic evidence for the past climate changes involving iron supply to the oceans?

Polar ice core data

Petit et al., (1999)

Glacial-interglacial CO2 problem

Antarctic ice coreLuthi et al., (2008)

Last glacial cycle

Since the last glacial maximum

Timescale

• 100 ppmv– Fossil fuel CO2 in the present atmosphere– De-glacial increase in the atmospheric CO2

• Current rate of increase in atmospheric CO2 is about 100 times faster than that during the “abrupt” end of last glacial period.

• Industrial carbon emission: decades• De-glaciation CO2 increase: 5,000 years

Theme II: Climate-Carbon relation

• The carbon cycle interacts with climate in fundamentally different ways between the two timescales

• Modern Ocean: the carbon cycle mediates climate warming (stabilizing feedback)

• Glacial Ocean: the carbon cycle enhanced climate cooling (de-stabilizing feedback)

Last Glacial Maximum

• Cold and dry climate• Increased albedo due to the land ice sheets– Some land vegetation was replaced by ice

• Global mean temperature was about 5°C cooler

• Sea level was lower by about 120m– Salinity was higher

Attribution of CO2 change

• Relatively well known effects– Land forest loss due to ice sheet– Solubility change due to temperature and salinity

Sigman and Boyle (2000)

Dust deposition over the Southern Ocean during LGM

Petit et al., (1999)

A simple theory• Theory predicts a strong

relationship between polar surface nutrient and atmospheric CO2

• About 50% consumption of current polar surface nutrient will lower atmospheric CO2 by 100 ppmv

Sarmiento and Toggweiler 1984

Can the iron hypothesis be the solution for the glacial CO2 problem?

• Scientists included iron cycling into the ocean climate-carbon models and simulated LGM condition– Bopp et al., (2003), 15 ppmv decrease– Parekh et al., (2006), 8 ppmv decrease

• Model prediction is much smaller than the observed 100 ppmv change!

Circulation and biology

• Dust deposition itself is unlikely the sole mechanism for glacial CO2 decrease

• Other mechanisms?– Circulation of the Southern Ocean– Sea ice and its impact on gas exchange in the Southern

Ocean– Silica and CaCO3 marine snow (silica leakage hypothesis)– More…