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)

NO 3 -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 NO 3 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 CO 2 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 CO 2 problem Antarctic ice core Luthi et al., (2008)

Last glacial cycle

Since the last glacial maximum

Timescale 100 ppmv – Fossil fuel CO 2 in the present atmosphere – De-glacial increase in the atmospheric CO 2 Current rate of increase in atmospheric CO 2 is about 100 times faster than that during the “abrupt” end of last glacial period. Industrial carbon emission: decades De-glaciation CO 2 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 CO 2 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 CO 2 About 50% consumption of current polar surface nutrient will lower atmospheric CO 2 by 100 ppmv Sarmiento and Toggweiler 1984

Can the iron hypothesis be the solution for the glacial CO 2 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 CO 2 decrease Other mechanisms? – Circulation of the Southern Ocean – Sea ice and its impact on gas exchange in the Southern Ocean – Silica and CaCO 3 marine snow (silica leakage hypothesis) – More…