Unresolved Issues

1
Unresolved Issues

• Cuffy and Vimeux (2001) show that
• 90 of DT can be explained by variations in CO2
  and CH4
  
• Reasonably firm grasp on causes of CH4 variations
  (Monsoon forcing)
  
• What produced CO2 variations?
• Variations are large 30
• Show rapid changes drop of 90 ppm from
  interglacial to glacial

2
Physical Oceanographic Changes in CO2

• During glaciations physical properties change
• Temperature and salinity
• Affect solubility of CO2(aq) and thus pCO2

3
Exchange of Carbon

• Carbon in rock reservoir exchanges slowly
• Cannot account for 90 ppm change in 103 y
• Rapid exchange of carbon must involve
  near-surface reservoirs

4
Changes in Soil Carbon

• Expansion of ice sheets
• Covered or displaced forests
• Coniferous and deciduous trees
• Displaced forests replaced by steppes and
  grasslands
  
• Have lower carbon biomass
• Pollen records in lakes
• Indicate glacial times were dryer and less
  vegetated than interglacial
  
• Estimates of total vegetation reduced by 25
  (15-30) during glacial maxima
  
• CO2 removed from atmosphere did not go into
  vegetation on land!

5
Where is the Missing Carbon?

• Carbon from reduced CO2 during glacial times
• Not explained by physical properties of surface
  ocean
  
• Did not go into biomass on land
• Must have gone into oceans
• Surface ocean not likely
• Exchanges carbon with atmosphere too rapidly
• Most areas of ocean within 30 ppm of atmosphere
• Glacial surface ocean must also have been lower,
  like atmosphere
  
• Deep ocean only likely remaining reservoir

6
Interglacial-Glacial Change in Carbon

• At LGM, reduction of carbon occurred in
  atmosphere, vegetation and soils on land and in
  surface ocean
  
• This carbon (1010 gigatons) must have been moved
  to deep ocean

7
Tracking Carbon

• d13C values can be used to determine how carbon
  moved from surface reservoirs to deep ocean
  
• Major carbon reservoirs have different amounts of
  organic and inorganic carbon
  
• Each with characteristic d13C values

8
d13C Changes During Photosynthesis

• Large KIE during carbon fixation by plants
• Magnitude depends on C-fixation pathway

9
d13C Tracks Carbon Transfer

• Isotope mass balance quantifies transfer of
  terrestrial Corg to deep ocean
  
• Cinorgd13Cinorg Corgd13Corg Ctotd13C
• (38,0000) (530-25) (38530x)
• Solving for x -0.34
• Just this transfer predicts a shift in deep ocean
  DIC of 0.34
  
• Isotopic change recorded in benthic foraminifera

10
Change in Benthic d13C

• Oscillations in benthic d13C correspond to
  benthic d18O
  
• 100,000 and 41,000 year cycles
• Confirm transfer of organic carbon to deep ocean
  during ice sheet expansion
  
• d13C shifts greater than 0.4
• Suggesting additional factors have affected
  oceanic d13C values

11
Increase the Ocean Carbon Pump

• If biological productivity and Corg export were
  higher in surface waters during glacial
  intervals
  
• Atmospheric CO2 could be fixed in shallow ocean
  by phytoplankton
  
• Sinking dead organic matter transfers that carbon
  to the deep ocean
  
• Biological productivity and export can only
  increase if essential nutrients increase in
  surface ocean
  
• Increases in wind-driven upwelling of deep,
  nutrient-rich water
  
• Increases in the nutrient concentration of deep
  water that is already upwelling

12
The Iron Hypothesis

• In the 1980s, the late John Martin suggested
  that
  
• Carbon uptake during plankton growth in many
  regions of the world's surface ocean
  
• Was limited not by light or the nutrients N and P
  
  
• But by the lack of the trace metal iron
• Iron is typically added to the open ocean as a
  component of dust particles

13
The Iron Hypothesis

• Correlations between dust and atmospheric carbon
  dioxide levels in ancient ice core records
  
• Suggest that the ocean would respond to natural
  changes in iron inputs
  
• Higher glacial winds would increase the amount of
  windblown dust containing Fe to oceans
  
• Stimulate phytoplankton growth
• Increasing carbon uptake and decrease atmospheric
  CO2
  
• Alter the greenhouse gas balance and climate of
  the earth

14
Evidence for Iron hypothesis

• Some areas of the ocean contain high amounts of
  essential nutrients (N, P)
  
• Yet low amounts of chlorophyll (HNLC)
• Phytoplankton require Fe in small amounts for
  growth
  
• Bottle experiments demonstrate conclusively
  that addition of Fe stimulates phytoplankton
  growth
  
• CO2 uptake

If Iron Hypothesis increased biological pump, ir
on addition
must increase production and export
15
Open-Ocean Iron Enrichment

• "Give me half a tanker full of iron and I'll give
  you an ice age (John Martin)
  
• Results of fertilizing large patches of the
  ocean with iron
  
• Showed strong biological response and chemical
  draw-down of CO2 in the water column
  
• But what was the fate of this carbon?
• Plant uptake of carbon in the ocean is generally
  followed by zooplankton bloom
  
• Grazers respond to the increased food supply
• Producing a blizzard of fecal pellets that
  descend through the water column
  
• Exporting the carbon to the deep sea

16
Quantifying Carbon Export

• Thorium is a naturally occurring element that by
  its chemical nature is "sticky"
  
• Due to its natural radioactive properties,
  relatively easy to measure.
  
• Analysis of a series of samples collected during
  the 1995 FeEx II
  
• Indicated that as iron was added
• Plant biomass increased
• Total thorium levels decreased indicating carbon
  export

17
Quantifying Carbon Export

• After some delay
• Particulate organic carbon export increased in
  the equatorial Pacific
  
• Relationship between uptake and export not 11
• The iron-stimulated biological community showed
• Very high ratios of export relative to carbon
  uptake
  
• Thus the efficiency of the biological pump had
  increased dramatically

18
Quantifying Carbon Export

• Results of similar iron fertilization of Southern
  Ocean
  
• Slower biological response
• Total thorium levels never responded
• The biological pump was not activated
• Speculate that difference
• Slowness of the biological community's response
  to stimulation in colder waters
  
• Biological pump may have turned on later

19
Persistence of Patch

• Sea surface color satellite image taken 32 days
  after the addition of Fe
  
• Colored ring indicates area of high chlorophyll
• Believed to be a result of the increased Fe

20
Iron Fertilization is Hot Topic

• Iron fertilization of the ocean captured
  attention of entrepreneurs and venture
  capitalists
  
• See potential for enhancing fisheries and gaining
  C credits through large-scale ocean
  manipulations

21
Marshall Islands

• Territorial waters of the Marshall Islands
• Leased to conduct an iron fertilization
  experiment
  
• The new businesses involved suggest that
• Iron fertilization process will reduce
  atmospheric CO2 levels
  
• Allowing Marshall Islands to profit by trading
  carbon credits with more industrialized nations
  
• Increased fisheries as a consequence of enhanced
  phytoplankton production
  
• Iron additions could alter the ocean in
  unforeseen ways
  
• Creating a polluted ocean with new opportunistic
  species that do not support enhanced fisheries

22
d13CDIC Tracks Productivity

• Photosynthesis removes 12C from surface ocean and
  exports it to deep ocean
  
• Close correlations between d13CDIC and nutrients

23
Measuring Changes in the Carbon Pump

• Greater productivity during glaciations pumps
  more Corg to deep sea, reduces atmospheric CO2
  
• Past changes in strength of carbon pump
• Recorded in planktic and benthic foraminifer

24
Past Changes in the Ocean Carbon Pump

• Dd13C planktic-benthic are tantalizingly large
  when CO2 is low and small when CO2 is high
  
• Correlation not perfect
• May explain as much as 25 ppm CO2 lowering
• Best documented in equatorial regions
• Worse in Southern Ocean
• Even HNLC regions
• Detailed records lacking

25
Changes in Deep Water Circulation

• d13C can be used to trace carbon transfer
• Photosynthetic rate
• Sets d13C and nutrient levels in surface waters
• Water gets down-welled and carry the signals
• These factors can produce regional differences in
  the d13CDIC
  
• Deep waters in different ocean basins
• Monitors changes in deep water circulation with
  time

26
Modern Deep Ocean Circulation

• High d13C values in N. Atlantic results from
• High production in surface waters in subtropical
  latitudes
  
• Transported north and sinks
• In contrast, intermediate waters originate in
  Antarctica
  
• Seasonal production produces lower 13C
  enrichment
  
• These contrasts allows water masses to be tracked

27
Atlantic Deep Water d13C

• Deep water formed in N. Atlantic have high d13C
  values and low nutrient concentrations
  
• Intermediate waters formed in the Southern Ocean
  have low d13C values and high nutrient
  concentrations

28
d13C Aging

• As the Corg in deep water is gradually oxidized
• 12C-rich CO2 released lowering d13CDIC
• Particularly evident in deep Pacific waters

29
Past Changes in d13CDIC

• d13C of benthic foraminifera indicate changes in
  Atlantic deep water flow at the LGM
  
• Northern water did not sink as deeply, not as
  dense
  
• Relative increase in water flowing from
  Antarctica
  
• Knowing the d13C of the source region (planktic
  foraminifera)
  
• Percent contribution from each region can be
  determined

30
Changing Sources of Atlantic Deep Water

• Long records of d13C indicate cyclic changes in
  deep water sources
  
• North sources dominate during interglacial
• Southern sources dominate during glacial
• 100,000 year cycle
• During glacial
• Low d13C water from Antarctica
• Increase flux of 12C carbon from continents
• Additive effects explains large shifts noted
  earlier

31
Summary

• d13C results indicate an important link
• Size of N. Hemisphere ice sheets
• Formation of deep water in N. Atlantic
• Less deep water formed in the N. Atlantic
• Every time ice sheets grew at a 100,000 year
  cycle
  
• Must have affected atmospheric CO2
  concentrations
  
• But how?

32
Changes in Ocean Chemistry

• CO2 levels in surface waters sensitive to
  carbonate ion concentration
  
• CO32- produced when corrosive bottom waters
  dissolve CaCO3
  
• When CO32- returned to surface waters
• Combine with CO2 to form HCO3-
• Thus reducing CO2 content of surface ocean
• The corrosiveness of deep water determined by the
  weight of foraminifer shells
  
• Depth of the CCD
• Southern Ocean particularly vulnerable to changes
  in carbon ion concentration

33
Carbon System Controls on CO2