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Unresolved Issues


... areas of ocean within 30 ppm of atmosphere. Glacial surface ocean must also have ... Suggest that the ocean would respond to natural changes in iron inputs ... – PowerPoint PPT presentation

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Title: Unresolved Issues

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

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

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

Changes in Soil Carbon
  • Expansion of ice sheets
  • Covered or displaced forests
  • Coniferous and deciduous trees
  • Displaced forests replaced by steppes and
  • 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!

Where is the Missing Carbon?
  • Carbon from reduced CO2 during glacial times
  • Not explained by physical properties of surface
  • 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

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

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

d13C Changes During Photosynthesis
  • Large KIE during carbon fixation by plants
  • Magnitude depends on C-fixation pathway

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

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

Increase the Ocean Carbon Pump
  • If biological productivity and Corg export were
    higher in surface waters during glacial
  • 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

The Iron Hypothesis
  • In the 1980s, the late John Martin suggested
  • 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

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
  • Alter the greenhouse gas balance and climate of
    the earth

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
  • Bottle experiments demonstrate conclusively
    that addition of Fe stimulates phytoplankton
  • CO2 uptake

If Iron Hypothesis increased biological pump, ir
on addition
must increase production and export
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

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

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
  • Thus the efficiency of the biological pump had
    increased dramatically

Quantifying Carbon Export
  • Results of similar iron fertilization of Southern
  • 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

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

Iron Fertilization is Hot Topic
  • Iron fertilization of the ocean captured
    attention of entrepreneurs and venture
  • See potential for enhancing fisheries and gaining
    C credits through large-scale ocean

Marshall Islands
  • Territorial waters of the Marshall Islands
  • Leased to conduct an iron fertilization
  • 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

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

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

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

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

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

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

d13C Aging
  • As the Corg in deep water is gradually oxidized
  • 12C-rich CO2 released lowering d13CDIC
  • Particularly evident in deep Pacific waters

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
  • Relative increase in water flowing from
  • Knowing the d13C of the source region (planktic
  • Percent contribution from each region can be

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

  • 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
  • Must have affected atmospheric CO2
  • But how?

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

Carbon System Controls on CO2
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