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
90% of the CO2
decrease unexplained
by physical processes
Exchange of Carbon


Carbon in rock reservoir exchanges slowly
 Cannot account for 90 ppm change in 103 y
Rapid exchange of carbon must involve nearsurface reservoirs
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!
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
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
Ice core data indicate
atmospheric CO2 30% lower
Terrestrial vegetation 25% lower
Mass balance indicates
2.7% increase in deep ocean
Ocean mixed layer in
equilibrium with atmosphere
so it too was lower by 30%
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
13
d C

Tracks Carbon Transfer
Isotope mass balance
quantifies transfer of
terrestrial Corg to deep
ocean
Cinorg*d13Cinorg + Corg*d13Corg
= Ctot*d13C
 (38,000*0‰) + (530*-25‰)
= (38530*x)

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


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
13
d C
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
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
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
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, iron 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 export
Quantifying Carbon Export


After some delay
 Particulate organic
carbon export increased
in the equatorial Pacific
Relationship between
uptake and export not 1:1
 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
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
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 capitalists
 See potential for enhancing fisheries and gaining “C
credits” through large-scale ocean manipulations
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

13
d CDIC 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
13
 Sets d C 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
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
Atlantic Deep Water


13
d C
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
13
d C

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


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
13
d CDIC
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
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?

Changes in Ocean Chemistry



CO2 levels in surface waters sensitive to carbonate
ion concentration
 CO32- produced when corrosive bottom waters
dissolve CaCO3
2- returned to surface waters
 When CO3
• 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
Increase carbon
pump in Antarctic
Change chemistry of shallow
Southern Ocean subsurface water
Increase biologic
carbon pump in
coastal and
tropical ocean
Change chemistry of
Antarctic surface water