Lecture 11: Glacial Cycles
and Greehouse Gases
(Chapter 10)
Atmospheric CO2 Evolution
Uplift weathering
BLAG spreading rate,
Carbon balance at tectonic time scales
• Carbon sinks:
chemical weathering
subduction
• Carbon source:
volcanic eruption
Atmospheric CO2 Evolution
Why in 100 yr cycle?
Uplift weathering
BLAG spreading rate,
What is atmospheric CO2
during glacial cycles?
How do we know?
Ice core: A two-mile time machine
location at the dome to obtain the oldest ice
Ice core dating:
annual layer counting
ice flow model
Ice coring project in
Greenland “summer”
Trapping gases in ice core
Greenhouse gases tend to
be globally uniform!
Ice core records
CO2
Charles David Keeling
CH4
CH4 and monsoon signal
An even longer record
CO2 change/Climate change:
100kyr cycle dominant
Question:
Chicken/egg ?
Vostok ice core
CO2 and climate: The last glacial cycle
Comparison of CO2 and CH4
Carbon Reservoir (0ka  LGM)
Glacial carbon go to deep ocean
Carbon exchange
How to track carbon cycle
during glacial cycles?
Carbon isotope as marker
13C
(99%), 12C(1%):
stable isotope (nonradioactive) naturally occurring
14C (small residual):
radioactive
( C / C ) sample  ( C / C ) s tan dard
13
 C
13
in o / oo
12
13
13
12
12
( C / C ) s tan dard
Organic carbon: living plants (mostly in plants/photoplantons) ~ -22
Inorganic carbon: HCO3-1, CO3-2 (water), CO2 (air) ~ +1,
Mostly in inorganic carbon (22 times more than organic carbon)
such that the mean is ~ 0.
Carbon reservoir, and their marker 13C values
Why organic δ13C more negative?
Photosynthesis and carbon isotope fractionation
Fractionation: Inorganic carbon (plant/plankton) form organic carbon
(tissue) with low 13C tissue, because plant/plankton favors 12C over 13C.
C3 and C4 pathways
Atmospheric inorganic carbon: δ13C ~ -7
C3 pathway: trees, shrubs, cool-climate grasses creates organic carbon: δ13C ~ -25
C4 pathway: warm-climate grasses creates organic carbon: δ13C ~ -13
Dominant C3 (trees) so mean plant δ13C ~ -25
Glacial cycle of carbon
Vostok ice core
Glacial-Interglacial change of Carbon (Oxygen) Isotopes
(a negative correlation)
(1) Ice sheet replace vegetation,
(2) Colder/drier climate forest
replaced by shrubs and grasses
 Less plants on continents
More negative d13C
Quantify glacial carbon sink
The Deep Ocean,
How?
=-180 GT
= -530 GT
Surf ocn CO2=Atms CO2 – 30ppm
= -300 GT
C13 verification
of missing carbon at glacial times are in deep ocean
= -530 GT
All glacial terrestrial carbon into the ocean lowers ocean C13 by -0.34o/oo
38000GT*0o/oo + 530GT*(-25o/oo)=(38530GT)*(-0.34o/oo)
Carbon and oxygen variation during glaciations
Pacific sediment core
-0.4
Glacial Bury Hypothesis?
(1) Ice sheet replace vegetation,
(2) Colder/drier climate forest
replaced by shrubs and grasses
Less plants on continents
But, can they be buried
underneath ice sheet?
(Ning et al. 2000,2010?)
?
Carbon Reservoir (0ka  LGM)
Glacial carbon go to deep ocean
C13 verification
of missing carbon at glacial times are in deep ocean
= -530 GT
= -180 GT
All glacial terrestrial carbon into the ocean lowers ocean C13 by -0.34o/oo
38000GT*0o/oo + 530GT*(-25o/oo)=(8530GT)*(-0.34o/oo)
atmosphere
A correction:?
+ 530GT*(-25o/oo) + 180GT*(-7o/oo) =(38530GT)*(-0.27o/oo)
Carbon and oxygen variation during glaciations
Pacific sediment core
-0.4
End of Lecture 11
Lecture 12: Carbon “Pumps”
into the Deep Ocean
(Chapter 10)
How is carbon pumped into deep ocean?
Pump I: Solubility pump
EQ
Pole
warm,
cold,
low solubility
high solubility
Glacial cooling about 2.5oC pumps atmospheric CO2 down by only about 10ppm
(20ppm, half balanced by a 1psu salinity increase)
Pump II: Biological Pump
(soft tissue pump, carbon pump)
Organic matter is produced in the uppermost sunlit layers of the ocean. A fraction of the
organic tissue (soft tissue) sinks to the deeper ocean through settling particles or advection of
dissolved organic carbon. This leads to a net consumtion of CO2 in these upper layer. Upon
reminerization of this organic matter in the deeper layers, this CO2 is returned to the
seawater. Thus, these biological processes lead to a net transfer of inorganic carbon from the
surface into the abyss. This process is termed the “soft tissue” pump.
Light + nutrients
The key to soft tissue biological pump is nutrients (light is infinite): increased
nutrient increases biological activity and in turn the downward pumping of carbon
Photosynthesis and Biological Pump
Primary Production and nutrients:
Annual carbon production in modern ocean: coastal,
equator, southern ocean
Tropical pump, enough
light, so nutrient (N, P)
limited
Southern ocean pump,
Not enough light, excess nutrients, but. iron limited.
Iron fertilization: enhancing biological pump
Geoenginering: The Iron Hypothesis
John Martin
How long the carbon can stay in the ocean?
Changes in Deep Ocean Circulation
Modern circulation and 13C
Two end members
Antarctic:
incomplete photosynthesis
less 12C to deep water
 lower 13C surface water
aging:
Downward more negative
due to the downward rain of
12C-rich carbon
Most clear where circulation
is weak, e.g. N. Pacifci
North Atlantic:
complete photosynthesis
more 12C to deep water
 high 13C surface water
Change of North Atlantic circulation and Biological Pump
Antarctic:
incomplete photosynthesis
less 12C to deep water
 lower 13C surface water
North Atlantic:
complete photosynthesis
more 12C to deep water
 high 13C surface water
 Reduced penetration of
North Atlantic Deep Water
Or could it be a surface
source change of 13C at
LGM?
LGM modeling
LGM: Older carbon, Younger deep water?
LGM
Holocene
Obs: Δ 13C
CCSM: Salinity
AMOC
Ideal Age
Evidence of changing deep circulation
History of NADW/AABW
Glacial:
stronger AABW,
weaker NADW
Interglacial:
weaker AABW,
stronger NADW
Using deep tropical
Atlantic 13C
Change of North Atlantic circulation and Biological Pump
Implication to CO2 reduction
Antarctic:
incomplete photosynthesis
less 12C to deep water
 lower 13C surface water
Enhanced Antactic
overturning delievers more
nutrient to the surface
Increase producitivyt
Increse biological pump
Reduce CO2
(Circulation Pump)
North Atlantic:
complete photosynthesis
more 12C to deep water
 high 13C surface water
 Reduced penetration of
North Atlantic Deep Water
How to measure the strengh of the soft tissue pump ?
Biological pump ~~ 13Csurf (+) - 13Cdeep (-) = 13C Vertical Difference >0
How to measure the strength of the biological pump
Nutrients and 13C vertical profile
Photosynthesis sends both 12C and nutrients (N,P) down
less nutrients
12
less C
13C

surf (+)
less nutrients
13Cdeep(-)
more 12C
more nutrients
more nutrients
How to measure the strengh of the soft tissue pump ?
Biological pump ~~ 13C (surface) - 13C (deep)
Vertical Difference of 13C: stronger photosynthesis  more organic 12C rain
down  13C (surface) positive/13C (deep) negative  large vertical difference
and stronger biological pump
Past change of the Biological Pump
Surface foram: surface 13C
Benthic foram: Bottom 13C
More nutrients to surface
 more Surface-Bottom >0
 stronger biological pump
 lower CO2
Stronger pump
lower CO2
Pump III: (Bio)Chemical Pump
(Carbonate pump, CaCO3 pump, Alkalinity pump)
Mineral calcium carbonate CaCO3 shells (formed in the upper layers of the ocean
mainly by 3 groups of organisms: Cocco-lithophorids (phytoplankton),
foraminifers, and pteropods (zooplankton)) raindown to the depth as they die,
eventually dissolve, either in the water column or in the sediments.
Deep water dissolution calcium carbonate
CaCO3 produces carbonate ion CO3-2 , which
when upwelled to the surface combines with
dissolved CO2 to produce bicarbonate ion
HCO3-1. This process removes CO2 from the
surface waters, pumping carbon to the deep
ocean.
Change of North Atlantic circulation and (Bio)Chemical Pump
Implication to CO2 reduction
Antarctic bottom water,
more corrosive
Enhanced Antactic
Bottom water
Increase corrosive and
dissolution of CaCO3
More carbonate ion
CO3-2 to the surface
Dissolves surface CO2
Reduce surface CO2
(up to 40ppm)
Polar Alkalinity hypothesis, Broecker and Peng,
North Atlantic deep water
less corrosive
 Reduced penetration of
North Atlantic Deep Water
Summary of Major Carbon Pumps
Solubility
pump
(10 ppm)
Soft tissue
Pump
(25 +?ppm)
Chemical
pump
(10+40ppm)
Solubility pump
Biological pump
Chemical pump
Methane (CH4)
Source:
Tropical wetland,
monsoon rainfall control
Boreal wetland,
summer warming control
Consistent with
CH4/July Inso correlation
23kyr signal dominates
Glacial-CO2 positive feedback
Colder
climate
Lower
CO2
Q1: A key for great 100 kyr glacial cycle?
Q2: Does it apply to anthropogenic global warming?
Glacial-CO2 positive feedback
CO2 decrease ==> Colder
Colder ==> CO2 decrease ?
Colder ==> solubility pump increases
==> soft tissue pump increases
(stronger wind-upwelling,
more nutrient, iron…)
==> chemical pump increases
(circulation, PH level…)
==> more sea-ice ==> reduces CO2 release to the atmos.
==>stronger stratification==> reduce upwelling of deeper
dissolved/reminirized carbon up)
Assessing Glacial-GHG feedback
Phase, lead/lag
21
23kyr
63
41kyr
23 kyr cycle, GHG leads ice volume,
forcing
41 kyr cycle, GHG in phase with ice volume,
Feedback
Why different?
Reference for Reading
• Brovkin et al., 2007: Lowering of glacial atmospheric CO2 in
response to changes in oceanic circulation and marine
biogeochemistry. Paleoceanography, 22, PA4202,
doi:10.1029/2006PA001380
End of Lecture 12
Pump III: Carbonate pump
(CaCO3 pump)
The lack of H+, CaCO3 pump is effectively

CO 2  CO 3  H 2 O  2 H 2 CO 3
Carbonate buffer