Carbon Cycle
Adapted in part from lectures by Dr. Gerardo Chin-Leo, TESC
Chautauqua UWA-6, Dr. E.J. Zita
9-11 July 2007
Fire, Air, and Water:
Effects of the Sun, Atmosphere, and Oceans in
Climate Change and Global Warming
Milankovitch Mechanism is not a complete
explanation for glaciation cycles…
Evidence for carbon feedback contribution to
long-term climate regulation
Feedback Loop Linking Glaciations, Atmospheric
CO2 and Phytoplankton C-burial
Interglacial Period
Small Ice Caps
Large Exposed
Continental Mass
More Nutrients to the Sea
From Land Erosion
High Productivity
High C-Burial
High CO2
Warm Temperatures
Few Nutrients to the Sea
From Land Erosion
Low Productivity
Low C-Burial
Low CO2
Cool Temperatures
Ice Age
Large Ice Caps
Small Exposed
Continental Mass
Chemistry of Inorganic C in Water
Carbonate buffering and pH
• CO2 + H2O <---> H2CO3- (carbonic acid)
• H2CO3 <---> H+ + HCO2- (bicarbonate)
• HCO3- <---> H+ + CO32+ (carbonate)
Atmospheric O2 and the C Cycle
Isotope composition of carbonate sediments reveals
the net production of O2
• Photosynthesis selects C12 over C13, thus organic material is
depleted (isotopically lighter) in C13
• During times of increased net O2 production, more organic C
is buried, thus atmospheric and oceanic C becomes richer
(isotopically heavier) in C13
• This enrichment of C13 in the environment is reflected in
carbonate sediments
• Assuming a constant total mass of C13 and C12, the faster the
organic C is buried (more O2 accumulates) the heavier
(enriched C13) the carbonates become
13C Isotope Signature Scale
(Del) C13 in ‰
(C13/ C12) sample - (C13/ C12) standard
=
--------------------------------------------------- X 1000
(C13/ C12) standard
Example:
[(0.001125 – 0.001123)/0.001123]*1000 = 1.78
(C13/ C12) standard is the ratio in a standard sample of the fossil invertebrate
Belemnitella americana (Cretaceous Peedee formation in South Carolina)
Evidence for the relative constancy of O2 in
the atmosphere
• There is evidence for the existence of forests dating
back to ~360 mya. These forests need O2 to exist
• There is evidence of forest fires ever since (charcoal in
sediment)
• Forest fires cannot occur if O2 < 13%
• If O2 > 35% fires burn so fiercely that all forests would
have disappeared
• Consequently, O2 is believed to have remained in the
range of 13-35% (current concentration is 21%)
13C
•
as an Indicator of Ancient CO2 Levels
13C
is taken by plants slower than 12C. Thus
organic matter is depleted in 13C compared to
CO2.
• However, when CO2 concentrations are low, plants
do not discriminate 13C from 12C as much as
when CO2 levels are high. Thus, the 13C: 12C ratio
in organic matter under low CO2 levels is higher
(more 13C relative to 12C) than during times of
high CO2 levels (more 13C relative to 12C) .
Use of 13C in Ecology
•
13C
is fractionated (or discriminated) by physical processes
(e.g evaporation/precipitation). There is less 13C in
atmospheric CO2 than in dissolved CO2 (bicarbonate)
• Consequently land plants are isotopically lighter than
aquatic plants
• There is another fractionation of 13C based on the
photosynthetic metabolism of the plant (C3, C4 or CAM).
• Consequently organic matter from C3, C4 or CAM plants
can be distinguished from each other based on their -13C
signatures.
Through condensation-evaporation, water gets isotopically
“lighter” as it moves to higher latitudes.
Polar ice is depleted in 18O.
18O
indicates ancient ice volumes and temperatures
• Volume:
Evaporation/precipitation and formation of polar ice excludes 18O.
During ice ages the concentration of 18O in the oceans
increases.
Benthic foraminifera fossils show the 18O:16O of ancient seawater
•
Paleothermometer: the formation of CaCO3 by foraminifera
excludes 18O as a function of temperature.
•
Benthic foraminifera form shells under constant temperature thus
their 18O:16O reflects the isotope composition of the water.
Planktonic foraminifera experience temperature fluctuations and
these are recorded as changes in their 18O:16O relative to benthic
forms
Relationship Between 18O Content and Temperature
Water 18O is derived from benthic (deep) foraminifera.
Carbonate 18O is derived from planktonic foraminifera
From: Introduction to Marine Biogeochemistry -Libes (1992)
Evidence that Increasing Atmospheric CO2 is
Anthropogenic
• Increase consistent with onset and development of
industrialization
• Magnitude and rate of increase consistent with
magnitude and rates of fossil fuels consumption
• Suess effect: Lowering of 14C:12C in CO2 by the
input of “old” carbon from fossil fuel burning
(Higher 14C when lower solar magnetic activity
shields Earth less from incoming cosmic rays)
Lower recent C14 /C12 from fossil fuel burning
Little Ice Age: low solar magnetic
activity throughout?
Evidence of anthropogenic source for greenhouse gases
Solar magnetic activity and C14 production
Cosmic rays excite N14 → decays to C14
Solar max: magnetic solar wind sweeps away
cosmic rays → less *N14 → less C14
http://www.nuclearonline.org/newsletter/Oct05.htm
http://www.dsri.dk/~hsv/Noter/solsys99.html
Car & Driver: Global warming is natural
• Earth has been getting steadily warmer since the
last ice age – a few more degrees would be nice
• All CO2 is the same, whether it comes from a
Porsche or your lungs – anthropogenic CO2 does no
extra harm
• There’s 30 times as much natural CO2 as man-made
• Water vapor is the dominant greenhouse gas, so
why worry so much about CO2?
• We can’t do anything about water vapor, so Kyoto
targeting CO2 is trivial.
Discuss and analyze these claims, given what we now know.
Inconvenient truth:
accelerated GW is anthropogenic
Bad news: we can’t do anything about
• Milankovitch cycles
• Increasing solar luminosity
• Increasing solar magnetic activity
Good news:
• We CAN do something about anthropogenic
emissions of greenhouse gases
• Oceans and plants will absorb as much CO2 as
they can.