The Biogeochemical
Carbon Cycle: CO2,the
greenhouse effect, &
climate feedbacks
Assigned Reading:
Kump et al. (1999) The Earth System, Chap. 7.
Overhead Transparencies
Faint
Young
Sun
Paradox
41H→4He
Incr.density =
Incr.luninosity
Liquid H2O existed
>3.5 Ga (sed. Rocks,
life, zircon 18O)
Simple Planetary Energy Balance
•Likely solution to
FYSP requires
understanding of
Earth’s energy
balance (& C cycle)
Energy
Absorbed
Neither
Albedo or
Geothermal
Heat Flux
Changes
Can Keep
the Earth
from
freezing w/
30% lower S
Lower S
compensated
by larger
greenhouse
effect
The Electromagnetic Spectrum
Incoming UV, Outgoing IR
“Greenhouse
Gases”
absorb
IR radiation
efficiently
Molecules
Acquire
Energy
when they
Absorb
Photons
1.CO2 Feedbacks: Geochemical Carbon cycle
‧Transfer of C between rocks and ocean/atmosphere
(> 106-yr) can perturb CO2 greenhouse effect
‧Ocean/atmosphere C reservoir small w.r.t. rock
reservoir and the transfer rates between them
2.Evidence for Long-Term CO2-Climate Link
3.Case Studies:
Permo-Carboniferous Glaciations
Warm Mesozoic Period
Late Cenozoic Cooling
Carbon
Cycle:
Strong
driver of
climate on
Geologic
timescales
The Geochemical Carbon Cycle
1. Organic Carbon Burial and Weathering
2. Tectonics: Seafloof spreading Rate
Mantle CO2 from Mid-Ocean Ridges
3. Carbonate-Silicate Geochemical Cycle
Chemical Weathering Consumes CO2
Carbonate Metamorphism Produces CO2
The BioGeochemical
carbon Cycle
Chemical Weathering = chemical attack
of rocks by dilute acid
1.
Carbonate Weathering:
2.
Silicate Weathering:
consumption for silicates
Carbonates weather faster than silicates
Geochemical
Carbon Cycle
#2
Carbonate
Rocks
Weather
faster than
Silicate
rocks!
Net Reaction of Rock Weathering
Carbonate and Silica Precipitation in Ocean
consumed
Would deplete atmospheric
Plate tectonics returns
via Volcanism
and Metamorphism
Carbonate Metamorphism
produced from subducted marine
sediments
Net reaction of
geochemical
carbon cycle
(Urey
Reaction)
Geologic record indicates climate has rarely reached or
maintained extreme Greenhouse or Icehouse conditions....
Negative feedbacks between climate andGeochemical
Carbon Cycle must exist
Thus far, only identified for Carbonate-Silicate
Geochemical Cycle:
Temp., rainfall enhance weathering rates
(Walker et al, 1981)
(I.e., no obvious climate dependence of tectonics or
organic carbon geochemical cycle.)
How are
CO2 levels
Kept in
Balance?
Feedbacks
Adapted from Kump
et al. (1999)
A Closer Look at
the Biogeochemical
Carbon & the
Organic Carbon
Sub-Cycle
BIOGEOCHEMICAL CARBON CYCLE
ATMOSPHERE CO2
dissolution sink
fixation
OCEANS
rock
weathering
sink
CONTINENTAL
EROSION
Exhalation
CO2
IGNEOUS ROCKS
Uplift
BIOSPHERE
SEDIMENTS
METAMORPHIC
ROCKS
lithification
Organic
matter
SEDIMENTARY
ROCKS
Corg
Earth's Carbon Budget
Biosphere, Oceans and Atmosphere
Crust
Mantle
Steady State & Residence Time
Steady State: Inflows = Outflows
Any imbalance in I or O leads to changes in reservoir size
Inflow:
Atmospheric
Respiration
Outflow:
Photosynthesis
The Residence time of a molecule is the average amount of
time it is expected to remain in a given reservoir.
Example
of atmospheric
Carbon Reservoirs, Fluxes and Residence Times
Species
Sedimentary carbonate-C
Sedimentary organic-C
Oceanic inorganic-C
Necrotic-C
Atmospheric-CO2
Living terrestrial biomass
Living marine biomass
Amount
Residence Time
Carbonate-Silicate Geochemical Cycle
• CO2 released from volcanism dissolves in H2O,
forming carbonic acid H2CO3
• CA dissolves rocks
• Weathering products transported to ocean by rivers
• CaCO3 precipitation in shallow & deep water
• Cycle closed when CaCO3 metamorphosed in
Simple Carbon Cycle Modeling
Total C entering atm. & oceans = Total C buried in sediments
closed system
13C
into atm. & oceans =13C being buried in sediments
conservation of isotopes
~5 the avg. value for crustal C
isotopic fifference between
inorganic and organic carbon
Hayes et al., Chem Geol. 161.37.1999; Des Mariais et al., Nature 359.605.1992
Carbon Isotopic Excursions
808-500Ma
More complete sediment record
+
Improved chronology
=
More detailed picture showing
Abrupt and extreme
C-isotopic shifts
A global composite of 13C data shows 4 excursions
Plus one at the pC-C boundary
Carbon Isotopic Excursions
808-500Ma
More complete sediment record
+
Improved chronology
=
More detailed picture showing
Abrupt and extreme
C-isotopic shifts
Modeling the Proterozoic Carbon Cycle
  carb and  org through time 2500-550 Ma in 100Ma increments
 note the constancy of  carbwhile  org decreased. Why? biochemistry
or pCO2
 forg increased through this time, was episodic and was linked to
periods of rifting and orogeny
 also associated with extreme glaciations
 Increase in the crustal inventory of C requires increases in the
inventories of crustal Fe3+ , crustal and marine sulfate, nitrate
and atmospheric oxygen.
 SO4 , NO3- and O2 increases changed patterns of respiration
 NO3- would have forced productivity changes