The Biogeochemical Carbon Cycle: CO 2,the greenhouse effect, & climate feedbacks Assigned Reading:...
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Transcript of The Biogeochemical Carbon Cycle: CO 2,the greenhouse effect, & climate feedbacks Assigned Reading:...
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→4HeIncr.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)
EnergyAbsorbed
Neither Albedo or
Geothermal Heat FluxChanges
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
MoleculesAcquireEnergy
when theyAbsorbPhotons
CarbonCycle:Strong
driver of climate onGeologic
timescales
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
The Bio-Geochemicalcarbon Cycle
The Geochemical Carbon Cycle
1. Organic Carbon Burial and Weathering
2. Tectonics: Seafloof spreading Rate
Mantle CO2 from Mid-Ocean Ridges
Chemical Weathering Consumes CO2
Carbonate Metamorphism Produces CO2
3. Carbonate-Silicate Geochemical Cycle
GeochemicalCarbon Cycle
#2
Chemical Weathering = chemical attackof rocks by dilute acid
1. Carbonate Weathering:
2. Silicate Weathering:
consumption for silicates
Carbonates weather faster than silicates
CarbonateRocks
Weatherfaster than
Silicaterocks!
Net Reaction of Rock Weathering
Carbonate and Silica Precipitation in Ocean
consumed
Would deplete atmospheric
Plate tectonics returns
and Metamorphism
via Volcanism
Carbonate Metamorphism
produced from subducted marinesediments
Net reaction ofgeochemicalcarbon cycle
(UreyReaction)
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 areCO2 levels
Kept inBalance?
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
OCEANS
rockweathering
sink
ExhalationCO2
fixation
SEDIMENTS BIOSPHERE
lithification
Organicmatter
SEDIMENTARYROCKS
Uplift
Corg
CONTINENTALEROSION
IGNEOUS ROCKS
METAMORPHICROCKS
Earth's Carbon Budget Biosphere, Oceans and Atmosphere
Crust
Mantle
Steady State & Residence Time
Steady State: Inflows = OutflowsAny imbalance in I or O leads to changes in reservoir size
AtmosphericOutflow:Inflow:
Respiration 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
Sedimentary carbonate-C
Sedimentary organic-C
Oceanic inorganic-C
Necrotic-C
Atmospheric-CO2
Living terrestrial biomass
Living marine biomass
Species 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
conservation of isotopes
13C into atm. & oceans =13C being buried in sediments
~5 the avg. value for crustal C
isotopic fifference betweeninorganic and organic carbon
Hayes et al., Chem Geol. 161.37.1999; Des Mariais et al., Nature 359.605.1992
Carbon Isotopic Excursions808-500Ma
More complete sediment record+
Improved chronology=
More detailed picture showingAbrupt and extreme
C-isotopic shifts
A global composite of 13C data shows 4 excursionsPlus one at the pC-C boundary
Carbon Isotopic Excursions808-500Ma
More complete sediment record +Improved chronology =More detailed picture showingAbrupt and extremeC-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