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1!
Simple view of controls on CO2
from last week!
CO2!Rat
e of
CO
2 i
n/o
ut
of
atm
osp
her
e!
In from volcanoes!
Out from w
eathering!
CO2 and Phanerozoic"
(550-0Ma) Climate!
•! Carbon reserviors, fluxes and isotopic
composition!
•! Modelled CO2 : GEOCARB III!
•! “Measured” paleo-CO2!
2!
Reservoirs!
and fluxes!
in 1015 gC and !
1015 gC/yr!
Green: size of reservoir (1018 gC)!
Red: flux from one reservoir to another (1018 gC/yr)!
3!
Isotopes of Carbon!
12C 98.93%!13C 1.07%!
!13C = (Rsample/Rstd - 1)x1000!
Rstd = 13C/12C PDB!
Isotopic fractionation:!
•!Physical transport!
•!Chemical reactions!
Imp: 12C isotope preferred by plants, so organic carbon!
is VERY ‘depleted’ in carbon isotope composition (!13C)!
relative to all other carbon reservoirs!
•! Organic matter 20‰ depleted relative to
inorganic carbon reservoir!
•! Isotopic composition of carbon entering from
volcanoes is the same as that of carbon being
buried in rocks!
4!
GEOCARB (Berner, “BLAG”)
Model !•! Aim: Reconstruct long term atmospheric
CO2 at 10 million year time resolution.!
•! Assumption: Flux of carbon into and out of the ocean/atmosphere reservoir are equal (steady state)!
•! Key component for stable output: weathering of silicate rocks increases with increasing temperature and CO2!
Berner and Kothavala (2001)!
Why assume steady state?!
5!
GEOCARB Model!
•! CO2 input to atmosphere!–! Thermal breakdown of
sedimentary carbonates and organic carbon (diagenesis, metamorphism and volcanism)!
–! Weathering of carbonates!
–! Oxidation of organic carbon!
•! CO2 drawdown from atmosphere!–! Silicate weathering
(carbonate burial)!
–! Organic carbon burial!
Urey reactions!
Organic carbon cycling!
6!
Ocean/atm/biosphere reservoir is
at steady state!
w = weathering!
m = metamorphism and volcanism!
b = burial!
c = carbonate!
g = organic matter!
mass balance of C!
mass balance of isotopes!
Organic carbon and Carbonate
rock reservoirs can grow and
shrink!
7!
What determines how fluxes (F)
change with time?!•! Weathering of carbonates (Fwc) and organic carbon (Fwg)
depends on amount stored in rocks (model variable) plus climate (model variable) and geographic variables (specified as input)!
•! Return of rock carbon to the atmosphere from volcanism (Fmg, Fmc) depends on spreading rate (specified) and whether they were buried on shelves or in deep sea (specified)!
•! Net burial of carbonate (by definition the silicate weathering flux) Fbc - Fwc depends on temperature, climate (model variables), geography, plants, etc. (input)!
•! Fbg depends on the chemistry and circulation of the ocean among other things - cannot be specified or parameterized based on model variables!
Weathering feedback term!
8!
Weathering uplift parameter!
Plant weathering!
•! Deep rooted large vascular plants (trees) rose during Devonian (350-300 Ma)!
•! Rise of Angiosperms (130-80 Ma)!
•! Extent to which plants enhance weathering rate is not well constrained experimentally and in this model is tunable parameter!
9!
Simplification for easier
understanding: G and C are big
and relatively unchangeable!
Fluxes on lhs depend on (1) spreading rate, (2) geography !
and (3) climate!
Fbc depends on (1) geography and evolution and (2) climate!
Fbg depends on the chemistry and circulation of the ocean
among other things!
2 equations, 2 unknowns (climate and burial of organic
carbon)!
Isotopes can be used to assess
fraction of carbon buried as
organic material!
10!
Model Results!
What about the data?!
11!
Royer et al 2004!
Paleosol (Cerling) Method!
•! Organic matter of land plants has lower !13C than the ratio in the atmosphere!
•! Plant organic matter (roots, fallen leaves) decays within the soil, releasing CO2 that is low in !13C!
•! Atmospheric CO2 (higher !13C ) also diffuses into the soil!
•! If atmospheric CO2 concentrations are high, the !13C in soil CO2 is close to the atmospheric ratio!
•! If atmospheric CO2 concentrations are low, the !13C in soil CO2 is close to the organic ratio!
•! CaCO3 precipitating in desert soils records the !13C in soil CO2.!
12!
soil
atmosphere
soil CO2 (low 13C/12C)
atmospheric CO2 (high 13C/12C)
no CO2 low CO2 high CO2
Stomatal density method!
•! Stomatal density (stomates/unit area) and Stomatal index(percentage of epidermal cells that are stomates) correlate inversely with pCO2!
•! Stomatal index less sensitve to changes in soil moisture humidity and temperature!
•! Calibration of extinct species?!
13!
Low CO2, continental glaciation!
Royer et al.,!
2004!
Why do we care that BLAG"
seems to work?!
14!
Why do we care that BLAG"
seems to work?!
1)! understand causes of high CO2 in past; understand the !
! timescales for CO2 removal from atmosphere!
2) !low CO2 and glaciations seem to be linked!
3)! can estimate climate sensitivity (definition: how much !
! temperature change occurs with a doubling of CO2?)!
Royer et al., 2007!
Ex: estimate climate sensitivity!
-!BLAG assumes feedbacks b/t CO2, !
temperature, and weathering!
-!if CO2 causes large T!
changes, weathering rates!
will skyrocket, and modeled!
CO2 will fall below proxy data!
-!if CO2 causes low T, weathering !
rates will be low, and CO2 will!
exceed proxy data!
-climate models give 1.5-6°C;!
-BLAG model gives ~3°C!
15!
For further reading!
Royer, Dana L., Robert A. Berner, Isabel P. Montañez, Neil J. Tabor, and David J.
Beerling (2004). "CO2 as a primary driver of Phanerozoic climate". GSA Today 14 (3):
4-10."
Berner, RA and Z. Kothavala (2001). "GEOCARB III: A revised model of atmospheric
CO2 over Phanerozoic time". American Journal of Science 304: 397–437."
16!
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