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