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 ParadoxFaint

Young Sun

Paradox

4 1H-->4He

Liquid H2O existed

life, zircon d18O)

Incr. density= incr. luminosity

>3.5 Ga (sed. rocks,

Simple Planetary Energy Balance •Likely solution to FYSP requires

understanding of Earth’s energy

balance (& C cycle)

Simple Planetary Energy Balance

EnergyAbsorbed

Neither

Geothermal

Changes Can Keep the Earth

from freezing w/

30% lower S

Albedo or

Heat Flux

Lower S compensated

by larger greenhouse

effect

The Electromagnetic Spectrum

Incoming UV, Outgoing IR

“Greenhouse

IR radiation efficiently

Gases” absorb

Moleculesacquireenergy

when theyabsorb

photons

CarbonCycle:Strong

driver of climate on geologic

timescales

geochemical The Bio-

carbon Cycle

Geochemical

#2

of rocks by dilute acid

C O2 + H2 2CO3

----------------------------------------------

1. Carbonate Weathering:

C a C O3 + H2CO3 2+ + 2 HCO3

-

2. Silicate Weathering:

C a S i O3 + 2 H2CO3 2+ + 2HCO3

- + SiO2 + H2O

-------------------------------------

•2x CO2 consumption for silicates

•

Carbon Cycle

Chemical Weathering = chemical attack

O <---> H

--> Ca

--> Ca

Carbonates weather faster than silicates

Carbonaterocks

weatherfaster than

silicaterocks!

geochemical

Reaction)

+ Carbonate and Silica Precipitation in Ocean

CaSiO3 + CO2 3 + SiO2

•CO2 consumed (~ 0.03 Gt C/yr)

• 2 in 20 kyr

• 2 via Volcanism and Metamorphism

-------------------------------------

C a C O3 + SiO2 --> CaSiO3 + CO2

•CO2 produced from subducted marine sediments

Net reaction of

carbon cycle (Urey

Net Reaction of Rock Weathering

--> CaCO

Would deplete atmospheric CO

Plate tectonics returns CO

Carbonate Metamorphism

•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 Kumpet al. (1999)

A Closer Look atthe Biogeochemical

Carbon & theOrganic Carbon

Sub-Cycle

ATMOSPHERE CO2

CONTINENTAL

EROSION

SEDIMENTS METAMORPHIC

ROCKS

BIOSPHERE

IGNEOUS ROCKS

SEDIMENTARY

ROCKS

OCEANS

exhalation

Uplift

lithification

organic

matter

DIC = HCO3 -+ CO3

2-

CaCO3

Corg

CO2rock

weathering

sink

dissolution sink

TECTONICS

fixation

BIOGEOCHEMICAL CARBON CYCLEBIOGEOCHEMICAL CARBON CYCLE

3.7x1018 moles

Crust

Corg

1100 x 1018

mole

Carbonate

5200 x 1018

mole

Mantle

18 mole

Earth's Carbon Budget

Biosphere, Oceans and Atmosphere

100,000 x 10

Steady State & Residence Time

Steady State: Inflows = OutflowsAny imbalance in I or O leads to changes in reservoir size

Inflow: Atmospheric CO2

760 Gton C

Outflow: 60 Gton C/yr 60 Gton C/yr

Respiration Photosynthesis

1 Gton = 109 * 1000 kg = 1015 g

The Residence time of a molecule is the average amount of time it is expected to remain in a given reservoir.

R of atmospheric CO2 = 760/60 = 13 yrExample: t

Carbon Reservoirs, Fluxes and Residence TimesCarbon Reservoirs, Fluxes and Residence Times

Species Amount

(in units of 1018 gC)

Residence Time (yr)* d 13 C

%o PDB**

Sedimentary carbonate-C 62400 342000000 ~ 0

Sedimentary organic-C 15600 342000000 ~ -24

Oceanic inorganic-C 42 385 ~ +0.46

Necrotic-C 4.0 20-40 ~ -27

Atmospheric-CO2 0.72 4 ~ -7.5

Living terrestrial biomass 0.56 16 ~ -27

Living marine biomass 0.007 0.1 ~ -22

•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

Carbonate-Silicate Geochemical Cycle

subduction zone or during orogeny.

closed system

13 13C being buried in sediments

conservation of isotopes

din = forg * dorg + f dcarb

fcarb + forg = 1

eTOC = dcarb – dorg

inorganic and organic carbon (Dc)

forg = (dcarb + 5) / eTOC

Total C entering atm. & oceans = Total C buried in sediments

C into atm. & oceans =

carb * ~ 5 the avg. value for crustal C

isotopic difference between

Hayes et al., Chem Geol. 161, 37, 1999; Des Marais et al., Nature 359, 605, 1992

Simple Carbon Cycle Modeling

Carbon Isotopic Excursions800-500Ma

More complete sediment record+

Improved chronology

=

More detailed picture showingabrupt and extreme

C-isotopic shifts

A global composite of d13C data shows 4 excursions

Plus one at the pC-C boundary

Carbon Isotopic Excursions800-500Ma

More complete sediment record

+

Improved chronology

=

More detailed picture showing

abrupt and extreme

C-isotopic shifts

Modeling the Proterozoic Carbon Cycleß dcarb and dorg through time 2500-550 Ma in 100Ma increments

ß note the constancy of dcarb while dorg decreased. Why? biochemistry or pCO2

ß forg increased through this time, was episodic and was linked to periods of rifting and orogeny ß also associated with extreme glaciations

ß Increase in the crustal inventory of C requires increases in the inventories of crustal Fe3+ , crustal and marine sulfate, nitrate and atmospheric oxygen.

-ß SO4 , NO3 and O2 increases changed patterns of respiration

-ß NO3 would have forced productivity changes