1. The Global Oxygen Cycle
Lecture 20
The Global Oxygen Cycle
Source/Sinks
Source - Organic Carbon Burial in sediments
Sink - Weathering
The Global Carbon Cycle
Source from rivers via weathering
Sink = CaCO3 and org C burial
Need Urey reaction

2. The Global Oxygen Balance
Earth is overall reducing
Separate O2; sequester reducing material
Large O2 linked to Small C
As P = R, O2 not affected by DP
tO2 = 4 my
solar UV
only non-cyclic
only w/o biology
P and R in balance
accelerated
weathering
Small imbalance in P-R
marine org C only, not terrestrial
tC = 20yr
80% in hemipelagic sediments
where %orgC = 0.5%
orgC includes H2S and Fe(II)
tC = 108 yr
Present is key to past
stoichiometric so use moles
Walker (1974) AJS

3. 1) If P ceased and R continued
org C would be consumed in 20 yr
O2 would decrease by 1%
2) If the only sink is weathering, O2 would go to 0
in 4 my. This is a short time geologically so controlling
balance must to strong.
3) Control on O2 = org C burial (O2 source) vs weathering (O2 sink)
4) Feedback mechanism
if atm O2
anoxic ocean
org C burial
atm O2
5) Control is with source rather than sink
Sedimentary org C reservoir has not changed with time

4. Galy et al, Nature, 2015
Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of 157 and 43 megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering.
200 MT C = 1.7 x 1010 mol C / yr
= 0.1% of org C buried

5. Hemipelagic sediments (org C > 0.5%)
200m to 3000m
80% of sediment orgC

6. CO2  and O2 

7. The long-term global carbon balance
weathering
CaCO3(s) + CO2(g) + H2O = 2HCO3- + Ca2+
2HCO3- + Ca2+ = CaCO3(s) + CO2(g) + H2O
deposition
A better example of reverse weathering!
Fig. 2.5 Emerson and Hedges

8. Chemical Weathering, the Geological Carbon Cycle, Control on CO2
1. CO2 is removed by weathering of silicate and carbonate rocks on land.
2. The weathering products are transported to the ocean by rivers where they
are removed to the sediments as CaCO3 and SiO2.
3. When these sediments are subducted and metamorphosed at high T and P,
CaCO3 and SiO2 are converted into CaSiO3 and CO2 is returned to the atmosphere.
Ittekkot (2003) Science 301, 56
For more detail see Berner (2004) The Phanerozoic Carbon Cycle: CO2 and O2. Oxford Press, 150pp.

9. TABLE 2 Oceanic fluxes of carbon Flux Atmospheric Demand
River input
Derived from atmosphere
Derived from carbonates
Hydrothermal input
Carbonate deposition
Deposited as carbonates
Lost to atmosphere
Net atmospheric demand
Units: 1012 mol/y
Some CO2 produced by carbonate deposition, but not enough!
The rest must come from the Urey reaction.
From McDuff and Morel (1980)

10. There must be a tight feedback control on atm CO2
1. The problem of the cool sun (Sagan and Mullen, 1972).
Solar luminosity has increased by 25% over the age of the solar system.
But liquid water has existed for 3.8 byr!
There must be a temperature buffer!
2. Was it NH3?? No. Most likely the greenhouse gas CO2.
3. CO2 is produced to the atmosphere by volcanoes and metamorphism.
Such as The “Urey Reaction” CaCO3(s) + SiO2(s) = CaSiO3(s) + CO2(g)
4. The important sink of CO2 is weathering of silicate minerals. Weathering of silicate rocks consumes CO2 and produces Ca2+ and Mg2+ to rivers. In the ocean this Ca2+ and Mg2+ is removed by formation of carbonate rocks which produces CO2. The rate of weathering is influenced by rock type, slope, temperature and runoff
5. The weathering and deposition of carbonate rocks alone is not sufficient. Need the Urey Reaction!
7. A negative feedback. If the earth became cooler, silicate weathering would decrease, atmospheric CO2 would increase and the earth would warm!