Download Major chemical cycles

Major chemical cycles
• GOAL:
Use changes in the ratios of isotopes of
carbon (C) and oxygen (O) to help
reconstruct Earth history
Earth History, Ch. 10
1
Major chemical cycles
• Changes in 13C/12C ratio in rocks and
fossils are related to changes in rate of
organic burial, which influence CO2 and O2
in Earth’s atmosphere
• Changes in 18O/16O ratio in rocks and
fossils are related to glacial ice volume (i.e.,
changes in Earth’s temperature
Earth History, Ch. 10
2
Carbon and Oxygen Isotopes
Element
Carbon
Oxygen
atomic
number
6
8
12
13
14
total
relative
abundance
(%)
98.890
1.110
trace
100.000
16
17
18
total
99.789
0.007
0.204
100.000
atomic
weight
Earth History, Ch. 10
3
Isotope ratios— “δ” notation
[(13C/12C)sample – (13C/12C)standard]
δ13C =
× 1000
(13C/12C)standard
[(18O/16O)sample – (18O/16O)standard]
δ18O =
× 1000
(18O/16O)standard
(13C/12C)standard is a piece of belemnite
(18O/16O)standard is “mean” ocean water
Earth History, Ch. 10
4
Mass spectrometer
Measures 18O/16O
and 13C/12C ratios
in CO2 gas, usually
derived from a mineral
grain
Earth History, Ch. 10
5
Isotope
fractionation
• CO2 and O2
continuously cycle
through the
atmosphere and
biosphere as a
consequence of
photosynthesis and
respiration, BUT…
Earth History, Ch. 10
6
Isotope fractionation (cont.)
• …some processes preferentially use certain
isotopes and not others (fractionation)
• CO2 in plants is enriched in 12C (plants are
“isotopically light”)
• H2O in water vapor is enriched in 16O (rain
and snow are “isotopically light”)
Earth History, Ch. 10
7
Isotope fractionation (cont.)
• Fractionation would be unimportant, except that
the C and O “budgets” can be thrown out of
balance by “sinks”
• Buried organic matter is a “sink” for light
carbon
– Accumulation of buried organic matter isotopically
heavier CO2 everywhere else
• Continental glaciers are a “sink” for light oxygen
– Accumulation of glacial ice isotopically heavier O2
everywhere else
Earth History, Ch. 10
8
A. Burial rate of carbon
balances oxidation rate
B. Burial rate of carbon
exceeds oxidation rate
Carbon “sink”
Earth History, Ch. 10
9
Oxygen “sink”
Earth History, Ch. 10
10
High rate of decomposition
(oxidation) consumes O2
High rate of burial
prevents decomposition
(oxidation), resulting in
more atmospheric O2
Earth History, Ch. 10
11
The Carbon isotope record
Burial of
organic carbon
in Carboniferous
coal swamps
Earth History, Ch. 10
12
Historical trends in atmospheric O2
Carboniferous peak in
abundance of oxygen
Large amount of buried
organic matter means
less oxygen being used
in organic decay (oxidation)
Earth History, Ch. 10
13
High rate of decomposition
(oxidation) releases CO2
Earth History, Ch. 10
High rate of organic burial
prevents decomposition,
which reduces the amount
of atmospheric CO2 14
Historical trends in atmospheric CO2
Devonian decline in
abundance of CO2
Diversification of land
plants caused increase
in rates of mineral
weathering processes,
which consume CO2
Decline in atmospheric
CO2 may have led to
global cooling
Earth History, Ch. 10
15
The Oxygen isotope record
glacial intervals
When glaciers lock up light oxygen, the residual
seawater is isotopically heavy. Thus, CaCO3 marine
shells will be isotopically heavy during ice ages
Earth History, Ch. 10
16