1. Triple Oxygen Isotopes 11/1/10
Lecture outline:
atmospheric O2 cycle
mass-independent
fractionation in the
stratosphere
Δ17O as
productivity proxy
- ocean today
- paleo-evidence
A C141 military aircraft is regularly used for atmospheric gas sampling, especially in polar regions

2. The O2 cycle Three main reactions:
photosynthesis – mass-dependent fractionation (δ17O = 0.5*δ18O)
respiration – mass-dependent fractionation
stratospheric O3 – mass-independent fractionation (δ17O = δ18O)
isotopic
composition
of O2 depends
on who’s
winning
Joe Berry, News & Views, 1999
Stratospheric rxns:

3. Mass-independent fractionation of O in stratosphere
This anomalous
O2 “leaks” into
the troposphere,
enters biosphere,
and becomes
fractionated along
mass-dependent
line.
Can define anomalous
17O signal as:
average air used
as standard,
reported as parts per meg
(1 in 106)
empirically
determined
Thiemens and Heidenreich, 1983; Theimens, 1999 (review)

4. Isotopic analysis of CO2 samples collected in S.H.
at various altitudes
Thiemens et al., GRL, 1995
Which are the high-altitude
samples? Why?
But I thought we said that
the stratospheric signal was
a depletion of 17O relative
to the mass-dependent line?
So Thiemens proved
that the Δ17O anomaly
is generated in the
upper stratosphere
ENDMEMBER #1
Why the large difference in analytical error
between 14CO2 and 17CO2?

5. Closed biological system – steady state Δ17O (wrt air)?
Luz et al., GRL, 1999
“steady state” mass-dependent O2
falls on this line (from samples above),
defines a mass-dependent end-member,
vs. the mass-independent stratsopheric
endmember
d17O
slope = 0.52
intercept
0.155
ENDMEMBER #2
d18O

6. Mass-independent oxygen fractionation schematic
NOTE: these authors are using tropospheric
air (HLA) as their standard for δ17O, δ18O, Δ17O
- contains signature of stratospheric 17O anomaly
All of these fluxes are in steady state.
Idea: A change in any of these fluxes will
result in a new steady state value of
tropospheric and surface ocean Δ17O of O2.
Luz et al., 1999

7. Triple oxygen Isotopes as measure of ocean productivity
Idea:
1) measure O2 conc in surface water
2) measure Δ17O in surface water
Three scenarios:
If photosynthesis > respiration,
then [O2]> saturation, Δ17O > water
in equilibrium w/air
2) If respiration > photosynthesis,
then [O2}< saturation, Δ17O > water
3) if no biological activity,
then [O2] = saturation, Δ17O = water
in equilibrium with air
How is ocean productivity
traditionally measured?
satellite photo coccolithophore bloom off Newfoundland

8. Ocean Net Productivity – model and results
Bender, Perspective, Science 2000
Luz and Barkan, 2000
Tracers for net and gross production. Δ17O and [O2]/[O2]sat values for seawater at equilibrium with air. Black lines: Coevolution of 17O and [O2]/[O2]sat for photosynthesis and respiration in a closed system. Coevolution lines are plotted for net/gross production > 0 [photosynthesis (P) > respiration (R)] and < 0 (P < R). Data points are for the euphotic zone of the Atlantic Ocean near Bermuda on 7 July The ratio of net/gross O2 production, which can be read from this diagram, is ~0.05 in the mixed layer (top 10 m) and 0.1 in the upper part of the euphotic zone (depth of 20 to 60 m).

9. Using Δ17O to calculate past biosphere production in ice cores
Luz et al., 1999
RESULTS
Hmmmm… productivity didn’t change by much during glacial
but terrestrial biosphere was significantly reduced …. ocean productivity
must have increased during the glacial to compensate the loss of terr. productivity

10. Using Δ17O to calculate past
biosphere production in ice cores: a continuous Δ17O record
Observations:
GISP and Antarctic values
similar within error
- glacial values are higher
Facts:
CO2 lower during
glacial
(should increase Δ17O)
land productivity
diminished (fewer
trees; should decrease
Δ17O)
So what about glacial
ocean productivity??
Crosses = GISP2
Triangles = Siple Dome
Blunier et al., 2002

11. Causes of glacial Δ17O enrichment
CO2 is the stepping stone for
anomalous O2 formation;
less CO2 = less anomalous O2;
values trend towards biologic (+155 per meg)
this effect clearly dominated glacial Δ17O values
model suggests that CO2 should have caused a much larger
increase in Δ17O (+70 per meg vs. observed +38 per meg)
-so imply an 80% reduction in productivity during the glacial
-if land reduced, then ocean productivity may not have changed
(-88 to +140%  poorly constrained)