Triple Oxygen Isotopes 11/1/10

Lecture outline:1) atmospheric O2 cycle

2) mass-independentfractionation in thestratosphere

3) Δ17O asproductivity proxy

- ocean today

- paleo-evidence

A C141 military aircraft is regularly used for atmospheric gas sampling, especially in polar

regions

The O2 cycle

Joe Berry, News & Views, 1999

Three main reactions:• photosynthesis – mass-dependent fractionation (δ17O = 0.5*δ18O)• respiration – mass-dependent fractionation• stratospheric O3 – mass-independent fractionation (δ17O = δ18O)

isotopiccompositionof O2 dependson who’swinning

Stratospheric rxns:

Thiemens and Heidenreich, 1983; Theimens, 1999 (review)

17 17 180.52*O O Oδ δΔ

Mass-independent fractionation of O in stratosphere

This anomalousO2 “leaks” intothe troposphere,enters biosphere,and becomesfractionated alongmass-dependent line.

17 0OΔ

17 0OΔ

Can define anomalous17O signal as:

empiricallydetermined

average air used as standard,reported as parts per meg(1 in 106)

Thiemens et al., GRL, 1995

Isotopic analysis of CO2 samples collected in S.H. at various altitudes

Which are the high-altitudesamples? Why?

But I thought we said that the stratospheric signal wasa depletion of 17O relativeto the mass-dependent line?

Why the large difference in analytical errorbetween 14CO2 and 17CO2?

So Thiemens provedthat the Δ17O anomalyis generated in theupper stratosphere

ENDMEMBER #1

Closed biological system – steady state Δ17O (wrt air)?

δ17O

δ18O

intercept0.155

slope = 0.52“steady state” mass-dependent O2

falls on this line (from samples above),defines a mass-dependent end-member,vs. the mass-independent stratsophericendmember

steady state

ENDMEMBER #2

Luz et al., GRL, 1999

Mass-independent oxygen fractionation schematic

Luz

et a

l., 1

999

NOTE: these authors are using troposphericair (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 willresult in a new steady state value oftropospheric and surface ocean Δ17O of O2.

Triple oxygen Isotopes as measure of ocean productivity

Idea: 1) measure O2 conc in surface water2) measure Δ17O in surface water

Three scenarios:1) If photosynthesis > respiration,

then [O2]> saturation, Δ17O > water in equilibrium w/air

2) If respiration > photosynthesis, then [O2}< saturation, Δ17O > water in equilibrium w/air

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

Bender, Perspective, Science 2000Ocean Net Productivity – model and results

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 1998. 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).

Luz and Barkan, 2000

Using Δ17O to calculate past biosphere production in ice cores

Luz et al., 1999

RESULTS

Hmmmm… productivity didn’t change by much during glacialbut terrestrial biosphere was significantly reduced …. ocean productivitymust have increased during the glacial to compensate the loss of terr. productivity

Using Δ17O to calculate past biosphere production in ice cores: a continuous Δ17O record

Crosses = GISP2Triangles = Siple Dome

Observations:-GISP and Antarctic values similar within error- glacial values are higher

Blunier et al., 2002

Facts:1)CO2 lower during

glacial(should increase Δ17O)

2)land productivitydiminished (fewer trees; should decreaseΔ17O)

So what about glacialocean productivity??

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)