Presentation on theme: "Triple Oxygen Isotopes 11/1/10 Lecture outline: 1)atmospheric O 2 cycle 2)mass-independent fractionation in the stratosphere Δ 17 O as productivity."— Presentation transcript:
Triple Oxygen Isotopes 11/1/10 Lecture outline: 1)atmospheric O 2 cycle 2)mass-independent fractionation in the stratosphere Δ 17 O as productivity proxy - ocean today - paleo-evidence A C141 military aircraft is regularly used for atmospheric gas sampling, especially in polar regions
The O 2 cycle Joe Berry, News & Views, 1999 Three main reactions: 1.photosynthesis – mass-dependent fractionation ( 17 O = 0.5* 18 O) 2.respiration – mass-dependent fractionation 3.stratospheric O 3 – mass-independent fractionation ( 17 O = 18 O) isotopic composition of O 2 depends on who’s winning Stratospheric rxns:
Thiemens and Heidenreich, 1983; Theimens, 1999 (review) Mass-independent fractionation of O in stratosphere This anomalous O 2 “leaks” into the troposphere, enters biosphere, and becomes fractionated along mass-dependent line. Can define anomalous 17 O signal as: empirically determined average air used as standard, reported as parts per meg (1 in 10 6 )
Thiemens et al., GRL, 1995 Isotopic analysis of CO 2 samples collected in S.H. at various altitudes Which are the high-altitude samples? Why? But I thought we said that the stratospheric signal was a depletion of 17 O relative to the mass-dependent line? Why the large difference in analytical error between 14 CO 2 and 17 CO 2 ? So Thiemens proved that the Δ 17 O anomaly is generated in the upper stratosphere ENDMEMBER #1
Closed biological system – steady state Δ 17 O (wrt air)? 17 O 18 O intercept slope = 0.52 “steady state” mass-dependent O 2 falls on this line (from samples above), defines a mass-dependent end-member, vs. the mass-independent stratsopheric endmember steady state ENDMEMBER #2 Luz et al., GRL, 1999
Mass-independent oxygen fractionation schematic Luz et al., 1999 NOTE: these authors are using tropospheric air (HLA) as their standard for 17 O, 18 O, Δ 17 O - contains signature of stratospheric 17 O 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 Δ 17 O of O 2.
Triple oxygen Isotopes as measure of ocean productivity Idea: 1) measure O 2 conc in surface water 2) measure Δ 17 O in surface water Three scenarios: 1)If photosynthesis > respiration, then [O 2 ]> saturation, Δ 17 O > water in equilibrium w/air 2) If respiration > photosynthesis, then [O 2 } water in equilibrium w/air 3) if no biological activity, then [O 2 ] = saturation, Δ 17 O = water in equilibrium with air How is ocean productivity traditionally measured? satellite photo coccolithophore bloom off Newfoundland
Bender, Perspective, Science 2000 Ocean Net Productivity – model and results Tracers for net and gross production. Δ 17 O and [O2]/[O2]sat values for seawater at equilibrium with air. Black lines: Coevolution of 17 O 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). Luz and Barkan, 2000
Using Δ 17 O 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
Using Δ 17 O to calculate past biosphere production in ice cores: a continuous Δ 17 O record Crosses = GISP2 Triangles = 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 Δ 17 O) 2)land productivity diminished (fewer trees; should decrease Δ 17 O) So what about glacial ocean productivity??
Causes of glacial Δ 17 O enrichment CO 2 is the stepping stone for anomalous O 2 formation; less CO 2 = less anomalous O 2 ; values trend towards biologic (+155 per meg) -this effect clearly dominated glacial Δ 17 O values -model suggests that CO 2 should have caused a much larger increase in Δ 17 O (+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)