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Oxygen Cycle: Triple Isotopes

• An anomalous isotopic composition of atmospheric O2 yields a very useful means to estimate photosynthesis rates.

• Potentially, this method could make a significant impact on our understanding of the ocean’s biological pump

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Anomalous 17O and 18O Composition of Stratospheric O2 and CO2

2O2 + energy O3 + O(1D) O(1D) + CO2 CO2 + O

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Isotopic Notation • Express the 17O/16O anomaly using 17Δ notation

17Δ = (17O – 0.516*18O)*1000

• Units are per meg, 1 per meg = 1 ‰ / 1000

• AIR is the standard and has a 17Δ = 0 per meg

• Since air is depleted in 17O/16O, most other species will have positive 17Δ values on this scale

• The coefficient of 0.516 was chosen to equal the slope of 17O vs 18O observed during respiration.

(Luz and Barkan, 2000)

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Slope of 17O vs 18O during Respiration

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17Δ of water equilibrated with Air

(Luz and Barkan, 2003) (Sarma et al, 2006)

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17Δ of Biologically Produced O2

(Luz and Barkan, 2000)

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17Δ of Photosynthetic O2

Lab Experiments 17Δ (per meg vs AIR)

Marine Plankton 244±20; 252±5

Sea of Galilee Plankton 159±10

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Ocean Range of 17Δ Values

Purely Photosynthetic O2 249 per meg

Purely Gas Exchange O216 per meg

Half Photo + Half Gas Exchange O2 130 per meg

Measuring 17Δ yields a direct estimate of the proportion of O2 from air and photosynthesis.

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Measured 17Δ in the Surface Ocean

• Oligotrophic N. Pacific (Juranek) 20-30• Oligotrophic N. Atlantic (Luz) 30-50• Southern Ocean (Bender) 20-50• Equatorial Pacific (Bender, Juranek) 50-90 • Sagami Bay (Sarma) 80-100 • California Current System (Munro) 25-100• Sea of Galilee (Luz and Barken) 100-140

17Δ (per meg)

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Near Hawaii Near Bermuda

L. Juranek (U.Washington) B. Luz (Hebrew U.)

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Mixed Layer O2 and 17Δ*O2 Budget

• dO2/dt = kam*Sol*pO2atm – kam*Sol*pO2ml + Photo – Resp

 

• d(17Δ*O2/dt) = kam*Sol*pO2atm*17Δair – kam*Sol*pO2ml*17Δdiss +

Photo*17Δphoto – Resp*17Δdiss

-         -assume respiration doesn’t change the 17Δ of the dissolved O2

• Substituting for Resp yields an expression for gross Photo:

Photo = kam*pO2atm*Sol*(17Δair – 17Δdiss)/(17Δphoto – 17Δdiss)

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• If one estimates air-sea O2 gas transfer rates (Kam) from wind speed measurements, then one can calculate the gross Primary Production rate from a single measurement (17Δ of dissolved O2)

PPg = Kam*Sol*pO2atm* (17air – 17diss)

(17diss – 17photo)

Estimating gross Photosynthesis rates from 17Δ

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Advantages over the 14C-PP Method

a. In situ PP rates not in vitro PP rates

                         -there are no bottle effects.

b. Much simpler field method

                         -no time consuming bottle incubations

c. Integrates over the lifetime of O2 in the mixed layer

                                -typically 10-20 days (i.e., 50-100m / 5m/d)

d. Measures gross PP rates

                                  -not an ambiguous rate between gross and net PP

                                  -recycling of 14C-labeled OC in the bottle and use of non-14C labeled CO2 during

photosynthesis yield biases in PP rates that are difficult to quantify

 

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Disadvantages of the 17Δ-PP Method

a. Measures gross PP rate integrated over the mixed layer depth, not the photic layer depth.

b. Uncertainty of method depends primarily on uncertainty of gas exchange rate (30%) and 17Δ measurement.

c. Need to convert from O2 production to organic carbon

production

         -a 10-20% reduction for Mehler reaction and photorespiration

   -divide O2 production by the Photosynthetic Quotient (PQ) of

~1.1 (NH4 based PP) to ~1.4 (NO3 based PP)

d. In some situations, upwelling or mixing can bias the 17Δ in the mixed layer usually causing an overestimation of gross PP.

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17Δ gross PP rates in the Surface Ocean

• Oligotrophic N. Pacific (Juranek) 800 - 1500• Oligotrophic N. Atlantic (Luz) 300 - 1000• Southern Ocean (Hendricks) 600 - 3000• Equatorial Pacific (Juranek) 1000 - 2000 • Sagami Bay (Sarma) 1500 - 3000 • California Current System (Munro) 100 - 3000• Sea of Galilee (Luz&Barkan) 1600 – 16000• Global Ocean (at 1gmC/m2/d) 130 PgC/yr

Gross PP (mg C m-2 d-1)

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Comparison of 17O-PP versus 14C-PP

BATS and HOTS = 1.6±0.4; CalCOFI = 2.7±1.6

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Estimating the ratio of net to gross PP

• Photo = kam*pO2atm*Sol*(17Δair – 17Δdiss)/(17Δphoto – 17Δdiss)

 

• dO2/dt = kam*pO2atm*Sol*(1 – pO2/pO2atm) + Photo – Resp

 

-assuming net community productivity (NCP) = gross Photosynthesis – total Respiration

• and substituting for kam*pO2atm*Sol yields:

NCP/ Photo = (O2/O2atm – 1)* (17Δphoto – 17Δdiss) / (17Δair – 17Δdiss)

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Estimates of NCP/PPg from 17Δ and O2/Ar Measurements

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Ratio of NCP/PPg in Surface Ocean

-at HOT and BATS: 0.13±0.03

-Southern Ocean: 0.17±.13

-Equatorial Pacific: 0.12±0.11

-California Current 0.16±0.12

• Coastal Ocean has NCP/PPg ratio that is similar to open oligotrophic ocean. (Unexpected).

• Could be our best estimate of export ratio and efficiency of biological pump.

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Estimates of Carbon Export (NCP) Rates

-at HOT and BATS: 10±5 mmols C m-2 d-1

-in the Southern Ocean: 13±4

-in the Equatorial Pacific: 6.9±6.2

-California Current (CalCOFI): 14±10

-Globally, at 10 mmols m-2 d-1, yields 16 Pg C/yr (higher than previous estimates of 6-10 Pg C/yr)

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Future of 17Δ + O2/Ar Ocean Research

• Improved ability to catch PP events.

• Obtain large scale synoptic surveys of ocean PP rates.

• Improve resolution of short spatial and temporal scale variability in marine PP.

• Validation of satellite PP rates.

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Repeated Survey of 17Δ-PPg (using Container Ship) vs Satellite PP

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VGPM sat PP

in situ GPPChl*10^4

August 2005