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
Anomalous d17O and d18O Composition of Stratospheric O2 and CO2 2O2 + energy O3 + O(1D) O(1D) + CO2 CO2 + O
Isotopic Notation • Express the 17O/16O anomaly using 17Δ notation 17Δ = (d17O – 0.516*d18O)*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 d17O vs d18O observed during respiration. (Luz and Barkan, 2000)
17Δ of water equilibrated with Air (Luz and Barkan, 2003) (Sarma et al, 2006)
17Δ of Biologically Produced O2 (Luz and Barkan, 2000)
17Δ of Photosynthetic O2 Lab Experiments17Δ (per meg vs AIR) Marine Plankton 244±20; 252±5 Sea of Galilee Plankton 159±10
Ocean Range of 17Δ Values Purely Photosynthetic O2 249 per meg Half Photo + Half Gas Exchange O2 130 per meg 16 per meg Purely Gas Exchange O2 Measuring 17Δ yields a direct estimate of the proportion of O2 from air and photosynthesis.
Measured 17Δ in the Surface Ocean 17Δ (per meg) • 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
Near Hawaii Near Bermuda L. Juranek (U.Washington) B. Luz (Hebrew U.)
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)
Estimating gross Photosynthesis rates from 17Δ • 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)
Advantages over the 14C-PP Method • In situ PP rates not in vitro PP rates • -there are no bottle effects. • Much simpler field method • -no time consuming bottle incubations • Integrates over the lifetime of O2 in the mixed layer • -typically 10-20 days (i.e., 50-100m / 5m/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
Disadvantages of the 17Δ-PP Method • Measures gross PP rate integrated over the mixed layer depth, not the photic layer depth. • Uncertainty of method depends primarily on uncertainty of gas exchange rate (30%) and 17Δ measurement. • 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) • In some situations, upwelling or mixing can bias the 17Δ in the mixed layer usually causing an overestimation of gross PP.
17Δ gross PP rates in the Surface Ocean Gross PP (mg C m-2 d-1) • 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
Comparison of 17O-PP versus 14C-PP BATS and HOTS = 1.6±0.4; CalCOFI = 2.7±1.6
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)
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.
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)
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.
Repeated Survey of 17Δ-PPg (using Container Ship)vs Satellite PP