1 / 52

Stable Isotopes and Microbial Biogeochemistry

Karen Casciotti Woods Hole Oceanographic Institution. Stable Isotopes and Microbial Biogeochemistry. Part 2: Isotopic fractionation and natural abundance level isotope variations. Marine Nitrogen Cycle. Atmospheric deposition and continental inputs. CO 2. Atmospheric N 2. N 2 O. N 2 O.

theolar
Télécharger la présentation

Stable Isotopes and Microbial Biogeochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Karen Casciotti Woods Hole Oceanographic Institution Stable Isotopes and Microbial Biogeochemistry Part 2: Isotopic fractionation and natural abundance level isotope variations.

  2. Marine Nitrogen Cycle Atmospheric deposition and continental inputs CO2 Atmospheric N2 N2O N2O Dissolved N2 + NH Particulate 4 DON Nitrogen Fixation N - Assimilation NO 3 Surface ocean Vertical mixing/ Deep ocean upwelling Nitrification NO3- + Remineralization NH 4 Sedimentary N2O Denitrification Water column Denitrification Sediment Burial

  3. Summary from Lecture 1 N2 • Isotopes provide a sensitive tracer of reactions mediated by the microbial community • SIP can be used to identify metabolic capabilities of uncultured organisms • Taxon-specific metabolic rates and activities can be determined by cell sorting and/or RNA capture techniques NH4+ 15NO3- Cell sorting Or RNA capture

  4. Sampling the ocean

  5. Overview of Lecture 2 • Applications of natural abundance isotopes to microbial (nitrogen) biogeochemistry: • Isotopic fractionation • Natural variations in isotope ratios--a record of microbial, chemical, and physical processes • Applications and interpretations • Example 1: Global N budgets • Example 2: Sources of N to the euphotic zone

  6. Notation • 15N: the isotope, its molar amount or concentration • 15F: Fractional abundance = 15N/(14N+15N) • 15Atom % = 15F x 100% • 15R: Isotope ratio = 15N/14N range: 0.0036581 to 0.0038236 (approx.) precision: 0.0000007 • d15N: delta value (‰) = (15R/15Rstd -1) x 1000 range: -5 to 40‰ (approx.) precision: 0.2‰ • ak: Kinetic fractionation factor = 14k/15k range: 0.985 to 1.040 (approx.) • ek: Kinetic isotope effect (‰) = (ak -1) x 1000 range: -15 to +40‰

  7. Stable Isotopic Fractionation 14k 14NO3- 14NO2- 15k 15NO3- 15NO2- ak = 14k/15k = 1 => no fractionation > 1 => normal fractionation: preference for 14N < 1 => inverse fractionation: preference for 15N

  8. Why fractionate? NO3- NO2- - - 16O 16O - 16O 14N 15N 14N 16O 16O 16O 16O 18O 16O 14NO3- 15NO3- N16O218O-

  9. Stable Isotopic Fractionation R 12 light 3 heavy No fractionation: p (heavy) = 4/20 0.25 11 light 4 heavy ‘Normal fractionation’: p (heavy) < 4/20 0.36 16 light 4 heavy 13 light 2 heavy ‘Inverse fractionation’: p (heavy) > 4/20 R = 4/16 = 0.25 0.15 Remove 5 out of 20; 15 remain

  10. Stable Isotopic Fractionation:Nitrate reduction NO3- NO2- Substrate (NO3-) Normal fractionation 75 Reaction progress 50 d15N 25 0 Product (NO2-) Normal fractionation 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO3-]/[NO3-]0

  11. Stable Isotopic Fractionation:Nitrogen fixation N2 Norg 1 Substrate (N2) No fractionation 0 Product (Norg) No fractionation -1 d15NNO3 Reaction progress -2 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO2-]/[NO2-]0

  12. Stable Isotopic Fractionation:Nitrite oxidation NO2- NO3- Product (NO3-) Inverse fractionation 25 0 -25 d15NNO3 Reaction progress Substrate (NO2-) Inverse fractionation -50 1.0 0.8 0.6 0.4 0.2 0.0 f = [NO2-]/[NO2-]0 Casciotti, submitted

  13. Notation • 15N: the isotope, its molar amount or concentration • 15F: Fractional abundance = 15N/(14N+15N) • 15Atom % = 15F x 100% • 15R: Isotope ratio = 15N/14N range: 0.0036581 to 0.0038236 (approx.) precision: 0.0000007 • d15N: delta value (‰) = (15R/15Rstd -1) x 1000 range: -5 to 40‰ (approx.) precision: 0.2‰ • ak: Kinetic fractionation factor = 14k/15k range: 0.985 to 1.040 (approx.) • ek: Kinetic isotope effect (‰) = (ak -1) x 1000 range: -15 to +40‰

  14. Microbial Nitrogen Cycle Nitrogen fixation N2O Denitrification N2 e ≈ 0‰ NO Anammox e ≈ 15‰ e = ? e < 5‰ e ≈ 20-30‰ Assimilation Org NH3 NO2- NO3- e ≈ 5-10‰ Nitrite oxidation e ≈ 10-20‰ e ≈ -15‰ Nitrification NH2OH Ammonia oxidation e ≈ 15-38‰ Oxidation state (-III) (-I) (0) (I) (II) (III) (V)

  15. Nitrate isotope variations in seawater d15NO3- Depth (m) Casciotti et al., 2007, Casciotti unpublished, Sigman et al., 2005

  16. Overview of Lecture 2 • Applications of natural abundance isotopes to microbial (nitrogen) biogeochemistry: • Isotopic fractionation • Natural variations in isotope ratios--a record of microbial, chemical, and physical processes • Applications and interpretations • Example 1: Global N budgets • Example 2: Sources of N to the euphotic zone

  17. Inputs and Outputs of Fixed N Atmospheric Deposition Continental Runoff N2 fixation NH4+, NO3-, DON Norg Water column Denitrification N2 Anammox N2 Sediment Burial Sedimentary Denitrification N2 Norg

  18. Inputs and Outputs of Fixed N Approaches to flux determination: • Direct or indirect rate measurements • Geochemical modeling • N:P Stoichiometry • Isotopes Denitrification Nitrogen fixation

  19. Nitrogen Budgets Is the N cycle in balance? Maybe not! Is it a moving target? What are the consequences?

  20. Marine N Isotope Budget Sigman, Karsh, and Casciotti, EOS in press

  21. Steady State Model for Ocean N Budget Sedimentary Denitrification dSD~ dNO3- eSD N losses dout~ 0‰ Mout NO3- N inputs din~ 0‰ Min dNO3= 5‰ Water column Denitrification dSD~ dNO3- eSD • N isotope budget insensitive to rate of N2 fixation, but very sensitive to the ratio of water column to sediment denitrification. • Suggest that given estimated fluxes of WC denit (75 Tg N yr-1), sedimentary denitrification should be ~ 280 Tg N yr-1. • Sum of sink terms far outweighs the estimated rate of N2 fixation (~125 Tg N yr-1)! • Shorter residence time for marine N. Brandes and Devol, 2002

  22. Global N budgets • Still out of balance! • Are we underestimating N2 fixation rates? • Are we overestimating water column denitrification? • Are we under/over estimating edenit? • What is the role of anammox? • Sedimentary nitrogen fixation? • Can we refine the budget using different isotope studies? • Can we really assume steady state?

  23. P* estimate of Nitrogen fixation rates Deutsch et al. (2007) Nature 445: 163-167

  24. New/Export Production Atmospheric deposition and continental inputs N2 CO2 NH4+ DON N2 Biomass NO3- Surface ocean NO3- Export of C, N Deep ocean

  25. Pacific Surface Nitrate 20 15 Nitrate (mM) 10 5 0 WOA05 data from NODC

  26. Approaches for quantifying the N2 fixation flux • Scaled microscopic enumeration: • 0.14 ± 0.07 mmol N m-2 d-1 (Karl et al., 1997) • Molecular analyses: • Abundance and diversity; qualitative activity • Extrapolated instantaneous rate measurement: • 0.08 ± 0.05 mmol N m-2 d-1 (Karl et al., 1997) • 0.07 ± 0.02 mmol N m-2 d-1 (Montoya et al., 2004) • Satellite observation: • ~ 5x1015 mmoles N d-1 (Westberry and Siegel, 2006) • Geochemical measurements • Nutrient stoichiometry: • 0.09 ± 0.04 mmol N m-2 d-1 (Karl et al., 1997) • 0.11 ± 0.02 mmol N m-2 d-1 (Deutsch et al., 2001) • Isotope balance: • 0.14 mmol N m-2 d-1 (Karl et al., 1997) • 0.13 ± 0.05 mmol N m-2 d-1 (Dore et al., 2002)

  27. HOT sinking d15NPN Deep NO3- N2 Dore et al., 2002 study d15N = ((15N/14N)sample/(15N/14N)std)-1)*1000

  28. HOT Sinking PN flux at 150 m Dore et al., 2002 study

  29. HOT sinking d15NPN Flux-weighted average d15NPN Dore et al., 2002 study

  30. Euphotic Zone Nitrogen Isotope Balance Nitrogen fixation (1) FN2fix + FNO3= FPN surface NO3- PN (2) FN2fix*d15NN2fix + FNO3*d15NNO3 = FPN*d15NPN 150 m (3) FN2fix/FPN = (d15NPN- d15NNO3)/(d15NN2fix- d15NNO3) Nitrate flux Export flux (Dore et al., 2002)

  31. Sample Types MULVFS (suspended particles) CTD Rosette (nitrate samples) NBST’s (sinking particles) 0 150 300 500 1000 3000 (Buesseler, Valdes, et al.) (Bishop)

  32. Euphotic Zone Mass and Isotope Balance FN2fix; d15NN2fix 0 NO3- PN 150 FNO3,150;d15NNO3,150 FPN,150;d15NPN,150

  33. VERTIGO Nitrate Isotopic Data Casciotti et al., 2008 in press DSRII

  34. VERTIGO Particulate N Isotopic Data Casciotti et al., 2008 in press DSRII

  35. Euphotic Zone Mass and Isotope Balance FN2fix = ? d15NN2fix = 0 ± 1 ‰ 0 NO3- PN 150 FNO3,150 = ? d15NNO3,150 = ? FPN,150 = 0.18 ±0.04 mmol m-2 d-1 d15NPN,150 = 2.5 ± 0.4‰

  36. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m b) 150-300 m 2) Pulse event a) reaching 200 m b) reaching 300 m Casciotti et al., 2008 in press DSRII

  37. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m: 3.0 ± 0.3‰ b) 150-300 m: 3.7 ± 0.3‰ ((d[15N]/dz ÷ 15Rstd )-1 ) *1000 d[14N]/dz Casciotti et al., 2008 in press DSRII

  38. VERTIGO Nitrate Isotopic Data Scenarios: 1) Diffusive flux a) 150-250 m: 3.0 ± 0.3‰ b) 150-300 m: 3.7 ± 0.3‰ 2) Pulse event a) reaching 200 m: 2.5 ± 0.5‰ b) reaching 300 m: 4.5 ± 0.5‰ Casciotti et al., 2008 in press DSRII

  39. Euphotic Zone Nitrogen Isotope Balance Nitrogen fixation (1) FN2fix + FNO3 = FPN surface NO3- PN (2) FN2fix*d15NN2fix + FNO3*d15NNO3 = FPN*d15NPN 150 m (3) FN2fix/FPN = (d15NPN - d15NNO3)/(d15NN2fix- d15NNO3) Nitrate flux Export flux

  40. PN flux during VERTIGO HOT Sinking PN flux at 150 m

  41. d15NPN during VERTIGO HOT sinking d15NPN

  42. But why the low d15NNO3?

  43. ALOHA Physical Properties (150m) (300m) (500m) (1000m) (>3000m) Casciotti et al., 2008 in press DSRII

  44. Mixing Curves for [NO3-] and d15NNO3 NPDW NPIW (500m) NPDW NPBW SSMW (300m) NPIW (500m) SSMW (300m) STSMW (150m) STSMW (150m) Casciotti et al., 2008 in press DSRII

  45. Scenario 1: 14N trapping by remineralization Low 15N/14N Lower 15N/14N Higher 15N/14N Lower 15N/14N Higher 15N/14N

  46. VERTIGO Particulate N Isotopic Data Casciotti et al., 2008 in press DSRII

  47. Modeled d15NNO3 Distribution Glover and Casciotti, unpublished

  48. Scenario 2: Long-term accumulation of N fixation signal NO3- uptake Low 15N/14N N2 fixation Low 15N/14N Low 15N/14N Lower 15N/14N Low 15N/14N High 15N/14N

  49. ALOHA 15N Mass Balance

  50. ALOHA 15N Mass Balance

More Related