1 / 39

Lecture 19 The Ocean Nitrogen Cycle

Lecture 19 The Ocean Nitrogen Cycle. Sinks/Sources Denitrification Reactions Distributions Nitrogen Fixation Reactions Distributions. The Global Oxygen Cycle. Source/Sinks Organic Carbon Burial Weathering. Main Ocean Source of N Nitrogen Fixation

clint
Download Presentation

Lecture 19 The Ocean Nitrogen Cycle

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. Lecture 19 The Ocean Nitrogen Cycle Sinks/Sources Denitrification Reactions Distributions Nitrogen Fixation Reactions Distributions The Global Oxygen Cycle Source/Sinks Organic Carbon Burial Weathering

  2. Main Ocean Source of N Nitrogen Fixation Enzyme catalyzed reduction of N2 N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16Pi Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase Inactivated when exposed to O2

  3. Main Ocean Sink of N Fixed Nitrogen (NO3-, NO2-, NH4+) is converted to N2 in low oxygen zones of the ocean Two Pathways Denitrification ( <2 to 10 mM O2): 2 NO3- + organic matter → N2 Anammox (<2 mM O2) NH4+ + NO2- → N2 + H2O

  4. Schematic of Ocean Nitrogen Cycle Gruber (2005) Nature 436, 786

  5. Global distribution of O2 at the depth of the oxygen minimum Gruber and Sarmiento, 1997

  6. Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163) Capone and Knapp (2007) Nature, 445, 159

  7. What is N*? How to calculate excess or deficient NO3- The solid line shows the linear equation P = 1/16 N + 0.345 (equivalent to N* = 0) Values to the right have positive N*, to the left have negative N* N* is defined as N* = [NO3] – 16 x [PO4] +2.9 PO4 versus Nitrate (GEOSECS data) Insert shows the effect of nitrification, photosynthesis, N2 fixation and denitrification.

  8. Vertical distribution of N* excess deficit N* is defined as N* = [NO3] – 16 x [PO4] +2.9

  9. Map View of N* N* at 200m in the Pacific (Gruber and Sarmiento, 1997)

  10. Why is N* negative - sinks Nitrogen Cycle w/ anammox Kuypers (2003) Nature 422: 608-611.

  11. Nitrogen species: NO3- ; NO2- ; N2O; N2 ; NH4+ (V) (III) (I) (0) (-III) Nitrogen Isotopes: 14N 99.634% 15N 0.366% Isotopic Composition: ‰ The standard is atmospheric N2

  12. Fractionation factors , where e is the isotopic enrichment factor Fractionation Heavier stable isotope forms stronger bond. Microbial Enzymes break light isotope bonds more easily. Reactants become heavier (enriched) (e.g. NO3-→ N2) Products become lighter (depleted) Partial versus total reaction (products have same values as reactants)

  13. The Global Nitrogen Budget-one example (Brandes et al, 2002)

  14. Why is this important for chemical oceanography? What controls ocean C, N, P? g ≈ 1.0 Mass Balance for whole ocean: C/ t = VRCR – f B CS = 0; CD = CD VU = VD = VMIX Negative Feedback Control: if VMIX↑ VUCD↑ B ↑ f B ↑ (assumes f will be constant!) assume VRCR  then CD↓ (because total ocean balance VUCD↓ has changed; sink > source) B ↓ The nutrient concentration of the deep ocean will adjust so that the fraction of B preserved in the sediments equals river input! CS CD if VMIX = m y-1 and C = mol m-3 flux = mol m-2 y-1

  15. Nitrogen Balance N2 Fix (100-330) CRCR (25) B Denitrification sed = 200-289 wc = 75 Net fluxes = -200 to 0 (sink > source; non-SS??) fB (25) Fluxes in Tg N y-1

  16. Earth is overall reducing Separate O2; sequester reducing material The Global Oxygen Balance Large O2 linked to Small C As P = R, O2 not affected by DP tO2 = 4 my solar UV only non-cyclic only w/o biology P and R in balance accelerated weathering Small imbalance in P-R marine org C only, not terrestrial tC = 20yr 80% in hemipelagic sediments where %orgC = 0.5% orgC includes H2S and Fe(II) tC = 108 yr stoichiometric so use moles Present is key to past Walker (1974) AJS

  17. 1) If P ceased and R continued org C would be consumed in 20 yr O2 would decrease by 1% 2) If only sink weathering, O2 would go to 0 in 4 my. This is short geologically so controlling balance must to strong. 3) Control on O2 = org C burial vs weathering 4) Feedback mechanism if atm O2 anoxic ocean org C burial atm O2 5) Control is with source rather than sink Sedimentary org C reservoir has not changed with time

  18. Hemipelagic sediments 200m to 3000m 80% of sediment orgC

  19. The Black Sea - Istanbul

  20. Rush hour on the Bosphorus

  21. Rumelihisarı

  22. Why is the Black Sea Interesting to Oceanographers? 1. The classic anoxic basin. Oxic layer over sulfidic layer. 2.Model for modern and ancient anoxic environments. 3. Well developed transition or suboxic zone. Model for world’s organic rich sediments. 4. Suboxic reactions easy to study here because of predictable depth locations. 5. An ideal location to study effect of climate forcing on ocean distributions. Climate Physical  Chemical Biological

  23. The Bosporus Strait connects the Black Sea with the Marmara Sea and is the only source of (relatively warm) salty water. Black Sea Marmara Sea

  24. Temperature and Salinity along the Bosporus CIL N. Sill MED S. Sill Gregg et al (1999)

  25. Bosporus Inflow and Entrainment of CIL results in the Bosporus Plume CIL (~50m) (from the surface in winter) Bosporus Inflow Avg CIL/BI ≈ 4 to 10 from Ozsoy et al.(1993)

  26. The Suboxic Zone: Oxygen – Sulfide Depth versus Density Total depth = 2200m

  27. Regional coupling of sources and sinks (Deutsch et al, 2007; Capone and Knapp, 2007) Potential for such coupling exists in the Black Sea

  28. Example of NO3-, NO2- and NH4+ for R/V Knorr 2003 CIL suboxic zone First seen during 1988 Expedition Data suggests anammox

  29. A combined geochemical and microbiological research approach consists of four approaches. • Determine net reactionsusing in situ distributions of nitrogen species and their 15N signatures • 2) Measure specific rates of reactions using labeled • nutrient spikes and selective inhibitors • 3) Culture organisms of interest • 4) Use DNA sequencing to determine the distributions • of attached and free living microorganisms • 5) Conduct mRNA analyses to determine which • bacteria are functionally active.

  30. Anammox in the Black Sea O2 Nutrients 15N spike  Ladderane Lipids 15N: Incubations of 15N-NH4+ and 14N-NO2- Kuypers. (2003) Nature 422: 608-611.

  31. There is a big N2 maximum but it is depleted in 15N-N2!! note: 2000 and 2001 note: 15N enriched near surface N2

  32. So The distributions of NO3- + NH4+ suggest anammox at sө = 15.95 There is a big maximum in N2/Ar at the right depth But d15N2 is quite variable and can be very depleted. If N2 was made by total consumption of NO3 + NH4 it’s d15N would be +6 to +8‰ Clearly there are additional fluxes or processes we need to understand.

  33. Conclusions from Geochemistry • Temporal variability in 15N-N2 • Anammox bacteria free-living • Denitrifiers particle attached • Our Hypothesis that that variability seen in • anammox / denitrification is due to variability • in organic rich particulate aggregates. • The redox state inside particles is • more reducing than outside

  34. % of Export Production (as N) at HOT derived from N2 Fixation (N-P mass balance model of Karl et al (1997) Nature 388, p. 533)

  35. Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)

  36. The Global Nitrogen Budget-one example (Brandes et al, 2002)

  37. Downcore records of 15N-orgN from several sites High values of 15N-OrgN suggest more extensive denitrification Deutsch et al, 2004)

  38. Deutsch et al, 2004

More Related