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Isotopic insights into the benthic N cycle, and its impact on the global marine N cycle.

Isotopic insights into the benthic N cycle, and its impact on the global marine N cycle. Start with a review of stable isotope behavior in general. Wind up looking at the d 15 N (and d 18 O) of pore water nitrate. Inputs Mixing Fractionation (photosynthesis, denitrification)

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Isotopic insights into the benthic N cycle, and its impact on the global marine N cycle.

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  1. Isotopic insights into the benthic N cycle, and its impact on the global marine N cycle. Start with a review of stable isotope behavior in general. Wind up looking at the d15N (and d18O) of pore water nitrate.

  2. Inputs Mixing Fractionation (photosynthesis, denitrification) Equilibrium fractionation – distribution of isotopes among coexisting phases at equilibrium. Can vary as a fn of T, P; temperature dependence forms the basis of isotopic thermometers (e.g., the 18O of CaCO3) Kinetic fractionation – difference in isotopic composition of reactants and products in a unidirectional reaction

  3. Kinetic fractionation: In general, less energy is required to break the bond of the lighter isotopic species, so the lighter species react faster. Zeebe and Wolf-Gladrow

  4. The trajectory in a delta – concentration plot reflects the amount of the compound added, and its isotopic composition. An approximate mass and isotope balance.

  5. A similar useful approximation applies to mixtures: The isotopic composition of a mixture reflects the concentration-weighted contributions of the components.

  6. Gramling

  7. Rayleigh fractionation -transformation (removal) with a constant fractionation and no replenishment of the reactant: Reactant pool: d(reactant) = d(initial reactant) – e x (ln(f)) where “f” is the fraction of reactant remaining and e is the kinetic isotope effect Instantaneous product: d(instantaneous product) = d(reactant) – e Integrated (accumulated) product: d(integrated product) = d(initial reactant) + e x {f/(1-f)} x ln(f) Rayleigh fractionation -transformation (removal) with a constant fractionation and no replenishment of the reactant: Reactant pool: d(reactant) = d(initial reactant) – e x (ln(f)) where “f” is the fraction of reactant remaining and e is the kinetic isotope effect Instantaneous product: d(instantaneous product) = d(reactant) – e Integrated (accumulated) product: d(integrated product) = d(initial reactant) + e x {f/(1-f)} x ln(f)

  8. Rayleigh fractionation Zeebe and Wolf-Gladrow

  9. Rayleigh-type fractionation as a model of DIC uptake during photosynthesis. The substrate (DIC) is never strongly depleted; the fractionation is expressed, and the product (organic carbon) is roughly 20 o/oo depleted relative to DIC.

  10. Rayleigh-type fractionation as a model of nitrate uptake during photosynthesis.

  11. Rayleigh-type fractionation as a model of nitrate uptake during photosynthesis. As utilization of the reactant nears completion, the accumulated product (integrated product) approaches the isotopic composition of the initial reactant.

  12. The instantaneous d15NO3- and [NO3-] trajectories of different marine N cycle processes, assuming an initial d15NO3- of 5‰. Sigman and Casciotti

  13. Depth profiles of the d15N and d18O of nitrate and of N*m along the North American Pacific margin, from Santa Barbara Basin to the tip of Baja California. N* is a measure of the nitrate deficit relative to the expected Redfield relationship with phosphate (N*m=nitrate-16*phosphate+2.9). The profiles from further south, where denitrification is occurring at high rates in the water column OMZ, are in darker shades. Sigman et al., 2003

  14. e ~ -17 e ~ 0 e ~ -25 Lehmann et al.

  15. Heavy NH4+ ? Light NO3- ? Heavy NO3- ? e ~ 0 e ~ -17 e ~ -25 Lehmann et al.

  16. Heavy NH4+ ? Light NO3- ? Heavy NO3- ? e ~ 0 e ~ -17 e ~ -25 Expression of these fractionations depends on the branching ratios; there is no net fractionation if a reactant is completely converted to a product.

  17. A low-flux cartoon of oxic respiration; values sort of realistic -O2:N = -170:16; DO2/DNO3 = 1.3; d15N OM = 0

  18. Both 14N and 15N diffuse out of sediments; 14N gradient proportionally steeper.

  19. Hoffmann Denitrification leaves the residual pore water nitrate strongly enriched in 15N and 18O.

  20. Hoffmann But the flux of nitrate (from nitrification and from bottom water) is down. Denitrifiers consume the high d15N and d18O “residual” nitrate, so that benthic denitrification has little effect on bottom water nitrate isotopic composition.

  21. Heavy NH4+ ? Light NO3- ? Heavy NO3- ? e ~ 0 e ~ -17 e ~ -25 Expression of these fractionations depends on the branching ratios; there is no net fractionation if a reactant is completely converted to a product.

  22. Pore water oxygen profiles and benthic oxygen fluxes (CH2O)106(NH3)16(H3PO4) + 138O2 => 106HCO3- + 16NO3- + HPO4-2 + 124H+ + 16H2O Integrated oxygen consumption => oxygen flux organic C decomposition (oxic) + reoxidation of reduced species (metals, sulfide) gradient at sediment-water interface, or fit profiles of oxygen or nitrate

  23. oxygen respiration (CH2O)106(NH3)16(H3PO4) + 138O2 => 106HCO3- + 16NO3- + HPO4-2 + 124H+ + 16H2O nitrate reduction (CH2O)106(NH3)16(H3PO4) + 94.4NO3- => 13.6CO2 + 92.4HCO3- + 55.2N2 + HPO4-2 + 84.8H2O MnO2reduction (CH2O)106(NH3)16(H3PO4) + 236MnO2 + 364H+ => 236Mn2+ + 106HCO3- + 8N2 + HPO4-2 + 260H2O Fe2O3reduction (CH2O)106(NH3)16(H3PO4) + 212Fe2O3 + 756H+ => 424 Fe2+ + 106HCO3- + 16NH4+ + HPO4-2 + 424H2O sulfatereduction (CH2O)106(NH3)16(H3PO4) + 53SO4-2 => 106HCO3- + 16NH4+ + HPO4-2 + 53HS- + 39H+ fermentation (CH2O)106(NH3)16(H3PO4) => 53CO2+ 53CH4 + 16NH3 + H3PO4

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