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  15 N (and  18 O) of pore water nitrate.

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 18 O of CaCO 3 ) Kinetic fractionation – difference in isotopic composition of reactants and products in a unidirectional reaction

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

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

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

Gramling

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

Zeebe and Wolf-Gladrow Rayleigh fractionation

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.

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.

The instantaneous   NO 3 - and [NO 3 - ] trajectories of different marine N cycle processes, assuming an initial   NO 3 - of 5‰. Sigman and Casciotti

Depth profiles of the  15 N and  18 O 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

 ~ 0  ~ -17  ~ -25 Lehmann et al.

 ~ 0  ~ -17  ~ -25 Lehmann et al. Heavy NH 4 + ?Light NO 3 - ?Heavy NO 3 - ?

 ~ 0  ~ -17  ~ -25 Expression of these fractionations depends on the branching ratios; there is no net fractionation if a reactant is completely converted to a product. Heavy NH 4 + ?Light NO 3 - ?Heavy NO 3 - ?

A low-flux cartoon of oxic respiration; values sort of realistic -O 2 :N = -170:16; DO 2 /DNO 3 = 1.3;  15 N OM = 0

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

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

Hoffmann But the flux of nitrate (from nitrification and from bottom water) is down. Denitrifiers consume the high  15 N and  18 O “residual” nitrate, so that benthic denitrification has little effect on bottom water nitrate isotopic composition.

 ~ 0  ~ -17  ~ -25 Expression of these fractionations depends on the branching ratios; there is no net fractionation if a reactant is completely converted to a product. Heavy NH 4 + ?Light NO 3 - ?Heavy NO 3 - ?

Pore water oxygen profiles and benthic oxygen fluxes (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO NO HPO H H 2 O 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

oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO NO HPO H H 2 O nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) NO 3 - => 13.6CO HCO N 2 + HPO H 2 O MnO 2 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 236MnO H + => 236Mn HCO N 2 + HPO H 2 O Fe 2 O 3 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 212Fe 2 O H + => 424 Fe HCO NH HPO H 2 O sulfate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106HCO NH HPO HS H + fermentation (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) => 53CO CH NH 3 + H 3 PO 4