Lecture 19 The Ocean Nitrogen Cycle Sinks/Sources Sink - Denitrification Reactions Distributions Source - Nitrogen Fixation Reactions Distributions The Global Oxygen Cycle Source/Sinks Source - Organic Carbon Burial in sediments Sink - Weathering The Global Carbon Cycle Source from rivers via weathering Sink = CaCO 3 and org C burial Need Urey reaction
Main Ocean Source of N Nitrogen Fixation Enzyme catalyzed reduction of N 2 N 2 + 8H + + 8e ATP → 2NH 3 + H ADP + 16P i Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase Inactivated when exposed to O 2 An excellent example of how paradigms change with time
Main Ocean Sink of N Fixed Nitrogen (NO 3 -, NO 2 -, NH 4 + ) is converted to N 2 in low oxygen zones of the ocean Two Pathways Denitrification ( <2 to 10 M O 2 ): 2 NO organic matter → N 2 Anammox (<2 M O 2 ) NH NO 2 - → N 2 + H 2 O
Global distribution of O 2 at the depth of the oxygen minimum Gruber and Sarmiento, 1997 Where are low oxygen zones?
Spatial Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163) Also, Capone and Knapp (2007) Nature, 445, 159
Spatial coupling of N 2 fixation and denitrification (Model results; Deutsch et al, 2007)
PO4 versus Nitrate (GEOSECS data) Insert shows the effect of nitrification, photosynthesis, N2 fixation and denitrification. The solid line shows the linear equation P = 1/16 N (equivalent to N* = 0) Values to the right have positive N*, to the left have negative N* What is N*? How to calculate excess or deficient NO 3 - N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9
N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9 Vertical distribution of N* deficit excess N2-Fix denitrif
N* at 200m in the Pacific (Gruber and Sarmiento, 1997) Map View of N*
N* on density of 26.5 Ryabenko 2013 Topics in Oceanography
Kuypers (2003) Nature 422: Nitrogen Cycle w/ anammox and denitrification Why is N* negative – two sinks
Nitrogen species: NO 3 - ; NO 2 - ; N 2 O; N 2 ; NH 4 + (V) (III) (I) (0) (-III) Nitrogen Isotopes: 14 N % 15 N 0.366% Isotopic Composition: ‰ The standard is atmospheric N 2
Fractionation factors, where 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. NO 3 - → N 2 ) Products become lighter (depleted) Partial versus total reaction (products have same values as reactants)
The Global Nitrogen Budget-one example (Brandes et al, 2002) Ocean could be at SS or not!
Why is this important for chemical oceanography? What controls ocean C, N, P? g ≈ 1.0 Mass Balance for whole ocean: C/ t = V R C R – f B C S = 0; C D = C D V U = V D = V MIX Negative Feedback Control: if V MIX ↑ V U C D ↑ B ↑ f B ↑ (assumes f will be constant!) assume V R C R then C D ↓ (because total ocean balance V U C D ↓ has changed; sink > source) B ↓ CSCS CDCD if V MIX = m y -1 and C = mol m -3 flux = mol m -2 y -1 The nutrient concentration of the deep ocean will adjust so that the fraction of B preserved in the sediments equals river input!
V R C R (25) N 2 Fix ( ) Denitrification sed = wc = 75 B fB (25) Fluxes in Tg N y -1 Brandes et al 2002 Net fluxes = -200 to 0 (sink > source; non-SS??) Nitrogen Balance Atm Input (25)
Walker (1974) AJS The Global Oxygen Balance solar UV only non-cyclic only w/o biology Earth is overall reducing Separate O 2 ; sequester reducing material Present is key to past P and R in balance Small imbalance in P-R marine org C only, not terrestrial 80% in hemipelagic sediments where %orgC = 0.5% orgC includes H 2 S and Fe(II) stoichiometric so use moles Large O 2 linked to Small C As P = R, O 2 not affected by P accelerated weathering C = 20yr O2 = 4 my C = 10 8 yr
1) If P ceased and R continued org C would be consumed in 20 yr O 2 would decrease by 1% 2) If the only sink is weathering, O2 would go to 0 in 4 my. This is a short time geologically so controlling balance must to strong. 3) Control on O 2 = org C burial (O2 source) vs weathering (O2 sink) 4) Feedback mechanism if atm O 2 anoxic ocean org C burial atm O 2 5) Control is with source rather than sink Sedimentary org C reservoir has not changed with time
Hemipelagic sediments (org C > 0.5%) 200m to 3000m 80% of sediment orgC
CO 2 and O 2
The long-term global carbon balance 2HCO Ca 2+ = CaCO 3 (s) + CO 2 (g) + H 2 O CaCO 3 (s) + CO 2 (g) + H 2 O = 2HCO Ca 2+ A better example of reverse weathering! Fig. 2.5 Emerson and Hedges weathering deposition
Chemical Weathering, the Geological Carbon Cycle, Control on CO 2 1. CO 2 is removed by weathering of silicate and carbonate rocks on land. 2. The weathering products are transported to the ocean by rivers where they are removed to the sediments as CaCO 3 and SiO When these sediments are subducted and metamorphosed at high T and P, CaCO 3 and SiO 2 are converted into CaSiO 3 and CO 2 is returned to the atmosphere. Ittekkot (2003) Science 301, 56 For more detail see Berner (2004) The Phanerozoic Carbon Cycle: CO 2 and O 2. Oxford Press, 150pp.
TABLE 2 Oceanic fluxes of carbon Flux Atmospheric Demand River input 43.2 Derived from atmosphere Derived from carbonates 8.7 Hydrothermal input 0.5 Carbonate deposition 49.4 Deposited as carbonates 24.7 Lost to atmosphere Net atmospheric demand Units: mol/y From McDuff and Morel (1980) 1.Some CO 2 produced by carbonate deposition, but not enough! 2.The rest must come from the Urey reaction.
1. The problem of the cool sun (Sagan and Mullen, 1972). Solar luminosity has increased by 25% over the age of the solar system. But liquid water has existed for 3.8 byr! There must be a temperature buffer! 2. Was it NH 3 ?? No. Most likely the greenhouse gas CO CO 2 is produced to the atmosphere by volcanoes and metamorphism. Such as The “Urey Reaction” CaCO 3 (s) + SiO 2 (s) = CaSiO 3 (s) + CO 2 (g) 4. The important sink of CO 2 is weathering of silicate minerals. Weathering of silicate rocks consumes CO 2 and produces Ca 2+ and Mg 2+ to rivers. In the ocean this Ca 2+ and Mg 2+ is removed by formation of carbonate rocks which produces CO 2. The rate of weathering is influenced by rock type, slope, temperature and runoff 5. The weathering and deposition of carbonate rocks alone is not sufficient. Need the Urey Reaction! 7. A negative feedback. If the earth became cooler, silicate weathering would decrease, atmospheric CO 2 would increase and the earth would warm! There must be a tight feedback control on atm CO 2
% of Export Production (as N) at HOT derived from N 2 Fixation (N-P mass balance model of Karl et al (1997) Nature 388, p. 533)
Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)
The Global Nitrogen Budget-one example (Brandes et al, 2002)
Deutsch et al, 2004) Downcore records of 15N-orgN from several sites High values of 15N-OrgN suggest more extensive denitrification