1. Organic Matter decomposition in marine sediments: Overview

2. Organic Matter decomposition in sediments First: O.M. as it reaches the sea floor is not necessarily “ready” for microbial respiration

3. Henrichs, 1992 On the path to CO 2, large molecules typically need to be broken down (extracellular hydrolysis) prior to uptake by bacteria.

4. Arnosti, 1995 Injected replicate cores with fluorescently-labeled polysaccharide (200kD). Sampled through time, and estimated the mw of the tagged polymers over 2 days. The mw decreased dramatically, indicating rapid hydrolysis of the high mw polysaccharide.

5. Arnosti, 1995 The mw decreased dramatically, indicating rapid hydrolysis of the high mw polysaccharide. The rates varied with location, sediment depth, and sediment type.

6. Organic matter oxidation Complex organic molecules Extracellular hydrolysis, oxidation CO 2 Microbial respiration

7. Oxidation / Reduction Reactions Example: “Reductant” “Oxidant” The Reductant loses electrons (is oxidized) ; The Oxidant gains electrons (is reduced) Formal oxidation states: C in CH 2 O : + 0 ; C in HCO 3 - : + IV O in O 2 : 0 ; O in HCO 3 - : - II

8. A “redox” view of sedimentary components

9. Electron acceptor use in marine sediments The fraction of o.m. oxidized by each electron acceptor is determined by: Order of use : can run out of reactive organic matter Abundance : can run out of electron acceptor

10. The energy yield of redox reactions Consider the “1/2 - reaction” : Formally: pH = - log (H + ) pe = - log (e - ) To use this relationship, we need to calculate K…. but : what’s (e - ) ? 0?

11. Combine 2 reactions: ∆G 0 = X ∆G 0 = 0 ∆G 0 = X + 0 0 …

12. Calculating pe 1)Write the 1/2 reaction 2)Calculate ∆G 0, omitting (e - ) and remembering G f 0 (H + )=0 : tables of G f 0 3)Calculate pe 0 from ∆G 0 4)Use the expression for pe to caculate pe under the desired conditions, e.g. :

13. Calculated Values

14. Graphically… To calculate ∆G for a redox Reaction, combine pe values:

15. The free energy yield of the oxidation of organic matter by different e - acceptors Order of use: O 2 NO 3 - Mn(IV) Fe(III) SO 4 2-

16. Abundance Solutes: SO 4 2- source : BW up to ~ 26 mM O 2 source: BW 0 - ~ 350 µM NO 3 - source: BW + oxic decomposition Solid phase: Fe and Mn …

17. Availability Riverine Particles Poorly crystalline oxides -- available -- Abundance of Fe oxides >> Mn oxides (almost always) -- Pelagic, deep-sea: low rain rates, little supply of either -- May be important in some margin & nearshore sites

18. oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO 3 - + 16NO 3 - + HPO 4 -2 + 124H + + 16H 2 O nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 94.4NO 3 - => 13.6CO 2 + 92.4HCO 3 - + 55.2N 2 + HPO 4 -2 + 84.8H 2 O MnO 2 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 236MnO 2 + 364H + => 236Mn 2+ + 106HCO 3 - + 8N 2 + HPO 4 -2 + 260H 2 O Fe 2 O 3 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 212Fe 2 O 3 + 756H + => 424 Fe 2+ + 106HCO 3 - + 16NH 4 + + HPO 4 -2 + 424H 2 O sulfate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106HCO 3 - + 16NH 4 + + HPO 4 -2 + 53HS - + 39H + fermentation (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) => 53CO 2 + 53CH 4 + 16NH 3 + H 3 PO 4

19. (Idealized) profiles reflect the sequence of electron acceptors predicted by the free energy yield calculations: O 2, NO 3 - / MnOx, FeOx, SO 4 2- Froelich et al., 1979

20. Low-flux sites have broad redox zones, with EA use in the top 10s of cm limited to oxygen, or oxygen and nitrate Jahnke et al., 1982 Central equatorial Pacific; oxygen not fully consumed, no evidence of denitrification.

21. Slow nitrate reduction and manganese oxide reduction (1 - >2 m), and manganese reoxidation below the sediment-water interface A higher-flux site, on the equator.

22. Which electron acceptors are used the most in sediments for organic matter oxidation?

23. Continental margin sediments: O2 --> 0 near the sediment-water interface !

24. R elative importance of oxidants in continental margin sediments Pore water profiles and benthic flux chamber deployments on the California margin Reimers et al., 1992

25. Pore water oxygen from in situ microelectrodes. Low bottom water oxygen in OMZ; oxygen penetration of millimeters at all these shallow sites. oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO 3 - + 16NO 3 - + HPO 4 -2 + 124H + + 16H 2 O

26. Steep nitrate gradients reflect rapid, shallow denitrification. Two-point gradient estimates at steepest part of profile. nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 94.4NO 3 - => 13.6CO 2 + 92.4HCO 3 - + 55.2N 2 + HPO 4 -2 + 84.8H 2 O

27. Mn 2+ (open) and Fe 2+ (filled) gradients (and MnOx and FeOx reduction rates) estimated from fits to upper part of each profile.

28. sulfate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106HCO 3 - + 16NH 4 + + HPO 4 -2 + 53HS - + 39H + The ammonia flux (corrected for Fe reduction) reflects sulfate reduction.

29. The oxygen fluxes are corrected for NH 3, Mn 2+ and Fe 2+ oxidation

30. O2O2 NO 3 - SO 4 2- Percent by oxygen 5 46 35 76 72 70 69 70 BW [O 2 ]

31. Example: pore water data

32. Which electron acceptors are used the most in sediments for organic matter oxidation?

33. And a shallow-water site dominated by sulfate reduction

34. Results from incubation studies Thamdrup, 2000 Summary of margin Site results: % of Corg ox O218±10% NO3small Fe(III)17±15% SO462±17% Mn(IV)very small ** Re-oxidation of reduced species is important part of O2 consumption ?Do incubations reflect in situ processes accurately?