1. Globally, O2 accounts for ~90% of OM decomposition at depths > 1000 m. Pore water profiles suggest:
Pelagic sediments:
O – 100 %
Continental margins (Reimers et al., Martin and Sayles) :
O2 65 – 90+ %
( 5 – 45 % in OMZ )
(But incubation studies of margin seds suggest
smaller role for O2, larger roles for FeOx and SO42- )

2. Pore water oxygen profiles and benthic oxygen fluxes
Oxic respiration / nitrification
Controls on oxygen penetration depth
Role of organic matter lability
Relative importance of diffusion and bioirrigation

3. 1. Oxic decomposition (nitrification)
(CH2O)106(NH3)16(H3PO4) + 138O2 =>
106HCO NO3- + HPO H+ + 16H2O
(at steady state)
Flux of oxygen across sediment-water interface =
net oxygen consumption in the sediments
organic C decomposition (oxic) + reoxidation of
reduced species (metals, sulfide)
Have they all seen Redfield?

4. How do we estimate the oxygen flux?
Pore water oxygen profiles.
Benthic flux chambers oxygen fluxes.
Pore water nitrate profiles.
Eddy correlation estimates
(vertical velocity & oxygen concentration).

5. in situ microelectrode O2 profiles in western North Atlantic
in situ microelectrode O2 profiles in western North Atlantic. Rates estimated by fitting the data with a diagenetic model.
Martin and Sayles, 2004

6. Sayles, Martin, and Deuser, 1994
Benthic oxygen demand off Bermuda estimated from month-long Benthic Flux Chamber experiments.
Sayles, Martin, and Deuser, 1994

7. Pore water oxygen-nitrate relationship – a flux balance that reflects reaction stoichiometry, and ratio of molecular diffusivities.
Why is this?
Jahnke et al., 1982

8. Predict oxygen profile from nitrate profile.
stoichiometry
diffusivity ratio
Predict oxygen profile from nitrate profile.
Predict dissolved inorganic carbon from oxygen or nitrate?

9. 2. O2 penetration depth – why do we care?
influences use of other electron acceptors
understand controls on C org preservation
(via “oxygen exposure time), and
downcore % C org variations
predict profile and rate of CO2 release, and
“metabolic” carbonate dissolution

10. What controls the O2 penetration depth?

12. This suggests that oxygen penetration depth is controlled by [O2]bw and the O2 flux (in turn driven by OM flux)

13. Found a good relationship between predicted O2 penetration depth (bottom water oxygen and oxygen flux) and observed OPD
Cai and Sayles, 1996

14. The relationship breaks down at higher penetration depths; non-constant decomposition rate?

15. This simple model provides a way to predict response of processes linked to the oxygen penetration depth – e.g., OM preservation and benthic denitrification – to changes in [O2]bw and surface ocean productivity (C flux, or O2 flux).
Why don’t OM quality, or bioturbation rate, seem to matter?

16. Martin and Sayles (2004) – test ability to predict OPD in cases where porosity is a function of depth, and over a range of [O2]bw and ko

17. Observed porosity (fn z)
in situ microelectrode O2 profiles in western North Atlantic. Rates estimated by fitting the data with a diagenetic model
Observed porosity (fn z)
Fitting the profiles, found that a constant O2 consumption rate (ko) was adequate for most sites
Martin and Sayles, 2004

18. Porosity variations can be a significant factor
But in general, very good agreement with predicted dependence on [O2]bw and O2 consumption rate ko

19. What is the physical meaning of the depth-independent oxygen consumption rate constant kox?

20. 3. Oxygen evidence of the lability (reaction rate) of sediment OM – how tight is the link between OM input (sediment trap OM flux) and benthic decomposition rate (oxygen consumption)?.

21. Laboratory evidence of a range in lability for marine OM – plankton decomposition experiments.
Westrich and Berner, 1984

22. Sayles, Martin, and Deuser, 1994
Benthic carbon flux and benthic oxygen demand off Bermuda.
Long-term sediment trap deployment, and a series of month-long BFC experiments
Sayles, Martin, and Deuser, 1994

23. Sediment trap carbon flux and observed benthic oxygen demand (circles) compared with trap flux and model-predicted oxygen demand for slow (0.2/y) and fast (5/y) decomposition rates.
Observations (little variation in O2 flux) more consistent with slow rates.

24. Contrast Sayles et al. Bermuda results (slow ko, lack of tight linkage between C flux and O2 demand) with results of K. Smith et al. off California.
Strong linkage off CA implies faster decomposition rates (5 to 10 / yr).

25. This pattern of strong linkage between C flux and O2 consumption off CA has persisted.

27. 4. Relative importance of diffusion and bio-irrigation to oxygen uptake in continental margin sediments
Compare pore water profiles and benthic flux chamber deployments on the California margin
Reimers et al., 1992

28. Pore water oxygen from in situ microelectrodes
Pore water oxygen from in situ microelectrodes. Low bottom water oxygen in OMZ; oxygen penetration of millimeters at all these shallow sites.
oxygen respiration (CH2O)106(NH3)16(H3PO4) + 138O2 =>
106HCO NO3- + HPO H+ + 16H2O

29. Linear gradient estimates from steepest part of each profile.
Deeper sites, with higher bottom water oxygen and deeper oxygen penetration.
Linear gradient estimates from steepest part of each profile.
Reimers et al.

30. Benthic flux chamber results; short (1-2 day) deployments at these high-flux, low O2 sites.
Jahnke et al.

32. This implies little irrigation-driven oxygen flux at these sites.
Reasonable agreement between O2 profiles (diffusive transport) and BFC results (total transport); no obvious pattern to the differences.
This implies little irrigation-driven oxygen flux at these sites.

33. Archer and Devol – WA margin

34. Microelectrode oxygen profiles

35. Benthic flux chamber O2

37. Total oxygen uptake substantially higher than diffusive uptake at the high oxygen (shelf) sites of Archer and Devol.
Bioirrigation is an important factor in the benthic oxygen flux at these high OM flux, high [O2]bw continental margin sites.