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

O2 penetration depth influence use of other electron acceptors understand controls on C org preservation interpret downcore % C org variations predict profile and rate of CO2 release “metabolic” carbonate dissolution Comparisons: benthic flux chamber : sediment trap (OM lability) pore water : benthic flux chamber (bioirrigation)

What controls the O2 penetration depth?

Suggests that oxygen penetration depth is controlled by [O2]bw and O2 flux (in turn driven by OM flux)

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

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

Simple model provides a way to predict response of processes linked to OPD – OM preservation and benthic denitrification – to changes in [O2]bw and surface ocean productivity (C flux, or O2 flux)

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

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

Porosity is a significant factor Good agreement with predicted dependence on [O2]bw and O2 consumption rate

The Cai and Sayles OPD - [O2]bw - O2 flux relationship is confirmed in the more-general (variable f) case. (OPD increases with [O2]bw , decreases with O2 flux)

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?

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

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

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 more consistent with slow rates.

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

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

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 => 106HCO3- + 16NO3- + HPO4-2 + 124H+ + 16H2O

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.

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

Reasonable agreement between O2 profiles (diffusive transport) and BFC results (total transport); no obvious pattern to the differences.

Archer and Devol – WA margin

Microelectrode oxygen profiles

Benthic flux chamber O2

Bioirrigation in high OM flux, high [O2]bw settings Total oxygen uptake substantially higher than diffusive uptake at the high oxygen (shelf) sites of Archer and Devol. Bioirrigation in high OM flux, high [O2]bw settings

A depth transect in the western Atlantic A depth transect in the western Atlantic. Shallowest site (250 m) in the O2 minimum (165 mM), and also characterized by coarse, low-porosity sediment. Martin and Sayles, 2004

Multiple in situ microelectrode O2 profiles at each site Multiple in situ microelectrode O2 profiles at each site. Rates estimated by fitting the data with a diagenetic model (not the “steepest slope” approach of Reimers et al.) O2 penetration 1 – 3 cm Martin and Sayles, 2004

Production and consumption rates of nitrate and ammonia also obtained by fitting the data with a diagenetic model. The ammonia flux reflects OM oxidation by sulfate and iron reduction.

O2 respiration dominates more strongly than on CA margin Martin and Sayles, 2004 O2 respiration dominates more strongly than on CA margin O2: 75 – 82 % NO3- : 5 – 6 % SO42- + Fe: 13 – 20 % O2: 91 % NO3- : 2 % SO42- + Fe: 7 %

Two-point gradient estimates at steepest part of profile. Steep nitrate gradients reflect rapid, shallow denitrification. nitrate reduction (CH2O)106(NH3)16(H3PO4) + 94.4NO3- => 13.6CO2 + 92.4HCO3- + 55.2N2 + HPO4-2 + 84.8H2O

Mn2+ (open) and Fe2+ (filled) gradients (and MnOx and FeOx reduction rates) estimated from fits to upper part of each profile.

Reimers et al., Mn2+ decrease? No Mn2+? Fe2+ decrease? MnO2 reduction (CH2O)106(NH3)16(H3PO4) + 236MnO2 + 364H+ => 236Mn2+ + 106HCO3- + 8N2 + HPO4-2 + 260H2O Fe2O3 reduction (CH2O)106(NH3)16(H3PO4) + 212Fe2O3 + 756H+ => 424 Fe2+ + 106HCO3- + 16NH4+ + HPO4-2 + 424H2O Fe2+ decrease?

The ammonia flux (corrected for Fe reduction) reflects sulfate reduction. sulfate reduction (CH2O)106(NH3)16(H3PO4) + 53SO4-2 => 106HCO3- + 16NH4+ + HPO4-2 + 53HS- + 39H+

The oxygen fluxes are corrected for NH3, Mn2+ and Fe2+ oxidation