Pore water oxygen profiles and benthic oxygen fluxes

Pore water oxygen profiles and benthic oxygen fluxes (CH 2 O)106(NH 3)16(H 3 PO 4) + 138 O 2 => 106 HCO 3 - + 16 NO 3 - + HPO 4 -2 + 124 H+ + 16 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

O 2 penetration depth influence use of other electron acceptors understand controls on C org preservation interpret downcore % C org variations predict profile and rate of CO 2 release “metabolic” carbonate dissolution Comparisons: benthic flux chamber : sediment trap (OM lability) pore water : benthic flux chamber (bioirrigation)

What controls the O 2 penetration depth?

Suggests that oxygen penetration depth is controlled by [O 2]bw and O 2 flux (in turn driven by OM flux)

Found a good relationship between predicted O 2 penetration depth (bottom water oxygen and oxygen flux) and observed OPD Cai and Sayles, 1996

The relationship breaks down at higher penetration depths; nonconstant decomposition rate?

Simple model provides a way to predict response of processes linked to OPD – OM preservation and benthic denitrification – to changes in [O 2]bw and surface ocean productivity (C flux, or O 2 flux)

Martin and Sayles (2004) – test ability to predict OPD in cases where porosity is a function of depth, and over a range of [O 2]bw and ko

in situ microelectrode O 2 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 O 2 consumption rate (ko) was adequate for most sites Martin and Sayles, 2004

Porosity is a significant factor Good agreement with predicted dependence on [O 2]bw and O 2 consumption rate

The Cai and Sayles OPD [O 2]bw - O 2 flux relationship is confirmed in the more-general (variable f) case. (OPD increases with [O 2]bw , decreases with O 2 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

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 modelpredicted 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 O 2 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. 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) + 138 O 2 => 106 HCO 3 - + 16 NO 3 - + HPO 4 -2 + 124 H+ + 16 H 2 O

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 O 2 sites. Jahnke et al.

Reasonable agreement between O 2 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 O 2

Total oxygen uptake substantially higher than diffusive uptake at the high oxygen (shelf) sites of Archer and Devol. Bioirrigation in high OM flux, high [O 2]bw settings

A depth transect in the western Atlantic. Shallowest site (250 m) in the O 2 minimum (165 m. M), and also characterized by coarse, lowporosity sediment. Martin and Sayles, 2004

Multiple in situ microelectrode O 2 profiles at each site. Rates estimated by fitting the data with a diagenetic model (not the “steepest slope” approach of Reimers et al. ) O 2 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.

Martin and Sayles, 2004 O 2 respiration dominates more strongly than on CA margin O 2 : NO 3 - : 75 – 82 % 5– 6% SO 42 - + Fe: 13 – 20 % O 2 : 91 % NO 3 - : 2% SO 42 - + Fe: 7%

Two-point gradient estimates at steepest part of profile. Steep nitrate gradients reflect rapid, shallow denitrification. nitrate reduction (CH 2 O)106(NH 3)16(H 3 PO 4) + 94. 4 NO 3 - => 13. 6 CO 2 + 92. 4 HCO 3 - + 55. 2 N 2 + HPO 4 -2 + 84. 8 H 2 O

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

Reimers et al. , Mn 2+ decrease? No Mn 2+? Fe 2+ decrease? Mn. O 2 reduction (CH 2 O)106(NH 3)16(H 3 PO 4) + 236 Mn. O 2 + 364 H+ => 236 Mn 2+ + 106 HCO 3 - + 8 N 2 + HPO 4 -2 + 260 H 2 O Fe 2 O 3 reduction (CH 2 O)106(NH 3)16(H 3 PO 4) + 212 Fe 2 O 3 + 756 H+ => 424 Fe 2+ + 106 HCO 3 - + 16 NH 4+ + HPO 4 -2 + 424 H 2 O

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

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