Tuesday: OM flux to the sea floor is: variable in space and time a very small fraction of primary production compositionally distinct from fresh plankton Today Electron acceptors for OM oxidation (order of use, relative importance) (Pore water oxygen profiles and benthic oxygen fluxes)

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

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.

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

OM decomposition: Organic matter ((CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )) + electron acceptor (O 2, NO 3 -, MnO 2, FeOOH, SO 4 -2 ) + (H +, H 2 O) => 106CO (NO 3 -, NH 3, or 0.5xN 2 ) + H 3 PO 4 + reduced electron acceptor (O(-II), N 2, Mn 2+, Fe 2+, S 2- ) + (H +, H 2 O) to balance

What determines the relative importance of the electron acceptors to overall benthic OM decomposition? 1) Order of use (can run out of labile OM) 2) EA abundance and # of electrons transferred per EA (can run out of EA)

Order of use: Oxidants are used in decreasing order of Gibbs free energy yield of the redox reaction. Free energy – the energy stored in the chemical bonds in a molecule (relative to some standard state) Free energy yield: (  G°) =  (free energy of products) –  (free energy of reactants) This is the maximum energy available from the reaction (and therefore the maximum chemical energy available to the respiring organism)

Free energy yield of OM decomposition redox couple:  G° = (  G° of OM oxidation) + (  G° of EA reduction) = G° f (106CO (NO 3 -, NH 3, or 0.5xN 2 ) + H 3 PO 4 ) - G° f ((CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )) (varies a little, depending on fate of N) + G° f (O(-II), N 2, Mn 2+, Fe 2+, S 2- ) - G° f (O 2, NO 3 -, MnO 2, FeOOH, SO 4 -2 ) (varies a lot, and leads to a predictable order of EA use)

Froelich et al., 1979 Suggests an order to oxidant use, which should lead to depth-zonation in pore water profiles of oxidation products and reactants (oxide phase) (fate of organic N)

oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO NO HPO H H 2 O nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) NO 3 - => 13.6CO HCO N 2 + HPO H 2 O MnO 2 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 236MnO H + => 236Mn HCO N 2 + HPO H 2 O Fe 2 O 3 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 212Fe 2 O H + => 424 Fe HCO NH HPO H 2 O sulfate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106HCO NH HPO HS H + fermentation (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) => 53CO CH NH 3 + H 3 PO 4

Froelich et al., 1979; deep eastern equatorial Atlantic gravity cores

By no means their worst-looking profiles!

(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

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.

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.

Shaw et al., CA borderland basins (low bottom water O 2, high C flux) EA series compressed. Mn reduction before (shallower than) Fe reduction Mn 2+ Fe 2+ NO 3 - Fe 2+ Mn 2+

Relative importance of EAs? Globally, O 2 accounts for ~90% of OM decomposition at depths > 1000 m. Pore water profiles suggest: Pelagic sediments: O 2 95 – 100 % Continental margins (Reimers et al, Martin and Sayles) : O 2 30 – 70 % NO – 15 % SO – 65 % But incubation studies of margin seds suggest smaller role for O 2, larger roles for FeOx and SO 4 2-

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

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 NO HPO H 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.

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 ) NO 3 - => 13.6CO HCO N 2 + HPO H 2 O

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

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

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

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

O2O2 NO 3 - SO 4 2- Percent by oxygen The standard view of the relative importance of EAs in continenal margin sediments.

O2O2 NO 3 - SO C org burial Compare total benthic carbon flux (oxidation + burial) with “typical” sediment trap profiles. A zone of enhanced C flux at the base of the continental slope. (downslope transport?)

A depth transect in the western Atlantic. Shallowest site (250 m) in the O 2 minimum (165  M), and also characterized by coarse, low- porosity 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 : 75 – 82 % NO 3 - : 5 – 6 % SO Fe: 13 – 20 % O 2 : 91 % NO 3 - : 2 % SO Fe: 7 % O 2 respiration dominates more strongly than on CA margin

Thamdrup, 2000 Incubation methods (most from nearshore seds) suggest a much smaller role for O2 (apart from reoxidation of reduced rxn products) and a larger role for iron and sulfate.