Biogeochemical Cycling in Anoxic Marine Sediments II

Biogeochemical cycling in anoxic sediments Consortia of bacteria are needed to degrade Complex organic mater Waste products of one bacteria serve as the substrate for another Major reactions are fermetation, sulfate reduction, and methanogenesis Biogeochemical zonation occurs due to differences in free energy of TEA yields C oxidation in CLB sediments show fluxes and processes are In balance, suggesting all major pathways are accounted for. Natural system closely resembles that expected from pure culture work. Sulfate Present Sulfate Present Sulfate Absent complex organic matter R Sulfate and Methane in CLB sediments (August) Molecular hydrogen as a control on organic matter oxidation in anoxic sediments Is C oxidation in anoxic sediments under thermodynamic or kinetic control? (CH2O)n + nH2O --> nCO2 +2nH2 2nH2 +mXox --> mX red + zH2O (e.g. Xox = SO42- Xred = S2-) ∆Grxn = ∆G(T)o + RT ln ( {Xred}m/{Xox}m (PH2)2n) and… PH2 = ({Xred}m/{Xox}m e(∆Grxn-∆G(t)o/RT))1/2n Oxidation of organic, rnatt:er in marive sediments Reaction Capacity (mrnolesJL sed) 0-85 Rapid cycling of H2 in anoxic sediments Hydrogen Concentration (nM) 120 Hydrogen Spiked Control 90 60 H2 has a lifetime of 4-5 sec in deep sections of CLB cores, 0.1 sec near the sed/water interface ! 30 0 0 4 2 Time (d) 6 TEAP Nitrate redn Sulfate redn Methanogenesis Acetogenesis [H2] (nM) 0.031 1.64 13.0 133 acetogenesis CO2 reduction Effect of TEA on H2 concentrations ∆G(kJ mol-1) <-180 -23 -20 -18 Effect of temperature on H2 concentrations ∆T from 10 to 30oC Will affect ∆Grxn by +15 kJmol-1 Theoretical effect Dependence of [H2] on [SO4 2-] Hydrogen Concentration (nM) 1.2 1.0 0.8 [H2] = 1.7[SO42-]-0.256 r2 = 0.996 0.6 0.4 0 30 60 90 120 Sulfate Concentration (mM) Response of hydrogen concentration to variations in porewater sulfate concentration. Error bars represent one standard deviation about the mean of triplicate sediment samples. A power function fit to the data indicates that hydrogen has an exponential dependence of -0.26 + 0.01 on sulfate (compare to theoretical value of -0.25). Profiles of hydrogen and sulfate in CLB and WOR sediments Depth (cm) 0 0 10 20 -10 0 0 10 20 -20 August -20 -30 0 -10 November -40 CLB 8/30/96 Temp. = 27 C 6 CLB 11/19/96 Temp. = 14.5 C A -60 12 Hydrogen Concentration (nM) Sulfate Concentration (mM) 0 0 2 -20 WOR 5/5/97 Temp. = 20 C B -30 4 0 C 4 8 Profiles of hydrogen and sulfate in CLB and WOR sediments Depth (cm) 0 0 10 20 -10 0 0 10 20 -20 -10 5-7x 5-7x August -20 -30 0 November -40 CLB 8/30/96 Temp. = 27 C 6 0 CLB 11/19/96 Temp. = 14.5 C A -60 12 Hydrogen Concentration (nM) Sulfate Concentration (mM) 0 2 -20 WOR 5/5/97 Temp. = 20 C B -30 4 0 C 4 8 TEAP Nitrate redn Sulfate redn Methanogenesis Acetogenesis [H2] (nM) 0.031 1.64 13.0 133 acetogenesis CO2 reduction Effect of TEA on H2 concentrations ∆G(kJ mol-1) <-180 -23 -20 -18 Profiles of hydrogen and sulfate in CLB and WOR sediments Depth (cm) 0 0 10 20 -10 0 0 10 20 -20 -10 5-7x 5-7x August -20 -30 0 November -40 CLB 8/30/96 Temp. = 27 C 6 0 CLB 11/19/96 Temp. = 14.5 C A -60 12 0 2 2.8x means vs 2.7x predicted -20 WOR 5/5/97 Temp. = 20 C B -30 4 0 C 4 8 Hydrogen Concentration (nM) Sulfate Concentration (mM) Effect of temperature on H2 concentrations ∆T from 10 to 30oC Will affect ∆Grxn by +15 kJmol-1 Theoretical effect Profiles of hydrogen and sulfate in CLB and WOR sediments 0 0 10 20 0 0 10 20 0 Depth (cm) e = -0.30 vs -0.26 -10 -20 -10 5-7x 5-7x August -20 CLB 8/30/96 Temp. = 27 C -30 0 November -40 6 CLB 11/19/96 Temp. = 14.5 C A -60 12 0 2 2.8x means vs 2.7x predicted -20 WOR 5/5/97 Temp. = 20 C B -30 4 0 C 4 8 Hydrogen Concentration (nM) Sulfate Concentration (mM) Effect of sulfate on H2 in CLB sediments Hydrogen (nM) -12 0 1.5 1 0.5 2 2 1.5 Hydrogen (nM) Depth (cm) -13 -14 -15 -16 [H2] = 0.6[SO42]0.30 r2 = 0.911 1 0.5 0 0.5 A 1 Sulfate (mM) 1.5 0 0 0.5 B 1 1.5 2 Sulfate (mM) The dependence of hydrogen concentrations on sulfate concentrations in the November core from Cape Lookout Bight . (A) blow-up of the 12-16 cm depth interval. Note that sulfate concentrations only reach threshold values below 16 cm: (B) plot of hydrogen concentration vs. sulfate concentration over the 12-16 cm interval. A power function fit to the data indicates that hydrogen has an exponential dependence of _ (compared to a lab value of 0.26 + 0.01 and a theoretical _ 0.30 + 0.04 on sulfate value of 0.25). Profiles of hydrogen and sulfate in CLB and WOR sediments 0 0 10 20 0 0 10 20 0 Depth (cm) e = -0.30 vs -0.26 -10 -20 -10 5-7x 5-7x pH effect August -20 CLB 8/30/96 Temp. = 27 C -30 0 November -40 6 CLB 11/19/96 Temp. = 14.5 C A -60 12 0 2 2.8x means vs 2.7x predicted -20 WOR 5/5/97 Temp. = 20 C B -30 4 0 C 4 8 Hydrogen Concentration (nM) Sulfate Concentration (mM) Hydrogen as a control on organic matter oxidation In anoxic sediments (fresh and marine) Hydrogen is a by-product of fermentation and is essential for sulfate reduction and methanogenesis. Hydrogen concentrations respond to T, [X], pH. Laboratory changes correspond well to field observations. Variations in H2 suggest maintenance of constant ∆G values of -10 to -15 kJ mol-1 . H2 has a very short lifetime in sediments- makes an Excellent E regulator. Small changes in H2 concentration Results in large changes in ∆G. Intense competition by bacteria regulate [H2] Methane and the Global greenhouse atms. Methane is increasing in concentration by about 1-2% per year. C isotopic changes in atms methane C isotopic changes in atmospheric methane How do we explain the increase in atmospheric ? Why is there a seasonal cycle in methane concentration? Why is there a seasonal cycle in methane C isotopes? (can C isotopes be used to understand and Quantify processes that lead to atms increase?) Freshwater I mean - 6 0 ° , ' ~ 1 mean -12%0 mean -7.9 Marine I There are two pathways that yield methane: Freshwater CH3COOH --> CH 4 + CO2 Marine CO2 + 4H2 --> CH 4 + 2H2O Isotope fractionation and methanogenesis -7 00 8 44 60 of Methane @er mil) Carbon isotope fractionation with methanogenesis Freshwater CH3COOH --> CH 4 + CO2 α = -48‰ Marine CO2 + 4H2 --> CH 4 + 2H2O α = -70‰ Carbon and Hydrogen isotopes fractionation with methanogenesis Production of methane from acetate and CO2 in CLB sediments. 14C tracer studies. Seasonal Changes in 13C for Methane and CO2 Cape Lookout Bight sediment gas bubble composition and d13C data. Values listed are means + SD for the number of samples bottle listed. Superscript indicate the number of samples for which compositional data wee obtained when different from the number of sample bottle listed. Date 6-June-1983 19-June-1983 3-August-1983 19-August-1983 15-September-1983 16-October-1983 20-November-1983 2-February-1984 7-April-1984 6-May-1984 31-May-1984 14-June-1984 2-July-1984 18-July-1984 11-August-1984 30-August-1984 22-September-1984 Methane sample bottles (no.) Methane content (%) d13C-CH4 (per mile) 5 6 5 5 5 6 4 4 4 4 5 5 4 4 5 4 5 97 + 2 95 + 4 96 + 4 94 + 2 97 + 2 95 + 3 93 + 2 98 + 3 94 + 33 90 + 6 94 + 5 94 + 3 97 + 42 98 + 22 98 + 34 94 + 1 99 + 02 -64.5 + 7 -62.2 + 0.4 -61.7 + 0.9 -57.5+ 0.3 -60.3 + 0.4 -60.0 + 0.5 -62.2 + 0.4 -63.4 + 0.6 -63.8 + 0.2 -63.8 + 0.4 -68.5 + 0.7 -64.1 + 0.6 -59.4 + 1.2 -60.6 + 1.6 -57.3 + 0.6 -57.9 + 1.0 -58.0 + 0.3 Carbon dioxide sample bottles (no.) 5 6 5 4 5 5 4 4 4 3 3 4 2 2 5 3 5 Carbon dioxide content (%) d13C-CO2 (per mil) 2.5 + 0.1 3.4 + 0.23 2.4 + 0.3 2.4 + 0.2 2.5 + 0.1 2.4 + 0.54 2.4 + 0.6 1.6 + 0.53 1.0 + 0.23 1.5 + 0.2 1.8 + 0.6 2.9 + 1.0 2.1 + 0.1 2.2 + 0.2 2.3 + 0.2 3.8 + 1.1 2.4 + 1.3 -6.8 + 1.1 -8.6 + 1.2 -8.8 + 1.0 -9.4 + 0.3 -8.3 + 0.5 -7.2 + 0.6 -8.0 + 0.2 -6.0 + 1.2 -5.1 + 0.7 -3.0 + 0.8 -7.0 + 2.0 -6.2 + 2.4 -10.0 + 0.7 -10.6 + 3.2 -7.6 + 1.2 -8.9 + 1.1 -8.1 + 1.0 Monthly flux and isotope data for methane flux from CLB Month January February March April May June July August September October November December Full year Monthly methane bubble flux* (mmol m-2) Annual Fluxa (%) 0 0 0 0 38 350 1270 1643 1095 409 47 0 0 0 0 0 0.8 7.2 26.2 33.9 22.6 8.4 1.0 0 4582 + 1277 100.0 d13C-CH4++ (per mil) -63.4 + 0.6 -63.4 + 0.2 -66.4 + 2.5 -64.3 + 0.7 -61.0 + 1.6 -58.7 + 2.0 -60.0 + 0.5 -62.2 + 0.4 -60.0 + 1.0 WAS Anaerobic methane oxidation…where has all the methane gone? Oceans have a huge reservoir of methane in sediments, but Contribute only 2% of the global atmospheric flux of methane. Several lines of evidence suggest methane is being efficiently Oxidized before it reaches the sediment water interface: curvature in methane profiles radiotracer experiments isotopic fractionation between methane and CO2 measured rates of methane oxidation in sulfate reduction zone. CH4 + SO4 2- --->> HCO3 - + HS- +H2O Energetically favorable, but ratio of SRR/MOR is very high ( >99.99). Anaerobic methane oxidation probably occurs as a consortia between SRB and MOB Coupled methane oxidation and sulfate reduction in CLB sediments CO2 Reduction Rate (µM/d) 0 2.5 5 7.5 10 Sulfate Concentration (mM) 0 Methane Oxidation Rate (µM/d) 0 2.5 5 7.5 10 20 10 0 0.5 1.0 Depth (cm) 20 2 February, 1990 40 Methane Concentration (mM) 0 40 30 2 February, 1990 1.5 2.0 Sulfate Methane 16 1.6 12 1.2 8 0.8 4 a 0.0 MOR CRR 100 8 80 6 60 4 40 2 b 0 20 0 0.12 MOR/CRR Ratio CH4 +2H2O --> CO2 + 4H2 Methane Oxidation Rate (mM/d) 0 10 0.4 0.09 0.06 0.03 c 0.00 0 20 Methane Concentation (mm) 2.0 40 60 Time (days) 80 100 120 CO2 Reduction Rate (mm/d) Methane oxidation and CO2 reduction to methane in CLB sediments Sulfate Concentation (mM) 20