Lecture 18 Redox Environments in the Oceans Diagenesis in Sediments Anoxic Basins

1. Lecture 18 Redox Environments in the Oceans Diagenesis in Sediments Anoxic Basins

2. Global Carbon Cycle – Fate of organic matter in sediments

3. Redox Half Reactions written as reductants in terms of 1 e-

4. Organic Matter Degradation (using Redfield stoichiometry) “OM” = (CH2O)106(NH3)16(H3PO4) Photosynthesis 106CO2 + 16 NO3- + HPO42- + 18H+ 122 H2O → “OM” + 138 O2 Respiration Aerobic Respiration 138 O2 + “OM” + 18 HCO3- → 124 CO2 + 16 NO3- + HPO42- + 140 H2O Denitrification 94.4 NO3- + “OM” → 13.6 CO2 + 92.4 HCO3- + 55.3 N2 + 84.8 H2O + HPO42- Manganese Oxide Reduction 236 MnO2 + “OM” + 364 CO2 + 104 H2O → 470 HCO3- + 8N2 + 236 Mn2+ + HPO42- Iron Oxide Reduction 212 Fe2O3 + “OM” + 742 CO2 + 318 H2O → 848 HCO3- + 16 NH3 + 424 Fe2+ + HPO42- Sulfate Reduction 53 SO42- + “OM” → 39 CO2 + 67 HCO3- + 16 NH4+ + 53 HS- + 39 H2O + HPO42- Methane Fermentation “OM” → 53 CO2 + 53 CH4 + 16 NH3 + HPO42- + 2H+ Indicator species are circled

5. Drilling Platform Blowout 2010 in the Deep Gulf of Mexico. Sampling stations The integrated dissolved oxygen anomaly was about 3.0 x 1010 moles O2. If this O2 anomaly was due to CH4 oxidation, how much CH4 would it account for? Contours of O2 anomaly J D Kessler et al. Science 2011;331:312-315

6. ¼ O2(g) + H+ + e- = ½ H2O log K = 20.75 1/8 CO2(g) + H+ + e- = 1/8 CH4 (g) + ¼ H2O log K = 2.87 Reverse 2nd reaction 1/8 CH4(g) + 1/4H2O = 1/8 CO2(g) + H+ + e- log K = -2.87 Add to first reaction 1/4O2(g) + 1/8 CH4(g) = 1/8 CO2(g) + ¼ H2O log K = 17.88 Multiply by x8 2O2(g) + CH4(g) = CO2(g) + 2 H2O log K = 143.04 So: 3.0 x 1010 mol O2 = 1.5 x 1010 mol CH4 (upper limit due to C2H6, C3H8)

7. Multi-colored sediments! What’s going on here???

8. Rarely see all reactions easily at the same site

9. Aerobic Respiration in Red Clay Sediments

10. Denitrification and MnO2 reduction – Hemipelagic Sediments Guatemala Basin, North Pacific Emerson et al

11. Iron reduction – Hemipelagic Sediments East Pacific Rise – North Pacific Emerson et al

12. Saanich Inlet, BC Sediments Sulfate Reduction Methane Production Anaerobic Methane Consumption Kuivila and Murray (1990)

13. The Black Sea

14. Why is the Black Sea Interesting to Oceanographers? 1. The classic anoxic basin. Oxic layer over sulfidic layer. 2.Model for modern and ancient anoxic environments. 3. Well developed transition or suboxic zone. Model for world’s organic rich sediments. 4. Suboxic reactions easy to study here because of predictable depth locations. 5. An ideal location to study effect of climate forcing on ocean distributions.

15. The Bosporus Strait connects the Black Sea with the Marmara Sea and is the only source of (relatively warm) salty water. Black Sea Marmara Sea

16. Temperature and Salinity along the Bosporus • Some CIW advected into strait in upper half of interface • T minimum in deep water erased over South Sill Gregg et al (1999)

17. Bosporus Inflow and Entrainment of CIL CIL (from NW Shelf in winter) Bosporus Inflow CIL/BI ≈ 4 for deep Black Sea from 50m to 2000m from Ozsoy et al.,(1993)

18. The Suboxic Zone: Oxygen – Sulfide Depth versus Density Total depth = 2200m

19. Example of NO3-, NO2- and NH4+ for R/V Knorr 2003 CIL suboxic zone First seen during 1988 Expedition Data suggests anammox

20. Results from Knorr 1988 Cruise (w/ Pump Profiling System) Oxygenated surface layer over a sulfide rich deep layer with a suboxic zone at the interface H2S O2 Suboxic Zone NO2 NO3 Murray et al (1995)

21. Fe Mn NH4 PO4