Week 6

• REDOX processes in seawater and sediments are discussed in Lecture 11 of the Ocean Biogeochemistry course.
• Sea Surface Temperature correlation
• Global calibration
• ’ determined in core-top sediment samples
• SST from
• ocean atlas
• Production ratio is linearly correlated with temperature
• There is no alteration of this ratio during sedimentation
• Indian Ocean
• REDOX concept revisited: Electrochemical potentials and Gibbs Free Energy change are major topics in REDOX processes in seawater and sediments.
• Major oxidants (terminal electron acceptors) in marine environments are also discussed in Lecture 11 of the Ocean Biogeochemistry course.
• Organic biomarkers in sediments are a topic in Lecture 11 of the Ocean Biogeochemistry course.
• REDOX reactions involve the transfer of electrons (OIL RIG: Oxidation is the loss of electrons, Reduction is the gain of electrons).
• An acid-base reaction is an example of a REDOX reaction.
• Copper-zinc electrolysis is another example of a REDOX reaction.
• In a REDOX reaction, the OXIDATION STATE changes.
• Some useful rules of thumb for the Oxidation State/ Oxidation Number include: Zero for any free elements such as Fe, O in O2, N in N2, equal to the charge of any simple, monatomic ion such as Na+, Cl-, sum of all oxidation numbers of the atoms in a neutral molecule must be equal to zero.
• In a REDOX reaction, if the oxidation state changes from +1 to -2, it is a redox reaction.
• If a reaction is not redox, the oxidation state does not change.
• Standard cell potential measured in a galvanic cell against a standard hydrogen electrode with platinum is denoted as SHE = standard hydrogen electrode.
• Oxidation number is used to analyse redox reactions: Cu2+ (aq) + Zn (s) → Cu (s) + Zn2+ (aq) +2 O2- +2 OH-.
• From the measured half-cell reducing potentials, the standard electrode potential for the reactions Cu2+ (aq) + 2e- → Cu (s) + Eh° = +0.34 V and Zn2+ (aq) + 2e- → Zn (s) + Eh° = -0.76 V can be calculated.
• The one with more positive Eh° will occur as a reduction; while the other half reaction is reversed and occurs as an oxidation.
• Seawater is thermodynamically unstable and contains 2 x 1016 moles of reduced organic carbon, making it a good electron donor.
• Seawater contains 3 x 1017 moles of molecular O2, making it a major electron acceptor.
• These two reactants are physically segregated by biological processes and gravity.
• The kinetics of these reactions are slow and enzymatic catalysis by organisms, mainly microbes, drives these reactions under normal oceanic conditions.
• NH4+ can be the oxidant and reactant, respectively, in many other reactions.
• Gravitational settling of particulate organic carbon (POC) is a process that segregates oxidants and reductants.
• Apparent oxygen utilisation (AOU) reflects the carbon flux.
• AOU = NAEC - [O2] in situ.
• This is offset by ventilation of oceans via thermohaline circulation.
• Gaseous equilibrium with atmosphere is reached when the water mass sank from surface.
• Photosynthesis→remineralisation is a global geochemical cycle of redox energy.
• Reactions proceed spontaneously in the direction that minimizes the energy of the reaction systems.
• Reactions with large equilibrium constants (K) are most likely to proceed spontaneously giving off energy (ΔG).
• In a REDOX reaction, the oxidation state changes.
• Some examples of reactions that are not redox include: Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2, Cu2+ (aq) + Zn (s) → Cu (s) + Zn2+ (aq), and Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2.
• To determine if a reaction is REDOX, work out all oxidation numbers to check for REDOX.
• In a REDOX reaction, the oxidation state changes from +1 to -2.