Week 5 Bio and Chem processes Flashcards
(25 cards)
What factors contribute to high sedimentation rates in estuaries?
Reduced river flow velocity, flocculation and aggregation of fine particles, high primary productivity, tidal trapping of sediments.
Why do estuarine sediments typically contain 3-7 wt% organic carbon compared to <1% in open ocean sediments?
High nutrient inputs promote phytoplankton blooms, terrestrial organic matter inputs from rivers, protected environment allows organic matter accumulation, rapid burial limits decomposition.
List the complete diagenetic sequence with Gibbs free energy values and electron acceptors.
O₂ aerobic respiration (-447 kJ/mol C), NO₃⁻ denitrification (-444), MnO₂ manganese reduction (-397), Fe(OH)₃ iron reduction (-131), SO₄²⁻ sulfate reduction (-76), CO₂ methanogenesis (+ variable).
Explain why the diagenetic sequence follows this specific order.
Microorganisms use the most energetically favorable electron acceptors first. The sequence is thermodynamically determined by ΔG° values. Each reaction yields less energy than the previous one.
Compare oxygen penetration depths in different marine environments.
Deep ocean: 10-50 cm, coastal sediments: 1-5 cm, estuarine muds: 0.1-2 cm, organic-rich marshes: <0.5 cm.
What causes the compressed redox zonation in estuarine sediments?
High organic matter loading, rapid microbial respiration, limited physical mixing, fine-grained sediment structure.
Describe three types of microniches that create redox heterogeneity.
Macrofaunal burrows (oxic halos), plant root channels (O₂ release), organic matter aggregates (anoxic cores), microbial mats (localized gradients).
Explain the difference between bioturbation and bioirrigation.
Bioturbation: Physical mixing of sediment particles by organisms. Bioirrigation: Active pumping of water through burrows. Both enhance solute exchange but via different mechanisms.
How does resuspension affect sediment chemistry?
Exposes anoxic sediments to O₂, releases porewater nutrients, redistributes particle-bound metals, alters microbial community structure.
Detail the iron-manganese shuttle mechanism for trace metals.
Metals adsorb to Fe/Mn oxides in oxic zones, oxides dissolve at redox boundary, metals release to porewater, diffuse upward and re-adsorb. Results in metal enrichment at redox interface.
List the key radionuclides used in sediment studies and their applications.
²¹⁰Pb - sedimentation rates, ²³⁴Th - short-term mixing, ²²⁶Ra - sediment sources, ¹³⁷Cs - anthropogenic marker.
Explain the difference between supported and unsupported ²¹⁰Pb.
Supported: Generated in situ from ²²⁶Ra decay. Unsupported: Atmospheric deposition (excess). Critical for dating calculations.
Describe the complete 8-step process for calculating sedimentation rates using ²¹⁰Pb.
Measure total ²¹⁰Pb activity profile, determine supported activity (deep layer), calculate excess activity at each depth, take natural log of excess activity, plot ln(A) vs. depth, identify mixed layer (exclude), measure slope of linear portion, calculate rate: S = -λ/slope.
What are the key assumptions of ²¹⁰Pb dating?
Constant sedimentation rate, no post-depositional mobility, initial excess activity uniform, no physical mixing below mixed layer.
How does the decay constant (λ) for ²¹⁰Pb factor into rate calculations?
λ = 0.0311 yr⁻¹ (half-life = 22.3 years). Converts activity gradient to accumulation rate. Critical for dimensional analysis.
Work through a sample calculation.
Given slope = -0.25 cm⁻¹, S = -(-0.25)/0.0311 = 0.25/0.0311 = 8.0 cm/yr.
What sediment features indicate anthropogenic impacts?
Increased sedimentation rates, changes in grain size distribution, metal enrichment profiles, shift in Pb isotope ratios, hydrocarbon markers.
How do lead isotopes fingerprint pollution sources?
²⁰⁶Pb/²⁰⁷Pb ratios vary by ore source: Australian ores: ~1.04, Canadian Shield: ~1.30, European: ~1.18, anthropogenic: distinct from local geology.
What chemical changes occurred post-1950 at Fawley refinery?
Sedimentation rate 4× increase, Pb isotope ratio shifts, hydrocarbon concentrations rose, heavy metal enrichment, loss of natural stratigraphy.
Why is sulfate reduction thermodynamically favored over methanogenesis?
SO₄²⁻ reduction yields -76 kJ/mol, CH₄ production yields ~-50 kJ/mol. Microbes use more favorable pathway first.
How does bioirrigation enhance solute exchange?
Creates 3D network of oxic-anoxic interfaces, increases effective diffusion rates, introduces fresh electron acceptors, accelerates organic matter remineralization.
What factors limit iron and manganese reduction in estuaries?
Low abundance of reactive oxides, competition with sulfate reducers, rapid sulfide precipitation, limited oxide regeneration.
Describe the complete metal release-readsorption cycle.
Oxic: Metals adsorb to oxides, suboxic: Mn/Fe oxides dissolve, anoxic: Metals release to solution, diffusion upward, re-adsorption at redox boundary.
What are the limitations of ²¹⁰Pb dating?
Mixing disturbs upper layers, variable initial activities, non-steady accumulation, diagenetic remobilization, limited to ~100-150 year record.
