Biogeochem processes Flashcards
(26 cards)
Why are there high rates of sedimentation in estuarine systems?
Reduction in river water flow, aggregation/flocculation, high rates of biological activity.
Up to what wt% in organic carbon can sediments in coastal waters and estuaries have?
~7 wt%
Means that for every 100 g of dry sediment, about 7 grams is organic carbon — which is considered very rich in OC, and common in productive, estuarine, or low-oxygen settings
What is the diagenetic sequence of electron acceptors?
O2, NO3-, Mn(IV), Fe(III), SO42-, CH2O.
Why is oxygen the most important oxidant in seawater?
High concentration and is used in microbial cellular processes.
What order are oxidants used up in?
Energy yield.
What is the distribution of energy yields by respiratory process in sediment?
Higher energy yields/more negative values occur in the upper layers where oxygen and nitrate are available, whereas lower energy-yielding processes such as sulfate reduction dominate in deeper anoxic zones.
What is the order of energy yields by respiratory process in sediment?
Surface-> Depth: Aerobic respiration, denitrification, manganese reduction, iron reduction, sulfate reduction.
Where is the depth of O2 penetration likely to be shallower?
The combination of lower oxygen supply and higher oxygen demand causes the oxygen penetration depth to be shallower in the Pacific Ocean compared to the Atlantic.
Why is the oxygen penetration depth only a few mm in estuarine and coastal sediments?
Organic carbon content is very high.
Why are redox zones compressed and occur simultaneously in estuarine and coastal environments?
Due to microenvironments - organic matter creates anoxic microniche after decomposition.
How may benthic fauna/flora influence redox zones in estuarine sediments?
Bioturbation and bioirrigation mixes sediment and causes oxygenation of deeper sediments. Sediment resuspension and bioirrigation may increase rate of solute exchange between pore waters and seawater. Root systems can cause redox stratification by transporting O2 to deeper sediments, influencing microbial respiration pathways.
What is the oxidation and reduction of metal species dependent on?
Oxygen availability and the presence of electron acceptors.
What happens to iron and manganese in oxic and anoxic conditions? What does this cause?
In oxic conditions, Fe and Mn oxides act as electron acceptors but in anoxic conditions, they dissolve which releases the metals into pore waters. Leads to cycling of metals at the oxic/anoxic boundary and can lead to accumulation of metals in sediments.
What are periodic redox oscillations driven by, and what can they enhance?
Driven by tidal dynamics or bioturbation. Enhance trace metal cycling through the promotion of alternating precipitation/dissolution processes.
What are the uses of primordial radionuclides?
Sediment accumulation rates and mixing, dating techniques, and bioturbation.
Why is sediment accumulation rate estimated using lead?
Sediment accumulation rates are estimated using ²¹⁰Pb because atmospheric lead (excess) (²¹⁰Pbₑₓc) settles onto sediments and decays over time, while additional “supported” ²¹⁰Pb comes from the in-sediment decay of ²²⁶Ra.
By measuring the decline of excess ²¹⁰Pb with depth, scientists can calculate how quickly sediments have built up over the past ~100–150 years.
Total lead in sediments = excess activity + supported activity from decay of 226Ra.
Why can data at the very surface not be used?
Organisms are bioturbating so not an active amount of sedimentation.
Why is the decrease in excess lead with depth a function of time?
Because if the sedimentation rate was constant, S = depth/time.
Describe the global distribution of sedimentation rates?
Sedimentation decreases with depth, except at marginal ocean basins as enclosed seas with smaller currents and are less affected by ocean conveyor.
How can estuarine sediments be records of pollution? (Fawley oil refinery)
Stable lead isotopes used to trace sources of pollutants: High sedimentation rates after development of refinery. Lower 206Pb/207Pb and 206Pb/208Pb ratios after development of refinery: consistent with use of Precambrian Australian/Canadian ores (used as catalyst in petrol).
How can infaunal organisms create localized oxic zones?
Leave behind burrow structures or tubes which act as conduits for oxygen penetration.
Why are estuaries CO₂ sources and shelves CO₂ sinks?
Estuaries: High organic decay (CO₂ release), turbidity limits photosynthesis.
Shelves: Phytoplankton blooms absorb CO₂; carbon exported to depth.
Exceptions: Upwelling, eutrophication, or river-dominated shelves may emit CO₂.
Describe a sediment trap setup for POC flux study.
Design: Moored funnel (0.5 m²) with rotating cups (monthly intervals), preservative (formalin), current meter.
Pros: Captures seasonal trends.
Cons: Swimmer contamination, current interference.
How does POC flux vary seasonally in the North Atlantic?
Climate at this latitude is temperate so highest fluxes would occur shortly after the spring bloom around June/July by time reaches the sea floor.
Fluxes would be lowest in winter.
