Biogeochemical processes in estuarine sediments

Question 1

Why are there high rates of sedimentation in estuaries?

Answer 1

• when there’s a reduction in river flow (has time to settle as it moves down the estuary)
• aggregation/flocculation (deposition of fine sediments) - flocculation of material at salinity around 5 where riverwater meets seawater
• leads to HIGH RATES OF BIOLOGICAL ACTIVITY

Question 2

Explain the bacterial decomposition of organic carbon.

Answer 2

• oxidation of organic matter is a key process in marine sediments
• most organic C is at the surface & decreases with depth (marine snow etc)
• there’s much higher organic C levels in coastal waters and estuaries than in the deep ocean, due to high rates of biological activity
• this organic material feeds bacteria and fauna at the sediment (they use it as their FOOD SOURCE)
• phytoplankton take up CO2 from atmosphere and produce org matter, and sediment orgs use this matter
• decomposition of organic matter is mainly carried out by bacteria

Question 3

What is the most important oxidant in seawater and why?

Answer 3

Oxygen, due to its high concentration & it’s used in microbial cellular processes.

However, oxygen in sediments may be used up more rapidly than it can be resupplied by diffusion, so other oxidising agent are used in sequence.

Question 4

What is the diagenetic sequence?

Answer 4

Oxidants are used up in order of their energy yields.
- O2 (aerobic respiration)
- NO3- (denitrification)
- Mn(IV) as Mn oxide (Mn reduction)
- Fe(III) as Fe oxide (oxyhydroxide) (Fe reduction)
- SO42- (sulphate reduction)
- CH2O (methane fermentation)
These are all microbially catalysed.

Question 5

What is the difference in sediment penetrating depth of oxygen in open ocean vs estuaries and shelf seas? Why?

Answer 5

• penetrating depth in open ocean can be 10s of cm
• in estuaries & shelf seas, supply of organic matter is much higher and oxygen is completely removed from sediment pore waters in upper few mm to cm
• meaning other oxidants used due to the amount of organic carbon

Question 6

Why are redox zones compressed in estuarine and coastal sediments?

Answer 6

• due to organic hotspots of carbon
• zones may even occur simultaneously
• the minute you get org carbon, oxygen declines

Question 7

What process drives the exchange of solutes between pore waters and overlying seawater? What may this rate of exchange be enhanced by?

Answer 7

Diffusion.

• sediment resuspension e.g. tides
• bioirrigation e.g. the orgs interrupting the zones

Question 8

What can primordial radionuclides (derived from rocks) be used to assess?

Answer 8

• sediment accumulation rates and mixing

- dating techniques (e.g. shell growth dating)

Question 9

What can 210Pb xs decay at known rate allow to be estimated?

Answer 9

sediment accumulation