Biogeochemical Processes

Question 1

Estuarine sediments

Answer 1

High rates of sedimentation
- reduction in river water flow
- aggregation/flocculation (deposition of fine sediments)
- High rates of biological activity

Question 2

Bacterial decomposition of organic C

Answer 2

Oxidation of OM is a key process in sediments
Decomposition of OM is mainly carried out by bacteria
Coastal seds have v high levels of organic C, due to high rates of biological productivity

Question 3

Use of O

Answer 3

Oxygen is the most important oxidant in seawater, high conc and used in cellular processes
In sediments oxygen may be used up more rapidly than it can be resupplied by diffusion, so other oxidising agents used

Question 4

Diagenetic sequence

Answer 4

Diagenetic sequence
O - aerobic respiration
NO3 - denitrification
Mn(IV) - Mn reduction
Fe(III) - Fe reduction
SO4 - sulphate reduction
CH2O - methane fermentation

Question 5

Oxygen penetration

Answer 5

May be 10s of cm in open ocean
In estuaries, supply of organic is higher, oxygen is completely removed from sediment pore waters in upper mms to cms.
Denitrification and sulphate oxidation are widespread
Reduction and Mn and Fe less important due to limited supply of reactive metal oxides

Question 6

Redox Zones

Answer 6

Redox Zones tend to be compressed in estuarine and coastal seds
May occur simultaneously due to microenvs, burrows/root tubes, organic hot spots

Question 7

Bioturbation

Answer 7

Exchange of solutes between pore waters and overlying seawater driven by diffusion
Rate of exchange may be enhanced by:
- sediment resuspension
- bioirrigation/turbation

Question 8

Cycling of trace metals in association with Fe/Mn oxyhydroxides

Answer 8

Metals adsorped on oxyhydroxides, will also be released into sediment pore waters if these phases are reduced
Leads to cycling of metals at the oxic/anoxic boundary and can lead to accumulation of metals in sediments

Question 9

Estuarine sedimentation rates

Answer 9

Primordial radionuclides (derived from rocks) can be used to assess:
- sediment accumulation rates and mixing (234Th and 210Pb)
- Dating techniques
- Bioturbation (234Th)

Question 10

210Pb dating

Answer 10

226Ra in soils and rocks decays to 222Rn
222Rn escapes and decays to 210Pb
210Pb removed by precipitation and incorporated into accumulating sediment.
Excess activity, 210Pbxs
Total 210Pb in seds is sum of excess activity.
Decays at known rate in sediment, accumulation estimated

Question 11

How to calculate a sedimentation rate

Answer 11

Identify background Activity, the value of A at greater depths where it is not changing with depth
Subtract the background activity from the observed activities at shallower depths
Take natural logarithm to get ln(A) = ln(Ameasured- A)
Plot depth z against ln(A)
Measure slope in middle region, negative
Multiply the minus the slope by the radioactive decay constant (0.0311/yr for 210Pb) = sed rate

Question 12

Estuarine seds as recorder of pollution

Answer 12

Seds close to Fawley oil refinery show evidence of pollution
High sed rates after dev of refinery
Lower 206Pb/207Pb and 206/208Pb ratios, after dev, consistent with use of precambrian ores used in petrol