Lakes, Peat Swamps and Coal Deposits

Question 1

Middle of lakes:

Answer 1

• Deep water, not a lot of energy
• Sediment raining out of the water column -> fine grained
• Sediment comes from organisms living in the water + a lot of dust blown in the air + fine grain muddy sediment that was suspended by the river flow
• Finest grain sizes (mud/clay)
• Suspended sediment, windblown dust, organisms living there

Question 2

Edges of Lake:

Answer 2

• More activity.

- Some lakes have braided rivers bring in heaps of sediment

Question 3

Shorelines (variety of environments):

Answer 3

• River deltas bring in most of sediment
• Waves can create beaches on shoreline
  (both methods providing sediment into the lake)

Question 4

Sediment settling

Answer 4

• Settling velocity = constant (based on grain density, gravity, fluid viscosity) * diameter squared
  Middle of the lake = sediment settling with all the fine grained sediment just going to blanket the bed.

Question 5

Lakes in the Eastern African Rift:

Answer 5

Can often record seasonal change due to different plankton living in each lake for each season

Question 6

Lakes Facies Model:

Answer 6

• Turbidity currents from shoreline
• Water temperature affects sediment suspension
• Sediment comes in with any flow that flows into the lake
• Lakes are thermally stratified (sediment can be trapped/suspended due to different layers of density)
• denser, coarser sediments hug at the bottom as turbidity currents

Question 7

Waves in lakes

Answer 7

• Away from deltas but on the shoreline you will see the effect of waves
• Wave size is determined by fetch (how long the wind has travelled for)
• Shorelines are reworked by waves (size controlled by fetch - winds that blows lengthwise of the wave will make larger waves compared to winds that blow across the lake)
• Wetting/drying -> mud cracks
• Shoreline marshes with roots
• Ice crystals.

Question 8

Lakes in Arid Environments:

Answer 8

Arid: more evaporation than precipitation
Arid climate lakes:
- could be very seasonal rain/dry
- evaporation > precipitation
- Dries out (ephemeral) and gets saltier
Arid regions are pretty much 30 degrees North and South or in Rain Shadow Regions
- When evaporation is greater than precipitation, get salt deposits (evaporites)
- Salt crusts
- Water is super-saturated with respect to dissolved minerals -> form crystals (crystals nucleate with something on the surface e.g. another crystal or dust)

Question 9

Bathtub ring:

Answer 9

• Precipitated minerals in flood plain
• Salt dissolves really easily and is in the middle (constantly in and out of solution)
• Things that are less easy to dissolve are on the outer edges as are left over when it floods heaps
• Corresponds to different amounts of flood levels.

Question 10

Lakes in cold environments:

Answer 10

• Cold environments you get ice -> ice blocks sediment at the surface
• most seasonal lacustrine deposits (Varves)
• Varves = seasonal layers from freezing and thawing
• End up with extremely seasonal deposits
• When the ice melts, all the sediment in/on the ice is released
• good to use for climate change and good dating technique

Question 11

Mires:

Answer 11

• Lakes infill with sediment and vegetation and over time become a low lying mire (swampy ground, but water level does not change)
• Low lying mire - water level is the same as the original lake (controlled by the ground water table elevation)
• Raised Mires -> vegetation growth is fast tom make a dome (sediment s are the vegetation, and produce coal)

Question 12

Ingredients of Organic coal

Answer 12

Woody plant materials tend to form coal as there is enough solid material to stay as a coherent carbon rich sediment)
- Algae and bacteria tend to form oil

Question 13

Peat Accumulation

Answer 13

• Woody plant growth is more rapid than decomposition - so stop the decomposition
  Physical:
• Protected from sediment and groundwater
• Rain - lots of it
• Subsidence - not too fast or slow
  Chemical:
• Low oxygen
• Low pH
  Low lying marginal marine swamps are not goods for accumulating peat - too much oxygen?

Question 14

Raised Mires produce purest coal due to:

Answer 14

• Raised enough so all the water flows away from the centre of the mire
• Are isolated from sediment and river input

Question 15

Lakes and Mires:

Answer 15

• Dominated by sediment settling (very fine grained)
• Temperature Stratified water -> different densities, different chemistry, different sediment load
• Sediment sources (wind-born dust, muddy surface water, aquatic organisms, down-slope turbidity currents - Bioturbation stirs together different layers
• organic matter accumulates if decomposition is limited