Lecture 9 - Lacustrine Processes

Question 1

an introduction to lakes:

Answer 1

• what do all lakes have in common? a closed depression, this distinguished a lake from a river.
• rivers erode downhill therefore closed depressions “shouldn’t” from.
• lakes are closed depressions within the river system.
• these depressions slow down the water.
• this fundamentally alters the physical and chemical properties of the water body.
• low flow velocity means low capacity for sediment erosion or transport.
• average stream flows are commonly in order 0.1-1 m/s, lake water flow velocities are more usually <0.01 m/s.
  sediment transported by the river will settle rapidly on entering still water.

Question 2

sediments and sedimentation:

Answer 2

• lakes are slow-moving parts of river systems, in which river-carried sediments accumulate.
• sediment = eroded soil, humic matter, biogenic particles, organic matter, biogenic silica, Fe and Mn oxyhydroxides (Fe2+ is soluble - forms in oxygen-free conditions e.g. waterlogged soil, Fe3+ is not - forms in presence of oxygen).
• so, eroded soil (coarse to fine) and particle humics, algae, Fe and Mn precipitates (very fine) make their way into lakes.
• then, coarse particles settle quickly near inflow or shoreline, fine particles mix throughout the lake and settle slowly (edge has coarse sediment, center has fine sediment).
• in between, expected profile across lake its sediment if all sediment comes only from uniform water column.
• however, margins are winnowed by wave action, steep slopes don’t retain sediment, therefore well-defined “mud limit” exists separating littoral (shoreline/shallow water) from profundal (deep water/far from shore) sediment.

Question 3

lake sediment records:

Answer 3

• lake edge has complicated, relatively disturbed sediment.
• but center (away from slumps and debris flows) has fine steadily accumulating sediment.
• this provides time-sequence with progressively younger ages up through sediment layers.
• layers “record” progressive changes in lake catchment system, all we need to do is: sample record, determine age of each layer, derive information about past lake-catchment environment from nature of sediment.
• this is subject of matter of palaeolimnology (the study of past lake environments by reading sediment record.

Question 4

lake coring:

Answer 4

can show base of the Holocene.

Question 5

sediment chronology:

Answer 5

1. changing erosion rates.
2. changing lake-water acidity.
   - Mackereth (1966) inferring soil erosion from K and Na concentration records.
   - Renberg (1990) reconstructing Holocene lake acidification from diatom record.