Week 2 - Reconstructing Quarternary Environments 3.9 Flashcards
3.9 Lake, Mire and Bog Sediments
Why are lakes, mires and bogs genetically related?
Because all lakes become infilled with sediment to form mires and bogs
In what 2 ways do lake, mire and bog deposits reveal information about past environmental conditions?
Fossil flora and fauna provide evidence of both local and regional ecological changes
The character of the sediments offers clues about former environmental conditions
In lake sediments, variations in chemical and physical properties reflects developments in the lake ____
Ecosystem
What do fossil lacustrine sediments and associated shoreline features reveal during the later part of the Quarternary?
A record of fluctuations in lake levels in response to climatic changes
How are Tectonic lakes formed and how long is the sediment record?
In areas of subsidence caused by folding or faulting, and include some of the largest lakes in the world
Some sediment sequences span the whole of Quarternary time
How are Volcanic lakes formed and how long is the sediment record?
In calderas and craters
From when volcanic activity ceased
How are Glacial lakes formed and how long is the sediment record?
Typically in glaciated regions - kettle lakes develop in hollows created by the melting of buried ice. And lakes damned behind or between glacial landforms as a result of the blocking of drainage outlets
Sediment records span shorts period of time
What is lithology?
The study of rocks - a rock formation having a particular set of characteristics
When sediments contain nekron mud (gyttja), what can that tell you about the conditions of the lake?
The condition is/was eutrophic - The lake is/was rich in mineral nutrients, organic productivity
What plant material will deep water nekron mud (gyttja) contain and what plant material will shallow water nekron mud contain?
Deep - Largely unrecognisable plant material
Shallow - detritus gyttja with recognisable plant macrofossils
Where the lake substrate is calcerous, what mineral may be precipitated from the water by aquatic plants and other organisms, and what will accumulate as a result?
Lime
A cream-white clay-rich sediment (marl)
In lake sediments, what conditions will result in predominately autochthonous material and what conditions will result in predominantly allochthonous material?
Auto - when sediments are deposited under eutrophic conditions
Allo - when the lake is poor in nutrients and organic productivity is low (oligotrophic)
How are diatomites formed?
Under eutrophic or oligotrophic conditions, depending on the ecological affinities of the diatom species
What 2 stratified ways can lakes be distinguished?
Chemically and thermally
What is epilimnion and hypolimnion?
Warm surface waters
Deeper cooler surface waters
When does thermal stratification occur in lakes?
When summer water temperatures exceed 4°C, the epiliminion becomes less dense than the hypolimnion
Also in winter due to surface freezing
What are diamictic lakes?
Temperate lakes experience vertical mixing (breakdown of thermal stratification) twice a year
What are Monomictic lakes?
High altitude, high latitude lakes where warming does not exceed 4°C. Similarly lakes in low altitude may not cool below 4°C.
Mixing occurs once a year.
What are meromicitc lakes?
Lakes that remain stable throughout the year
Large and deep lakes tend to be ____ whereas shallow or small lakes tend to be ____
Stratified
Unstratified
Why might there be differences in lake chemistry, lake biota and sediment type between epilimnion and hypolimnion?
Because Epilimnion is well oxygenated and Hypolimnion is anoxic (anaerobic)
Why might there be large scale die off of algae and diatoms in diamictic lakes and how can this help scientists reconstruct past environments?
Diamictic lakes experience seasonal turnover
What are organic varves?
Consists of lateral concentrations of organic matter
What are chemical varves?
Seasonal alternation between epiliminion and hypolimnion may lead to seasonal production of calcareous deposits, iron oxides or sulphur-rich sediments
