Lecture 4 – Evidence for past Climates 3 Flashcards by Jamie Payne

Source area definition

where sediment particles are created

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Sink area definition

where sediment is deposited in layers

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Weathering definition

the general process by which rocks are broken down at the Earth’s surface

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Erosion definition

processes that dislodge particles of rock produced by weathering

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Transportation definition

processes by which sediment particles are moved to sink areas

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Deposition/sedimentation definition

processes by which sediment particles settle out as water currents slow

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What is sediment burial?

occurs as layer of sediment accumulate in a sink area on top of one another

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What is diagenesis

physical or chemical changes caused by heat, pressure and chemical reactions

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How do chemical and physical weathering reinforce each other?

smaller fragments produced by physical weathering creates a greater surface area exposed to chemical weathering

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How much sediment do rivers carry yearly?

Approx 25 billion tons

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What counts as a strong current?

More than 50cm/s

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What can strong currents carry?

Gravel and smaller particles

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What counts as a moderately strong current?

20-50cm/s

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What do moderately strong currents lay down

Sand beds

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What can weak currents carry?

The finest particles

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What counts as a weak current?

Slower than 20cm/s

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How does a sedimentary basin form?

Rift develops as hot mantle wells up and lithosphere heats, stretches and thins
Seafloor spreading begins
Evaporites, deltaic sediments and carbonates are deposited
Deposits are buried by accumulation of further sediment

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What are the characteristics of sedimentary environments?

Type and amount of water
Type and strength of transport agents
Topography
Biological activity
Plate tectonic settings of sediment source area
Climate

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What is cross bedding?

beds deposited by wind or water and inclined at angles up to 35 degrees from horizontal

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When does cross bedding occur?

When sediment particles are deposited on the steeper, down current slopes of sand dunes

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`What are ripples?

small ridges of sand or silt whose long dimensions are at right angles to the current

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What are valve deposits?

Regular alternations in glacial lake sediment layers

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What is one valve composed of?

one light and one dark layer, deposited in one annual cycle

How do valve deposits form?

Couplets/pairs represent annual seasonal deposition in fresh or brackish water

What are the darker layers in valve deposits and when do they occur?

coarse grained, silt layer, occur in spring and summer (stronger currents so carry larger sediment)

What are the lighter layers in valve deposits and when do they occur?

Fine grained, clay layers, occur in autumn/winter (river is frozen to less power to carry sediment)

Can valve deposits occur in salt water?

They do not occur in salt water because the clays coagulate (turn solid)

How do we use valve deposits in climate reconstruction?

Characteristics of an individual varve layer may provide an indicator of the climate at the time of deposition e.g. thickness, pollen and diatom content

What is palynology?

Study of pollen

How do we use palynology for climate reconstruction?

Different types of pollen give an indication of different types of plants indicates the vegetation type Vegetation type then indicates the climate

What % of life forms are beetles?

25%

Where are beetles found?

in most habitats except sea or polar regions

How do we use beetles for climate reconstruction?

We know the distribution/climate type most beetles settle in | Therefore, we can use their fossils to give an indication of the climate type where they live

What are diatoms?

Aquatic microscopic algae (plants)

How sensitive are diatoms to environmental conditions?

Very

How do we use diatoms in climate reconstruction?

Analysis of abundance/types provides a picture of water quality – e.g. salinity, nutrients and temperature Can be combined with varve deposit analysis to tell us the rough timeframe as well

How long was the lake core in Lake Tanganyika??

109m long

What was the Tanganyika lake core shielded from and why?

Terrigenous input - to stop the cores from being overwhelmed by external inputs

How did we date the lake in core in Lake Tanganyika?

Radiocarbon dating

What intervals was the Tanganyika core sampled at?

5cm intervals

What changes did they find in the Tanganyika core and what did it suggest?

5 abrupt changes, suggesting that the lake is sensitive to climate fluctuations

What did we find in the zone 5 of the Tanganyika core and what did this suggest?

``` Cyclostephanos dominated (a type of species with a very large silica shell) High silica may indicate high run off, implying high precipitation ```

What did we find in the zone 3 of the Tanganyika core and what did this suggest?

``` Aulacoseriaa dominated (a large species requiring high wave action to keep them in the photic zone) May indicate increased windiness ```

Give an example of diatoms use?

Lake Tanganyika

What is the process of deep sea cores?

Core brought up on deck Split in half – one half is the archive half that can only be used in exceptional circumstances Working half – worked on by scientists

What is the sediment record restricted to on the sea bed and why?

180 million years - and older and they'll be destroyed by subduction

Is the sea bed better than terrigenous cores and why?

Sea floor offers more continuous stratigraphic record than terrestrial sections, and better dated sediment

What are foraminifera?

microscope aquatic organisms with a carbon shell

How sensitive are foraminifera to water temperature and salinity?

Especially sensitive to water temperature and salinity

What are the two ways we can use foraminifera?

either looking at the frequency of species or by measuring the ratio of oxygen 18 to oxygen 16

What does the shells of foraminifera show?

the ratio of oxygen 18 to oxygen 16 in the water at the time the shell formed

What happens to the ratio of oxygen 18 to oxygen 16 in cold environments?

Oxygen 16 evaporates and is stored as snow/ice, increasing the ratio of Oxygen 18 to 16

What happens to the ratio of oxygen 18 to oxygen 16 in warm environments?

evaporated Oxygen 16 is put back into the system via precipitation, which means the ratio stays the same

What is an isoptope?

different types of atoms of the same chemical element, with the same atomic number but a different number of neutrons

What type of record do we want for ice cores?

we want the longest record possible (where ice flow is slowest)

How can we date ice cores?

Annual variations in snow properties related to temperature Radiometric dating of dust layers Can occasionally get a volcanic eruption, with ash stored in the ice cores They can also contain ‘bubbles’ which store the atmospheric gases as the time, and give us an idea of the atmosphere