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EA1300 World of Dynamic Environments > China > Flashcards

China Flashcards

1
Q

Who was Xu Xiake?

A
  • Father of modern speleology (study of caves) and karstology.
  • Coined term Fenglin: ‘Peal Forest’.
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2
Q

What is the difference between Chinese geomorphologists and Western geomorphologists?

A

Chinese distinguish karst types by presence or absence of karst plain between hills, Western use shapes of hills.

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3
Q

What is a Fenglin Karst?

A
  • Isolated limestone hills rise up above flat alluvial plain.
  • Western geomorphologists call this ‘tower karst’.
  • Fenglin is a very mature form of karst terrain, requires very thick limestone sequences.
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4
Q

What is a Fengcong Karst?

A
  • Conical hills separated by deep, closed depressions, forms continuous terrain of steep slopes and significant relief.
  • Western geomorphologists call this ‘Cone Karst’.
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5
Q

How are carbonate rocks (e.g. limestone) formed?

A
  • Many marine organisms use ions dissolved in seawater to precipitate calcium carbonate as mineral calcite/aragonite.
  • Minerals can form rocks called limestone.
  • Ions product of chemical weathering reactions on land, transported to oceans via rivers.
  • Solubility of limestone means it can produce karstic landscapes.
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6
Q

How are carbonates different to siliciclastic sediments?

A

Carbonate: ions produced by chemical weathering, used in inorganic/biogenic precipitation of calcite (CaCO3).

Clastic: physical weathering + transport + deposition of pre-existing grains. Roundness and sorting of grains reflects transport and depositional environment.

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7
Q

What are some properties of calcite?

A
  • More soluble in low temperatures, high pressures, more acidic water.
  • Components of limestone: non-skeletal grains (inorganic), skeletal grains (biogenic), matrix and cements.
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8
Q

What are non-skeletal grains?

A
  • Ooids ( <2mm diameter) and pisoids (>2mm diameter).
  • Spherical-subspherical grains, consist of 1+ regular concentric lamellae around nucleus.
  • Tiny particles of CaCO3 precipitated from seawater.
  • Particles accreted onto ooid during agitation.
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9
Q

What are peloids?

A
  • Spherical, ellipsoidal, angular grains.
  • Microcrystalline carbonate.
  • No internal structure.
  • Mostly faecal pellets.
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10
Q

What are skeletal grains?

A

Bivalves, cephalopods, corals, echinoids, sponges, algae, cyanobacteria etc.

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11
Q

What are matrix?

A

Grains in many limestones sit within matrix of calcareous mud (micrite).

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12
Q

What is micrite?

A

Generally <4 microns.
- Disaggregation of calcareous algae.
- Bioerosion of carbonates.
- Mechanical disaggregation of carbonates.
- Disaggregation of peloids.
- Inorganic/biochemical precipitates.

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13
Q

What is cementation?

A

Major process producing hard limestone from loose sediment.

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14
Q

What are Karstic Landscapes?

A
  • Karst is topography formed from dissolution of soluble rocks like limestone.
  • Same chemical reaction that causes dissolution of limestone statues.
  • Initial karstification process is formation of sinkholes or dolines.
  • Doline karst field often contains caves and subterranean drainage systems.
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15
Q

Why Guilin landscape is so different to our own: climate

A
  • Guilin is a tropical climate influenced by East Asian Monsoon.
  • Fengcong and Fenglin karst requires hot and wet climate, with significant rates of carbonate dissolution in regime of abundant biogenic carbon dioxide.
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16
Q

Why Guilin landscape is so different to our own: thick limestone sequence

A
  • Allows massive lowering of surface without reaching base of limestone.
  • Area along Li River contains limestone 2.6km thick, continuously deposited in Devonian and Carboniferous.
  • During Carboniferous, China was at low latitude and largely covered by shallow seaway.
17
Q

Why Guilin landscape is so different to ours: tectonic uplift

A
  • Enables doline karst to evolve into fengcong karst.
  • Will continue to evolve towards higher relief until base of dolines reach base of limestone sequence or regional base level.
18
Q

Why Guilin landscape is so different to our own: lateral

A
  • ‘Egg-box’ Fengcong karst can evolve into ‘tower’ Fenglin karst.
  • Depends on balance between: rate of tectonic uplift, rate of surface lowering due to chemical weathering, rate of lateral planation.
19
Q

What is lateral planation?

A
  • Requires enhanced dissolution by corrosive water at level of water table, close to alluvial plane.
  • Leads to under-cutting of rounded hills, facilitating formation of tower-like structures.
  • Input from clastic sediment from neighbouring regions important to maintain flat, alluvial plain between towers.
20
Q

What is limestone dissolution?

A

Water passing through limestone dissolves limestone = ions in solution.

21
Q

What happens when water re-enters air?

A

1) Releases dissolved CO2 so can hold less dissolved carbonate.
2) Evaporate, concentrating dissolved ions. CaCO3 precipitated.

22
Q

What are speleothems?

A
  • Good archives of past climate change.
  • Help to understand different components of climate system interact.
  • Dated using radiometric dating, analysed for oxygen isotope composition.
  • Reflects change in hydrological cycle.
23
Q

How are speleothems dated?

A
  • CaCO3 contain tiny amounts of uranium.
  • Part of 238U-260Pb decay series often used to date speleothems.
  • Over time, uranium-234 decays to thorium-230. Ratio of U to Th decreases.
  • Ratio can be analysed using mass spectrometer to calculate age of sample.
24
Q

Is the half life of 234U longer or 14C?

A

234U is longer (245,000 yrs), 14C is 5700 yrs.

25
Q

Oxygen isotopes in water cycle

A
  • Oceans largest source of water vapour.
  • 16O preferentially evaporated from seawater.
  • 18O preferentially condensed in clouds.
  • Leads to progressive enrichment in 16O with latitude and altitude.
  • Seawater (8)18O varies spatially and temporally.
26
Q

What is the amount effect?

A

Heavy rainfall tends to have lighter (8)18O (larger raindrops enable more 16O from higher in atmosphere to be rained out).

27
Q

Dayu Cave, Central China

A
  • Further north than karstic scenery, still strongly influenced by East Asian Summer Monsoon.
  • Qinling Mountains important refuge for Giant Pandas.
  • East Asian Summer Monsoon brings huge amounts of water vapour to continent.
  • Due to amount effect, when rainfall is heaviest, rain has light (8)18O.
  • Scientists used variations in speleothem (8)18O to reconstruct past changes in rainfall intensity.

EA1300 World of Dynamic Environments (4 decks)

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