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OS102 > Marine sediments > Flashcards

Marine sediments Flashcards

1
Q

What are hydrogenous sediments?

A

Minerals that have directly precipitated from seawater — e.g. sulphides from hydrothermal vents,

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2
Q

What is an example of biogenic production?

A

Shells of plankton

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3
Q

What are the main types of sediment?

A

• Terrigenous
• Biogenic
• Authigenic

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4
Q

What is terrigenous sediment?

A

Land derived

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5
Q

What is biogenic sediment?

A

Biological material

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6
Q

What is authigenic sediment?

A

Created in situ

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7
Q

What is an example of terrigenous sediment?

A

• Hemi-pelagic muds — muds sliding off continental shelf into the open ocean
• Abyssal (red) clays

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8
Q

What is an example of biogenic sediment?

A

• Calcareous ooze
• Siliceous ooze

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9
Q

What is an example of authigenic sediment?

A

• manganese nodules

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10
Q

What controls the accumulation of ocean sediments?

A

• Deposition rate from surface
• Degree of preservation

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11
Q

What is true of deposition rate from surface?

A

• Terrigenous sediments — pretty much constant
• Biogenic sediments — Depends on productivity in the surface waters which in turn is linked to nutrient availability

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12
Q

What is true of degree of preservation?

A

• Biological sediments will dissolve and rate depends on conditions — e.g. depth, pressure, water temperature etc.
• Extent if dissolution depends on time exposed — e.g. how long between shell leaving the surface to it being covered by a layer of sediment?

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13
Q

What is true of calcium carbonate saturation?

A

• Warm, shallow ocean = saturated
• Cool, deep ocean = undersaturated

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14
Q

What is true of biological sediment accumulation?

A

Biological sediments will accumulate where water us not too deep and there is sufficient primary production at surface
— to provide material which can reach bottom and be covered before dissolving

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15
Q

What is true of Abyssal/pelagic (red) clay accumulation?

A

Will form in deep areas of open ocean away from coast.
— where there is reduced primary production on surface waters and extensive opportunity for dissolution of biological materials

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16
Q

What is the composition of pelagic (red) clays?

A

< 30% biogenius
50% < um in diameter

17
Q

What is the rate of accumulation of pelagic (red) clays?

A

<2mm/1000yr

18
Q

What are the characteristics of pelagic (red) clays?

A

Enriched with metals including lead, manganese and iron
— hence red

19
Q

What is the composition of calcareous oozes?

A

• >30% biogenic

20
Q

What is the rate of accumulation of calcareous oozes?

A

1-15mm/1000yr

21
Q

What are the characteristics of Calcareous oozes?

A

Foraminifera or pteropod

22
Q

What is the composition of siliceous oozez?

A

> 30% biogenic

23
Q

What is the rate of accumulation of siliceous oozes?

A

1-15mm/ 1000 yr

24
Q

What are the characteristics of siliceous oozes?

A

Radiolarian or diatom

25
Q

What is the composition of hemipelagic muds?

A

> 50% by weight lithogenous
<30% biogenous
50% >um in diameter

26
Q

What is the rate of accumulation of hemipelagic muds?

A

It is erratic due to transport mechanism (muds sliding off continental shelf into open ocean)

27
Q

What is a characteristic of hemipelagic muds?

A

Moves from estuaries to shelf and slumps down the shelf edge; may be ice rafted

28
Q

What does authigenic mean?

A

Self originating

29
Q

What is true of authigenic processes?

A

• Occurs after deposition
• Can include precipitation of metals onto particles, precipitation of salts or calcium carbonate in some conditions
• Most significant and widely known process is creation of manganese nodules

30
Q

What is the typical structure of sediment? (Top to bottom)

A
  1. ⬇️Organic matter accumulating with abyssal clay
  2. Oxygenated surface layer
  3. Anoxic layer
  4. Old chemically and physically stable sediment
31
Q

What is true of the oxygenated surface layer?

A

It may vary in thickness

32
Q

What happens when bacteria is breaking down organic matter?

A
  1. They may de-oxygenate the sediment
  2. O2 diffusion is slow
  3. So bacteria use SO4^2- for oxygen, creating H2S and changing chemical
33
Q

How do manganese nodules form?

A

• Deep abyssal plains — sediment accumulates very slowly

34
Q

How many common chemical forms does manganese have?

A

• 2 — Mn(II) and Mn(IV)

35
Q

What is Mn(II)?

A

Mn^2+ is in the 2+ oxidation state

36
Q

What is Mn(IV)?

A

Mn^4+ is in the 4+ oxidation state

37
Q

What is true of Mn(IV)?

A

It is the more oxidised form and tends to precipitate as manganese oxide (MnO^2)

38
Q

What is true of Mn(II)?

A

It is found in reducing conditions (low O2 conc) and is more soluble
— i.e. low oxygen concentrations = dissolved manganese concentrations will increase

OS102 (13 decks)

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