Marine sediments

Question 1

What are hydrogenous sediments?

Answer 1

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

Question 2

What is an example of biogenic production?

Answer 2

Shells of plankton

Question 3

What are the main types of sediment?

Answer 3

• Terrigenous
• Biogenic
• Authigenic

Question 4

What is terrigenous sediment?

Answer 4

Land derived

Question 5

What is biogenic sediment?

Answer 5

Biological material

Question 6

What is authigenic sediment?

Answer 6

Created in situ

Question 7

What is an example of terrigenous sediment?

Answer 7

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

Question 8

What is an example of biogenic sediment?

Answer 8

• Calcareous ooze
• Siliceous ooze

Question 9

What is an example of authigenic sediment?

Answer 9

• manganese nodules

Question 10

What controls the accumulation of ocean sediments?

Answer 10

• Deposition rate from surface
• Degree of preservation

Question 11

What is true of deposition rate from surface?

Answer 11

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

Question 12

What is true of degree of preservation?

Answer 12

• 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?

Question 13

What is true of calcium carbonate saturation?

Answer 13

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

Question 14

What is true of biological sediment accumulation?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

What is the composition of pelagic (red) clays?

Answer 16

< 30% biogenius
50% < um in diameter

Question 17

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

Answer 17

<2mm/1000yr

Question 18

What are the characteristics of pelagic (red) clays?

Answer 18

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

Question 19

What is the composition of calcareous oozes?

Answer 19

• >30% biogenic

Question 20

What is the rate of accumulation of calcareous oozes?

Answer 20

1-15mm/1000yr

Question 21

What are the characteristics of Calcareous oozes?

Answer 21

Foraminifera or pteropod

Question 22

What is the composition of siliceous oozez?

Answer 22

> 30% biogenic

Question 23

What is the rate of accumulation of siliceous oozes?

Answer 23

1-15mm/ 1000 yr

Question 24

What are the characteristics of siliceous oozes?

Answer 24

Radiolarian or diatom

Question 25

What is the composition of hemipelagic muds?

Answer 25

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

Question 26

What is the rate of accumulation of hemipelagic muds?

Answer 26

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

Question 27

What is a characteristic of hemipelagic muds?

Answer 27

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

Question 28

What does authigenic mean?

Answer 28

Self originating

Question 29

What is true of authigenic processes?

Answer 29

• 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

Question 30

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

Answer 30

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

Question 31

What is true of the oxygenated surface layer?

Answer 31

It may vary in thickness

Question 32

What happens when bacteria is breaking down organic matter?

Answer 32

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

Question 33

How do manganese nodules form?

Answer 33

• Deep abyssal plains — sediment accumulates very slowly

Question 34

How many common chemical forms does manganese have?

Answer 34

• 2 — Mn(II) and Mn(IV)

Question 35

What is Mn(II)?

Answer 35

Mn^2+ is in the 2+ oxidation state

Question 36

What is Mn(IV)?

Answer 36

Mn^4+ is in the 4+ oxidation state

Question 37

What is true of Mn(IV)?

Answer 37

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

Question 38

What is true of Mn(II)?

Answer 38

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