Marine sediments Flashcards
What are hydrogenous sediments?
Minerals that have directly precipitated from seawater — e.g. sulphides from hydrothermal vents,
What is an example of biogenic production?
Shells of plankton
What are the main types of sediment?
• Terrigenous
• Biogenic
• Authigenic
What is terrigenous sediment?
Land derived
What is biogenic sediment?
Biological material
What is authigenic sediment?
Created in situ
What is an example of terrigenous sediment?
• Hemi-pelagic muds — muds sliding off continental shelf into the open ocean
• Abyssal (red) clays
What is an example of biogenic sediment?
• Calcareous ooze
• Siliceous ooze
What is an example of authigenic sediment?
• manganese nodules
What controls the accumulation of ocean sediments?
• Deposition rate from surface
• Degree of preservation
What is true of deposition rate from surface?
• Terrigenous sediments — pretty much constant
• Biogenic sediments — Depends on productivity in the surface waters which in turn is linked to nutrient availability
What is true of degree of preservation?
• 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?
What is true of calcium carbonate saturation?
• Warm, shallow ocean = saturated
• Cool, deep ocean = undersaturated
What is true of biological sediment accumulation?
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
What is true of Abyssal/pelagic (red) clay accumulation?
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
What is the composition of pelagic (red) clays?
< 30% biogenius
50% < um in diameter
What is the rate of accumulation of pelagic (red) clays?
<2mm/1000yr
What are the characteristics of pelagic (red) clays?
Enriched with metals including lead, manganese and iron
— hence red
What is the composition of calcareous oozes?
• >30% biogenic
What is the rate of accumulation of calcareous oozes?
1-15mm/1000yr
What are the characteristics of Calcareous oozes?
Foraminifera or pteropod
What is the composition of siliceous oozez?
> 30% biogenic
What is the rate of accumulation of siliceous oozes?
1-15mm/ 1000 yr
What are the characteristics of siliceous oozes?
Radiolarian or diatom
What is the composition of hemipelagic muds?
> 50% by weight lithogenous
<30% biogenous
50% >um in diameter
What is the rate of accumulation of hemipelagic muds?
It is erratic due to transport mechanism (muds sliding off continental shelf into open ocean)
What is a characteristic of hemipelagic muds?
Moves from estuaries to shelf and slumps down the shelf edge; may be ice rafted
What does authigenic mean?
Self originating
What is true of authigenic processes?
• 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
What is the typical structure of sediment? (Top to bottom)
- ⬇️Organic matter accumulating with abyssal clay
- Oxygenated surface layer
- Anoxic layer
- Old chemically and physically stable sediment
What is true of the oxygenated surface layer?
It may vary in thickness
What happens when bacteria is breaking down organic matter?
- They may de-oxygenate the sediment
- O2 diffusion is slow
- So bacteria use SO4^2- for oxygen, creating H2S and changing chemical
How do manganese nodules form?
• Deep abyssal plains — sediment accumulates very slowly
How many common chemical forms does manganese have?
• 2 — Mn(II) and Mn(IV)
What is Mn(II)?
Mn^2+ is in the 2+ oxidation state
What is Mn(IV)?
Mn^4+ is in the 4+ oxidation state
What is true of Mn(IV)?
It is the more oxidised form and tends to precipitate as manganese oxide (MnO^2)
What is true of Mn(II)?
It is found in reducing conditions (low O2 conc) and is more soluble
— i.e. low oxygen concentrations = dissolved manganese concentrations will increase
