Seds - Deep Oceans

Question 1

Deep ocean sedimentation: (timing)

Answer 1

Takes 20-50 days for pelagic sediments to settle – Stokes Law says it should take years

• Fecal pellets
o Stick sediment together, so aggregated particles fall faster
 Faster than the single particles calculated using Stokes Law
o Not going to fall straight down – ocean currents
o Marine snow – fact we can see it shows aggregation
• Other aggregation methods:
o Differential settling rates.
o Brownian motion driven collision.
o Bacterial growth.
o Aggregation due to mucus from bacterial mats.

Question 2

Deep Ocean Water Structure:

Answer 2

• Caballing
o Drives downwelling which increases rate of fall
o Two waters (A+B) have different pressure, temperature
o Combined they have a higher salinity which increases their density
o Leads to faster particle fall
• Water is stratified

• Thermohaline global conveyer belt
o Driven by cabaling

Question 3

Wind Driven Currents:

Answer 3

• Requires stratification of ocean waters
o Different densities and temperatures cause this
o Means what happens at top in not the same as at the bottom
• Pynocline, thermocline, halocline.
• Net transport is 90o to wind direction
o Means wind does not show transport

Question 4

Deep Sea sediment distribution:

Answer 4

Deep Sea sediment distribution:
• Continental shelves = more sediment
o Rivers carry large amounts of continental material
 Bengal Mega-fan
• Calcerous
o Death of carbonate organism and fall of body – dependent of CCD
o oligotrophic = low nutrients
o Less preservation in cold water so not found at poles
• Siliceaous
o More eutrophic because calcareous sediment wont form here
o Equator – upwelling = high conditions – no calcareous – therefore siliceous
o Tropics – good conditions for calcareous but is actually siliceous – same reason
o Poles – cold water so little preservation of calcareous
• Deep sea sediments
o Deep sea clays
o Too deep for calcareous – below CCD
o No nutrients for siliceous
o Fine material from sky
 Very slow deposition
 Pelagic rain
o Very slow deposition in absence of any other preservation

Thickness:
• More thick near continents
o Driven by rivers
• Thin in deep oceans – red clay blankets

Question 5

Pelagic ooze components:

Answer 5

• Oozes contain 30% or greater biogenic material.
• Biological productivty falls off with distance from land.
• Terrigineous influx falls off faster than biological productivity though
Calcareous:

Foraminifera (carbonate)
•	Forams
o	Single celled organisms that grow to cm lengths
o	Multi-chambered skeletons (shells)
•	Cocoliths
o	Too small to see
o	Chalk
o	Main component of deep-sea sediments

Siliceous:
•	Radiolaria
o	Smaller multichambered skeletons
•	Diatoms
o	Like coccoliths but siliceous

Question 6

Nutrient availability as a control:

Answer 6

• More nutrients = more siliceous ooze
• No biogenic production + too deep for calcareous on bottom left = just deep-sea clays
• Cold water = more nutrients

Question 7

CCD:

Answer 7

Current CCD is quite deep because is quite warm
Colder + acidic = shallower
More continental material = increased acidity = shallower = CCD – Atlantic

Question 8

Red Clay:

Answer 8

• Sedimentation when other sediments are not being preserved
• Very fine grained.
• Insoluble components.
• Volcanic debris.
• Hydrothermal minerals.
• Covers approx. 38% of the sea floor.

Question 9

Rhythms in pelagic sediments:

Answer 9

• Processes are fairly continuous – the duration of them are not which allows beds to form
• Productivity = sandy material dominated
• Dilution dominant = mostly background clays

Question 10

Glacial Dropstones:

Answer 10

• Mark temperature isoclines within the ocean, so often aligned with the boundaries between silaceous and carbonate oozes.
• Use cross-cutting relationships to date stones.
• Use deformation to determine lithification state of underlying sediment.
• Difference to rule
• Only defomation around stone, not on bottom which will show its not been dragged up

Question 11

Heinrich events/Ice-rafted debris

Answer 11

• Sudden warming
o Causes smaller dropstones
o Large melting of glaciers
o Well-defined pebbles which won’t be rounded off

Question 12

Deep Sea Sediment type breakdown

Answer 12

+ Terrigineous - river influx and Aeolian dust - quartz sand and siliclastic mud - continental margins and abyssal plains
+ Biogenic - Marine organism skeletons - calcareous/ siliceous ooze - deep sea flood mediated by CCD
+ Hydrogenous (authigenic) - bacterial mediated precipitation of dissolved minerals - manganese nodules/ phosphorite deposits - mid ocean ridges
+ Cosmogenous - extraterrestrial dusts - Tekite sphere - global distribution

Question 13

Stokes Law

Answer 13

V =gd^2 (σ-ρ)/18μ

V is velocity in m/s
▪ g is acceleration due to gravity (9.81 m/s2 on the Earth)
▪ d is the diameter of the falling particle given in m
▪ σ is the density of the falling particle in kg/m3
▪ ρ is the density of the fluid the particle is falling through in kg/m3 ▪ μ is the viscosity of the fluid the particle is falling through in Pa s

Question 14

Seds ref

Answer 14

Ocean floor above the CCD = biogenic calcareous oozes dominate - cocolithophore
Ocean floor below CCD = red clay
Leader 2013