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BENTHIC ECOLOGY > 1 SEDIMENTS AS HABITATS > Flashcards

1 SEDIMENTS AS HABITATS Flashcards

1
Q

What are sediment rates in the deep oceans, continental margins and bays/deltas?

A
  1. 0.5-1cm per 1000 years
  2. 10-50cm per 1000 years
  3. > 500cm per 1000 years
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2
Q

What is the difference in thickness of sediment in mid-oceanic ridges vs continental ridges?

A

MO ridges = very little
CR = up to 10,000m

Average is around 500m

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3
Q

Why is thickness in the Atlantic twice that of the pacific?

A

Major rivers flowing into atlantic extend over more land and carry a much greater sediment load than the equivalent rivers of pacific

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4
Q

What is the most important factor in determining sediment thickness?

A

Time

Sediment thickness = sedimentation rate x time for accumulation

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5
Q

What is sediment?

A

Matter composed of particles which by gravitation fall to the bottom of a liquid. (simplistic version)

Mixture of inorganic and org particles, dissolved interstitial gas & fluid phases, an active biota & the mineral solid phase.

Dynamic 4D systems that are compositionally complex, act as biogeochem bioreactors & form habitat for microbial & invertebrate org’s.

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6
Q

What are the 5 phases of sediment systems?

A
  1. mineral phase: the mineral or solid phase of the deposit (sediment particles)
  2. vital phase: the organisms which inhabit the sediment environment, both within and on top of the deposit
  3. non-living organic phase: products of secretion, such as mucous and detritus. Far more subtle remnants incl polymeric material, enzymes, & cellular remains
  4. free aqueous phase: seawater, which may vary in salinity and dissolved oxygen, will carry range of dissolved nutrients, metabolites and other constituents
  5. gas phase: gas bubbles, consisting of air if intertidal, otherwise microbial process by-products like CO2 and methane

Also: air water interface. In intertidal areas, the air-water interface may be below the sediment surface. Sediment fluids and gases can persist above the sediment water interface.

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7
Q

Sediments are derived from … … sources, and are always a mixture of … components and … in composition and …

A

numerous different
different
heterogeneous

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8
Q

What are the 4 main types of sediment?

A

LITHOGENOUS: derived from rocks (75% of all sediments)

BIOGENOUS: remnants and fragments of organisms (shells, bones etc)

HYDROGENOUS (authigenic): inorganic precipitation

COSMOGENOUS: cosmic derived, extraterrestrial origin; silicate (like mantle); metal (like core); or both.

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9
Q

How is lithogenous sediment formed?

A

Erosion of continental rocks, broken down by physical and chem weathering.

Exposure to frost/heat cycles
Water/ice cycles
Biological activity

Nature of parent rock & climatic conditions determines intensity of this erosion

Also inputs like volcanic from land

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10
Q

What are the transport forms of eroded lithogenous sediment?

A
  1. wind driven transport (dust storms)

2. water driven transport (esp large discharges in asia) (rivers biggest flux, glaciers, sea ice, icebergs)

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11
Q

What are biogenous sediments?

A
  • bioclastic
  • remnants & fragments of shells and tests e.g. caco3, silicon, phosphate
  • few groups occur in sufficient abundance to form sediments
  • when at least 30% of sediment has biogenic origin, refer to it as biogenous sediment/ooze
  • e.g. coral reefs, antarctic shelf (sponges= siliceous ooze), tropical waters (less silica), open ocean (coccoliths, forams and diatoms = more calcareous and siliceous oozes)
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12
Q

What is maerl?

A

Biogenous sediment derived from algae

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13
Q

What are the major groups contributing to biogenous sediment?

A
  • spicules, other parts
  • coccoliths
  • forams
  • diatom
  • radilarian
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14
Q

Why are biogenic oozes potentially not found close to shore?

A

large input of terrigenous sediment to the continental margin overwhelms the biogenous component

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15
Q

What are hydrogenous sediments?

A
  • widely distributed but insignificant in terms of quantity
  • formation of solid material in the sea by inorganic reactions
  • e.g. evaporites
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16
Q

What is distribution of carbonate-rich and silica-rich deposits influenced by?

A

Water circulation patterns

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17
Q

What is the lysocline?

A

Point where seawater becomes undersaturated with calcium carbonate.
Below this depth, carbonate dissolves

18
Q

What is the CaCO3 compensation depth?

A

Where carbonate input from surface waters balanced by dissolution in corrosive deep waters.
varies betw ~3km in polar and 5km in tropical regions.
Warm surface waters more easily saturated, cooler deeper waters undersaturated.
High productivity regions, ccd might be below the lysocline.

19
Q

What is the standard way of measuring sediment particle size?

A
  • use graded sieves & quantify the vol of sediment retained on standard mesh sizes.
  • wet sieving most common
  • then split of continuum of particle sizes into diff divisions/grades to aid description e.g. Udden-Wentworth grade scale, Krumbein phi scale
20
Q

What are the major size categories of sediment?

A

Gravel - boulders; cobbles; pebbles
Sand
Mud - silt; clay

21
Q

What are the descriptors for distribution of particle size data?

A
  • form
  • sphericity
  • permeability
  • colour
  • texture
22
Q

How to calculate the particle size?

A
  1. convert returns of particle analysis into mm.
  2. convert the values in mm to phi; phi = -log2 D where D = particle size in mm
  3. use Folks equations to calc mean, Md, Mz, IGSD and IGS
23
Q

What is sediment deposition? What does it help to explain?

A

Settling velocity (cm s-1)
Helps explain why sediment is found where it is
Large particulates settle quickly, small particles settle slowly
Particle shape, conc, density & efficiency of dispersion all influential

24
Q

Large particles require … energy to move; they behave … because need a lot of … to move them.

A

more
independently
inertia

25
Q

Below … microns, particles do not behave independently

A
  1. Cohesion occurs.
26
Q

What are non cohesive sediments (examples)

A

Sands
Not changed by erosion
Large inertial forces transporting particles

27
Q

What are cohesive sediments (examples)?

A

Muds/clays
Cohesive forces important, not inertial
Depends on mineralogy, pore fluid salinity, microbiology, time & history

28
Q

Will sand particles at low velocities transport, erode or deposit?

A

Deposit; they’re heavier, denser, fall out of solution

29
Q

What is the Hjulstrom curve?

A

Shows the relationship between size of sediment and velocity required to erode, transport or deposit it.
Shows why sediment is where it is in estuary.
But precise shape of diagram depends on particle size and shape; drag vs lift forces; particle density (many particles decrease turbulence); current speed faster = more energy = more resuspension and transport of sediment

30
Q

What are biostabilisers?

A

Microphytobenthos, algae, diatoms

Exude mucous that binds particles together and protects the bed

31
Q

What is flocculation?

A

Condition where small charged particles become attached and form a fragile structure, a floc

Interplay of 2 opposing forces
Van der Waals forces - attractive
Electrostatic - repulsion

Shape of particle influences distribution of particles
Spheres: Va proportional d-7 = broken bonds
Plates: Va proportional d-3 = unbalanced ionic substitution

32
Q

What is bedload?

A

Big particles being transported along sediment

Enough inertia to move them along but not enough to lift them into water column and become suspended

33
Q

What is saltation?

A

Enough inertia to pick particle up and throw it into water column
They’re too large to remain in water column through suspension processes so v quickly falls back down into bed

34
Q

What is the force of water movement affected by? (2 things)

A
  • drag (rel to cross sectional area exposed to flow in particle, increases w velocity)
  • friction (= shear stress) (when force on bed is tangential, shear proportional to square of the velocity of water)
35
Q

What is the no slip condition?

A

As water passes over surface, water molecules adjacent to surface do not move.

Slows down next pocket/layer of water above it, but they can still move

36
Q

What is each progression of a discrete travelling water pocket called?

A

Lamella

37
Q

What is the benthic boundary layer?

A

When 99% of the free stream velocity is attained, the height at which that is.

38
Q

Where is laminar flow restricted to?

A

Only at the leading edge, where water comes in contact w the surface (with a smooth boundary)

39
Q

What is the very small layer that remains laminar because of the no slip condition?

A

Viscous sublayer

This is why you can get some small species living in extremely fast water; occupying stable viscous sublayer

40
Q

When does a viscous sub layer exist? What sort of beds have no viscous sublayer?

A

When D > t/3, smooth flow begins to break down
When D > 7t, smooth flow destroyed

D = particle size diameter
t = thickness of viscous sublayer

There is no viscous sublayer over gravel or rippled beds.

41
Q

What influences does the sedimentary matrix exert onto organisms (e.g. tubeworms)?

A
  • Transport processes (BBL, SVL, flow; deposition/resuspension/stability)
  • Pore space (meiofauna; macrofauna; permeability; disturbance)
  • Vertical transitions (oxygen; redox; sulphide; org matter)
  • Particle size (microbes; chemistry)
  • Protection (predators; variable environment)

BENTHIC ECOLOGY (10 decks)

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