1 SEDIMENTS AS HABITATS Flashcards
What are sediment rates in the deep oceans, continental margins and bays/deltas?
- 0.5-1cm per 1000 years
- 10-50cm per 1000 years
- > 500cm per 1000 years
What is the difference in thickness of sediment in mid-oceanic ridges vs continental ridges?
MO ridges = very little
CR = up to 10,000m
Average is around 500m
Why is thickness in the Atlantic twice that of the pacific?
Major rivers flowing into atlantic extend over more land and carry a much greater sediment load than the equivalent rivers of pacific
What is the most important factor in determining sediment thickness?
Time
Sediment thickness = sedimentation rate x time for accumulation
What is sediment?
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.
What are the 5 phases of sediment systems?
- mineral phase: the mineral or solid phase of the deposit (sediment particles)
- vital phase: the organisms which inhabit the sediment environment, both within and on top of the deposit
- non-living organic phase: products of secretion, such as mucous and detritus. Far more subtle remnants incl polymeric material, enzymes, & cellular remains
- free aqueous phase: seawater, which may vary in salinity and dissolved oxygen, will carry range of dissolved nutrients, metabolites and other constituents
- 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.
Sediments are derived from … … sources, and are always a mixture of … components and … in composition and …
numerous different
different
heterogeneous
What are the 4 main types of sediment?
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.
How is lithogenous sediment formed?
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
What are the transport forms of eroded lithogenous sediment?
- wind driven transport (dust storms)
2. water driven transport (esp large discharges in asia) (rivers biggest flux, glaciers, sea ice, icebergs)
What are biogenous sediments?
- 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)
What is maerl?
Biogenous sediment derived from algae
What are the major groups contributing to biogenous sediment?
- spicules, other parts
- coccoliths
- forams
- diatom
- radilarian
Why are biogenic oozes potentially not found close to shore?
large input of terrigenous sediment to the continental margin overwhelms the biogenous component
What are hydrogenous sediments?
- widely distributed but insignificant in terms of quantity
- formation of solid material in the sea by inorganic reactions
- e.g. evaporites
What is distribution of carbonate-rich and silica-rich deposits influenced by?
Water circulation patterns
What is the lysocline?
Point where seawater becomes undersaturated with calcium carbonate.
Below this depth, carbonate dissolves
What is the CaCO3 compensation depth?
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.
What is the standard way of measuring sediment particle size?
- 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
What are the major size categories of sediment?
Gravel - boulders; cobbles; pebbles
Sand
Mud - silt; clay
What are the descriptors for distribution of particle size data?
- form
- sphericity
- permeability
- colour
- texture
How to calculate the particle size?
- convert returns of particle analysis into mm.
- convert the values in mm to phi; phi = -log2 D where D = particle size in mm
- use Folks equations to calc mean, Md, Mz, IGSD and IGS
What is sediment deposition? What does it help to explain?
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
Large particles require … energy to move; they behave … because need a lot of … to move them.
more
independently
inertia
