Transcript ocean_11_lab_4

CHAPTER 4
Marine Sediment
Classification:
A. Shape, Size, Variation
B. Formation Processes:
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Lithogenic (rock)
Biogenic (organic based)
Authogenic/Hydrogenous
(precipitated from water)
Volcanic
Cosmogenic (outer space)
Sediment Transport
Fluid velocity
determines
the
size of the
particles that
can be moved
Sediment Texture
• Grain size sorting
– Indication of selectivity of transportation and
deposition processes
• Textural maturity
– Increasing maturity if
• Clay content decreases
• Sorting increases
• Non-quartz minerals decrease
• Grains are more rounded (abraded)
Sediments
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Reflect composition of rock from which derived
Coarser sediments closer to shore
Finer sediments farther from shore
Mainly mineral quartz (SiO2)
Terrigenous & Lithogenic sediments
(from land)
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Rivers
Winds (aeolian)
Glaciers (ice-rafted debris, IRD)
Turbidites
Sea level changes
Terrigenous Sediments:
• derived from weathering of rocks
at or above sea level (e.g.,
continents, islands)
• two distinct chemical
compositions
– ferromagnesian, or ironmagnesium bearing minerals
– non-ferromagnesian minerals
– e.g., quartz, feldspar, micas
• largest deposits on continental
margins (less than 40% reach
abyssal plains)
• transported by water, wind,
gravity, and ice
• transported as dissolved and
suspended loads in rivers, waves,
longshore currents
River sediment loads (~109 tons/yr)
Sediment Distribution
• Neritic
– Shallow-water deposits
– Close to land
– Dominantly lithogenous
– Typically deposited quickly
• Pelagic
– Deeper-water deposits
– Finer-grained sediments
– Deposited slowly
Neritic Lithogenous Sediments
• Beach deposits
– wave-deposited sand
• Continental shelf deposits
• Turbidite deposits
• Glacial deposits
– High latitude continental shelf
– Currently forming by ice rafting
Pelagic Deposits
• Fine-grained material
• Accumulates slowly on deep ocean floor
• Pelagic lithogenous sediment from
– Volcanic ash (volcanic eruptions)
– Wind-blown (aeolian) dust
– Fine-grained material transported by deep sea
currents
• Dust (LANDSAT image).
• Dust comprise much of
the fine-grained deposits
in remote open-ocean
areas (red clays)
• primary dust source is
deserts in Asia and North
Africa
Distribution of Sediment on
Continental Shelf by Grain Size
Seafloor Features:
Continental Margins
Submarine canyons
(cut into the c. slope)
Abyssal
plain
Continental shelf
Continental rise
Abyssal
plain
Continental
slope
Glacial (Ice-rafted debris)
• boulder to clay size
particles also eroded
and transported to
oceans via glacial ice
• glacier termination in
circum-polar oceans
results in calving and
iceberg formation
• as ice (or icebergs)
melt, entrained
material is deposited
on the ocean floor
• termed 'ice-rafted'
debris
Biogenic sediments
(from living things)
Forams
Calcareous (CaCO3)
Foraminifera -- animals
Coccolithophores -- plants
Siliceous (SiO2)
Radiolaria -- animals
Diatoms -- plants
Diatoms
Radiolarian
Biogenic Sediment
• Two major types:
– Macroscopic
• Visible to naked eye
• Shells, bones, teeth
– Microscopic
• Tiny shells or tests
• Biogenic ooze
• Mainly algae and protozoans
mm = micron = millionth of a meter!
mm = micron = millionth of a meter!
mm = micron = millionth of a meter!
mm = micron = millionth of a meter!
Biogenic Sediments:
• composed primarily of
marine microfossil remains
• median grain size typically
less than 0.005 mm (i.e.,
silt or clay size particles)
• characterized as CaCO3
(calcium carbonate) or SiO2
(silica) dominated systems
• sediment with biogenic
component less than 30%
termed calcareous,
siliceous clay
• calcareous or siliceous
'oozes' if biogenic
component greater than
30%
• siliceous oozes
(primarily diatom
oozes) cover ~15%
of the ocean floor
– distribution
mirrors regions
of high
productivity
– common at
high latitudes,
and zones of
upwelling
– radiolarian
oozes more
common in
equatorial
regions
• calcareous oozes
(foraminifera,
coccolithophores) cover
~50% of the ocean floor
• level below which no
CaCO3 is preserved is the
'carbonate compensation
depth‘ (CCD)
• This change in dissolution
rate is called the lysocline.
Below the lysocline, more
and more calcium
carbonate dissolves, until
eventually, there is none
left. The lysocline typically
occurs at a depth of 3000
to 4000 m
Sediment Accumulation
Calcium Carbonate Content in Pelagic
Oceanic Sediment
Rates of Deposition of Marine Sediment
Temporal Succession of Pelagic Sediment at Spreading Centers
Sediment Succession in Absence of
Siliceous Ooze
Cosmogenous Marine Sediments
• Macroscopic meteor
debris
• Microscopic ironnickel and silicate
spherules (small
globular masses)
– Tektites
– Space dust
• Overall, insignificant
proportion of marine
sediments
Marine Sediment Mixtures
• Usually mixture of different sediment types
• Typically one sediment type dominates in different
areas of the sea floor.
Distribution of Marine
Sediments:
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sediments thickest along
continental margins, thin at midocean ridges
coastlines
– dominated by river-borne and
wave reworked terrigenous
sediments
– shelf and slope characterized
by turbidites and authigenic
carbonate deposits
– glacial deposits and ice-rafted
debris common at high
latitudes
– high input of terrigenous
sediments 'dilutes' biogenous
components
deep-sea (pelagic) basins
– abyssal clays (wind blown
deposits) common
– lower quantities of biogenic
material
distribution of biogenous sediments
dependent upon three primary
factors
– production in surface waters
– dissolution in deep waters
– dilution by other sediments
types