Transcript Sediments

Sediments
Sediments
• 4 Main Concepts to Discuss
– Sediments as historical records
– 2 dominating types of sediment
– Marine sediments on land
– Sediments ages
Sediment Cycle
• Over geological time, mountains rise as lithospheric
(crustal) plates collide, fuse, and subduct.
• Water and wind erode the mountains and transport
resulting sediment to the sea.
• The sediments are deposited on the seafloor, where
they travel with the plate and are either uplifted or
subducted. Thus, the material is made into
mountains again.
Sedimentary Environments: Continental Margins
Sedimentary conditions vary according to:
Tectonic Status:
Climate:
Active or passive
Glacial:
Scour & moraine
dumped sed.
Large River Flux: Large sediment
source & delivery
(river systems)
Low Sed. & Warm: Carbonates
Margin Comparison
• Active Margins
• Tend to be dominated by
erosional processes
• Erosional products are
often attached to the
overriding plate & may
include those from the
subduction ocean crust,
as well as terrigenous
sediments transported
by gravitational
movements down the
slope.
• Passive Margins
•
Tend to be dominated by
depositional processes
• Shelf surface sediments are
the result of present day
depositional processes (rivers),
but also reflect depositional
conditions of the recent lower
sea level (~15,000 years ago).
Known as relict sediments. At
that time in higher latitudes,
glaciers scoured shelves &
deposited sediments
(moraines). In lower latitudes
rivers flowed farther across
the shelves & soils formed on
the shelves.
Terrigenous Sediments are transported to the ocean
via:
1. Rivers in their
particulate
loads
Ions are also
transported to
the oceans via
rivers &
contribute to
the salt mass
balance.
Deep-sea sediments: sedimentation rates: 1-20 mm/1000 yr
Slope sediments: accumulate at rate: 100 mm/1000 yr
Terrigenous sediments are generally distributed
across the shelf based on their grain size
Grain Size: Gravel, sand, silt & clay
In general there is a gradation of grain sizes from source to
place of deposition (i.e bigger heavier grains remain nearer
source).
Clay Sized
Characteristic of
continental slope,
rise, and deep sea.
Found on shelf
where energy is
low (away from
waves & currents)
Silt Sized
Characteristic of
continental slope &
rise. Found on
shelf where energy
is low (away from
waves & currents)
Sand Sized
Characteristic of
beach & shelf
Marine sedimentologists describe sediment by its grain size,
shape, mineralogy, & sorting in order to interpret the source
area & transportation process.
Quartz:
Is the most resistant mineral
to abrasion & leaching so dominant
mineral in most sands.
Gulf of Mexico heavy
mineral provinces are
used to interpret
sediment distribution
processes and
transportation routes.
Where water is quiet (little energy) silt & clay settle to the
floor.
Silt-Sized: Characteristic of continental slope & rise. Found
on shelf where energy is low (away from waves and currents).
Clay-Sized – ubiquitous on shelves, slopes, & deep-sea
deposits.
• Indicates low energy environments when abundant
• Have high surface area (flat), so they readily absorb
substances & can also form aggregates.
• Clays are commonly associated with organic matter,
organics cling to the clay.
• Also deposited together in fecal pellets (matter).
Falls as Marine Snow
Larvacean fecal
pellets and
marine snow
from a sediment
trap sample in
the North Pacific
Ocean.
Terrigenous Sediments also transported to
the ocean via:
2. Glaciers: Glacial marine sediments – around Antarctica are
deposited in a belt about 300-1000 km wide.
During glacial times the ice shelf advances toward the
ocean; during warmer times, glaciers retreat & the
sediments are reworked by ocean processes (waves and
currents)
Terrigenous Sediments also transported to the
ocean via:
3. Wind: Dust falls into the ocean,
especially in large dust storms
 Sediments known as eolian
 Small component of sediment
delivered to the ocean: North
Pacific accumulation 1
mm/1000 years & Atlantic
accumulation rates ~2.5
mm/1000 years
 Accumulation probably vary
through time, depending on
changes in global climate
Significant amounts of dust are transported to
the Caribbean from Africa via dust storms
• N. Hemisphere winter greatest concentration of dust settles
over Caribbean and equatorial S. America
• N. Hemisphere summer the greatest concentration moves north
over Florida
• Changes caused by seasonally shifting ITCZ
Millions of tons of dust are transported from
Africa, across the Atlantic to N. America & the
Caribbean, via dust storms each year
Dust storms deposit
more than African soil
when they settle. Iron
& phosphate from the
soil is thought to
promote
phytoplankton
growth in the marine
environments of the
Caribbean & eastern
Atlantic. (Live insects
as well)
Millions of tons of dust are transported from
Africa, across the Atlantic to N. America & the
Caribbean, via dust storms each year
• Viable live microbes (bacteria, fungi & viruses) are bring found
in the dust. Estimates suggest there may be as many as 10
quadrillion microbes in the dust each year.
• Of the identified microbes 30% are known pathogens!
• Description: Microbes
isolated from African dust
aerosols grow on a filter
placed on agar medium. The
dark, fuzzy colonies are fungi
and the colored, shiny
colonies are bacteria.
Photographed December 13,
2001. Air sample collected in
Mali, Africa.
• USGS Publication
Terrigenous Sediments also transported to the
ocean via:
4. Volcanic Eruptions: Volcanic sediments, such as tephra
and ash-fall from air
•Used to correlate and absolute age
date
•Ash can be injected into the
stratosphere, >10-15 km, causing
tephra transport from 3000-6000 km.
Down slope transportation of Terrigenous
Sediments
Turbidity currents - mass movement of water
Sand & mud are dislodged from continental shelf &
slope by a disturbance, such as an earthquake or
landslide.
Turbidity Current
• The material is thrown into suspension, creating a mixture
that is denser than water.
• The dense mixture flows down the slope, eroding at the
flows head.
• The flow generally continues across the continental rise,
coming to rest as the slope decreases, causing flow to loose
momentum.
• Head moving past erodes pelagic mud
• Coarse sediment is deposited first, followed by successively
finer deposits
• Distinct deposits called turbidites are formed.
• They are characterized by graded bedding.
The bulk of terrigenous sediments are deposited on the
continental shelf, slope and rise. Thickness is a reflection of
supply (elevation coupled with water availability (climate) &
distance from land.
Beyond the Continental Shelf:
• Most sediments found in the deep ocean are deposited from the pelagic
environment (upper water column of the open ocean).
• Pelagic sediments are very fine grained (clays), the median grain size is
<5µm (1mm=1000µm) with <25% of the >5µm fraction being
terrigenous
• Sediments on older abyssal plains, are typically 1000 m thick.
Some pelagic sediments contain biogenic grains: grains that are
biological in origin. These include calcareous, siliceous, &
phosphatic biological hard parts (shells).
Small floating (planktonic) organisms live in the upper water
column & fall to the ocean floor after death. These biogenic grains
are especially common on the slope & deep-sea.
Biogenic oozes -
>50% by weight of the
grains are < 5µm & >30% of
the grains are composed of
skeletal remains (shells).
Biogenic ooze can be classified more specifically by
shell mineralogy & then by majority type of the organism
making the shells
1. Calcareous Ooze – microfossils
made of CaCO3 shells
Types of Calcareous (carbonate, CaCO3) Ooze
Foraminiferal ooze – dominated by tests of
planktonic foraminifera (forams-animal like
Protist, most 50-500 µm)
Nannofossil ooze – dominated by calcareous fossils
(plant-like Protist, 5-50µm)-More resistant to
dissolution thank planktonic forams
Pteropod Ooze – dominated by planktonic
gastropod molluscs with argonite shells
(animals) – Easily destroyed by dissolution, so
generally occur above 3000 m
2. Siliceous Ooze – microfossils made of SiO2 (silica)
opaline shells (amorphous, hydrated form) comprise >30% of
Sediment
Types of Siliceous Ooze
Diatom Ooze (plant like
protist, 5-50µm): >30%
diatoms, typical of high
latitudes & some cont.
margins
Radiolarian Ooze (animal
like protist, 40-150µm) >30%
radiolarians – typical of
equatorial divergence
(upwelling) areas of high
biologic fertility
How do such small objects sink to the deep
sea floor (1000s of m depth)?
• Tiny particles should sink very slowly or not at all due to turbulence within
the water.
• Particles of 5-10 nanometers (nm - 109), would require 100 years to sink to
ocean floor.
• Sinking rate is accelerated by fine particles combining into small
bundles (zooplankton fecal pellets, 50-250 nm size
• A single pellet may contain 105 coccoliths
• Sinking rate of pellets = 40 to 400 meters/day
Calcite Compensation Depth (CCD)
Depth in ocean that marks where rate of dissolution of carbonate
balances (=) rate of accumulation (supply)
-marked by a transition from carbonate ooze to red clay
CO2 in water makes carbonic acid which dissolves CaCO3
 CO2 is produced by respiration of organisms
 Increased pressure increases the solubility of gases in liquids, so
deeper water can hold more gas.
 CO2 remains close to source (respiration or where organic matter is
oxidized
CCD – mean depth of 4.5 km
About half distance between crests of mid ocean ridge &
deepest (non-trench) part of ocean
Pacific – CCD typically at shallower depths, 4200-4500 m
-Because water is older which causes it to collect more decaying matter and
respiration
N. Atlantic – CCD typically ≥ 5000 m
Red or brown clay may contain: Cosmogenic grains
Extraterrestrial Sources: Insignificant fraction of deep sea sediments are
micrometeorites called: Microtektites – small glassy bodies, 1 mm to 30
µm in diameter. Several different shapes: ovoid, teardrop, dumbbell –
usually yellow brown in color
Accumulation rates - .00002 mm/1000 yrs
Tecktites – 2-4 cm, found in strewn fields, high velocity impacts
Classification of sediment formed “in
place”: Authigenic
• Authigenic or Hydrogenous source – ions
solution in seawater from river input and
hydrothermal or volcanic input.
– Sediments from in-situ on the ocean floor through
inorganic precipitation
– Most form from slow precipitation of minerals
from seawater
More specific types of authigenic or hydrogenous sediments
1. Metal Rich Sediments & iron oxides – associated with
spreading centers (hydrothermal vents)
2. Manganese Nodules – rich in nickel, copper, cobalt, iron, &
traces of others
3. Evaporites – sediments which form from evaporation of
seawater
- Restriction from main body of the ocean is required to raise the
concentration of salts to the point of precipitation. Generally takes
place in an arid area or area of consistent wind.
How do we study ocean sediments?
• Tools
– Clamshell Sampler
• Takes shallow water samples
– Piston Corer
• Used to take deeper samples. Can punch through 25 m of sediment
– JOIDES Resolution drilling ship
• Returned cores over 1000 m
Why do we study ocean sediments?
• The sediments give us a glimpse into recent (≤ 180 106 yrs)
ocean history
• Stratigraphy – Analysis of layered sedimentary deposits,
ocean or land.
– Deep-Sea Stratigraphy – utilizes variations in rock composition,
microfossils, depositional patterns, geochemical and physical
characteristics to trace or correlate distinctive sedimentary layers
from place to place, establish the age of the deposits, and
interpret changes in ocean and atmospheric circulation,
productivity, and other aspects of past ocean behavior
– This has led to an emerging new field in oceanography –
PALEOCEANOGRAPHY!!!
– The study of the ocean’s past by interpreting data from cores and
by other means, (ice cores, rock outcrops, fossil records, etc.)
Conclusions
1. Sediments are loose accumulations of particulate
material. Their depth and composition tell us of relatively
recent events in the ocean basin above.
2. The most abundant sediments are terrigenous (land) and
biogenous (living organisms). The volume of terrigenous
sediment exceeds that of biogenous sediment, but
biogenous material covers a greater area of seabed.
3. Marine sediments have been uplifted and exposed on
land. Arizona’s Grand Canyon is made of marine
sediment.
4. Because marine sediments are usually subducted along
with the seabed onwhich they lie, the oldest sediments
are relatively young – rarely older than 180 million years.