Marine Sedimentation

• Sediment Defined:

• unconsolidated organic and inorganic particles that accumulate on the ocean floor

• originate from numerous sources

– weathering and

erosion of the continents

– volcanic eruptions

– biological activity

– chemical processes

within the oceanic crust and seawater

– impacts of extra-terrestrial objects

• classified by size according to the Wentworth scale

• grain size indicates condition under which sediment is deposited

– high energy environments characteristically yield sediments larger

in size

– small particles (silts, clays) indicate low energy environments

• considered well-sorted if most particles appear in the

same size classification

• poorly sorted sediments comprised of multiple sizes

• sediment maturity is indicated by several factors

– decreased silt and clay content

– increased sorting

– increased rounding of grains, as a result of weathering and abrasion

• particle transport is controlled by grain size and velocity

of transporting medium

•

• Average grain size reflects

the energy of

the depositional

environment.

• Hjulstrom’s

Diagram graphs the

relationship

between particle size

and energy for erosion,

transportation

and deposition.

4-1 Sediment in the Sea

Classification of marine sediments can be based upon size or origin.

• Size classification divides sediment by grain size into gravel, sand and clay.

– Mud is a mixture of silt

and clay.

• Origin classification divides sediment into

five categories: Terrigenous sediments, Biogenic sediments, Authigenic sediments,

Volcanogenic sediments and Cosmogenic sediments.

4-1 Sediment in the Sea

• Terrigenous (or Lithogenous Sediments):

• derived from weathering of rocks at or above sea level (e.g., continents, islands)

• two distinct chemical compositions

– ferromagnesian, or iron-magnesium 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

• (LANDSAT images adapted from Geospace Images catalog).

• sediment delivered to the open-ocean by wind activity as particulate

matter (dust)

• primary dust source is

deserts in Asia and North Africa

• comprise much of the fine-grained deposits in remote open-ocean areas (red clays)

• volcanic eruptions contribute ash to the atmosphere which

settles within the oceans

• sediment also transported to the open-ocean by gravity-driven turbidity currents

• dense 'slurries' of suspended sediment moved as turbulent underflows

• typically initiated by storm activity or earthquakes

– first identified during 1929 Grand Banks earthquake

– seismic activity triggered turbidity current which severed telegraph lines

• initial flow often confined to submarine canyons of the continental shelf and slope

• form deep-sea fans where the mouth of the canyon opens onto the continental rise

20 m s-1

near Grand

Banks

• 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

• Biogenous Sediments:

• composed primarily of marine microfossil remains

• shells of one-celled plants and animals, skeletal fragments

• 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

– distribution controlled largely by dissolution processes

– cold, deep waters are undersaturated with respect to CaCO3

– deep water is slightly acidic as a result of elevated CO2 concentrations

– solubility of CaCO3 also increases in colder water and at greater pressures

– CaCO3 therefore readily dissolved at depth

• level below which no CaCO3 is preserved is the 'carbonate compensation depth'

• typically occurs at a depth of 3000 to 4000 m

•

Microfossils in

Paleoclimatology/

Paleoceanography

• Dissolution Calcium carbonate dissolves better in colder water, in acidic

water, and at higher pressures. In the deep ocean, all three of these conditions exist. Therefore, the

dissolution rate of calcium carbonate increases greatly below the thermocline. 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 depth below which all calcium carbonate is dissolved is called the carbonate

compensation depth or CCD.

• Hydrogenous (or Authigenic) Sediments:

• produced by chemical processes in seawater

• essentially solid chemical precipitates of several common

forms

• non-biogenous carbonates

– form in surface waters supersaturated with calcium carbonate

– common forms include short aragonite crystals and oolites

• phosphorites

– phosphate crusts (containing greater than 30% P2O5) occurring as nodules

– formed as large quantities of organic phosphorous settle to the

ocean floor

– unoxidized material is transformed to phosphorite deposits

– found on continental shelf and upper slope in regions of high productivity

• manganese

nodules

– surficial

deposits of manganese,

iron, copper,

cobalt, and

nickel

– accumulate

only in areas

of low sedimentation

rate (e.g., the

Pacific)

– develop

extremely

slowly (1 to 10

mm/million years)

•

• The term evaporites is used for all deposits, such

as salt deposits, mainly

chemical sediments that are composed of minerals

that precipitated from saline solutions

concentrated by

evaporation. Evaporite deposits are composed

dominantly of varying proportions of halite (rock

salt) (NaCl), anhydrite

(CaSo4) and gypsum (CaSo4.2H2O). Evaporites

may be classified as chlorides, sulfates or

carbonates on the basis of

their chemical composition (Tucker, 1991).

evaporites ('salt'

deposits')

occur in regions

of enhanced

evaporation

(e.g., marginal

seas)

evaporative

process removes

water and leaves

a salty brine

e.g.,

Mediterranean

'Salinity Crisis'

between 5 and 6

million years

ago

• Cosmogenous Sediments:

• sediments derived from extraterrestrial materials

• includes micrometeorites

and tektites

• tektites result from

collisions with extraterrestrial materials

– fragments of earth's

crust melt and spray

outward from impact

crater

– crustal material re-

melts as it falls back

through the atmosphere

– forms 'glassy' tektites

• Distribution of Marine Sediments:

• sediments thickest along continental margins, thin at mid-ocean 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

• high productivity in zones of upwelling and nutrient-rich high latitude waters

• calcareous oozes more common in warmer or

shallower water

• siliceous oozes more

common in colder or deeper water

• terrigenous sedimentation rates range from ~1 mm to 10's cm/1000 years

• biogenous sedimentation rates typically ~1 cm/1000

years

Nearshore sediments, turbidites:Up to

km/my (kilometers/million years)

Hemipelagic deposits: Tens to hundreds

of m/myDrift deposits40-400 m/my

Mid-latitude eolian deposits: 3 to 10

m/my

Ice rafted material: 10+ m/my

Carbonate oozes: Up to 50 m/my

Siliceous oozes: Up to 10 m/my

Hydrothermal deposits: (off ridge

axes)About 0.5 m/my

Hydrogenous sediments: Rarely exceed

0.2 m/my

Ferromanganese nodules: 0.0002 to

0.005 m/my (0.2 to 5 mm/my)

Shelf sedimentation is strongly controlled by tides, waves and currents, but their influence decreases with depth.

• Shoreline turbulence prevents small particles from settling and transports them seaward where they

are deposited in deeper water.

• Particle size decreases seaward for recent sediments.

• Past fluctuations of sea level has stranded coarse sediment (relict sediment) across the shelf including

most areas where only fine sediments are deposited today.

4-2 Sedimentation in the Ocean

Geologic controls of continental shelf sedimentation must be considered in terms of a time frame.

• For a time frame up to 1000 years, waves, currents and tides control sedimentation.

• For a time frame up to 1,000,000 years, sea level lowered by glaciation controlled sedimentation and caused rivers to

deposit their sediments at the shelf edge and onto the

upper continental slope.

• For a time frame up to 100,000,000 years, plate tectonics

has determined the type of margin that developed and controlled sedimentation.

4-2 Sedimentation in the Ocean

60% of the

world’s shelves

are covered

with relict

sediments that

were formed

about 15,000 y

BP under a

different energy

regime.

• Gas Methane Hydrates

(Clathrates)

• Hydrates store immense

amounts of methane, with major implications for

energy resources and

climate, but the natural

controls on hydrates and

their impacts on the environment are very

poorly understood

• The worldwide amounts of carbon bound in gas

hydrates is conservatively

estimated to total twice the

amount of carbon to be

found in all known fossil fuels on Earth (USGS).

• Methane bound in hydrates

amounts to approximately 3,000 times the volume of

methane in the

atmosphere.