5 sediments kd

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OceanographyGEO 9

Kirk Domke

Winter 2013

Lecture 5

Sediments:Records of Earth's past

Sediments are particles of rocks or organisms

that are weathered, transported, and deposited

by a fluid (air or water).

Sediment composition, particle size, and particlepattern reflect their source and depositional

conditions.

The age of sediments is determined by relative

and absolute dating.

Ocean sediments record Earth history

(uniformitarianism!)e.g. environmental events, climate change

Earth is dynamic :

constantly in a state of change!

The sedimentary record in theocean and of marine rocks

preserved on land contain evidenceof these changes.



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So, what are sediments?

Ocean sediments are the particles of rocks &organisms that cover much of the sea floor.

- e.g. sand, mud, sea shells, etc.

Not quite ...



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Sediments in the ocean can be thin,

smooth & covered by tracks, or rippled by currents

or thick,

Sources of ocean sediment are:

A. Materials derived from land (terrigenous)

B. Materials produced by organisms (biogenous)

C. Minerals precipitated directly from seawater

(hydrogenous or authigenic)

D. Particles from space (cosmogenous)

(E. Anthropogenic)

A. Terrigenous sediments are the most aboundant marinesediments by volume (>87% volume, 45% of area).

Rivers are the primary source of terrigenoussediments



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Terrigenous sediments also comefrom windblown dust, volcanic ashor lava, and local erosion.

B. Biogenous sediments arethe most abundant marinesediments by area (55% of area).

Come from:- sea shells- skeletons

- microscopic algae 5 cm

0.0002 cm0.02 cm

Most biogenous sediment is producedby microscopic algae that drift near thesurface until death, when their shells

sink.

Oozes are a type of biogenous sediment (>30% biologicalmaterial) that form from accumulations of plankton “tests”(microscopic shells).

Silica (SiO2) Siliceous Ooze

Calcium carbonate (CaCO3) Calcareous ooze

Coccolithophores& Foraminifera

Diatoms &

Radiolarians



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Calcareous ooze is not found everywhere... Carbonate Compensation Depth (CCD)

As we'll see: colder water and water at higher pressures candissolve more CO2, making the water slightly acidic.

Below the CCD, (avg. 4500 m) calcium carbonatedissolves!

Does that mean we can use the presence of carbonate/calcareous ooze to estimate water depth?

What is the average depth of the ocean?What type of sediment is likely dominant over much of the ocean?

At high latitudes (polar regions) not only is CaCO3 more likely todissolve, but diatom productivity is higher.

C. Hydrogenoussediments are formed in place –

precipitated directly from seawater *

(*Technically, this does include biogenous sediments, but manganese nodules,

phosphorites, and evaporite deposits are the main examples.)

Precipitation: chemical process in which ions in solution combine

and produce a solid.For example: Ca2+ + CO3

2- → CaCO3

or: Na+ + Cl- → NaCl



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Evaporite deposits form when seawater dries up

First calcium carbonate(limestone)

Then calcium sulfate

(gypsum/wallboard)

Finally potassium chloride,sodium chloride (table salt).

Layers of evaporites in therock record indicate hot,dry climates.

D. Cosmogenousparticles constantly rain down from

space but are a very small proportion of marinesediment (<<1%).

These are mostly interplanetary dust andmicrometeorites.

Some are

microtectites, rare,glassy particlesformed during ameteor impact.

Generally raindrop-

shaped, <1.5 mmlong.

Spherical microtectites provide some of the evidencefor a major meteor impact at the end of theCretaceous period, coincident with the extinction of

the dinosaurs.

The distribution of marine sediment types reflects physical transport , biological activity, and water chemistry

(sometimes all at once!)



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Continental shelf (neritic)

sediments: mostly terrigenous

(sand, silt, clay)



(sand, silt, clay)

Open ocean (pelagic)

sediments: terrigenous clays,

biogenous oozes



(sand, silt, clay)

Some shelf sediments make iteven to the continental slope

Open ocean (pelagic)

sediments: terrigenous clays,

biogenous oozes

The nature of the sediments we find

on the seafloor are determined by:

1. Distance from sediment source

2. Energy of deposition location

3. Source material



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Distance from sediment sourceWhich is easier to move: a boulder or a grain of sand?

Usually, only the smaller particles travel the furthest.

It takes high energy to movelarge rocks.

Big chunks tend to break upduring the trip.

Distance from sediment source

Distance from sediment source Since particle size tells us so much about depositional

environment, it is one of the ways we classify sediments:

Which of these make it to the ocean?



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3. Source material

Once in the ocean, sediments are

transported by currents.

Near-shore sediment transport is mostlyaccomplished by longshore drift.

Longshore drift occurs

when waves strike the

coast at an angle.

Sediments may be transported to deeper water by turbiditycurrents, gravity-driven flows of suspended sediment & water.

Turbidity currents can travel many miles downslope and providemuch of the fine terrigenous clay that reaches the deep basins



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Transport in the deep ocean is much less, but some deepcurrents may still move sediments.

Unlike near-shore environments, the energy regime of deep sea currents stays fairly constant over relatively longtime scales

- The strength & direction of deep ocean currents are

primarily a function of global processes such as climate

We're beginning to see how to read the rock record…

We're beginning to see how to read the rock record…

...but in order to begin understanding the

information preserved in rocks we also need to

know how old they are.

There are two general ways to date a rock:

1. Its age relative to other rocks

2. Its absolute age in years

Relative dating is qualitative: “younger” vs “older”

mother

father

oldest son

youngest son daughter



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... the Principle of Superposition

- in a normal setting, the material on the bottomwas deposited first and the layers of material ontop get successively younger

… and the Principle of Cross-Cutting Relationships

Relative dating relies on:

Principle of Original Horizontality

- gravity requires that material is deposited ingenerally horizontal layers.

- material that cuts across other material must be younger than the other material

A

B

C

D

Absolute dating is quantitative: “my dog is 2 yrs old”

Absolute ages for marine rocks are usually derived fromradiometric dating performed on volcanic ashes.

Radiometric dating cannot be performed on sedimentparticles themselves. To explain why, let's look at howradiometric dating is done...

Sometimes sediments have annual layers that can becounted like tree rings – but usually only in particular environments (e.g. glacial lakes)



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Radioactive elements are unstable and decay:

The half-life is the amount of time for ½ of the radioactive parent

element to decay into its daughter product

Parent Isotope Daughter isotope Half-life

Uranium 238 → Lead 206 4.5 Gyr

Potassium 40 → Argon 40 8.4 Gyr

Carbon 14 → Nitrogen 14 5,700 yr

Because of the different half-lives, different Parent/Daughter pairs are better for some measurements than others.

14Carbon dating (radiocarbon) is good for “recent” sediments since its half -life is 5,700 yrs

Very ancient ages, like the age of the Earth, aredetermined using the longer lived pairs, like U-Pb, or K-Ar.

So, why can't we do radiometric dating on the sediments directly?

What are sediments made of?

Particles of other rocks!

If you date a (terrigenous) sediment you'll get the age of therock that was eroded to produce the sediment!

Fortunately, volcanic ash beds – which can be datedradiometrically – are sometimes preserved in marinesediments.

So, we've seen how different sediments reflect

their source and depositional environment .

UNIFORMITARIANSM

Now we can use the principle that processes todayoperate in the same manner as in the distant past:

/ /



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For example: Turbidite deposits- similar particle size- similar layering

- similar composition

This does not prove* that the rocks record turbidity currents;it only allows us to conclude that it is the most reasonableexplanation.

*Things are only disproved in science

Marine records: Heinrich Events, Glacier Retreat and OceanTemp.

Reef deposits in Eastern Canada Evaporite deposits and the Messinian Salinity Crisis (when theMediterranean almost dried up)

5 sediments kd

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