sedimentation and stratigraphy

7/28/2019 Sedimentation and Stratigraphy

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GEOL342: Sedimentation and Stratigraphy

Spring 2011

Chronostratigraphy:

Numerical or absolute dating There are many methods, each with its own strengths

and limitations:

Varves

Dendrochronology

Thermoluminescence dating

Fission track dating

Cosmogenic nuclide dating Amino acid racemization

Tephrachronology

Astronomical dating/Milankovitch cycles

Radiometric dating

Radiometric dating



Antoine Becquerel (1852-1908): Discovered natural radioactivity (1896). In the

following years, a large number of radioactive isotopes and their daughter products

became known.

Pierre (1859-1906) and Marie (1867-1934) Curie: Discovered that the radioactive

element radium continuously releases newly generated heat - radiogenic heat. With

this discovery, it became clear that the decay of radioactive substances provided a

continuous source of new heat that Thomson hadn't accounted for. The Earth might,

indeed, be much older than his calculations indicated. But how old?

History:

At the beginning of the 20th century, Ernest Rutherford and Frederick

Soddy developed the concept of the half-life - For any radioactive substance,

there is a specific period of time in which half of a sample will decay to adaughter substance. E.G., if we have a newly created 1 kg. sample of a

substance whose half-life is 10 years, then ten years from its creation, half of

the radioactive material will remain in the sample. The other half will be the

daughter product. After twenty years, 0.25 kg. will remain (with the rest being

daughter product), and after thirty years, 0.125 kg. of the original radioactive

substance will remain in the sample.

In 1904, Rutherford made the first attempt to use this principle to estimate the

age of a rock. His analysis was technically problematic because of his choice of

a gas, helium as a radioactive product (gasses have a way of migrating out of

rocks), but it was a start.

http://www.nobel.se/physics/laureates/1903/becquerel-bio.html


http://www.nobel.se/physics/articles/curie/






http://www.rutherford.org.nz/



http://en.wikipedia.org/wiki/Frederick_Soddy











In 1905, Bertram Boltwood noted a specific parent-daughter relationship

between an isotope of uranium, 235U, a radioactive isotope, and lead (Pb)

suggesting that one decayed into the other - the uranium-lead system. Because

lead is usually found as a solid, this method was more promising. Like

Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks

was technically flawed but a step forward. Beginning in 1911, Arthur Holmes began a long career of applying the

concept of radiometric dating to rocks, and is given credit for ironing out the

technical issues that hampered earlier attempts.

After a century of applying the method we now know that thet oldest known

Earth rocks are aprox 4.2 billion years old (abbreviated "ga"). The oldest in the

Solar System are 4.56 ga.

The current understanding:

Radioactive decay - unstable parent atoms change into more stable daughter atoms.

This involves one of the following transformations:

Loss of neutron(s)

Loss of proton(s)

Loss of alpha (α) particles (= 2 neutrons, 2 protons, i.e. He atom)

Loss of beta (β) particle - 1 neutron and 1 electron

Electron capture - 1 electron joins with a proton to form a neutron (i.e. gamma

particle γ)

Decay constant (λ) - The probability that a given nucleus will decay at a given time.

This is unique to each element. If one assumes that the parent:daughter ratio present in

a crystal is determined only by the elapsed time since the parent and daughter were

locked into the crystal and neither have escaped

N = N0e-λt

Where:

N = # radioactive nuclei present

t = time elapsed

N0 = # radioactive nuclei present at t=0

http://www.pbs.org/wgbh/aso/databank/entries/do07ra.html



http://www.pbs.org/wgbh/aso/databank/entries/boholm.html



http://news.bbc.co.uk/2/hi/science/nature/7639024.stm








Half-life (t1/2) - increment of time

needed for half the parent atoms to decay to daughters

t1/2 = 0.693/λ

t = (1/λ) ln(d/p +1)

where t is the age of the rock/mineral

Caveats:

Radiometric dating records the closure time when a crystal cooled to solid state

and locked radiogenic elements into its structure.



Most dating is done on igneous and metamorphic rocks. Sediments are remnants of

other rocks - radiometric ages obtained from sedimentary rocks are often the age of

the protolith, not the sedimentary rock. Thus, for sediments, we typically rely on

igneous marker beds that constrain the ages of adjacent sediments.

Some parent or daughter atoms can escape if the system is not fully closed. (This is

why we don't continue to use Rutherford's He system.)

Potassium (40K) → Argon (40Ar) by electron capture and γ decay.

t1/2 = 1.3 billion years

Feldspars, micas, ashes

Benefits: K is extremely common

Limitations: Ar is an inert gas and diffuses out of minerals. Ar is common in

atmosphere and must be accounted for. It is sensitive to metamorphic resetting

and weathering (allows Ar to escape).

Uranium (238

U) → Lead (206

Pb) by series of α and β decays.

t1/2 = 4.5 billion years

Zircon, monazite, badellyite, apatite

Benefits: Zircons are very stable and withstand weathering, can be sedimentary

minerals. Thus we can date things from Earth's earliest times.

Limitations: detrital zircon records mineral formation age, not sedimentary rock

age.

Uranium (234

U) → Thorium (230

Th) by α decay.

t1/2 = 250 kyr

Carbonates

Benefits: Abundances can be used to measure sedimentation rates (U

preferentially stays in solution)

Limitations: 230Th decays rapidly to 232Th. Because the decay pathway of U is

so complex multiple isotopes have to be taken into account.

Several other systems are useful for dating igneous and metamorphic rocks, including:

87Rb→87Sr

147Sm→143 Nd

176Lu→176Hf

various U/Pb/Th systems



Several other methods operate along the same general principles as rediometric

dating. Suppose, for example, we want to the age of a sample since it was last heated.

14C Dating

This method is not used on minerals. Rather, it exploits the fractionation of

radioactive 14C and stable 12C by plants during photosynthesis.14C is produced in the

upper atmosphere by bombardment of 14 N by cosmogenic neutrons and incorporated

in plant tissue in a fixed ratio to 12C. This fractionation is conserved across green

plants and tells us the initial ratio of these isotopes when the plant was growing.Because of its short half-life, 14C dating is useful only as far back as 40,000 yrs.

Note:

Well calibrated samples show that the rate of 14C generation has varied slightly

over time, thus, 14Cdates must be adjusted to take these variations into account.

The application to geochronology comes in when datable plant material is

found in association with sediments.



Thermoluminescence

The elements uranium and thorium in minerals, like zircon, quartz, and fluorite

(right) decay to produce alpha particles. These can get trapped in imperfections of the

crystal lattice in quantum-mechanically stable states. If this goes on long enough,

ultimately the crystal can become saturated. Since this is a background process the

accumulation of alpha particles can be used to constrain the age of ambient minerals.

Heat will release the trapped particles' energy as light, producing luminescence, whichcan be quantified. If the minerals are pristine, one can expose them to heat and

measure their luminescence to get an age. Because heating releases the trapped

energy, it effectively "resets" a sample's thermoluminescence clock. Thus,

archeologists use the method on items like fired pottery.

Sedimentologists have used thermoluminsecence as a method for tracking

the migration rates of different sizes of beach and river sands.

Range: 5,000 - 300,000 yrs. Saturation of the crstal sets upper limit on recoverable

ages.

http://upload.wikimedia.org/wikipedia/commons/5/52/Keizars_lossduringmigration.jpg






Fission track

dating

α particles resulting from the decay of 238U make tracks (holes) in crystals as they

escape (10-20μm long). These tracks can be thought of as if they were the "daughter

products" of radioactive decay and can be used for dating provided:

They can be counted

The concentration of 238U in the sample is known.

So:

Fission tracks are physically revealed by chemical etching, then counted under

a microscope.

The concentration of When the concentration of 238U is determined by placing

the specimen in a nuclear reactor along with a calibrated standard material and

bombarding it with neutrons, inducing the formation of new fission tracks. The

ratio of the concentration of 238U to the concentration of fission tracks provides

an estimate of the mineral's age.

Fission tracks close when the crystal is heated to modest temperatures (74-200ºC

depending on the mineral). Thus, fission track dating provides the sample's age since

its last episode of heating, when the crystal experienced closure - the immobilization

of its crystal lattice.



This spiraling, or chirality, requires active maintenance on the part of the

organism. When it dies, changes through molecular kinetics cause amino acids

randomly to switch to a right-handed configuration.

In principle, one could measure the chirality of amino acids in a biological

sample (mollusk shell, vertebrate bone, wood, etc.) to determine its age.

Alas, the rate of racemization is very sensitive to:o temperature

o pH

o humidity

o characteristics of the enclosing matrix

but if these can be constrained, (and this is possible in very predictable

environments like deserts and deep oceans) the smaple's age can be determined

by calculating a ratio between left- and right-handed amino acids.

Typically, for a given site and geologic interval, amino-acid dates must becalibrated using more reliable indicators like 14C.

This technique's advantage is that it can be used on a wider range of biologic

samples than 14C (wood only) so is good for dating shells and uplift terraces,

although the uncertainties are often large.

Range: 10,000 - 100,000 yrs.

Tephrachronology



Here we apply the concepts of chemostratigraphy to datable material.

Ash layers represent a single geologically instantaneous event that can

be correlated with datable deposits of crystalline igneous rock , even when

the ash, itself, contains no crystals amenable to radiometric dating.

Ash often spreads globally if the eruption is large enough Each ash layer has a unique chemical signature:

o Trace element abundances

o stable isotope ratios, etc.

Range 0-2MA. Upper limit typically results from diagenetic alteration of the

ash. Eg. Bishop Tuff (0.78MA) and Mazama ash(6000 yrs)

Some methods involve simply counting seasonal units that display some kind of

identifiable secular variability in thickness or chemical composition.

Varves and ice cores

Lake sediments that record seasonal variations enabling years to be counted. Within

a basin, patterns of variation between seasons can be correlated.

http://2.bp.blogspot.com/_EQitk5rqTcs/S_f4ilc6TnI/AAAAAAAABgc/xIyiL840yto/s1600/Mazama-Ash-Snake-River.jpg






While physically different, ice cores can be employed similarly. The deep cores

from Greenland and Antarctic ice date back 180,000 yrs.

Dendrochronology

Here, the seasonal units in question are layers of wood laid down in growing trees.

Annual variability in tree ring widths has been used to create a global record

that extends back to 8000 yrs.

Also provides info on temperature, runoff, precipitation, and soil moisture.

Astronomical dating

Milankovitch cycles: In the 1920s, the Yugoslavian meteorologist Milutin

Milankovitch realized The Earth's movement through space is subject to three kinds

of cycles:

http://www.sciencedaily.com/releases/2008/06/080619142112.htm



http://i392.photobucket.com/albums/pp6/SerbianCulture/milankovitch_portrait.jpg









Orbital eccentricity: The orbit around the Sun is an ellipse that changes shape

(becoming more and less circular) in a cycle of 100,000 years.



Axial inclination: The axis of rotation is tilted. The angle of tilt varies from

21.5 deg. to 24.5 deg. in a cycle of 41,000.

Axial precession: The axis of rotation wobbles around an axis like that of a toy

top. So, today the axis points toward Polaris, the north star, but in earlier times,

it didn't. One full precessional wobble takes 23,000 years.

Solar forcing: The

sum of the effects of these cycles gives the general tendency for glaciers to



form. Note: Solar forcings are different at different latitudes and in different

hemispheres.

Solar forcings can stimulate positive feedback processes that result in global

climate changes, tipping climate systems into glacial and interglacial modes. These

cycles can be seen in records of ice and sediments.

Unique insolation character associated with a given period of time Milankovitch

cycles control ice cover, eustatic sea level, and accommodation space.

Weathering rates, sediment supply, ocean circulation, and sediment accumulation

change in response to these cycles.

Milankovitch cycles can be tracked in carbonates, deep sea, and lake sedimentary

packages.

Range as far back as 10MA

Chronostratigraphy: The web of correlation

Establishing the time relationships among geologic units by means of integrated

methods including:

Irreversible processes that operate continuously in one direction:

biostratigraphy and geochronology

Cyclic processes (pattern recognition and placement):

o lithostratigraphy

o sequence stratigraphyo chemostratigraphy

o seismic stratigraphy

o magnetostratigraphy

Considerations and caveats:

Precision - repeatability of measurements. We can assess this more easily than

we can accuracy - the degree to which they approach the unattainable ideal of

"truth."

Difficult to obtain a numeric value for some methods, like biostratigraphy.Avoidance of circularity. E.G.:

o The primitive Triassic Ichthyosaur Thaisaurus Mazin et. al 1991 came

from poorly constrained sediments of Thailand that were assumed to be

of Early Triassic age because of the presence of a primitive ichthyosaur.

o A biostratigrapher 1995 employs Thaisaurus as an index taxon for the

Olenekian stage (Early Triassic) based on the above presumption.



Resolution - ability to discriminate between two closely spaced events in

geologic time. Radiometric methods lose resolution with increasing age

because of increasing margins of error. Magnetostratigraphic methods don't.

Can vary within a method (eg. Radiometric dating)

sedimentation and stratigraphy

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