Download 1-magnetostratigraphyppt

Paleomagnatism and
Dendrochronology
DatingTechniques
• What are magnetic fields?
Regions around magnetic material or a
moving electric charge within which the force of
magnetism acts.
Magnetostratigraphy: uses records of changes in polarity of the
geomagnetic field preserved in sedimentary sequences to correlate
between wells and to date the sediment.
(GPTS) geomagnetic polarity time scale
Paleomagnetism: is the study of the record of the Earth's
magnetic field in rocks, sediment, or archeological materials.
Curie point: the critical temperature of about 500°C - 600°C
(for magnetite) where iron-bearing minerals become magnetized in
alignment with the Earth’s magnetic field. When cooled further, the
magnetite molecules will retain this orientation unless subsequently
heated past Curie point. This semipermanent alignment is referred
to as thermal remnant magnetism.
Detrital remnant magnetism: When the preferred orientation of
magnetic minerals in sedimentary rocks imparts bulk magnetic
properties to the rocks.
How?
During deposition of sediments, small magnetic mineral grains are
able to rotate in the loose unconsolidated sediment of the
depositional surface and thus align themselves mechanically with
Earth’s magnetic field.
In a lab, rock samples are tested with
a magnetometer.
Black Stripes represent normal
polarity while the White Stripes
represent reversed polarity.
In order to tie the patterned measured
in an individual succession to be able
to estimate the polarity of the
stratigraphy it is essential to have a
tie point to the geologic time scale.
This may be provided by absolute
dating of the unit. Examples: Lava,
and biostratigraphy
Individual normal and reverse polarity intervals ("Chrons")
typically range from ~10 thousand to 10 million years in
duration.
One problem with this time of dating
is that microscopic iron particles in
some sediments undergo chemical
changes after they have settled
through the water column. This
causes the iron particle to realign
with the Earth’s magnetic field at the
time of the chemical change.
Global magnetic reversal patterns from
Spreading ridges.
Dedrochronology:
The word comes from
1. Dendros – using trees
2. Chronos – time
3. Ology – Study of
Lab Processes:
Two Processes
1.
The Fundamental Technique
- Match ring-growth samples.
2.
Skelton Process
- Marking trees ring width variation.
Problems:
Why not Just count rings?
Because of locally absent rings and false bands
What do tree rings tell us?
1.
To put the present into proper historical context.
2.
To better understand current environment process
and conditioning.
3. To improve understanding of possible future issues.