Download INSTRUCTOR GUIDE Chapter 4 Paleomagnetism and

Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
INSTRUCTOR GUIDE
Chapter 4 Paleomagnetism and Magnetostratigraphy
SUMMARY
Reversals of our planet’s magnetic field throughout geologic time have provided Earth
scientists with a distinct ‘barcode’ record of normal and reversed polarity that is
preserved in deep-sea sediments and in oceanic crust. In Part 4.1, you will consider the
nature of Earth’s magnetic field today and explore the nature of a paleomagnetic record
preserved in deep-sea sediments. Part 4.2 explores the nature of the paleomagnetic record
preserved in ocean crust as positive and negative magnetic anomalies and relates the
anomaly pattern (‘barcode’) observed in the ocean basins to the paleomagnetic record
preserved in deep-sea sediments. In Part 4.3, you will see how paleomagnetism was used to
test the Seafloor Spreading Hypothesis. In Part 4.4, you will learn how paleomagnetism
has been used to create the geomagnetic polarity timescale of the past 160 million years
and how paleomagnetic, radiometric, and biostratigraphic data are integrated to provide
methods of age determination.
FIGURE 4.1. The Earth’s magnetic field protects our planet from much of the deadly radiation and charged
particles (solar wind) streaming away from the Sun, as depicted in this NASA diagram. This magnetosphere is
compressed on the day (Sun) side of the Earth and smeared out on the night side. Times of strong solar wind
interact with the magnetosphere near the north and south magnetic poles and are observable on Earth as aurora
(“Northern” and “Southern Lights”). Figure courtesy of NASA.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
Goal: to explore the nature of Earth’s magnetic field, the paleomagnetic record of
deep-sea cores, and its applications for age determination.
Objectives: After completing this exercise your students should be able to:
1. Describe the nature of the Earth’s magnetic field, polarity, and the relationship
between magnetic inclination and latitude.
2. Compare and contrast magnetostratigraphic records from the Northern and
Southern Hemispheres.
3. Describe the nature of magnetic reversals using evidence from marine
sediments and ocean crust.
4. Explain how paleomagnetic evidence was used to support the sea floor
spreading hypothesis.
5. Explain how the Geomagnetic Polarity Timescale (GPTS) was developed, its
value, and its organization.
6. Determine the age of a sedimentary sequence by interpreting a paleomagnetic
record and correlating it to the GPTS.
I. How Can I Use All or Parts of this Exercise in my Class?
(based on Project 2061 instructional materials design.)
Title (of each part)
How much class time will
I need? (per part)
Can this be done
independently (i.e., as
homework)?
Part 4.1
Part 4.2
Part 4.3
Part 4.4
Earth’s Magnetic
Field &
Paleomag
Record of Deep
Sea Seds
15-20 minutes
Paleomagnetism
in Oceanic Crust
Testing Seafloor
Spreading
Hypothesis
Geomagnetic
Polarity
Timescale
20 minutes
30 minutes
25 minutes
Yes, but best to
Yes, with followYes, with followdo as an in-class up discussion
up discussion
scaffolding intro
to paloemag
What content will students be introduced to in this exercise?
Science as human endeavor
X
X
X
Science as evolving
X
X
X
process/nature of science
Earth history archives (nature
X
X
X
of the sedimentary record)
Stratigraphic principles
X
X
X
Magnetism as an age
X
X
X
indicator
Geologic timescale and/or
Geomagnetic polarity
timescale
Sed accum rates, age-depth
X
plots
Calibration and correlation of
X
X
different types of data
What types of transportable skills will students practice in this exercise?
Make observations (describe
X
X
X
what you see)
Yes, with
follow-up
discussion
X
X
X
X
X
X
X
X
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Ch 4 Paleomagneticm and Magnetostratigraphy
Plot data, determine lines of
best fit, interpret graphs,
diagrams, photos, tables
Pose hypotheses or
predictions
Perform calculations &
develop quantitative skills
Decision-making, problem
solving & pattern recognition
What general prerequisite
knowledge & skills are
required?
What Anchor Exercises (or
Parts of Exercises) should
be done prior to this to
guide
student
interpretation
&
reasoning?
Instructor Guide
X
X
X
X
X
X
X
X
X
X
X
X
X
Basic geography
Basic geography
Basic math and
geography
None
Chapter 1 Intro
to Paleoclimate
records;
Chapter 2
Seafloor
Sediments
Part 4.1
Part 4.2
Parts 4.2 and
4.3; Chapter 3
Microfossils
and Biostrat.
What other resources or
materials do I need? (e.g.,
internet access to show online video; access to maps,
colored pencils)
What
student
misconception does this
exercise address?
None
None
None
None
Earth’s magnetic
field has not
been stable
through geologic
time
The seafloor does
not have black
and white stripes
What forms of data are
used in this? (e.g., graphs,
tables, photos, maps)
Bathymetric
Bathymetric
Plots of
profiles with data; map; x-y data
magnetostrat
plots
magnetostrat in
data vs. depth
deep sea drill
in deep sea drill
sites
sites
NW and SW
East Pacific and
South Atlantic
Pacific Ocean
NW Pacific
Part 1 of this exercise module is designed as an initial inquiry aimed at
drawing out student beliefs and prior knowledge. This can be very
effective as an in-class (lecture) exercise. Parts 2-4 can be done in class
but are probably best assigned as homework, or used in a laboratory
section. The various parts often lead with tasks or questions that can
further identify student prior knowledge.
Exercise Parts can be concluded by asking: On note card (with or
without name) to turn in, answer:
What did you find most
interesting/helpful in the exercise we did above? Does what we did
model scientific practice? If so, how and if not, why not?
What geographic locations
are these datasets from?
How can
I use
this
exercise to identify my
students’ prior knowledge
(i.e.,
student
misconceptions,
commonly held beliefs)?
How can I encourage
students to reflect on
what they have learned in
this exercise? [Formative
Assessment]
How can I assess student
learning
after
they
complete all or part of the
exercise?
[Summative
Assessment]
Paleomagnetic
‘barcode’ can
be used to
build a
geologic time
scale
Geomagnetic
Polarity Time
Scale;
magnetostrat
in drill sites
NW Pacific
See suggestions in Summative Assessment section below.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Where can I go to for
more information on the
science in this exercise?
Instructor Guide
See the Supplemental Materials and Reference sections below.
II. Annotated Student Worksheets (i.e., the ANSWER KEY)
This section includes the annotated copy of the student worksheets with answers for each Part
of this chapter. This instructor guide contains the same sections as the student book chapter,
but also includes additional information such as: useful tips, discussion points, notes on
places where students might get stuck, what specific points students should come
away with from an exercise so as to be prepared for further work, as well as ideas
and/or material for mini-lectures.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
Part 4.1. Earth’s Magnetic Field Today and the Paleomagnetic
Record of Deep-Sea Sediments
1 Do the following to describe the Earth’s magnetic field as you currently understand it:
(a) The circle below represents the Earth. Draw a sketch of the Earth and its magnetic field
by putting in the magnetic lines of force; label the equator and the North and South poles.
http://earthsci.org/education/teacher/basicgeol/platec/platec.html
You can ask for a volunteer to go to the board or overhead projector and draw a sketch of the
Earth with its magnetic field. Then ask if anyone wants to add to, or modify the first
volunteer’s interpretation/representation. You can expect a range of possibilities, but below is
a sketch that can help you focus a classroom discussion. The class is likely to grasp the
concept of dipolar field with north and south magnetic poles. But two other important
concepts: 1) the Earth’s magnetic field lies close to the spin/rotational axis of Earth, and 2)
magnetic lines of force strike the Earth at different angles (= magnetic inclination or magnetic
dip); this latter concept will be explored more below in Question 2.
(b) Do the magnetic lines of force intersect the Earth’s surface everywhere at the same
angle?
No. The magnetic lines of force intersect the Earth’s surface at various angles.
inclination at the equator is 0º and at the poles it is 90º.
The
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
EARTH’S MAGNETIC FIELD
Figure 4.1 depicts the Earth’s magnetosphere and Figure 4.2 depicts the behavior of iron
filings in the presence of a magnetic field (you may remember doing this little experiment in
elementary or middle school). Note the general similarity in the shape of the magnetic field in
these two diagrams. The Earth’s magnetic field approximates a dipole bar magnet and is
generated within the liquid outer core owing to our planet’s rotation about an axis defined
by the North and South geographic (true) poles. The magnetic poles are close to Earth’s
rotational axis (i.e., the magnetic field approximates the spin axis of the Earth), but today the
magnetic north pole and magnetic south pole are offset from true North and South by
approximately 11°. The Earth’s magnetic field is dynamic and complex and the magnetic
poles are not stationary. In 2005, the north magnetic pole was at 82.7°N, 114.4°W, moving
northwest at approximately 40 km/yr, while the south magnetic pole (2001) was at 64.7°S,
138.0°E (NOAA National Geophysical Center, http://www.ngdc.noaa.gov/geomag/, see also
“Frequently Asked Questions on the NGDC site”). A compass points to the magnetic north
pole in response to the magnetic lines of force generated by our magnetic field, similar to the
bar magnet and iron filings (Figure 4.2). Likewise, magnetic minerals in sediments and
rocks will behave like tiny compass needles that align with the Earth’s magnetic field at the
time they are deposited (in the case of sediments), or crystallized as molten magma cools (in
the case of igneous rocks). In other words, magnetic minerals in sediments and rocks
become locked in and therefore capture and preserve the nature of the magnetic
field at that particular location for that particular time in Earth history. The study of
the geologic record of Earth’s magnetic field and its changes is called Paleomagnetism.
FIGURE 4.2. Bar magnet with iron filings showing magnetic lines of force. From
http://www.askamathematician.com/?p = 4129 - Here is another link - http://geekbeat.tv/ferrofluids/magneticfield-lines-around-a-bar-magnet-2/.
2 Inclination (or magnetic dip) is the angle between the orientation of a magnetic mineral
grain in a rock or in sediments and the (horizontal) surface of the Earth at a particular
location. Based on the similarity in behavior of the Earth’s magnetic field and the magnetic
lines of force generated by a bar magnet, predict the following:
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
(a) Where on Earth’s surface would you expect magnetic minerals found in sediments and
igneous rocks to have very little or no inclination? Why?
Little or no inclination: near the equator, where the magnetic lines of force are roughly
parallel to the surface of the Earth.
(b) Where on Earth’s surface would you expect the magnetic minerals to have the steepest
inclination? Why?
Steepest inclination: near the (magnetic) poles, as observed in the bar magnet analogy.
See diagram from 1(a) above.
3 If the inclination is into the Earth at or near the magnetic north pole, what is the nature of
the inclination at or near the south magnetic pole? Explain.
Today, the magnetic lines of force are going into the Earth near the magnetic north pole
(= positive values), therefore they must be coming out of the Earth near the magnetic
south pole (= negative values). This idea of inclination into and out of Earth will become
relevant in the questions and discussion that follow regarding the preserved record of
magnetic field reversals.
4 Drawing from your answers in Questions 2 and 3 above, and your general knowledge of
plate tectonics, predict how the inclination (magnetic dip) preserved in older rocks might be
used to determine past lithospheric plate positions?
Inclination is a function of latitude (tan I = 2 tanλ, where I = inclination, and λ = latitude
at the time of rock formation; solve for λ), therefore past plate positions in a north-south
reference frame can be inferred, assuming a dipolar field similar to today’s field.
Natural Remanent Magnetization in Sediments and Igneous Rocks
The magnetic signal that geoscientists are interested in measuring is called natural
remanent magnetization (NRM). This signal is carried by magnetic minerals such as
magnetite. The NRM is preserved in ocean crust (basalt) as it cools and crystallizes past the
Curie temperature (approximately 580°C for the mineral magnetite), or in sediments as
detrital magnetic mineral grains (eroded from another source) accumulate on the seafloor. In
both situations, the magnetic minerals become aligned with the Earth’s magnetic field at the
time of crystallization and deposition, respectively. Not all rocks are good carriers of a
magnetic signal. The basalt that makes up oceanic crust is a magnetite-bearing rock, while
granite, a typical igneous rock of continental crust, has a generally poor content of magnetic
minerals. Likewise, terrigenous sediments (e.g., sand, mud, and clay) derived from the
erosion of continental rocks have a much higher concentration of detrital magnetic mineral
grains than does a pure biogenic sediment, such as calcareous or siliceous ooze.
Figures 4.3(a) and (b) depict the magnetic character of two deep-sea sediment cores. Site
1149 (Figure 4.3a) is located in the northwest Pacific (approximately 31°N latitude)
near the Marianas Trench. Site 1172 (Figure 4.3b) is located in the southwest Pacific
(approximately 44°S latitude) near Tasmania (Figure 4.4). Both sedimentary sequences
represent continuous sediment deposition over the past 6.5 million years or so. The x-axis
shows inclination (magnetic dip); positive inclination values are into the Earth, negative
values are out of the Earth. The y-axis is depth in the core in meters below seafloor
(mbsf).
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
Remind students that in this exercise they are examining the paleomagnetic signal
preserved in sediments cored in the deep-sea; the youngest sediments are at the top (0
= present day seafloor), with sediment age increasing with increasing depth in the core. Each
of these records represents just part of the sedimentary sequence cored at these two sites.
Also please note: these plots come straight from their sources; they have not been redrafted
here. This emulates how a scientist goes to the primary literature and finds that different
authors present their data in different ways (e.g., style of graph, or scale used, etc.).
5 Make a list of observations about the nature of the paleomagnetic record in these two
deep-sea sediment sequences. How are they similar and how are they different?
Observations:
• Earth’s magnetic field has reversed polarity many times in the past 6.5 Ma
• These changes in polarity, called reversals, are very rapid (i.e., change in inclination
from positive values to negative values, or vise versa).
• The two sites are mirror images of each other. In other words, both sites reveal a
similar pattern of reversals, but in an opposite sense in the Northern and Southern
Hemisphere.
• The inclination angle is steeper at Site 1172 (the site at higher latitude, therefore
inclination is expected to be greater).
• If the pattern at the two sites records the same reversals, then the sedimentation
accumulation rates of the two sites are different.
FIGURE 4.3. Two paleomagnetic records from deep-sea
sediments cored in the Pacific Ocean. Magnetic inclination
(degrees; °) is plotted against depth in hole (meters below
seafloor; mbsf). (a) ODP Hole 1149A is located southeast of
Japan near the north end of the Marianas Trench
(31°20.52′N, 143°21.07′E). From Shipboard Scientific Party,
2000. (b) ODP Hole 1172B is located east of Tasmania
(43°57.57′S, 149°55.70′E). From Shipboard Scientific Party,
2001a.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
Similarities
Differences
There have been many reversals
of polarity at both sites during
the past 6.5 Ma
Inclination values are greater at
Site 1172
Reversals of polarity appear
very rapid geologicially speaking
The topmost polarity at both
sites is opposite; at Hole 1149A
(left) the inclination values are
positive, at Hole 1172B (right)
the values are negative
Same pattern of polarity
reversals, except that they are a
mirror image
Page 9 of 40
Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
FIGURE 4.4. World map showing all Ocean Drilling Program sites (1985 to 2003; Legs 100-203). Black boxes
show locations of Site 1149 (North Pacific) and Site 1172 (South Pacific). Blue box shows location of Site 1208
(Figure 4.11). Courtesy of IPDP.
6 How do your observations of the modern sediments (those closest to the seafloor and at
the shallowest depths) at these two sites compare with the predictions that you made for
Questions 2 and 3?
Inclinations are positive (into the Earth) at the Northern Hemisphere Site 1149 in the most
recent sediments, and inclinations are negative (out of the Earth) at the Southern Hemisphere
Site 1172. Also important to notice the steeper inclination values at the higher latitude site
1172 off Tasmania.
7 Assuming that magnetic reversals are geologically instantaneous, the sediments at
approximately 25–33 mbsf at Site 1149 were deposited at the same time as what interval
at Site 1172? (By doing this you are essentially correlating an interval at Site 1149 with an
age equivalent interval at Site 1172, just as paleomagnetists do!)
~12-17 mbsf; thus, sediments at 25-33 mbsf at Site 1149 are same age as those at 12-17
mbsf at Sitel 1172
8 (a) What is the average inclination of this interval (25-33 mbsf) at Site 1149?
-50º
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
(b) What is the average inclination of this interval (12-17 mbsf) at Site 1172?
+65º
(c) How does the average inclination of this interval at each site compare with the average
inclination of the “modern” sediments (those at the top of these cores) at each site?
1149 modern inclination = +50º
1172 modern inclination = -65º
It has the same inclination values, but with the opposite polarity.
reversed.
The magnetic field
(d) Do these observations support or refute the notion that the Earth’s magnetic north and
south poles generally approximate the geographic (true) north and south poles over geologic
time?
Support. Explain. The Earth’s magnetic field at the two intervals is reversed (opposite sign)
from the nature of the field today indicating that magnetic north during this time was located
near geographic south.
NOTE: This series of questions also illustrates how we can use paleomagnetism
(magnetostratigraphy) to correlate from one site to another. In addition, the differences
in depth in the core and differences in thickness in the interval between the two sites
illustrates that the rate of sediment accumulation is not the same at the two sites.
PALEOMAGNETIC REVERSALS (REVERSALS OF MAGNETIC POLARITY
The Earth’s magnetic field episodically reverses polarity. Prior to a reversal of the magnetic
field, the field fluctuates and becomes weaker and there may be multiple poles for a short
time. The process is geologically rapid, taking several thousand years for the field to
completely reverse polarity and stabilize again (Figure 4.5). Today’s field is referred to as
“normal polarity”. The last reversal of the magnetic field occurred approximately 780,000
years ago. Refer to http://www.ngdc.noaa.gov/geomag/ for additional information about
Earth’s magnetic field (see also “Frequently asked questions”).
between reversals
during a reversal
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
FIGURE 4.5. Earth’s magnetic field simulated with a supercomputer model of the geodynamo by G.A. Glatzmaier
and P.H. Roberts, University of California (es.ucsc.edu/~glatz/geodynamo). Yellow magnetic field lines are directed
outward and blue magnetic field lines are directed inward. The stable normal dipolar magnetic field is shown in
the picture on the left between reversals. The picture on the right during a reversal provides an idea of the
complicated, but rapid nature of a reversal of magnetic polarity. Note: the field does not go away during a
reversal.
PALEOMAGNETISM IN SEDIMENT CORES
Paleomagnetic records (i.e., the ancient or fossil magnetic signal preserved in sedimentary
layers; Figure 4.3) are measured in deep-sea sediments cored by the Ocean Drilling Program.
These data were collected using a cryogenic magnetometer (Figure 4.6). Sediments cored
from the seafloor are passed through the magnetometer and the inclination, or magnetic
dip, of the magnetic minerals is measured, as well as the magnetic intensity. In Figure
4.3, the data are plotted with the x-axis showing magnetic inclination (in degrees of
inclination, or dip, from the horizontal) and the y-axis as depth in the drill hole (in meters
seafloor, mbsf). See http://www-odp.tamu.edu/sciops/labs/pmag/ (or
http://www.odplegacy.org/operations/labs/paleomagnetism/) for a detailed discussion of how
shipboard paleomagnetic data are collected.
FIGURE 4.6. Cryogenic magnetometer aboard the JOIDES Resolution drillship. The 2G 760-R superconducting
rock (cryogenic) magnetometer is used primarily for continuous measurements of magnetic properties on 1.5-m
long core halves. The wide silver cylinder shields the magnetometer from the present day magnetic field. The
photo shows the tray where the core half is placed and the opening into the magnetometer. Photo courtesy of
IODP.
Part 4.2. Paleomagnetism in Ocean Crust
Figure 4.7 shows two transects of ship-towed magnetometer data across the East Pacific
Rise, a volcanically active ridge in the eastern Pacific (Pitman and Heirtzler, 1966). Each
transect displays two types of data: magnetic intensity (in gammas) and bathymetry
(water depth, in kilometers) plotted on the y-axes, and distance (in km) on either side of the
ocean ridge plotted on the x-axes. The ridge crest at 0 km is the axis of the ocean ridge. 1
and 1′, 2 and 2′, and so on correspond to distinctive measurements of the magnetic character
of seafloor rocks (called “positive magnetic anomalies”) on either side of the ridge.
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Ch 4 Paleomagneticm and Magnetostratigraphy
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See next page.
FIGURE 4.7. Two transects (numbers 21 and 19) of magnetometer data collected by the research vessel
Eltanin. The East Pacific Rise is the crest (bathymetric high) in both transects. Stronger magnetic intensity
values are called positive magnetic anomalies; several examples on either side of the ridge are number 1–4
and 1′–4′, respectively. From Pitman and Heirtzler, 1966.
1 (a) What common features do you see in the magnetic intensity data from these two
transects across the East Pacific Rise (Figure 4.7)?
The same distinctive pattern of positive magnetic anomalies is seen in both transects. In
addition, there is a mirror-image symmetry in the magnetic intensity pattern on each side of
the ridge with increasing distance from the ridge axis (0 km).
(b) Read the box below on seafloor magnetic anomalies. What do paleomagnetic records
from such places as the East Pacific Rise (Figure 4.7) tell us about the nature of Earth’s
magnetic field preserved in oceanic crust?
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Ch 4 Paleomagneticm and Magnetostratigraphy
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Suggests that reversals of the magnetic field are preserved in the crust, as well as in
sediments.
SEAFLOOR MAGNETIC ANOMALIES
Figure 4.8 depicts a paleomagnetic record of ocean crust across a volcanically active ocean
ridge. A ship towing a magnetometer records signals of stronger (positive magnetic
anomalies) and weaker (negative magnetic anomalies) magnetic intensities. What the
magnetometer measures is the magnetic character of the oceanic crust, consisting of the
igneous rock called basalt (which is relatively enriched in magnetic minerals such as
magnetite), as well as the much weaker overlying sedimentary cover and its detrital
magnetic minerals. Positive magnetic anomalies correspond to ocean crust that has
normal polarity, like today (today’s field plus the ancient normal field result in a stronger
signal); negative magnetic anomalies correspond to ocean crust with reversed polarity
(today’s field plus the ancient reversed field result in a weaker signal). The magnetic
anomalies are mapped as magnetic “stripes”, which depict an alternating pattern of
positive and negative magnetic anomalies preserved in the ocean crust.
FIGURE 4.8. An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite
well by a calculated profile (red) based on the Earth’s magnetic reversals for the past 4 million years and an
assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The
remarkable similarity of these two profiles provided one of the decisive arguments in support of the seafloor
spreading hypothesis. From http://library.thinkquest.org/18282/lesson4.html. Reproduced from USGS.
Figure 4.9 shows an interpretation of the pattern of positive and negative magnetic
anomalies measured by the ship-towed magnetometer on either side of East Pacific Rise and
a corresponding geomagnetic polarity time scale (Figure 4.10). Polarity epochs
represent times of dominantly normal or dominantly reversed polarity. This “barcode”
pattern of polarity changes led to the development of the geomagnetic polarity time scale.
See next page.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
FIGURE 4.9. Magnetic profiles depicting positive and negative magnetic anomalies across the East Pacific
Rise at latitude 52°S, longitude 118°W (notice the similarity to magnetic profiles in Figure 4.7). The heavy arrow
shows the location of the crest of the East Pacific Rise (i.e., spreading center = zero-age crust). 1 and 1′ and 2
and 2′ correspond to distinctive positive anomalies. M-B, G-M, G-G = boundaries between the Bruhnes,
Matuyama, Gauss, and Gilbert polarity epochs. From Cox, 1969.
2 Describe the nature of the magnetic record preserved in the ocean crust of the East Pacific
Rise shown in Figure 4.9 by answering the questions below:
(a) What pattern do you see in magnetic anomaly data preserved at the crest of the ocean
ridge?
The pattern of magnetic anomalies is symmetrical about the spreading center.
In other words, we hope that the students will observe that each side of the ocean ridge has a
distinct pattern of anomalies (positive and negative excursions of magnetic field intensity),
and that one side of the ridge is a mirror image of the other side. If the students don’t pick
up on this observation themselves, point out that the ridge axis is the site of present-day
spreading and the point of symmetry for the magnetic anomalies on the flanks of the
spreading center.
(b) How does this pattern of magnetic anomaly data relate to distance from the crest of the
ocean ridge?
The pattern refers to the general peak or trough shape of the positive and negative
anomalies. The pattern is not cyclical or predictable, but is distinctly random or episodic
with increasing distance from the ridge crest. In addition, this same pattern is observed on
both sides of the ridge. Polarity (normal or reversed) represented by each anomaly is the
same on either side of ridge with increasing distance from ridge; i.e., the pattern is
symmetrical on each side of the ridge axis.
(c) Provide an explanation for the observed pattern of magnetic anomalies on either side of
the ocean ridge.
The students should notice that the anomaly closest to the ridge axis is positive, and
corresponds with our current ‘normal’ polarity. Normal polarity (like today) is shown in
black, and ‘reversed’ polarity is shown in white. The important teachable point here is that
the spreading centers are the sites of ocean crust production. As new crust is added at
the ridge, the previously formed crust moves away from the spreading axis like a conveyor
belt. Ocean crust cools and subsides as it moves away from the hot active spreading center.
This is why the spreading centers/oceanic ridges stand high relative to the abyssal plains on
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
their flanks. ‘Seafloor spreading’ results in the symmetrical (mirror image) pattern of
anomalies on either side of the ridge.
Lastly, again emphasize that the Earth’s magnetic field has reversed polarity episodically
many, many times throughout our planet’s history.
THE GEOMAGNETIC POLARITY TIME SCALE
Figure 4.10 shows the first geomagnetic polarity time scale (GPTS), which was derived from
continental lava flows with paleomagnetic records from numerous locations (Cox, 1969).
These lava flows and the polarity reversals that they contained were radiometrically dated.
This compilation demonstrated the potential of using magnetic reversals as a means of
building a geologic time scale. More details on the GPTS are explored in Part 4.4.
to
FIGURE 4.10. Time scale for geomagnetic reversals. Normal
polarity intervals are shown by the black portions and correspond
positive magnetic anomalies on the seafloor; reversed polarity
intervals are shown by the white portions and correspond to
negative polarity intervals. Note that the Bruhnes Epoch is
normal polarity, the Matuyama Epoch is dominantly reversed
polarity, the Gauss Epoch is dominantly normal polarity and the
Gilbert Epoch is dominantly reversed polarity. From Cox, 1969.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
Bruhnes
~42.5 mbsf
Matuyama
Matuyama
~120 mbsf
Gauss
FIGURE 4.11. Paleomagnetic record preserved in the upper 155 m of Hole 1208A, located east of Japan on
Shatsky Rise (36°7.6301′N, 158°12.0952′E). From Shipboard Scientific Party, 2002.
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3 Figure 4.11 depicts the paleomagnetic record from ODP Site 1208 in the northwest Pacific
Ocean (site location shown in Figure 4.4).
(a) How do the paleomagnetic data preserved in the vertical sequence of deep-sea
sediments cored at ODP Site 1208 (Figure 4.11) compare with the horizontal pattern of
seafloor magnetic anomaly data from the eastern Pacific shown in Figures 4.7 & 4.9? List the
similarities and differences.
Similarities
Differences
Changes in magnetic field
are rapid in both sediments
and ocean crust.
Sediment
paleomagnetic
data (fig. 4.11) are shown as
inclination (in
degrees
positive
or
negative),
whereas seafloor data are
shown as intensity (in
gammas)
On close inspection, the
pattern of reversals is
very
similar
(more
resolution in the sediment
record).
The sediment record
is
displayed vertically with
increasing depth into the
seafloor (in m), whereas the
seafloor data are shown
horizontally with increasing
distance from the ridge axis
(in km).
There is greater detail
(resolution)
in
the
sediment record compared
with the seafloor record
(b) The Bruhnes-Matuyama and Matuyama-Gauss boundaries are defined in Figures 4.9 &
4.10. At what depths would you place these boundaries in Site 1208?
Bruhnes-Matuyama ~ 42.5 mbsf
Matuyama-Gauss ~ 120 mbsf
The pattern of positive values (‘normal’ polarity) and negative values (‘reversed’ polarity) in
the Site 1208 sedimentary sequence bears striking resemblance to marine magnetic anomaly
patterns shown and discussed above. For example, the Bruhnes-Matuyama boundary occurs
at ~43 mbsf at Site 1208, and the Matuyama-Gauss boundary occurs at ~120 mbsf.
The pattern is similar, but the ocean crust anomalies are shown horizontally because they are
produced as new ocean crust is formed at the spreading center then moves like a conveyor
belt away from the ridge axis, while the sediment reversals are shown vertically because they
are deposited in this manner by gravity.
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(c) What assumptions did you have to make to assign these two paleomagnetic boundaries to
specific depths at Site 1208?
Since figures 4.9 and 4.10 depict ocean spreading centers, we know that the crest is where
new ocean crust is formed. As we have learned, modern deposits in the Northern Hemisphere
yield a positive magnetic anomaly. Site 1208 (Figure 4.11) is not a spreading center, but
rather a place where sediments accumulate due to the rain down of material. The shallower
deposits will therefore be the youngest, and as a result will reflect the present day positive
magnetic anomaly. Since the Bruhnes-Matuyama boundary occurs from the transition of a
negative anomaly to the present day positive anomaly (as seen in figure 4.9 and 4.10), we
know that the B-H boundary has to be at about 43 mbsf in figure 4.11 where we see such a
transition. From that point, we can match the pattern of magnetic reversals in figures 4.9,
4.10 with that in figure 4.11 to determine where to place the G-M boundary.
(d) Using the geomagnetic polarity time scale shown in Figure 4.10, what is the age of the
sedimentary sequence at 150 mbsf?
~ 3.1 Ma; The two short-lived reversals below the Matuyama/Gauss boundary in the Site
1208 data (Figure 4.11) correlate with the Kaena Event and Mammouth Event within the
Gauss Normal Epoch (Figure 4.10). The base of the Mammouth Event at Site 1208 is at 150
mbsf, which correlates to ~3.1 Ma on the Geomagnetic Polarity Time Scale shown in Figure
4.10.
4 Reflecting on the paleomagnetic data you have worked with in this exercise, explain how
paleomagnetic records preserved in sedimentary sequences and/or ocean crust can be used
to construct a geologic time scale (e.g., Figure 4.10). In your response be sure to include a
description of the assumptions you would need to make, as well as reference to other data
that would be needed to support the construction of this time scale.
The sequence of magnetic reversals preserved in ocean crust as it forms, and polarity
changes preserved in sediments as they accumulate on the seafloor are natural tape
recorders of unique events in Earth history. This ‘tape’ or ‘barcode’ record could form the
basis for a geologic time scale.
What do you need to know? We need to be able to date the magnetic anomalies. Ocean crust
can be dated radiometrically by measuring the ratio of radiogenic isotopes to their stable
isotopic decay products in minerals that constitute the basalt. Some of the marine magnetic
anomalies have been calibrated by datable volcanic ash-fall deposits in marine sedimentary
sequences with good microfossil biostratigraphy and magnetostratigraphy. Both of these
methods provide an ‘absolute’ age for the anomalies. Correlation between radiometricallydated ocean crust and its contained magnetic anomalies, and fossiliferous marine sediments
and its preserved magnetostratigraphy provides a means of dating fossil first and last
occurrences. The integration of magnetostratigraphy and biostratigraphy provides the basis of
the Geomagnetic Polarity Time Scale (discussed in more detail in Part 4 of this module).
Assumptions? The main assumptions are that there are no breaks (unconformities) in
sedimentary sequences, and that we can accurately identify all the marine magnetic
anomalies. Identification of polarity events and chrons in marine sediments is accomplished in
close cooperation with microfossil biostratigraphy.
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Part 4.3. Using Paleomagnetism to Test the Seafloor Spreading
Hypothesis
In 1969, Deep Sea Drilling Project (DSDP) Leg 3 set out to test the Seafloor Spreading
Hypothesis. The drillship Glomar Challenger cored nine sites in the South Atlantic, seven to
the west of the Mid-Atlantic Ridge and two to the east of the ridge (Figure 4.12).
FIGURE 4.12. Map of the South Atlantic showing the
cruise path and sites drilled during DSDP Leg 3. Notice
that Sites 14-16 and 19-22 were drilled to the west of
the Mid-Atlantic Ridge and Sites 17 and 18 were
drilled to the east of the ridge. From Maxwell et al.,
1970.
crust (“basement”) at each of the drill sites
the sediment overlying ocean crust at the
(from Maxwell et al., 1970). The distance
(linear) value, as well as a distance along a
spherical surface.
Table 4.1 summarizes the age of ocean
based on the paleomagnetic age, plus the age of
bottom of each drill site based on fossil evidence
from the ridge axis is given as a straight-line
curve, which is based on an axis of rotation on a
Note: DSDP Sites 17 and 18 are on the east flank of the Mid-Atlantic Ridge, while the other
sites are on the west flank and adjoining seafloor.
1 What two types of data are used to determine age of each location on the seafloor? See
Table 4.1.
The marine anomaly pattern on the seafloor at the location of each drill site is compared
(correlated) with the Geomagnetic Polarity Time Scale that was available at the time (1970).
In addition, a paleontological age assignment is based on the microfossils that directly overlie
the basalt at each of the sites. Paleontological ages are also correlated with the Geomagnetic
Polarity Time Scale.
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TABLE 4.1. Estimates for the age of ocean crust (basement) at the DSDP Leg 3 drill sites and distance of each
site from the Mid-Atlantic Ridge axis. From Maxwell et al., 1970.
Site No.
Magnetic age of
basement (million
years)
Paleontological age
sediment above basement
(million years)
Distance from ridge axis (km)
Linear
Rotation at 62°N, 36°W
16
9
11±1
191±5
211±20
15
21
24 ±1
380±10
422±20
26 ±1
506±0
506±20
18
17
34–38
33±2
643±20
718±20
14
38–39
40±1.5
727±10
745±10
19
53
49±1
990±10
1010±10
20
70–72
67±1
1270±20
1303±10
≥76
1617±20
1686±10
21
2 On the graph paper provided, plot paleomagnetic age of basement (oceanic crust) vs.
distance from the ridge axis using solid circles. On the same graph, plot the
paleontological age of sediment overlying basement vs. distance from the ridge axis
using open circles. Draw a visual best-fit line through the data points.
See plots on next page. First is hand drawn as students might do and second is the
age-distance plot as shown in DSDP Volume 3.
NOTE: If you don’t specify the x and y axes, you will find that some students will put km on
the x-axis and others age. This can result in a classroom discussion about the pros and cons
of both ways of presenting the data. Placing distance from the ridge on the x-axis depicts the
ocean floor as a horizontal surface, which may seem more intuitive (as seen above from the
original DSDP volume), but it yields a graphical slope (y = mx + b) that is inverse of the
spreading rate. Placing distance on the y-axis and age on the x-axis, on the other hand,
results in a graphical slope value that represents the spreading rate.
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3 What is the relationship between the age of basement and distance from ridge axis?
Basement rocks increase in age with increasing distance from the ridge. In other words, there
is a direct relationship between distance from the ridge axis and age of the oceanic crust.
4 What is the relationship between the age of sediment overlying basement and distance
from ridge axis?
The sediment (and its contained microfossils) sitting directly on the basalt at each site
increases in age with increasing distance from the ridge. So, like basement age, there is a
direct relationship between distance from the ridge axis and age of the sediments in contact
with the underlying oceanic crust.
The basalt that makes up oceanic crust basement crystallized at the ridge axis as volcanic
lava cooled. This is also the time when the ocean crust acquired its magnetic character. Over
time, the ocean crust at each of the drill sites has moved away from the ridge axis with slow
conveyor-like motion to its present geographic location on the adjacent seafloor.
5 Hypothesize why the ages of sediment immediately overlying the basement (i.e., the
basalt) also show increasing age from the ridge axis:
Pelagic sediment (microfossils, silt, and clays that have settled out of suspension) can only
accumulate on the oceanic crust (basalt) once the basalt actually exists. In other words, the
basalt has to first form at the ridge before the sediment can accumulate on it. As the
sediment-draped basalt moves away from the ridge due to seafloor spreading, sediment
continues to accumulate on the moving seafloor so that the sediment section typically gets
thicker and thicker with increasing ocean crust age and distance from the ridge. In addition,
the oldest (basal) sediment overlying the basalt is older the further the drill site is now from
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the ridge crest because the basalt is also older with increasing distance from the spreading
center.
6 The average rate of seafloor spreading in the South Atlantic can be calculated from the
data provided in Table 4.1 and in the graphs you made. In preparation for this calculation, do
the following:
• In the space below sketch a profile across the Mid-Atlantic Ridge in the South Atlantic
depicting the relationships of distance and age that you plotted.
• Label the spreading axis with its age.
• Provide units on your profile (distance in km, age in myr)
• Based on this sketch, write a formula to calculate the average spreading rate.
Students can think of this rate term as being analogous to the velocity of their car in
miles/hour or km/hour. Simply stated: distance (d) divided by time (t): d/t. Therefore,
spreading rate = kilometres per million years (km/myr) (or centimetres per year; cm/yr).
7 Use your graph with best-fit line through the data points (Question 2 above) to calculate
seafloor spreading rates:
(a) What was the average rate of seafloor spreading on the western side of the Mid-Atlantic
Ridge in km/myr (kilometers per million years)? Show your work.
In the plot above, an approximate best-fit line through the data (simple rise/run) reveals a
slope of 1200 km/65 myr, or 18.46 km/myr.
(b) What was the average rate of seafloor spreading in cm/yr (centimeters per year)? Show
your work.
Spreading rates are more typically described in units of cm/yr, so, an exercise of unit
conversion is helpful here:
18.46 km/myr x 1 myr/106 yr x 103 m/1 km x 102 cm/1 m = 1.846 cm/yr
This number reflects the spreading rate on one side of the ridge, the so-called half
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spreading rate.
8 Calculate how fast the South Atlantic Ocean is widening (i.e., opening). Show your work.
Half-spreading rate x 2 = 1.846 cm/yr x 2 = 3.692 cm/yr, which is the full
spreading rate.
THE SEAFLOOR SPREADING HYPOTHESIS
In 1960, Professor Harry Hess of Princeton University advanced the hypothesis that new
ocean crust is produced at the mid-ocean ridges such as the Mid-Atlantic Ridge and the
East Pacific Rise. The youngest ocean crust should lie in the axis of each volcanically active
ocean ridge and the oldest ocean crust should lie farthest from the ridge axis. According to his
hypothesis, the seafloor moves laterally away from these spreading centers and explains
why North and South America have moved away from Europe and Africa over time. This
became known as the seafloor spreading hypothesis.
By the early 1960s, abundant paleomagnetic data from ship-towed magnetometers had
demonstrated the existence of alternating positive (normal polarity) and negative (reversed
polarity) magnetic anomalies on the seafloor (e.g., Vine and Matthews, 1963). When
mapped, they reveal a pattern of magnetic “stripes” that form a symmetrical pattern on
either side of a spreading center (i.e., the pattern on one side is a mirror image of the pattern
on the other side). For example, Figure 4.13 is a seafloor map showing magnetic anomalies
on the Mid-Atlantic Ridge south of Iceland.
FIGURE 4.13. Map of magnetic “stripes” on the seafloor south of Iceland. The straight lines mark the ridge axis
and the central positive magnetic anomaly. From Vine, 1966.
9 How could these magnetic “stripes” be used to test the hypothesis of seafloor spreading?
Make a list of observations based on the patterns of magnetic stripes that would be useful for
evaluating this hypothesis.
Recognizing the mirror-image symmetry of the anomalies on either side of the mid-ocean
ridges was a critical observation. The students should note the linearity of the anomalies,
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which parallel the ridge axis. The pattern of anomalies can be compared to the anomalies
preserved in other ocean basins or spreading centers.
10 What additional data would you like to collect in order to test the seafloor spreading
hypothesis?
Additional data needed would include age of the individual magnetic anomalies. This could be
accomplished by radiometric dating of oceanic crust. Marine magnetic anomalies can also be
calibrated by datable volcanic ash-fall deposits in marine sedimentary sequences with good
microfossil biostratigraphy and magnetostratigraphy. For example, the Joides Resolution could
sail to the location of one of these magnetic anomalies, core through the sediments to the
underlying ocean crust, and then sample the basalt for radiometric analysis. The microfossils
in the sediments overlying the basalt will also provide some relative age information. Both of
these methods provide an ‘absolute’ age for the anomalies. Correlation between
radiometrically-dated ocean crust and its contained magnetic anomalies, and fossiliferous
marine sediments and its preserved magnetostratigraphy provides a means of dating fossil
first and last occurrences. The integration of magnetostratigraphy and biostratigraphy
provides the basis of the Geomagnetic Polarity Time Scale (discussed in more detail in
Part 4 of this module).
Part 4.4. The Geomagnetic Polarity Time Scale
The Geomagnetic Polarity Time Scale (GPTS) is a composite geomagnetic polarity sequence
that has been calibrated with radiometric age dates. The Late Cretaceous and Cenozoic (83 to
0 Ma) GPTS is based on an analysis of marine magnetic profiles from the Atlantic, Pacific, and
Indian ocean basins (Cande and Kent, 1992, 1995). The marine magnetic anomaly sequence
recorded in the South Atlantic Ocean serves as the principal polarity sequence for this interval
(Heirtzler et al., 1968; Cande and Kent, 1992). Radiometric dating of volcanic ash fall
deposits (bentonites) in land-based and marine sections with reliable biostratigraphic and
magnetostratigraphic control provides the numerical calibration for most of the anomaly
pattern (paleomagnetic ‘barcode’). An orbitally-tuned age provides the calibration age of the
Gauss/Matuyama boundary (Figure 4.14). Nine age calibration points, plus the zero-age ridge
axis, provide the age control on the Late Cretaceous-Cenozoic GPTS (Cande and Kent, 1992).
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FIGURE 4.14. GPTS for the past 5 myr. Ages (in Ma) of the chron and subchron boundaries are shown as well as
the names of the chrons and subchron events. From Mankinen and Wentworth, 2003.
Figure 4.15 shows a portion of the geomagnetic polarity time scale (GPTS). The GPTS is
divided into magnetic epochs, which are themselves divided into chrons. Distinctive normal
polarity events occur within the chrons. The suffix “n” after the anomaly number refers to
normal polarity and the “r” refers to reversed polarity.
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FIGURE 4.15. Portion of the geomagnetic polarity time scale used during ODP Leg 191. From Shipboard Scientific
Party, 2001b.
In the investigation that follows, you will use the GPTS to make an interpretation of the
paleomagnetic data recovered from ODP Site 1208 in the northwest Pacific (Figure 4.4). In
other words, you will correlate the Site 1208 data to the GPTS (Figure 4.15).
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1 Interpret the paleomagnetic record of the upper 300 m of sediments cored at ODP Site
1208 (Figure 4.16) by correlating with the GPTS shown in Figure 4.15. Use the blank columns
on the right of each panel of paleomagnetic inclination data to interpret normal polarity
(color in) or reversed polarity (leave blank/white). As much as possible, determine the
magnetic chrons and the polarity events and label these on your polarity interpretation.
Assume that the top of the sequence at 0 mbsf is the top of the Brunhes Magnetic Epoch (=
Chron C1n). Note that there are missing data at 225–230, 235–238, and 273–277 mbsf.
NOTE: Interpreted magnetostratigraphy of ODP Site 1208, from the Leg 198 Initial Reports
volume (http://www-odp.tamu.edu/publications/198_IR/chap_04/chap_04.htm). Note that
the image from the URL has a typo in the depth scale of the right panel; instead of 50, 100,
and 150 those values should be 200, 250, and 300 mbsf. It has been corrected here. The
gray areas are intervals of uncertain polarity due to unrecovered section or poor data quality.
See next page.
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Figure 4.16. Paleomagnetic data from ODP Hole 1208A. Left graph: 0–155 mbsf; right graph: 155–305 mbsf.
From Shipboard Scientific Party, 2002.
2 What assumption(s) must you make in order to correlate the Site 1208 paleomagnetic
data with the GPTS?
That the sediment section is continuous and complete (i.e., no breaks in the section; no
unconformities). Physical sedimentology, biostratigraphy, physical properties, and sediment
geochemistry can all provide clues about the presence of an unconformity in a sedimentary
section.
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3 What is the approximate age of the sedimentary sequence at 150 and 300 mbsf (meters
below seafloor)? What paleomagnetic chrons do these levels correlate with?
Referring to the Magnetic Epoch column (figure 4.15):
150 mbsf
300 mbsf
Age
~3.33 Ma
~12.775 Ma
Chrons
C2An
C5Ar.2n
Notice that the age at 150 mbsf is older than the ~3.1 Ma age determined in Part 4.2,
question 3d. This is because the geomagnetic polarity time scale used for that question is
based on the time scale from 1970 (Figure 4.10). There have been improvements in the
geomagnetic polarity time scale since then.
4 As a summary of what you have learned in this chapter, list several ways that magnetic
reversal data can be useful to Earth scientists.
Magnetostratigraphy is very valuable for establishing the age of sedimentary sequences
provided there is additional first-order age information such as biostratigraphy to initially
calibrate the normal-reversed polarity pattern. Highly resolved magnetostratigraphic records
like Site 1208 (or Sites 1149 and 1172 examined in Part 4.1) provide reliable correlation to
other sites around the world. High-resolution magnetostratigraphic records also provide
valuable age control for calculation of rates, such as sediment accumulation rates (see below;
from http://www-odp.tamu.edu/publications/198_IR/chap_04/chap_04.htm).
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Integrating Paleomagnetic and Biostratigraphic Age Control Data
THE GEOMAGNETIC POLARITY TIME SCALE (GPTS)
Figure 4.17 shows the record of Earth’s magnetic polarity changes back approximately
160 Ma compiled from seafloor magnetic anomalies and uplifted marine sequences.
Black=normal polarity, white=reversed polarity. The left plot is from Lowrie (1982). The
right plot is a composite record of seafloor magnetic anomalies that extends back to the top
of the ‘Cretaceous Long Normal’, approximately 83.0 Ma, compiled by Cande and Kent
(1995). This is the currently accepted geomagnetic polarity time scale or GPTS for the
later part of the Cretaceous Period and Cenozoic Era (Gradstein et al., 2004). Nine
radiometric age tie points, plus the zero-age ridge axis, are used to calibrate this part of
the time scale (ages for the marine magnetic anomalies between the calibration points are
interpolated by a cubic spline function; Cande and Kent, 1992; Berggren et al., 1995).
For the Late Cretaceous to present, the prominent positive magnetic anomalies (i.e., those
corresponding to time intervals, or chrons, of normal geomagnetic polarity) have been
numbered from Chron 1n in the central axis of the spreading centers to Chron 34n at the
younger end of the Cretaceous Long Normal (also called the Cretaceous Quiet Zone; 83.0
Ma). Many of the younger chrons are divided into shorter polarity intervals, or subchrons.
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The GPTS based on marine magnetic anomalies extends back through the late Middle Jurassic
(Callovian stage, approximately 162 Ma; e.g., Kent and Gradstein, 1986), while land-based
polarity records go back through the Triassic (e.g., Gradstein et al., 1995). A nomenclature
called the “M-sequence” is used for marine magnetic polarity chrons of the Early Cretaceous
to late Middle Jurassic extending from Chron M0r at the base the Cretaceous Long Normal
(Chron 34n; base of the Aptian stage in the Early Cretaceous, approximately 121 Ma) to
Chron M39 in the Jurassic (Callovian stage).
5 Make a list of observations about the characteristics of the paleomagnetic record for the
past 160 million years based on Figure 4.17 (next page).
Students will notice that the polarity changes are random. They will also notice that the
frequency of reversals has been increasing from the Late Cretaceous through the Paleogene
and through the Neogene. The randomness of the ‘barcode’ is a major reason why the
paleomagnetic record is such a powerful tool for correlation and chronostratigraphy.
6 Have magnetic reversals occurred in a regular or cyclic fashion (i.e., periodic), or have the
reversals occurred in an irregular or random fashion (i.e., episodic)?
Students should recognize the random or episodic nature of polarity changes. Explain. This
randomness yields a record of reversals that resembles a familiar barcode. The distinct
pattern is useful for correlation, but it may also provide important insights into the long-term
nature of Earth’s magnetic field. For example, the ‘Cretaceous Long Normal’ was initiated
~120 Ma and is contemporaneous with the emplacement of the Large Igneous Province
preserved in the western equatorial Pacific as Ontong Java Plateau. This feature formed
from a mantle plume. Such deep-seated mantle plumes may be related to thermal
discontinuities at or near the Core-Mantle boundary, which are suspected to have somehow
disrupted Earth’s magnetic field. Hopefully the students will also recognize the increasing
frequency of reversals since ~83.5 Ma in the Late Cretaceous.
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FIGURE 4.17. Geomagnetic polarity time scale. Plot on left shows magnetic polarity changes back to the Middle
Jurassic (nearly 160 Ma; from Lowrie, 1982). Note the long interval in the Cretaceous Period with no magnetic
reversals; this is called the “Cretaceous Long Normal” chron. The plot on the right shows the details of the marine
anomaly record of the South Atlantic (from Cande and Kent, 1995). Red lines show how the South Atlantic
anomaly record correlates with the longer Lowrie (1982) record.
Since the late 1960s, the geomagnetic polarity time scale has been extended further back
into the Cenozoic and Mesozoic Eras, refined with greater resolution (i.e., recognition of
subchrons and polarity events; Figure 4.15) and calibrated with radiometric age dates. In
addition, biostratigraphic datum levels (first occurrences or bases and last occurrences or
tops) have been integrated (correlated) with the geomagnetic polarity time scale (Figure
4.18). In doing so, microfossil datum levels have become primary indicators of
geologic age in deep-sea sediments. Recall from Chapter 3 that biostratigraphic datum
levels define the boundaries of biostratigraphic zones, or biozones.
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FIGURE 4.18. Portion of
geomagnetic polarity
time scale from 0 to 25
Ma, including geologic
epochs and ages (see time
scale inside front cover of
book), and showing
integration with
microfossil biozones for
planktic forams (N zones)
and calcareous
nannofossils (CN zones).
Microfossil datum levels
are shown with ages in
parentheses. B = base
or first occurrence;
T = top or last
occurrence. Shipboard
Scientific Party, 2002,
Explanatory notes.
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7 Drawing from your work in Chapters 3 and 4, explain how biostratigraphic datum levels can
be dated using the geomagnetic polarity time scale. In other words, how can ages be
assigned to microfossil first occurrences (bases) and last occurrences (tops) as
illustrated in Figure 4.18?
It’s based on relative age dating.
The age of oceanic crust can be determined by radiometric dating. Any sedimentary material
deposited on top of this will have a relative age to the oceanic crust. Spreading centers
capture magnetic field normal and reverse polarities that can all be dated using radiometric
dating. Because microfossils are deposited shortly after the crust is created, wherever
scientists find first or last occurrences of the microfossil, they can determine a relative age of
their occurrence in relation to the radiometric age of the crust. Also, depending on which
polarity interval the microfossil is found, one can correlate its age to whichever chron it lies
in.
8 Reconsider Question 2 (Part 4.4) which asked about the assumption(s) you needed to
make in order to correlate the Site 1208 paleomagnetic data with the GPTS. How could you
test your assumption(s) using microfossils?
The relative age of the microfossils at Site 1208 can be used to correlate Site 1208 with the
GPTS.
III. Summative Assessment
The following questions could be used to assess student learning after completing this
exercise:
1. The Earth has a magnetic field that protects our planet from dangerous cosmic energy.
What is the nature of the magnetic lines of force relative to the surface of the Earth?
(A sketch may be helpful in answering this question.)
a. Magnetic lines of force are parallel to the Earth’s surface at all latitudes
b. Magnetic lines of force are perpendicular to the Earth’s surface at all latitudes
c. Magnetic lines of force are parallel at the equator
d. Magnetic lines of force are perpendicular near the poles (Arctic and Antarctic)
e. C and D only
2. Earth’s magnetic field has episodically (randomly) and rapidly switched polarity. Such
magnetic reversals have occurred throughout Earth history. These ancient magnetic lines
of force were "frozen" (recorded) into igneous and sedimentary rocks. How might the
orientation of these "paleomagnetic" lines of force be useful to geoscientists?
a. They can help determine age
b. They can help determine longitude
c. The can help determine latitude
d. All of the above
e. A and C only
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3. Examine the magnetic inclination record (figure on right)
measured from a sedimentary sequence in the North Pacific
Ocean. At what depth in the core was a polarity change
(magnetic field reversal) recorded?
a. at 7 mbsf
b. at 20 mbsf
c. at 27 mbsf
d. at 39 mbsf
4. The Earth’s magnetic field
a. has maintained the same polarity throughout Earth’s history
b. has reversed polarity over regularly spaced intervals of
time, about every 50 million years.
c. has reversed polarity many times, but these reversal
do not occur in any regularly spaced intervals of time.
5. Which of the following statements about interpreting
paleomagnetic data is true?
a. Rocks of the same core depth and latitude in the North
Atlantic should show the same change in paleomagnetic
inclination.
b. Rocks of the same age and latitude in the North
Atlantic should show the same change in paleomagnetic
inclination.
c. Rocks of the same age and longitude in the South Pacific
should show the same change in paleomagnetic inclination
d. All of the above
Short Answer:
6. How is the Earth’s magnetic field like a bar magnet?
7. What is Declination? Inclination?
8. How would a freely moving compass needle (or a magnetic mineral) align with the
magnetic field if it was at the equator? At the north pole? The south pole? 40˚S? 40˚N?
9. How does inclination relate to latitude at the time of rock formation? Does it have any
relationship to longitude at the time of rock formation?
10.What types of materials record the Earth’s magnetic signature?
11.What is Natural Remanent Magnetism (NRM)?
12.What is magnetic intensity? How does magnetic intensity change perpendicular to a midocean ridge? How do these changes relate to NRM?
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
13.What is a magnetic reversal? Are these fast or slow?
Global events or regional? Periodic (repeated at a
regular interval) or random in time?
14.How are paleomagnetic records for the same time
period from the northern and southern hemisphere
similar and how would they be different?
15.What is the process for determining age using
paleomagnetic data? What assumptions must be
made?
16.Why is it important to integrate paleomagnetic data
with both radiometric age data and biostratigraphic
data?
17.Examine the figure (to the right) showing a segment
the magnetic polarity timescale. Note that “normal”
polarity (i.e., like today’s magnetic field) is depicted
black, and “reversed” polarity is depicted as white.
What was the dominant polarity in the middle
Pliocene?
of
as
IV. Supplemental Materials
•
•
•
•
•
•
•
To learn more about the Earth’s magnetic field go to the “frequently asked questions
page”
on
the
National
Geophysical
Data
Center
site
http://www.ngdc.noaa.gov/geomag/.
To view a model of the convection and magnetic field generation in the fluid outer core
surrounding
the
solid
inner
core
go
to:
http://www.es.ucsc.edu/~glatz/geodynamo.html.
In 2003, NOVA produced a 1-hour public television show, Magnetic Storms, which
explores the generation of Earth’s magnetic field, ancient and recent changes in Earth’s
magnetic field, and its role in protecting Earth from cosmic radiation:
http://www.pbs.org/wgbh/nova/magnetic/about.html.
To take a panoramic virtual tour of the paleomagnetic lab on board the JOIDES
Resolution go to: http://iodp.tamu.edu/labs/ship/Cdeck_paleomagnetism.html.
For a nice review of our current understanding about the unstable nature of Earth’s
magnetic field: Gubbins, D., 2008. Geomagnetic reversals. Nature, 452 (13 March
2008):165-167.
To learn more about magnetic anomalies in the crust:
http://denali.gsfc.nasa.gov/research/crustal_mag/
For a video tour of the paleomagnetic laboratory aboard the JOIDES Resolution go to:
http://www.schooltube.com/video/b5c68583947cbf2567a1/
and select tour the Cryogenic Magnetometer Lab w/ Klayton Curtis.
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Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
V. References
Berggren, et al., 1995, Late Neogene chronology; new perspectives in high-resolution
stratigraphy. Geological Society of America Bulletin, 107(11), 1272–87.
Cande, S.C. and Kent, D.V., 1992, A new geomagnetic polarity time scale for the Late
Cretaceous and Cenozoic: J. Geophys. Res., vol. 97(B10), p.13, 917–13, 951.
Cande, S.C. and Kent, D.V., 1995, Revised calibration of the geomagnetic polarity timescale
for the Late Cretaceous and Cenozoic. Journal of Geophysical Research, 100(B4), 6093–5.
Cox, A., 1969, Geomagnetic reversals. Science, 163, 239–45.
Gradstein, F.M., et al., 1995, A Triassic, Jurassic and Cretaceous time scale. In
Geochronology, Time Scales and Global Stratigraphic Correlation, Berggren, W.A., et al.
(eds), Special Publication of the Society of Economic Paleontologists and Mineralogists, vol.
54, pp. 95–126.
Gradstein, F.M., Ogg, J.G. and Smith, A.G. (eds), 2004, A Geologic Time Scale 2004.
Cambridge University Press, Cambridge, UK, 589 pp.
Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pitman III, W.W., and LePichon, X., 1968, Marine
magnetic anomalies, geomagnetic field reversals, and motions of the ocean floor and
continents: J. Geophys. Res., vol. 73, p. 2119–2136.
Kent, D. and Gradstein, F.M., 1986, A Jurassic to Recent geochronology. The Western North
Atlantic Region: The Geology of North America, vol. M, Geological Society of America,
Boulder, CO, pp. 45–50.
Lowrie, W., 1982, A revised magnetic polarity timescale for the Cretaceous and Cainozoic.
Philosophical Transactions Royal Society of London, 306, 129–36.
Mankinen, E., and Wentworth, C., 2003, Preliminary paleomagnetic results from the Coyote
Creek outdoor classroom drill hole, Santa Clara Valley, California, USGS Open-File Report
03-187, 32p. http://pubs.usgs.gov/of/2003/of03-187/of03-187.pdf.
Maxwell, A.E., et al., 1970, Initial Reports of the Deep Sea Drilling Project, vol. 3, US
Government
Printing
Office,
Washington,
DC.
http://www.deepseadrilling.org/03/dsdp_toc.htm
Pitman, W.C., III and Heirtzler, J.R., 1966, Magnetic anomalies over the Pacific-Antarctic
Ridge. Science, 154, 1164–71.
Vine, F.J., 1966, Spreading of the ocean floor: New evidence. Science, 154, 1405–15.
Vine, F.J., and Matthews, D.H., 1963, Magnetic anomalies over oceanic ridges: Nature, vol.
199, p. 947–949.
Shipboard Scientific Party, 2000. Site 1149. In Proceedings ODP, Initial Reports, vol. 185,
Plank, T., et al. (eds), College Station, TX, Ocean Drilling Program. http://wwwodp.tamu.edu/publications/185_IR/chap_04/chap_04.htm
Shipboard Scientific Party, 2001a. Site 1172. In Proceedings ODP, Initial Reports, vol. 189,
Exon, N.F., et al. (eds), College Station, TX, Ocean Drilling Program. http://wwwodp.tamu.edu/publications/189_IR/chap_07/chap_07.htm
Shipboard Scientific Party, 2001b. Explanatory notes. In Proceedings ODP, Initial Reports, vol.
191, Kanazawa, T., et al. (eds), College Station, TX, Ocean Drilling Program. http://wwwPage 39 of 40
Ch 4 Paleomagneticm and Magnetostratigraphy
Instructor Guide
odp.tamu.edu/publications/191_IR/chap_02/chap_02.htm
Shipboard Scientific Party, 2002, Explanatory Notes. In Proceedings ODP, Initial Reports of
the Ocean Drilling Program, vol. 198, Bralower, T.J., et al., College Station, TX, Ocean
Drilling
Program.
pp.
1–63.
doi:10.2973/odp.proc.ir.198.102.2002.
http://wwwodp.tamu.edu/publications/198_IR/198TOC.HTM
Shipboard Scientific Party, 2002. Site 1208. In Proceedings ODP, Initial Reports, vol. 198,
Bralower, T.J., et al. (eds), College Station, TX, Ocean Drilling Program. http://wwwodp.tamu.edu/publications/198_IR/chap_04/chap_04.htm
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