Teacher Guide

Instructors’ Guide
How Old Is It?
Part 2 – Magnetostratigraphy
Paleomagnetic Stratigraphy Exercise
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In this exercise, you will interpret the
polarity chrons (geomagnetic reversal
stratigraphy) for ODP Site 1237 in
the southeast Pacific, located on the
continental margin of Peru (16°0.421’S,
76°22.685’W; Figures 11 and 12). First,
notice the latitude of this site. In the
Northern Hemisphere, normal polarity is
represented by positive inclination. Since
this site is in the Southern Hemisphere,
normal polarity will be represented by
negative inclination (positive upwards,
out of the Earth). Second, well-preserved
magnetic data are either normal polarity
or reversed polarity, and there have been
many reversals during the past 30 million
years represented by the sequence
cored at Site 1237. You will need some
independent means of age control in order
to interpret the black and white stripes of
normal and reversed polarity, respectively.
Biostratigraphy based on the established
sequence of first and last occurrences
of microfossil species provides that firstorder age control.
1. Using the Site 1237 biostratigraphic
data provided (Table 1), interpret the
polarity chrons for ODP Site 1237
(Holes 1237B, 1237C, and 1237D).
The cored sequence is shown in four
separate figures: Figure 13 (upper 100
meters composite depth, mcd); Figure
14 (100-200 mcd); Figure 15 (200300 mcd); and Figure 16 (300-360
mcd). Construct your paleomagnetic
stratigraphy to the right of the data
presented in Figures 13-16. Color in
segments representing normal polarity,
and leave segments of reverse polarity
blank (white).
Polarity interpretations are shown
in Figures 16-19. The shipboard
paleomagnetists had less confidence
in the interpretation of the polarity
chrons below ~200 meters composite
depth (mcd) (C5An; ~12.4 Ma).
Compare the clarity of the magnetic
reversals in the upper 200 mcd of Site
1237 with the 200-360 mcd interval.
2. Plot an age versus depth profile based
on your paleomagnetic interpretations
(you can plot it on the same graph
as your biostratigraphic datums
from the biostrat exercise). Use the
graph paper provided. How well do
the calcareous nannofossil datums
compare with the polarity chron
boundaries?
Have students plot the age-depth
points for the upper 200 mcd of
section only. The paleomagnetic age
control points are shown in Table 2.
62
Figure F29. Inclination after demagnetization at peak alternating fields of 25 mT for the upper 100 mcd of
(A) Hole 1237B, (B) Hole 1237C, and (C) Hole 1237D with the accompanying polarity interpretations.
-30
30
90 -90
C
Hole 1237C
Inclination (°)
-30
30
90 -90
Hole 1237D
Inclination (°)
-30
30
Polarity zone
interpretation
90
C1n
1n
20
1r
1r.1n
2n
2r.1n
2r
Depth (mcd)
Matuyama
1r
40
C1r
C2n
C2r
C2An
Jaramillo
-90
0
B
2Ar
3n.2n
3n.3n
3n.3n
3r
100
Gilbert
80
C3r
C3An
C3Ar/C3Bn/
C3Br
C4n
C4Ar/C4An/
C4Ar
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
C4ACn/C5ACr/
C5ADn
Figure 16. Polarity interpretation of the upper 100 m of Site 1237 (Figure 12). From ODP Leg 202 Initial
Reports volume, Site 1237 chapter, http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm
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3n.1n
Nunivak
Sifurfall
Cochiti
3n
Thvera
2n
Mammoth
Kaena
2An.1n
60
C2Ar
C3n
Reunion
Oldavai
Hole 1237B
Inclination (°)
Brunhes
A
Gauss
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
2
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
63
Figure F30. Inclination after demagnetization at peak alternating fields of 25 mT for the 100- to 200-mcd
interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the accompanying polarity interpretations.
A
Hole 1237B
B
Inclination (°)
-90 -60 -30 0 30 60 90
100
-90
C
Hole 1237C
Inclination (°)
-30
30
90
MCD stack
Inclination (°)
-90
-30
30
Polarity zone
interpretation
90
C1n
C1r
C2n
C2r
3r
C2An
3An.1n ?
120
C2Ar
C3n
3An.2n?
3Ar
C3r
3Bn/3Br/4n
Depth (mcd)
140
160
4r
4An
C3An
C3Ar/C3Bn/
C3Br
C4n
C4Ar/C4An/
C4Ar
4Ar
180
5r
3n.3n
5An.1n
5An.2n
200
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
C4ACn/C5ACr/
C5ADn
Figure 17. Polarity interpretation of the 100-200 m interval of Site 1237 (Figure 13). From ODP
Leg 202 Initial Reports volume, Site 1237 chapter, http://www-odp.tamu.edu/publications/202_IR/
chap_08/chap_08.htm
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5n
3
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4
Depth (mcd)
300
280
260
240
220
-90 -60 -30 0 30 60 90
200
Hole 1237B
Inclination (°)
Hole 1237C
Inclination (°)
-90 -60 -30 0 30 60 90
B
Hole 1237D
Inclination (°)
-90 -60 -30 0 30 60 90
C
Stack
Inclination (°)
-90 -60 -30 0 30 60 90
D
6n
5En
5Dr
5Dn
5Cr
5Br
5Ar
6Cn
6Br
6Bn
Not interpreted
C4ACn/C5ACr/
C5ADn
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
C5Ar
C5An
C5r
C5n
C4Ar/C4An/
C4Ar
C4n
C3Ar/C3Bn/
C3Br
C3An
C3r
C3n
C2Ar
C2An
C2r
C1r
C2n
C1n
Polarity zone
interpretation
AAn
to 5ADn
5Cn
A
Figure F31. Inclination after demagnetization at peak alternating fields of 25 mT for the 200- to 300-mcd interval of (A) Hole 1237B,
1237C, (C) Hole 1237D, and (D) the stacked and smoothed record with the accompanying polarity interpretations.
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
Figure 18. Polarity interpretation of the 200 to 300 m interval of Site 1237 (Figure 14).
From ODP Leg 202 Initial Reports volume, Site 1237 chapter, http://www-odp.tamu.edu/
publications/202_IR/chap_08/chap_08.htm
65
Figure F32. Inclination after demagnetization at peak alternating fields of 25 mT for the 300- to 360-mcd
interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the accompanying polarity interpretations.
A
Hole 1237B
Inclination (°)
-90 -60 -30 0 30 60 90
300
B
-90
Hole 1237C
Inclination (°)
-30
30
C
90
MCD Stack
Inclination (°)
-90
-30
30
Polarity zone
interpretation
90
C1n
6Cn.3n
6Cr
C1r
C2n
C2r
C2An
310
7N
Depth (mcd)
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
8N
320
C2Ar
C3n
C3r
9N
C3An
330
C3Ar/C3Bn/
C3Br
C4n
350
C4Ar/C4An/
C4Ar
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
360
C4ACn/C5ACr/
C5ADn
Figure 19. Polarity interpretation of the 300 to 360 m interval of Site 1237 (Figure 15).
From ODP Leg 202 Initial Reports volume, Site 1237 chapter, http://www-odp.tamu.edu/
publications/202_IR/chap_08/chap_08.htm
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Not interpreted
340
5
A
Site 1237
Core recovery
Hole
Hole Hole
1237B 1237C 1237D
1H
2H
3H
50
5H
5H
6H
9H
14H
15H
16H
17H
18H
19H
20H
21H
22H
23H
250
24H
25H
26H
27H
28H
300
29H
30H
TM
32H
33H
34H
middle
Oligocene
early
1H
IA
2H
3H
4H
IB
11H
12H
5H
6H
7H
8H
IIA
13H
14H
15H
16H
17H
18H
19H
20H
21H
22H
23H
24H
25H
26H
27H
IIB
9H
10H
Lithologic unit boundaries
Litho subunit boundaries
Calcareous nannofossils FO
Calcareous nannofossils LO
Planktonic foraminifers FO
Planktonic foraminifers LO
Diatoms FO
Diatoms LO
Magnetostratigraphy
11H
28H
29H
30H
31H
32H
33H
B
3
Mass accumulation
rate (g/cm2/k.y.)
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31H
350
8H
10H
13H
Miocene
late
7H
10H
11H
Plio.
6H
9H
12H
200
3H
4H
8H
150
2H
4H
7H
100
1H
Pleist.
Total MAR
CaCO3 MAR
Noncarbonate MAR
Corrected LSR
2
20
10
1
0
Sedimentation
rate (m/m.y.)
0
Depth (mcd)
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
horizontal lines = lithologic unit boundaries, dashed horizontal lines = lithologic subunit boundaries.
B. Corrected linear sedimentation rates (LSR), total mass accumulation rates (MAR), and carbonate mass
accumulation rates are calculated from the smooth age model, average dry density, and calcium carbonate
concentrations at 1-m.y. intervals. FO = first occurrence, LO = last occurrence.
0
0
5
10
15
20
25
30
Age (Ma)
Figure 20. Sedimentation rate history of Site 1237 based on the microfossil data presented in Table 1 and
reversal stratigraphy presented in Figures 16 to 19. From ODP Leg 202 Initial Reports volume, Site 1237
chapter, http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm
6
How Old Is It? Part 2 – Magnetostratigraphy: Instructors’ Guide
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
Table T16. Paleomagnetic age control points based
on polarity chron boundary and polarity chron interpretation, Site 1237.
Polarity chron boundary
interpretation
Polarity chron
interpretation
C1n (b) Matuyama/Brunhes
C1r.1n (t) Jaramillo
C1r.1n (b) Jaramillo
C2n (t) Olduvai
C2n (b) Olduvai
C2r.1n (t) Reunion
C2r.1n (b) Reunion
C2An.1n (t) Gauss/Matuyama
C2An.1n (b)
C2An.2n (t)
C2An.2n (b)
C2An.3n (t)
C2An.3n (b) Gauss/Gilbert
C3n.1n (t)
C3n.1n (b)
C3n.2n (t)
C3n.2n (b)
C3n.3n (t)
C3n.3n (b)
C3n.4n (t)
C3n.4n (b)
C3r
C3An.1n
C3An.2n
C3Ar
C3Bn/C3Br/4n
C4r
C4An
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C4An (b)
C4Ar.1n (t)
C4Ar.1n (b)
C4Ar.2n (t)
C4Ar.2n (b)
C5n.1n (t)
C5n.1n (b)
C5n.2n (t)
C5n.2n (b)
C5r.1n (t)
C5r.1n (b)
C5r.2n (t)
C5r.2n (b)
C5An.1n (t)
C5An.1n (b)
C5An.2n (t)
C5An.2n (b)
Age
(Ma)
0.78
0.99
1.07
1.77
1.95
2.14
2.15
2.581
3.04
3.11
3.22
3.33
3.58
4.18
4.29
4.48
4.62
4.8
4.89
4.98
5.23
5.64
6.1
6.4
6.8
7.5
8.4
8.9
9.025
9.23
9.308
9.58
9.642
9.74
9.88
9.92
10.949
11.052
11.099
11.476
11.531
11.935
12.078
12.184
12.401
Depth
(mcd)
Error
(±m)
20.3
24.2
25.6
36.4
38.5
41.1
41.9
47.9
56.5
57.5
58.1
60.9
65.7
75.5
77.4
81.3
83.3
86.8
89.2
91.6
97.3
107
122
131
139
145
161
166
168.2
169.6
170.5
172.1
172.5
173.1
174.0
174.4
181.2
182.5
182.7
186.1
187.2
190.5
193.5
195.4
197.9
0.6
0.2
0.2
0.4
0.3
0.2
0.3
0.2
0.2
0.2
0.2
0.2
0.3
0.2
0.2
0.2
0.5
0.5
0.3
0.4
0.3
9.2
5.8
3.8
3.5
13.0
3.0
2.3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.4
Note: (b) = bottom, (t) = top.
Table 2. Paleomagnetic age control points based on polarity chron
boundary and polarity chron interpretation for Site 1237. From ODP
Leg 202 Initial Reports volume, Site 1237 chapter, http://www-odp.
tamu.edu/publications/202_IR/chap_08/chap_08.htm
7